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International audience; Highlights:This paper reviews ways to improve water use efficiency beyond the plant level, across time and space scales, from cells, organs, and plants, to field, farm, and landscape.Abstract:Water scarcity will be one of the main issues of the 21 st century, because of competing needs between civil, industrial, and agriculture use. While agriculture is the largest user of water, its share is bound to decrease as societies develop. Clearly, agriculture needs to become more water efficient. Improving water use efficiency (WUE) at the plant level is important although there is a long way into translating this at the farm/landscape level. As we move up from a cell/organ/plant scale to more integrated scales such as plot, field, farm system, and landscape, other factors need to be considered, including trade-offs, to possibly improve WUE. These include choices of crop variety/species, farm management, landscape design, infrastructure development, ecosystem functions, where human decisions matter. This review is a cross-disciplinary attempt to analyze ways to address WUE at these different scales where metrics of analysis are defined and trade-offs considered. The equations in this perspective paper use similar metrics across scales for an easier connection and are developed to highlight which levers, at different scales, can improve WUE. We also refer to models operating at these different scales to assess WUE. While our entry point is plants and crops, we scale up the analysis of WUE to farm systems and landscapes.

Sustainable field crop management has been considered to reach the food security issue due to global warming and water scarcity. The effect of deficit irrigation and nitrogen rates on quinoa yield is a challenging issue in those areas. In this regard, the interaction effects of different N rates (0, 125, 250, and 375 kg N ha−1) and irrigation regimes [full irrigation (FI) and deficit irrigation at 0.75 FI and 0.5 FI] on quinoa yield and water and nitrogen efficiencies were evaluated with a two-year field experiment. Increasing nitrogen fertilizer application levels from 250 to 375 kg N ha−1 under FI and deficit irrigation did not cause a significant difference in seed yield and the total dry matter of quinoa. Furthermore, 20% and 34% reductions were observed for nitrogen use efficiency (NUE) and nitrogen yield efficiency with the application of 375 kg N ha−1 compared with that obtained in 250 kg N ha−1 nitrogen fertilizer, respectively. Therefore, a Nitrogen application rate of 250 kg ha−1 and applying 0.75 FI is suggested as the optimum rate to reach the highest seed water use efficiency (0.7 kg m−3) and NUE (0.28 kg m−3) to gain 4.12 Mg ha−1 quinoa seed yield. Under non–limited water resource conditions, an FI and N application rate of 375 kg ha−1 could be used for higher seed yield; however, under water-deficit regimes, an N application rate of 250 kg ha−1 could be adequate. However, questions about which environmental factors impressively restricted the quinoa growth for optimizing the potential yield need further investigation.

Declining water tables and shrinking irrigation supply are two major challenges for agricultural production that increase production costs of high delta crops in Pakistan. Crop producers may tackle these threats by optimizing the use of available resources. To devise policies for improved resource use and cost reduction, assessing current efficiency levels and their determinants can provide useful information. This research assessed the technical, allocative, and groundwater-irrigation efficiency of smallholder sugarcane farms in Faisalabad, Pakistan using primary data for 56 tubewell solo owners, 37 tubewell shareholders, and 47 water buyers and applying the data envelopment analysis technique. The factor affecting the allocative efficiency were explored using the tobit regression. We found that the groundwater-irrigation efficiency ranged from 92% for the solo owners, 92% for shareholders, and to 83% for water buyers. On average, technical efficiency at sugarcane farms was more than 90%. Solo owners, shareholders, and water buyers achieved only 40%, 28%, and 37% of allocative efficiency, respectively. By improving the allocative efficiency through optimal allocation of inputs, farms can considerably decrease their costs for a given output level. The allocative efficiency was negatively associated with farmers' age, farm size, tubewell depth, discharge rate and salinity in groundwater. Our findings imply that sugarcane farms can substantially reduce their production costs by installing smaller tubewells that match their crop irrigation requirement and groundwater depth. Further, improving the allocative efficiency of relatively large sugarcane farms, monitoring and regulating groundwater extraction to prevent further increase in groundwater and irrigation and soil management practices to reduce salinity can improve the efficiency.

Viticulture covers a large part of arable lands in the Mediterranean regions. However, maintaining this emblematic crop in such drought-prone environments is critically challenged by climate change. Exploring large genetic collections can help to breed cultivars with enhanced water use efficiency. Leaf stomata, which are tiny epidermal pores delineated by guard cells that can open and close, play a pivotal role in this process as they regulate gas exchange – photosynthesis and transpiration – between the plant and the atmosphere. Due to stomatal behaviour, enhanced photosynthesis generally coincides with enhanced transpiration. Accounting for this process, lowering transpiration at night has been recently proposed as a promising breeding strategy to reduced water loss without penalty on photosynthesis. However, daytime and nighttime transpiration are interrelated to some extent. Here, we explored the genetic and physiological bases of diel (24-h) dynamics of transpiration in grapevine (Vitis vinifera). A pipeline was developed for high-throughput phenotyping of transpiration dynamics on detached leaves. This enabled genome wide association studies (GWAS) on a diversity panel made of 279 cultivars, which generated candidate genes that were further addressed by different approaches. Quantitative genetic analyses revealed several single nucleotide polymorphisms (SNPs) associated with different aspects of the diel transpiration dynamics. Notably, we found an association for nighttime transpiration that co-localized with the abundance of prostrate trichomes and cuticular nanoridges on the leaf epidermis. A MYB transcription factor was identified as a good candidate involved in the production of both structures, showing a complex interplay with stomatal development. Based on genomic and coexpression analyses, we propose a new model of interacting genes involved in the development of epidermal structures and impacting nighttime transpiration. Unexpectedly, we also found an association for daytime transpiration that was close to a gene that is essential for nighttime starch metabolism. The corresponding mutant in Arabidopsis thaliana was impaired in light-induced stomatal opening and endogenous nighttime reopening. Comparison with other mutants of the pathway revealed that the role of this gene in controlling stomatal movements extends beyond starch metabolism. Its cell-autonomous action was addressed, without unequivocal conclusion. Overall, this work provided several targets of interest to save water at night and favouring higher water use efficiency in grapevine., La viticulture couvre une grande partie des terres arables dans les régions méditerranéennes. Cependant, le maintien de cette culture emblématique dans des environnements sujets à la sécheresse est menacé par le changement climatique. L'exploration de larges collections génétiques peut aider à sélectionner des cultivars plus efficaces en termes d'utilisation de l'eau. Les stomates, de minuscules pores sur l’épiderme des feuilles délimités par des cellules de garde pouvant s'ouvrir et se fermer, jouent un rôle essentiel dans ce processus car ils régulent les échanges gazeux – photosynthèse et transpiration – entre la plante et l'atmosphère. En raison du comportement stomatique, une photosynthèse accrue coïncide généralement avec une transpiration accrue. Ainsi, réduire la transpiration nocturne apparait comme une stratégie de sélection prometteuse pour réduire les pertes en eau sans pénaliser la photosynthèse. Cependant, la transpiration diurne et nocturne sont corrélés dans une certaine mesure. Ici, nous avons exploré les bases génétiques et physiologiques de la dynamique journalière (24 h) de la transpiration chez la vigne (Vitis vinifera). Un protocole a été développé pour le phénotypage à haut débit de la dynamique de la transpiration sur feuilles détachées. Cela a permis de réaliser des études d'association pangénomique (GWAS) sur un panel de diversité composé de 279 cultivars, ayant généré des gènes candidats qui ont ensuite été étudiés par différentes approches. Des analyses génétiques quantitatives ont révélé plusieurs polymorphismes nucléotidiques uniques (SNP) associés à différents aspects de la dynamique de la transpiration. Nous avons notamment trouvé une association pour la transpiration nocturne qui co-localise avec l'abondance de trichomes couchés et de stries cuticulaires sur l'épiderme des feuilles. Un facteur de transcription MYB a été identifié comme un candidat probant impliqué dans la production des deux structures, montrant une interaction complexe avec le développement stomatique. Sur la base d'analyses génomiques et de coexpression, nous proposons un nouveau modèle de gènes en interaction, impliqués dans le développement des structures épidermiques et ayant un impact sur la transpiration nocturne. De manière inattendue, nous avons également trouvé une association pour la transpiration diurne proche d'un gène essentiel au métabolisme nocturne de l'amidon. Le mutant correspondant chez Arabidopsis thaliana était altéré dans l'ouverture stomatique à la lumière et la réouverture nocturne endogène. La comparaison avec d'autres mutants de la voie a révélé que le rôle de ce gène dans le contrôle des mouvements stomatiques s'étend au-delà du métabolisme de l'amidon. Son action cellule-autonome a été étudiée, avec des conclusions équivoques. Dans l'ensemble, ces travaux ont fourni plusieurs cibles intéressantes pour économiser l'eau la nuit et favoriser une meilleure efficience d'utilisation de l'eau chez la vigne.

Climate change is expected to increase the frequency of droughts in most tropical regions in the coming decades. A passive phenomenon called hydraulic redistribution (HR) allows some plant species to take up water from deep and wet soil layers and redistribute it in the upper dry layers where other plants and soil biota can benefit from it. In addition, soil fertilization, particularly potassium (K), may also affect drought-adaptive mechanisms and increase water use efficiency (WUE) on poor and acidic tropical soils. The present study aimed at quantifying the role of HR and K fertilization on both wood productivity and WUE for stemwood production (WUEp) of Eucalyptus grandis plantations in Brazil under ambient and reduced (−37%) throughfall conditions. Tree transpiration was measured using trunk sap flow sensors over 21 months, and HR was estimated from the reverse sap flow (RF) observed in shallow roots over 18 months. Tree biomass, hydraulic conductance, soil water storage from surface to the water table (down to 17 m), and leaf photosynthetic capacity were also assessed. Significant HR was detected over the whole year, even during the rainy seasons. Neither potassium fertilization nor throughfall exclusion affected the velocity of water transported by HR, probably because most trees reached water table. Nonetheless, some photosynthetic capacity parameters, including the maximum photosynthetic rate (Amax), increased in treatments with K addition. This higher Amax combined with an increased sapwood area index, was associated with an increase in water uptake by 30 %–50 % and WUEp by 300 % relative to K-deficient trees. We postulate that the increase in WUEp promoted by potassium fertilization was partly driven by an increase in biomass allocation to wood, at the expense of foraging organs (leaves and roots), because K addition alleviated constraints on light and water use. Our results indicate that fertilizing E. grandis plantations with K is beneficial to both wood biomass production and WUEp.

Taro (Colocasia esculenta (L.) Schott) is a substantial staple food in most of the tropical regions. Prolonged exposure to drought impairs crop production worldwide. Tolerant crops have the best capability to cope and avoid drought, through phenotypic flexibility mechanisms. The water use efficiency (WUE) is well known in taro crops, but very scarce information is available relating to their nutrient efficiency (NER) in drought conditions. Our work provided pertinent information about the physiological variation of seven taro accessions subjected to seven months of drought, by recording the differences for nutrient allocation, chlorophyll canopy, biomass loss, and stress intensity. Significant relationships between control and drought treatments on WUE (+85%), total plant biomass (TPB, -26.8%), chlorophyll content index (CCI, +1.8%), and nutrient harvest index (NHI, +0.2%) were detected. Drought led to a generalized loss of TPB as drought avoidance strategy, although distinct phenotypic flexibility was observed through the root:shoot ratio (R:S) and stress index (SI) from the corm and shoot organs. The nutrient allocation from the corms to shoots, with NER increase registered in drought conditions, can be a valuable tool to complement the TPB and WUE productivity traits, to be used in taro breeding programs.

Key Message : Needle anatomic plasticity enables fast-growing maritime pine trees to cope with water limitations by enhancing both water-use efficiency and embolism resistance.Abstract : Maritime pine is a major forest tree grown for wood production in Southern Europe. A breeding program for the selection of fast-growing varieties was established in the 1960s, in France. In the context of climate change, the magnitude of phenotypic plasticity is a key issue for the prediction of putative decreases in productivity in the improved genetic material. We characterized the phenotypic plasticity of anatomical and physiological traits in two families with contrasting growth rates, under different water regimes, at the juvenile stage. An analysis of 38 traits showed that the fastest growing family had the greatest phenotypic plasticity for morphological, anatomic, chemical and physiological traits, enabling it to increase its water use efficiency and embolism resistance in response to water deficit. The observed modifications to the extravascular (proportion of spongy parenchyma in needles) and vascular (xylem in the needles and stem) compartments in response to water constraints were consistent with a higher water use efficiency and greater embolism resistance. The ability to optimize meresis and auxesis according to environmental conditions during needle development could be related to growth performance over time in different environmental conditions. These results suggest that selection for growth in maritime pine leads to the selection of individuals with greater phenotypic plasticity related to higher performances in non-limited conditions.

International audience; The selection of ideotypes combining high biomass production, high water use efficiency (WUE) and adequate wood properties is a major challenge in improving forest trees for industrial plantations. This issue was addressed in a field experiment evaluating 1130 clones of Eucalyptus urophylla * Eucalyptus grandis. Genomic selection using 3303 SNPs and the GBLUP model was carried out at age 55 months to select genitors and clones for stem volume (V55), WUE (using stable carbon isotope composition in stemwood, δ13C as a proxy, transformed into intrinsic WUE, Wi), as well as lignin (KL) and holocellulose (HCEL) contents. The variance components were mainly additive for δ13C(Wi), KL and HCEL, while they were highly non-additive for V55 (the ratio of dominance to additive variance was 130%). The narrow sense heritability was higher for Wi (h2 = 0.704) than for volume and wood traits (

The availability of water and nutrients are two main factors limiting tree growth and forest productivity. Despite the strong legacy of research on forest nutrition, the role of nutrients in modulating tree drought responses remains largely unknown. This thesis aimed to evaluate the role of nutrient availability on key traits related to water and carbon relations such as growth and its determinants, water-use efficiency (WUE), vulnerability to cavitation, or non-structural carbohydrate (NSC) contents, in poplars under contrasting drought intensities. Research experiments relied on two complementary experimental designs, one in a greenhouse and the other in an outdoor common garden, and considered a maximum of four genotypes primarily selected for their contrasting growth and stomatal regulation. The main findings are as follows: 1) a higher nutrient availability increases growth and WUE under moderate drought, 2) a higher nutrient availability does not necessarily increase the risk of xylem hydraulic failure, 3) NSC depletion, in particular starch, co-occurs with massive xylem embolism under lethal drought, and 4) a higher nutrient availability reduces NSC storage, especially under moderate drought. This study shows for the first time the temporal covariance between the dynamics of xylem hydraulic failure and carbon starvation in a fast-growing but highly vulnerable tree species, and gives an example of how these relationships are modulated by nutrient availability.; La disponibilité en eau et en nutriments sont deux facteurs limitant la croissance et la productivité des forêts. Malgré un historique important de travaux sur le rôle physiologique des principaux éléments minéraux, l’impact du statut nutritif des arbres sur leur capacité de réponse à la sécheresse demeure très largement inconnu. Cette thèse visait à évaluer le rôle de la disponibilité en nutriments sur des caractères clés indicateurs du fonctionnement hydrique et carboné comme la croissance et ses déterminants architecturaux, l’efficience d’utilisation de l’eau (WUE), la vulnérabilité à la cavitation ou encore les teneurs en glucides non structuraux, en réponse à des sécheresses d’intensités variées chez le peuplier. Deux dispositifs expérimentaux complémentaires ont été support des expériences, l’un en serre et l’autre en pépinière, et les travaux se sont concentrés sur un maximum de quatre génotypes sélectionnés pour leur comportement contrasté en termes de croissance et de régulation stomatique. Les résultats majeurs sont les suivants : 1) une disponibilité en nutriments plus importante augmente la croissance et WUE en conditions hydriques limitantes, 2) une disponibilité en nutriments plus importante n’augmente pas nécessairement les risques de dysfonctionnements hydrauliques, 3) l’épuisement des réserves carbonées, notamment de l’amidon, coïncide avec l’embolie massive du xylème à mortalité en cas de sécheresse sévère, et 4) une plus grande disponibilité en azote diminue la mise en réserve glucidique, notamment en condition de sécheresse modérée. Cette étude illustre pour la première fois de façon explicite la covariance au cours du temps entre les dysfonctionnements hydrauliques et l’épuisement des réserves chez une essence à croissance rapide très vulnérable à la sécheresse, et donne un exemple de la façon dont ces relations sont modulées par la disponibilité en nutriments.

The number of drought events is expected to increase in intensity and frequency as a result of climate change. Since poplar productivity is closely linked to water availability, there is an increasing risk of decline in wood production from poplar plantations. Optimization of the ratio of biomass production to water used (i.e. water use efficiency, WUE) appears therefore as a relevant target for poplar research. Previous studies have shown the clonal diversity of WUE in poplar is driven mainly by stomatal conductance (gs). However gs and photosynthesis are not always tightly coupled which can result in large variations of WUE at leaf level. Additionally, because transpiration efficiency (TE) is laborious to measure, experiments are often conducted in pots in glasshouses. However in controlled conditions the environment is widely different than in the field and comparisons of WUE in controlled and field conditions are scarce in the literature. We assessed the diversity of stomatal dynamics among poplar genotypes under control or drought conditions grown in a glasshouse and in the field. We investigated the link between physiological, morphological and molecular factors and stomatal dynamics, and their influence on TE. Furthermore, we examined the relation between different estimators of WUE and its components between controlled and field conditions. Element content and candidate gene expression in the guard cells were also quantified at two times during the day to analyze their link to stomatal conductance. We found among the four genotypes studied significant genotypic variability of stomatal dynamics to irradiance and VPD which was altered by drought and growing conditions. Stomatal size and density as well as water use, but not WUE, were correlated to stomatal dynamics, emphasizing the importance and complexity of such mechanisms at the whole plant scale. Good agreements between leaf-level and whole-plant WUE among genotypes and between growing conditions were also found. Finally, distinct guard cell element contents and candidate gene expression, between leaf sides and time of day, linked with stomatal conductance draw attention to the diversity of components contributing to TE. These findings provides valuable information to better understand the diverse, sometimes unsuspected, leaf-level mechanisms driving water use efficiency at the whole plant scale.; Il est prévu une augmentation de l’intensité et de la fréquence des sécheresses dans les années à venir à cause des changements climatiques. Puisque la productivité des peupliers est étroitement liée à la disponibilité en eau, il existe un risque de déclin de la production de bois dans les peupleraies. L’optimisation de la biomasse produite en regard de l’eau consommée (efficience d’utilisation de l’eau, WUE) apparaît alors être une question de recherche prometteuse. Des études précédentes ont montré une diversité clonale de WUE chez les peupliers, pilotée principalement par la conductance stomatique (gs). Cependant, gs et l’assimilation en CO2 ne sont pas toujours connectés, ce qui peut conduire à de fortes variations de WUE au niveau foliaire. De plus, puisque la mesure de l’efficience d’utilisation de l’eau au niveau de la plante entière (TE) est laborieuse à mesurer, les expérimentations sont souvent réalisées en serre. Toutefois, les conditions contrôlées d’une serre conduisent à un environnement très différent des conditions naturelles, et les comparaisons de WUE entre conditions contrôlées et naturelles sont rares dans la littérature. Nous avons évalué la diversité des dynamiques stomatiques au sein de génotypes de peupliers sous conditions témoins et sous sécheresse en serre et en pépinière. Nous avons examiné le lien entre différents facteurs physiologiques, morphologiques et moléculaires et les dynamiques stomatiques, ainsi que leur influence sur TE. De plus, nous avons étudié la relation entre différents estimateurs de WUE et ses composantes entre des conditions contrôlées et naturelles. Le contenu en éléments minéraux et l’expression de gènes candidats ont également été quantifiés à deux moments de la journée pour analyser leur relation avec gs. Nous avons observé une variabilité génotypique significative des dynamiques stomatiques à la fois en réponse à l’irradiance et au VPD, de plus modifiée par la sécheresse et les conditions de croissance. La taille et la densité des stomates ainsi que la transpiration foliaire étaient fortement corrélées aux dynamiques stomatiques en serre, mais très peu en pépinière. Ces résultats soulignent l’importance et la complexité de ces mécanismes à l’échelle de la plante entière. WUE au niveau de la feuille et de la plante entière étaient relativement stables au sein des génotypes et entre conditions de croissance, mais bien moins avec la sécheresse. Enfin, des contenus en éléments et des expressions géniques distinctes ont été observées entre faces de la feuille et enter moments de la journée, en lien avec gs. Ces résultats fournissent de précieuses informations pour mieux comprendre les divers mécanismes foliaires pilotant WUE au niveau de la plante entière.

Poplar plantations, widely used for the production of woody biomass, might be at high risk from the climate change-induced increase in the frequency of drought periods. Therefore, selecting improved genotypes, which are highly productive but with a high water use efficiency (WUE), is becoming a major target. The use of automated weighing systems in controlled environments facilitates the estimation of cumulated water loss and whole plant transpiration efficiency (TE). Differences in TE and leaf level intrinsic WUE as well as the contribution of underlying ecophysiological traits were determined in three contrasting P. nigra genotypes. Strong differences in TE among the selected genotypes were congruent with differences in leaf level intrinsic WUE. Our data show that a high total leaf area was overcompensated by a low per leaf area transpiration rate, leading to higher TE in highly productive genotypes originating from cool locations. Nocturnal water loss was relatively low but contributed to variations in TE among genotypes. In response to drought, leaf level WUE increased but not TE, suggesting that carbon losses due to whole plant respiration could offset the drought-induced increase in intrinsic WUE.

Quercus robur L. and Quercus petraea (Matt.) Liebl. are two sympatric oak species occupying distinctive ecological niches as well as presenting disparate drought tolerances. Available litterature reports the existence of stable inter-specific differences of water use efficiency (WUE) between the two species, Q. petraea displaying higher values than Q. robur. Water use efficiency can be studied at several integration scales both spatially and temporally. Based on instantaneous foliar gaz exchange, the intrinsic water use efficiency can be determined (Wi). On the other hand, water use can be characterised by measurements of the carbon isotopic composition of plant tissues (13C) or by estimation of the whole plant transpiration efficiency (TE), thus allowing a time-integrated estimate of water use. The characterisation of WUE through these different estimators allowed us to put in evidence in oak seedlings grown under controlled conditions a higher transpiration efficiency in Q. robur even though no differences were observed between the two species regarding instantaneous measurements (Wi). TE variations were mainly linked to the variations of the biomass production by the plants while Wi variations were essentially associated to the stomatal conductance. Both intra and inter specific variations of the dynamics of stomatal response to step changes of environmental factors (light, CO2, VPD) could not be clearly linked to the variations of the different WUE estimates nor their respective components. Nevertheless, drought conditions impacted the stomatal dynamics in both species towards faster stomatal responses, especially for stomatal closing. Furthermore, the main observed differences between species were linked to biomass production. Both species displayed different carbon allocation strategies, especially regarding the root compartment. Thus, Q. petraea produced a significantly more ramified and thinner root system than Q. robur. Such behaviour was accentuated under drought stress, under which Q. robur was more sensitive than Q. petraea in term of biomass production. The complexity of the relationships between water use efficiency and all of these morphological and physiological traits as well as the possible drought tolerance implications in both oak species are discussed in this PhD thesis., Quercus robur et Quercus petraea sont deux espèces de chêne sympatriques occupant des niches écologiques distinctes et présentant des différences de sensibilités à la sécheresse. La littérature disponible suggère qu’il existe des différences inter-spécifiques stables de l’efficience d’utilisation entre ces deux espèces, Q.petraea présentant des valeurs supérieures à Q.robur. L’efficience d’utilisation de l’eau peut être étudiées à différentes échelles d’intégration spatiales et temporelle. Sur la base de mesures instantanées d’échanges gazeux foliaires, l’efficience d’utilisation de l’eau intrinsèque (Wi) peut être déterminée. D’autre part, l’efficience d’utilisation de l’eau peut être caractérisée par des mesures de composition isotopique du carbone (δ13C) au sein des tissus végétaux ou par l’estimation de l’efficience de transpiration plante entière (TE), permettant ainsi une estimation de l’efficience intégrée dans le temps. La caractérisation de l’efficience à travers ces différents estimateurs a permis de mettre en évidence chez des semis croissant en conditions contrôlées une efficience de transpiration supérieure chez Q.robur bien qu’aucune différence ne soit détectable entre les deux espèces sur la base des mesures instantanées (Wi). Ces variations de TE étaient essentiellement liées aux variations de la production de biomasse par les plants tandis que les variations de Wi qui ont été observées étaient principalement associées à la conductance stomatique. Les variations intra et inter espèces de la dynamique de réponse stomatique au changement abrupte d’un facteur environnemental (lumière, CO2, VPD) n’ont pu être clairement reliées aux variations des différents estimateurs de l’efficience ou leur composantes respectives. Par ailleurs, les conditions de stress hydriques impactaient la dynamique de réponse stomatique chez les deux espèces de chênes. Cet impact se traduit par des réponses stomatiques plus rapides. Les principales différences observées entre les espèces étaient liées à la production de biomasse. Les deux espèces affichaient en effet des stratégies d’allocation du carbone divergentes notamment au niveau du compartiment racinaire. Ainsi Q.petraea produisait un système racinaire plus fin et ramifié que Q.robur. Par ailleurs, cet état de fait était plus marqué sous conditions de stress, conditions à laquelle Q.robur est plus sensible que Q.petraea en terme de production de biomasse. La complexité des relations entre efficience d’utilisation de l’eau et l’ensemble des traits physiologiques et anatomiques étudiés ainsi que leurs possibles implications dans la tolérance à la sécheresse des deux espèces de chênes sont discutées dans cette synthèse.

International audience; One of the challenges of eco-efficient agriculture is the development of operational farming practices to increase the level of agricultural production, maximize the efficiency of resource use and reduce environmental impacts. Based on the efficiency frontier concept and the decomposition of resource use efficiency, we used a three-quadrant framework allowing to carry a functional analysis of the cropping system. Using a data envelope approach, we established boundary curves which represent the maximum achievable performances (yield, N uptake) when N is the only limiting factor. This framework has been first implemented and tested using published data from 112 agronomic situations of rainfed durum wheat in experimental fields in northern Syria and then further applied on a data set of 245 agronomic situations of durum wheat in farmers’ fields in two grain-producing regions of Tunisia. The results demonstrated the impact of preceding crops: durum wheat following legumes or vegetable showed a higher potential for N uptake but with only a minor effect on its conversion into grains. This positive effect of diversified rotation on potential N uptake by durum wheat is partly of-set by increased N uptake gaps in farmer’s fields indicating a higher effect of other limiting factors.

Key message A genomic segment on maize chromosome 7 influences carbon isotope composition, water use efficiency, and leaf growth sensitivity to drought, possibly by affecting stomatal properties. Abstract Climate change is expected to decrease water availability in many agricultural production areas around the globe. Therefore, plants with improved ability to grow under water deficit are urgently needed. We combined genetic, phenomic, and physiological approaches to understand the relationship between growth, stomatal conductance, water use efficiency, and carbon isotope composition in maize (Zea mays L.). Using near-isogenic lines derived from a maize introgression library, we analysed the effects of a genomic region previously identified as affecting carbon isotope composition. We show stability of trait expression over several years of field trials and demonstrate in the phenotyping platform Phenodyn that the same genomic region also influences the sensitivity of leaf growth to evaporative demand and soil water potential. Our results suggest that the studied genomic region affecting carbon isotope discrimination also harbours quantitative trait loci playing a role in maize drought sensitivity possibly via stomatal behaviour and development. We propose that the observed phenotypes collectively originate from altered stomatal conductance, presumably via abscisic acid. Electronic supplementary material The online version of this article (10.1007/s00122-018-3193-4) contains supplementary material, which is available to authorized users.

Natural selection driven by water availability has resulted in considerable variation for traits associated with drought tolerance and leaf‐level water‐use efficiency (WUE). In Arabidopsis, little is known about the variation of whole‐plant water use (PWU) and whole‐plant WUE (transpiration efficiency). To investigate the genetic basis of PWU, we developed a novel proxy trait by combining flowering time and rosette water use to estimate lifetime PWU. We validated its usefulness for large‐scale screening of mapping populations in a subset of ecotypes. This parameter subsequently facilitated the screening of water use and drought tolerance traits in a recombinant inbred line population derived from two Arabidopsis accessions with distinct water‐use strategies, namely, C24 (low PWU) and Col‐0 (high PWU). Subsequent quantitative trait loci mapping and validation through near‐isogenic lines identified two causal quantitative trait loci, which showed that a combination of weak and nonfunctional alleles of the FRIGIDA (FRI) and FLOWERING LOCUS C (FLC) genes substantially reduced plant water use due to their control of flowering time. Crucially, we observed that reducing flowering time and consequently water use did not penalize reproductive performance, as such water productivity (seed produced per unit of water transpired) improved. Natural polymorphisms of FRI and FLC have previously been elucidated as key determinants of natural variation in intrinsic WUE (δ13C). However, in the genetic backgrounds tested here, drought tolerance traits, stomatal conductance, δ13C. and rosette water use were independent of allelic variation at FRI and FLC, suggesting that flowering is critical in determining lifetime PWU but not always leaf‐level traits., Accelerated flowering time achieved through nonfunctional and weak alleles of flowering time genes FRIGIDA (FRI) and FLOWERING LOCUS C (FLC) reduce water use in Arabidopsis thaliana without detrimentally impacting reproductive output.

As all perennial plants, fruit trees must cope with multiple environmental stresses during their life. The current progressive increase in air temperature makes fruit trees particularly fragile because of their long life during which they might accumulate stresses and the long duration of the selection process to search for more adapted materials. The progressive temperature increase directly impacts the tree phenology, especially their flowering time which results from the regulation of dormancy duration. The ongoing changes endanger the flower quality and the synchronicity of flowering among varieties that is necessary for cross-pollination. Moreover, during summer, despite the irrigation of most orchards, trees must face periods of soil water deficit and/or high temperatures during which the regulation of stomatal closure may impact the tree water use efficiency. Fruit development also depends on the climatic conditions during formation and maturation whereas floral induction occurs for the next season. In this talk, we will briefly review the threats for fruit production and the physiological mechanisms possibly involved that would deserve deep investigations for provid ing a more comprehensive knowledge of fruit tree temperature perception and responses. This knowledge is indeed crucial to draw strategies for the creation of new plant material better adapted to future climatic conditions.

To cope with increasing water scarcity, breeding programs targeting for more efficient crop water use are crucial. This requires comprehensive knowledge of plant responses to drought, including the regulation of water status (by stomatal closure) and water - use efficiency (the ratio of carbon gain to water use). These questions have been scarcely studied in fruit trees, yet are of major economic importance. A main limitation results from the lack of high throughput techniques to characterize the physiological responses of hundreds of trees in the field as required to perform genetic analyses. We screened the diversity of European apple tree responses to drought, by unique combination of hight hroughput and in planta measurements applied on two core - collections in Montpellier (France) and Bologna (Italy), respectively. A progressive soil water deficit was ensured and continuously monitored in both orchards, through summer irrigation withholding. In the whole collections (>250 varieties), a semi-empirical index (IPL index), computed from chlorophyll fluorescence measurements, was used as an indicator of leaf photosynthesis, whereas the variability of stomatal regulation and canopy structure was characterized by airborne imagery. The validity of high-throughput indices was assessed through fine measurements of water potential, stomatal conductance and photosynthesis on 6 varieties common to both collections. A large variability of IPL index was observed, with highly significant effects of the genotype and watering scenario (well-watered vs water stress). A genome wide association study (GWAS) was undertaken to identify the genomic regions controlling the variations detected. In parallel, the effect of interaction between Genotype and Environment (GxE) was dissected to highlight contrasted strategies of water use under drought across the varieties studied.

National audience; Plasma membrane intrinsic proteins (PIPs) are aquaporins that facilitate the passive movement of water through biological membranes. These channels are involved in numerous physiological and cellular processes such as hydraulic conductivity, transpiration, photosynthesis and osmoregulation in plants. In a context of climatic changes, they could turn out to be key targets for selection of crop varieties optimized for water use efficiency. Whereas PIP mRNA levels in maize tissues vary according to the developmental stages and under varying environmental conditions, the molecular actors and mechanisms regulating the transcription of these genes remain largely unknown. Relying on the genetic material (254 maize genotypes and ~1000 K SNPs collection) and expertise of the European DROPS (DROught tolerant yielding PlantS) project, we performed GWAS (genome-wide association study) analyses in order to identify promising eQTLs (expression quantitative trait loci) associated with variation in the expression of five PIP genes in the mature and elongation zones of the leaf. In most cases, highly significant eQTLs were identified close to the PIP genes themselves suggesting the large impact of cis-regulation, as well as numerous promising distal eQTLs that appear to be mainly zone-specific. Conditional GWAS, avoiding the cis-eQTLs effects, allowed the identification of even more proximal and distal eQTLs, highlighting the complex regulation of the PIP gene expression. Deeper analyses and functional validation of several eQTLs are currently ongoing.

There is increasing concern that due to land pressure and the need to maximize income, smallholder coffee farmers are increasingly being forced to cultivate in areas which are considered to be sub-optimal for coffee. Little is known about optimal coffee and tree combinations in these conditions and the degree to which crops and trees compete or are synergistic. In environmental conditions which were sub optimal for coffee cultivation in Nicaragua (1470 mm annual rainfall, 27 °C mean annual temperature and 455 m altitude compared to optima of 2000 mm, 23–24 °C and altitude between 1000 and 1400 m at that latitude, respectively), coffee and shade tree transpiration and soil evaporation were directly and separately measured in agroforestry (AFS) and full sun systems (FS). AFS was found to be a more efficient water user than FS because a greater proportion of rainfall was used by plant transpiration rather than being lost by soil evaporation. Plant transpiration accounted for 83% and 69% of evapotranspiration while soil evaporation represented 17% and 31%, in AFS and FS respectively. In AFS most of the water transpiration was due to coffee (72.5%) and much less by deciduous Tabebuia rosea (19%) and evergreen Simarouba glauca shade trees (8.5%). Furthermore, the study demonstrated the vastly different behaviour in water use by the shade trees. When in leaf, Tabebuia rosea transpired at four to six times the rate of evergreen Simarouba glauca, although crown sizes were similar. Contrasting precipitation between two consecutive years of study demonstrated that competition for water between coffee and shade tree occurred only in a severe dry season when coffee leaf water potential (LWP) reached its lowest values of −2.33 MPa in AFS. It was concluded that in most circumstances there was sufficient water for both coffee and trees, that coffee in AFS was a more efficient user of water than FS coffee, and that evergreen Simarouba glauca was more suitable as coffee shade tree compared to deciduous Tabebuia rosea in the sub optimal environmental condition studied. (Resume d'auteur)

En Tunisie, les études sur les associations d’irrigants ont souvent diagnostiqué des performances médiocres, mais n’ont proposé que des analyses incomplètes des facteurs influençant ces performances. Dans la présente étude, les acteurs intervenant dans une association d’irrigants ont été invités à exprimer leurs points de vue sur les objectifs de l’association et les facteurs jouant sur ses performances, en réagissant à des exemples inspirés de trois théories de l’action collective et adaptés à leur situation. Ces acteurs sont les agriculteurs, le conseil d’administration, les salariés de l’association et les agents de l’administration publique. Les analyses différenciées faites par les acteurs ont permis une meilleure compréhension de leurs attentes vis-à-vis de l’association et des relations entre ces acteurs. Cet usage des théories de l’action collective s’est avéré être un outil d’élicitation innovant pour révéler les points de blocage et améliorer les performances de l’association.

International audience; Drought is the major constraint causing considerable yieldreduction in chickpea. Roots play a critical role in enhancingdrought tolerance. The main objective of the study was to introgressdrought tolerant root traits into Kenyan chickpea varietiesthrough marker assisted breeding (MABC). Eight simplesequence repeat (SSR) markers, linked to quantitative trait loci(QTL) for root and yield traits, were used to screen the parentsat ICRISAT, India. In addition, 1144 single nucleotide polymorphicmarkers (SNPs) were also used in genotyping theseparents at Legume Genomics Center, United Kingdom. Crosseswere made between two selected varieties, ICCV 92944 (ChaniaDesi II) and ICCV 00108 (LDT 068) and ‘QTL-hotspot’ donorparent ICC 4958 that has extensive rooting system. Polymorphic phic SSR and SNP markers were used to select progenies withroot QTL at F1, BC1F1 and BC2F1 that were later advanced toBC2F3. The BC2F3 populations were evaluated for root traitsat Egerton University in randomized complete block designwith two replications in pot experiment. The BC2F3 familieswere significantly different for root dry weight (RDW), shootdry weight (SDW), total plant dry weight (PDW) and root toshoot dry weight (R/S) ratio (R/S) for Chania Desi II x ICC 4958and R/S for LDT 068 x ICC 4958. These lineshad significantlyimproved root traits compared the recurrent parents. MABCis aneffective and efficient method of introgressing complexdrought tolerant traits which leads to improvement in yield especiallyunder drought conditions.

Drought is the major constraint causing considerable yieldreduction in chickpea. Roots play a critical role in enhancingdrought tolerance. The main objective of the study was to introgressdrought tolerant root traits into Kenyan chickpea varietiesthrough marker assisted breeding (MABC). Eight simplesequence repeat (SSR) markers, linked to quantitative trait loci(QTL) for root and yield traits, were used to screen the parentsat ICRISAT, India. In addition, 1144 single nucleotide polymorphicmarkers (SNPs) were also used in genotyping theseparents at Legume Genomics Center, United Kingdom. Crosseswere made between two selected varieties, ICCV 92944 (ChaniaDesi II) and ICCV 00108 (LDT 068) and ‘QTL-hotspot’ donorparent ICC 4958 that has extensive rooting system. Polymorphic phic SSR and SNP markers were used to select progenies withroot QTL at F1, BC1F1 and BC2F1 that were later advanced toBC2F3. The BC2F3 populations were evaluated for root traitsat Egerton University in randomized complete block designwith two replications in pot experiment. The BC2F3 familieswere significantly different for root dry weight (RDW), shootdry weight (SDW), total plant dry weight (PDW) and root toshoot dry weight (R/S) ratio (R/S) for Chania Desi II x ICC 4958and R/S for LDT 068 x ICC 4958. These lineshad significantlyimproved root traits compared the recurrent parents. MABCis aneffective and efficient method of introgressing complexdrought tolerant traits which leads to improvement in yield especiallyunder drought conditions.

The study of the genetic basis of grain yield is one of the major challenges of the scientific community because of both its complex genetic control and the strong interaction with environment and management practices. Furthermore, such interactions may affect yield during the entire life cycle of the plant. It is therefore crucial to consider yield as the result of multiple simpler traits and thus study their genetic control separately. In this study, we used the phenotyping platform PhenoArch in order to identify growth and water use related traits that may explain the segregation for yield and plant height observed at the QTL Qyld.idw-3B by Graziani et al. (2010). Four pairs of durum wheat near-isogenic lines (NILs) for the QTL Qyld.idw-3B were grown at three levels of drought stress: no stress (soil water potential > -1 bar), mild stress (soil water potential of -5/-8 bar) and severe stress (soil water potential ≈ -13 bar). The stress was applied at the beginning of stem elongation until the end of the experiment (late milk stage, Zadok 77) on eight replicates per genotype per treatment. We recorded two main types of phenotypic data: (i) canopy images and (ii) weight measurements: every night, digital RGB images were collected. From these images we estimated several growth related phenotypes like biomass, leaf expansion and plant height; every plant was weighted to estimate the evapotranspiration at least once per day. Combining these data, it was possible to evaluate key physiological parameters like water use efficiency (WUE) and leaf transpiration. The QTL seemed to not affect vegetative behaviour and water use of plants during the early vegetative stages while majorly differentiating the NILs couples during the reproductive and earl ripening stages. This explains the segregation for final plant height previously observed by Graziani et al (2010). These results may provide useful information for further phenotypic as well as physiological and genetic characterization of the QTL Qyld.idw-3B with a main focus on mid to late stem elongation and reproductive/maturity stages.

The introduction to this book sets out to question why the belief that drip irrigation is a promising technology to save water, modernize agriculture and help the poor remains powerfully intact, despite growing evidence of more contrasted field situations. It pleads for going beyond these narratives to understand the multiple realities of drip irrigation-in-use. This allows showing that drip irrigation is not only a technology but part of broader societal and environmental changes. Unraveling these untold stories raises questions about the accountability and responsibilities over these changes.

The terrestrial carbon and water cycles are intimately linked: the carbon cycle is driven by photosynthesis, while the water balance is dominated by transpiration, and both fluxes are controlled by plant stomatal conductance. The ratio between these fluxes, the plant water-use efficiency (WUE), is a useful indicator of vegetation function.WUE can be estimated using several techniques, including leaf gas exchange, stable isotope discrimination, and eddy covariance. Here we compare global compilations of data for each of these three techniques.We show that patterns of variation in WUE across plant functional types (PFTs) are not consistent among the three datasets. Key discrepancies include the following: leaf-scale data indicate differences between needleleaf and broadleaf forests, but ecosystem-scale data do not; leaf-scale data indicate differences between C3 and C4 species, whereas at ecosystem scale there is a difference between C3 and C4 crops but not grasslands; and isotope-based estimates of WUE are higher than estimates based on gas exchange for most PFTs.Our study quantifies the uncertainty associated with different methods of measuring WUE, indicates potential for bias when using WUE measures to parameterize or validate models, and indicates key research directions needed to reconcile alternative measures of WUE.

The interactions between hydrological and ecological processes are key issues to improve our predictions of ecosystem responses to increasing droughts. However, predicting the dynamics and the impacts of vegetation water stress remains challenging because of complex ecohydrological feedbacks. The ecohydrological optimality approach proposes that functional adjustments within plant communities may buffer the increase in vegetation water stress despite local water shortage. This study aimed to test whether vegetation water stress may be invariant across contrasting plant communities, reflecting possible optimality processes. We addressed the following question: does a lower soil water storage capacity under the same climate generate greater vegetation water stress over time? We hypothesized that vegetation water stress would be buffered around a low and constant level through the adjustment of vegetation biomass productivity net primary productivity (NPP), evapotranspiration (ET) and/or water-use efficiency (WUE) in relation with local soil water storage capacity. We monitored 12 native plant communities distributed along a gradient of soil water storage capacity (ranging from 20 mm to 120 mm) during five successive years. Net primary productivity, ET, WUE as well as soil water dynamics were assessed and modelled for each plant community throughout the 5 years of study. Vegetation water stress was determined for each plant community as the deviation of between actual ET and their maximum ETm rate achieved under non-limiting conditions. We found that NPP and ET were together proportionally related to local soil water storage capacity across the 5 years of study while WUE did not differ between plant communities. Vegetation water stress was found quite similar for all plant communities whatever the soil water storage capacity. These results suggested that vegetation water stress was strongly buffered by the community-level plant growth rates and total water use along the soil gradient, but not by WUE. Our results suggest that stressful environments rarely exist for plant communities. A dynamic scaling relationship between NPP and ET may underpin the control of vegetation water stress over seasonal and pluriannual time-scales. Such results could contribute to better understanding processes associated with ecohydrological optimality and improve the predictions of vegetation dynamics under increasing droughts. (Resume d'auteur)

International audience; Pour obtenir et maintenir une eau potable qui réponde aux normes, les collectivités locales ont le choix entre deux types d’actions : préventives par la mise en place de périmètres de protection de captages d’eau potable (PPC) ou d’aires d’alimentation de captages (AAC), ou/et curatives qui consistent à traiter les eaux pour satisfaire les exigences réglementaires. La mise en place des actions préventives aboutit à l’instauration de prescriptions sur les activités existantes et futures. Cependant, à l’heure actuelle, la procédure de mise en place des périmètres de protection des captages d’eau potable ne prend pas en compte les impacts financiers et économiques des prescriptions, ce qui pose des problèmes d’acceptabilité de cet instrument mais aussi des difficultés méthodologiques pour l’évaluation des impacts des prescriptions, en particulier celles appliquées au secteur agricole. Le but de cette étude est de répondre à la nécessité de prendre en compte le volet financier et économique dans le cadre des procédures préventives. Ainsi, l’étude propose un protocole d’évaluation socio-économique pour améliorer la procédure PPC. Dans le cadre de ce protocole, nous avons mis en place, d’une part, un ensemble complet de données sur les coûts financiers des mesures agricoles et non agricoles appliquées dans le cadre de ces PPC et, d’autre part, une méthode qualitative basée sur l’analyse des coûts et bénéfices supportés par les acteurs impactés par les PPC.

The high diversity of tropical forests complicates predictions of how canopy trees and lianas will respond to drought. Detail ed physiological measurements are necessary to improve predictions, yet these measurements are tedious and time-consuming, so relatively little data is available. The DROUGHT Project, initiated in 2016, aims to elucidate how hydraulic traits related to drought vulnerability are coordinated among canopy species across the Neotropics by increasing the scope of data available. Method: So far, we have performed detailed measures of hydraulic conductivity, xylem vulnerability curves, sapwood pressure-volume curves, and wood density on emergent branches of 35 common liana and tree species from French Guiana and Panama. Our measurements were stratified across a broad range of wood density. Objectives included determining the range of hydraulic traits, evaluating coordination among hydraulic traits to identify strategies, and providing trait values for inclusion in predictive models. Result: Wood density was correlated with sapwood pressure-volume traits, including capacitance, sapwood water content and turgor loss point. Xylem hydraulic conductivity was related to xylem vulnerability to embolism at some sites, but not others, indicating variation in the xylem safety versus efficiency trade-off. Relationships between wood density and xylem vulnerability to embolism varied among sites. In general, wood density showed greater correlation with sapwood pressure-volume traits than with xylem vulnerability to embolism. Conclusion: Detailed physiological traits offer a way forward for predicting tropical liana and tree responses to drought. An understanding of the trade-offs and coordinated functions that separate tropical canopy species along an axis of drought vulnerability may be an important component for parameterizing Earth system models.

Initially associated with hi-tech irrigated agriculture, drip irrigation is now being used by a much wider range of farmers in emerging and developing countries. This book documents the enthusiasm, spread and use of drip irrigation systems by smallholders but also some disappointments and disillusion faced in the global South. It explores and explains under which conditions it works, for whom and with what effects. The book deals with drip irrigation 'behind the scenes', showcasing what largely remain 'untold stories'.Most research on drip irrigation use plot-level studies to demonstrate the technology’s ability to save water or improve efficiencies and use a narrow and rather prescriptive engineering or economic language. They tend to be grounded in a firm belief in the technology and focus on the identification of ways to improve or better realize its potential. The technology also figures prominently in poverty alleviation or agricultural modernization narratives, figuring as a tool to help smallholders become more innovative, entrepreneurial and business minded. Instead of focusing on its potential, this book looks at drip irrigation-in-use, making sense of what it does from the perspectives of the farmers who use it, and of the development workers and agencies, policymakers, private companies, local craftsmen, engineers, extension agents or researchers who engage with it for a diversity of reasons and to realize a multiplicity of objectives. While anchored in a sound engineering understanding of the design and operating principles of the technology, the book extends the analysis beyond engineering and hydraulics to understand drip irrigation as a sociotechnical phenomenon that not only changes the way water is supplied to crops but also transforms agricultural farming systems and even how society is organized. The book provides field evidence from a diversity of interdisciplinary case studies in sub-Saharan Africa, the Mediterranean, Latin America, and South Asia, thus revealing some of the untold stories of drip irrigation.

Le potassium (K) est un élément majeur connu pour contribuer à la résistance des plantes à la sècheresse. L'étudede son influence sur la réponse physiologique du maïs (Zea mays L.) sous contrainte hydrique est essentiellepour prédire la future productivité dans un contexte de changements climatiques, en particulier de la diminutiondes précipitations. Des modalités d'apports en K et en eau ont été croisées et soumises à des plants de maïs,élevés en condition contrôlées ou cultivés au champ. La croissance (biomasses aériennes et racinaires,rendements en grain) ainsi que les mécanismes écophysiologiques du métabolisme carboné (photosynthèse,transport des sucres) et du statut hydrique (transpiration, conductance stomatique, potentiels hydriques) ont étéétudiés. L'apport de K a contribué à l'augmentation de la croissance, le développement et le rendement grain quelque soit le régime hydrique imposé au maïs et les conditions d'expérimentation. Les résultats attendus sur lameilleure régulation stomatique en cas de déficit hydrique sont moins évidents. L'effet du stress hydrique ou dela déficience en K tendent à diminuer la photosynthèse. Cependant, ces effets ressortent plus sur les feuillesâgées que sur les feuilles jeunes. Dans ces mêmes conditions, le transport des sucres ne semble pas être unélément limitant de la croissance. Plusieurs résultats convergent pour attribuer au K un rôle dans la maîtrise despertes en eau (par unité de surface foliaire) et sur la meilleure efficience d'utilisation de l'eau. Néanmoins, cetteefficience est imputée à des meilleurs rendements, liés à une surface foliaire plus importante, et non pas à unemoindre consommation de l'eau., Potassium (K) is a major nutrient known to help plants resist drought. In the context of climate change,quantifying the role of K on maize physiological acclimation to reduced precipitations is essential to betterpredict future productivity. Maize (Zea mays L.) plants grown under controlled or field conditions weresubmitted to different K and water levels. Plant growth (shoot and root biomass, grain yield) as well as plantwater status (transpiration, stomatal conductance, water potential) and ecophysiological mechanisms of Carbonmetabolism (photosynthesis, sugar transport) were studied. Regardless of the water regime and experimentalconditions, K nutrition increased growth and whole-plant development and improved grain yield. The effect ofwater stress on stomatal regulation was not straightforward and depended on the level of K fertilization. Theeffects of water or K deficit tend to decrease photosynthesis. Drought or K nutrition affected more leafphotosynthesis in old than in young leaves, and sugar transport did not seem to be a growth limiting factor. Ourresults demonstrated a strong effect of K on biomass production and a higher water use efficiency with less of animpact on leaf-level physiology. This better water use was mainly the consequence of the positive effect of leafarea on yield, and not due to a reduce water use.