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The functional reasons for isoprene emission are still a matter of hot debate. It was hypothesized that isoprene biosynthesis evolved as an ancestral mechanism in plants adapted to high water availability, to cope with transient and recurrent oxidative stresses during their water-to-land transition. There is a tight association between isoprene emission and species hygrophily, suggesting that isoprene emission may be a favorable trait to cope with occasional exposure to stresses in mesic environments. The suite of morpho-anatomical traits does not allow a conservative water use in hygrophilic mesophytes challenged by the environmental pressures imposed or exacerbated by drought and heat stress. There is evidence that in stressed plants the biosynthesis of isoprene is uncoupled from photosynthesis. Because the biosynthesis of isoprene is costly, the great investment of carbon and energy into isoprene must have relevant functional reasons. Isoprene is effective in preserving the integrity of thylakoid membranes, not only through direct interaction with their lipid acyl chains, but also by up-regulating proteins associated with photosynthetic complexes and enhancing the biosynthesis of relevant membrane components, such as mono- and di-galactosyl-diacyl glycerols and unsaturated fatty acids. Isoprene may additionally protect photosynthetic membranes by scavenging reactive oxygen species. Here we explore the mode of actions and the potential significance of isoprene in the response of hygrophilic plants when challenged by severe stress conditions associated to rapid climate change in temperate climates, with special emphasis to the concomitant effect of drought and heat. We suggest that isoprene emission may be not a good estimate for its biosynthesis and concentration in severely droughted leaves, being the internal concentration of isoprene the important trait for stress protection. [ABSTRACT FROM AUTHOR]

The argan tree (Argania spinosa) occurs in a restricted area of Southwestern Morocco characterized by low water availability and high evapotranspirative demand. Despite the adaptation of the argan tree to drought stress, the extent of the argan forest has declined markedly due to increased aridity, land use changes and the expansion of olive cultivation. The oil of the argan seed is used for cooking and as the basis for numerous cosmetics. The identification of argan tree varieties with enhanced drought tolerance may minimize the economic losses associated with the decline of the argan forest and constrain the spread of desertification. In this study we collected argan ecotypes from four contrasting habitats and grew them under identical controlled environment conditions to investigate their response to drought. Leaf gas exchange analysis indicated that the argan ecotypes showed a high degree of adaptation to drought stress, maintaining photosynthetic activity at low levels of foliar water content and co-ordinating photosynthesis, stomatal behavior and metabolism. The stomata of the argan trees were highly sensitive to increased leaf to air vapor pressure deficit, representing an adaptation to growth in an arid environment where potential evapotranspiration is high. However, despite originating in contrasting environments, the four argan ecotypes exhibited similar gas exchange characteristics under both fully irrigated and water deficit conditions. Population genetic analyses using microsatellite markers indicated a high degree of relatedness between the four ecotypes; indicative of both artificial selection and the transport of ecotypes between different provinces throughout centuries of management of the argan forest. The majority of genetic variation across the four populations (71%) was observed between individuals, suggesting that improvement of argan is possible. Phenotypic screening of physiological responses to drought may prove effective in identifying individuals and then developing varieties with enhanced drought tolerance to enable the maintenance of argan production as climate change results in more frequent and severe drought events in Northern Africa. [ABSTRACT FROM AUTHOR]

Arundo donax exhibits rapid growth and requires little nutrient input, making it an ideal perennial biomass crop species. However, this growth is accompanied by high rates of water use, potentially restricting the use of A. donax in rain-fed marginal lands. Here, we investigated the physiological and morphological responses to drought in two ecotypes of A. donax from contrasting habitats: one from an arid environment in Morocco, and the second from a warm humid sub-Mediterranean climate in central Italy. Prolonged drought resulted in identical reductions in leaf-level photosynthesis ( PN) and stomatal conductance ( Gs) in the two ecotypes. However, water deficit induced an increase in xylem vessel diameter in the Moroccan plants, improving the movement of water along the stem, but also likely reducing the resistance to embolism. In contrast, the Italian ecotype reduced xylem vessel area, thus increasing resistance to water transport and xylem embolism. The increased xylem vessel size and associated vulnerability to embolism in the Moroccan plants may have contributed to an increase in the loss of leaf numbers, but also to higher relative water content ( RWC) in the remaining leaves in comparison to the Italian ecotype, where a greater number of leaves persisted. Despite the Moroccan plants possessing stems with a lower basal area than their Italian counterparts, both ecotypes exhibited identical leaf to supporting stem area ratios under both control and water deficit conditions. This may account for the similarities observed in leaf area measures of PN and Gs in this and previous studies of different A. donax ecotypes. Selection of A. donax ecotypes on the basis of xylem responses to drought may facilitate the development of varieties suited to arid environments prone to severe drought and wetter habitats where prolonged droughts occur less frequently. [ABSTRACT FROM AUTHOR]

Genetic analyses have suggested that the clonal reproduction of Arundo donax has resulted in low genetic diversity. However, an earlier common garden phenotyping experiment identified specimens of A. donax with contrasting biomass yields (ecotypes 6 and 20). We utilized the same well-established stands to investigate the photosynthetic and stress physiology of the A. donax ecotypes under irrigated and drought conditions. Ecotype 6 produced the largest yields in both treatments. The A. donax ecotypes exhibited identical high leaf-level rates of photosynthesis ( PN) and stomatal conductance ( Gs) in the well-watered treatment. Soil drying induced reductions in PN and Gs, decreased use of light energy for photochemistry, impaired function of photosystem II and increased heat dissipation similarly in the two ecotypes. Levels of biologically active free-abscisic acid ( ABA) and fixed glycosylated- ABA increased earlier in response to the onset of water deficit in ecotype 6; however, as drought progressed, the ecotypes showed similar increases in both forms of ABA. This may suggest that because of the low genetic variability in A. donax the genes responding to drought might have been activated similarly in the two ecotypes, resulting in identical physiological responses to water deficit. Despite the lack of physiological ecotypic differences that could be associated with yield, A. donax retained a high degree of PN and biomass gain under water deficit stress conditions. This may enable utilization of A. donax as a fast growing biomass crop in rain-fed marginal lands in hot drought prone climates. [ABSTRACT FROM AUTHOR]

• The mechanism uncoupling isoprene emission and photosynthesis under drought was investigated in Populus alba saplings. • Isoprene emission, incorporation of 13C into the isoprene molecule, isoprene synthase (ISPS) activity, concentration and gene expression, and photosynthesis were measured as a function of the fraction of transpirable soil water (FTSW) and in plants recovering from drought. • Photosynthesis sharply declined below FTSW30 (a FTSW of 30%) and its inhibition was not caused by metabolic factors. A decline in isoprene emission was only evident towards the FTSW endpoint. 13C incorporation into isoprene was lower when photosynthesis was constrained by drought. ISPS activity was inhibited by mild drought, while ISPS gene expression and concentration declined in concert with isoprene emission at the FTSW endpoint. Following rewatering, isoprene emission was higher than, and photosynthesis was similar to, prestress values. ISPS activity and concentration, and 13C incorporation into isoprene, also rapidly recovered to prestress values, while ISPS gene expression remained low in rewatered plants. • Our experiment revealed a larger contribution of alternative carbon sources to isoprene emission only when photosynthesis was dramatically constrained by drought. Isoprene emission was likely controlled at the posttranscriptional level under severe drought. [ABSTRACT FROM AUTHOR]

Heat waves are predicted to increase in frequency and duration in many regions as global temperatures rise. These transient increases in temperature above normal average values will have pronounced impacts upon the photosynthetic and stomatal physiology of plants. During the summer of 2017, much of the Mediterranean experienced a severe heat wave. Here, we report photosynthetic leaf gas exchange and chlorophyll fluorescence parameters of olive (Olea europaea cv. Leccino) grown under water deficit and full irrigation over the course of the heat wave as midday temperatures rose over 40 °C in Central Italy. Heat stress induced a decline in the photosynthetic capacity of the olives consistent with reduced ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) activity. Damage to photosystem II was more apparent in plants subject to water deficit. In contrast to previous studies, higher temperatures induced reductions in stomatal conductance. Heat stress adversely affected the carbon efficiency of olive. The selection of olive varieties with enhanced tolerance to heat stress and/or strategies to mitigate the impact of higher temperatures will become increasingly important in developing sustainable agriculture in the Mediterranean as global temperatures rise. [ABSTRACT FROM AUTHOR]

Seaweed-based extracts have been recently employed as sustainable tools to improve abiotic stress tolerance and increase grape quality. However, the effect of these extracts on secondary metabolism compounds, that are fundamental for grape and wine quality, is still scarce. In the present study, the effects of foliar treatments with an Ascophyllum nodosum extract on physiological and biochemical parameters of Vitis vinifera (cv. Sangiovese) were investigated. We hypothesized an enhancement in the biosynthesis of secondary metabolites in berry skins and in leaves in response to these treatments, effective in improve grape quality and help vines to cope with abiotic stresses. Gas exchanges, chlorophyll fluorescence and midday stem water potential on leaves treated with A. nodosum extract and non-treated control leaves, were monitored over two growing seasons at three phenological stages: full véraison , mid maturation and full maturation. In addition, anthocyanins, flavonols and hydroxycinnamic acids were quantified both in berry skins and in leaves. The foliar treatments with A. nodosum increased photosynthesis and stomatal conductance in treated compared to control plants. Furthermore, extract-treated vines were able to maintain the potential efficiency of photosystem II close to the optimal value even during the hottest periods. No effect of A. nodosum extract treatments was observed on stem water potential. A. nodosum applications delayed berry ripening, leading to a lower sugar content and a higher anthocyanin content in treated berry skins. Interestingly, treatments also affected the content and the partitioning of secondary metabolites in berry skins, as anthocyanins and flavonols contents were higher in treated compared to control plants, while the ratio of methoxylated to non-methoxylated anthocyanins was lower in treated than in control vines. Furthermore, A. nodosum extract-treated plants also had higher content of flavonols and hydroxycinnamic acids both in berry skins and in leaves and showed a reduction in the biosynthesis of methoxylated anthocyanins, which are usually accumulated in grapes under environmental constraints. Considering the challenges posed by climate change in the Mediterranean basin, the use of seaweed extracts might represent a sustainable tool to mitigate the increasing severity of drought, often associated to heat-waves, on the viticulture sector. • A. nodosum extract improves leaf gas exchanges and prevents leaf photo-inhibition. • A. nodosum extract delays grape ripening and improves the balance of berry traits. • A. nodosum extract affects the content and partitioning of anthocyanins in berry. • A. nodosum extract promotes the accumulation of phenolics in leaf and berry skin. • Biostimulants might represent a sustainable tool to mitigate environmental stresses. [ABSTRACT FROM AUTHOR]

The proportion of the leaf epidermis allocated to stomata ( EP % ) and stomatal function (the capacity to adjust stomatal pore area to regulate stomatal conductance: G s ) are key components in leaf gas exchange, and have likely played a major role in plant evolution. We examined the velocity of change in G s ( G s50% ) during a transition from steady state conditions in the light to darkness and EP % in 31 vascular plants with diverse evolutionary origins. Across all species, EP % correlated to G s50% and the magnitude of G s reduction ( G sLIGHT - G sDARK ) after the cessation of illumination. Those species with higher absolute and relative G s50% values tended to distribute stomata more evenly over the abaxial and adaxial leaf surfaces, whereas species with lower G s50% utilised only one leaf surface for gas exchange. Groups that diverged at relatively early stages in plant phylogeny, including ferns, gymnosperms and basal angiosperms, exhibited lower EP % and G s50% , and took longer to achieve the initial 50% reduction in G s ( T 50% ) than the more recently diverging angiosperms; in particular, the amphistomatous monocot grasses, which also showed higher absolute rates of photosynthesis and G s . We propose that selective pressures induced by declining [CO 2 ] over the past 100 Myr have favoured greater allocation of the epidermis to stomata, increased amphistomaty (the presence of stomata on the abaxial and adaxial surfaces) and faster control of G s in the more recently derived angiosperm groups. Modification of photosynthesis to enhance the carbon and water use efficiencies of C3 crops may therefore require concurrent increases in stomatal density and in the capacity of stomata to react quickly to environmental pressures. [ABSTRACT FROM AUTHOR]

The rapid growth of the giant reed ( Arundo donax L.) is sustained by high rates of photosynthesis ( P N ) and stomatal conductance ( G s ). High rates of G s would render A. donax vulnerable to desiccation during episodes of high evapotranspirative demand and/or low water availability if not accompanied by effective stomatal control. Stomatal control involves the adjustment of stomatal pore aperture to the prevailing environmental conditions and physiological status of the plant to optimise water use efficiency. We assessed stomatal response to environmental signals (light intensity, [CO 2 ] and leaf to air vapour pressure deficit − VPD) and the foliar concentration of abscisic acid ([ABA]) of field grown A. donax under irrigated (control) and rain-fed (drought) conditions. Drought-stressed A. donax showed more rapid reductions in G s to lower light intensity/darkness, a slower rise in G s following increased light and enhanced sensitivity to variations in [CO 2 ]. The stomatal response to leaf to air VPD was unaffected by the water status of the plant. The rates of stomatal response to light/dark and [CO 2 ] were strongly correlated with the concentration of free-ABA within the cytosol but not with the relative water content of the leaves. When exposed to drought, stomata became increasingly sensitive to [CO 2 ] in comparison to PAR and leaf to air VPD. This pronounced increase in stomatal sensitivity to [CO 2 ] was replicated by supplying exogenous ABA to cut leaves from a well-watered control plant. The results of this study indicate that the high P N of A. donax is underpinned by highly effective stomatal control. The stomata of A. donax respond rapidly to changes in environmental conditions and their behaviour is sensitive to the concentration of ABA within the leaf. The high potential for gas exchange and stomatal control observed in A. donax makes it a suitable model species for enhanced stomatal control of P N and the optimisation of stomatal behaviour. [ABSTRACT FROM AUTHOR]

We assessed the response to drought in Corymbia citriodora, a fast growing wood-crop with high levels of water-loss, and Ziziphus nummularia, a tree species that occurs in arid areas of Asia and a potential alternative wood-crop plantation species in drought-prone regions. Z. nummularia was able to sustain higher levels of stomatal conductance at lower levels of soil water availability than C. citriodora. The leaves of Z. nummularia also contained higher levels of the antioxidant peroxidase, offering enhanced protection from drought induced oxidative stress. The carbon isotopic and nitrogen concentration of C. citriodora foliage was strongly affected by decreasing soil water availability, but a compositional effect was only apparent in Z. nummularia leaves at the lowest level of soil water. The higher leaf levels of stomatal conductance and nitrogen are indicative of relatively high assimilation rates in Z. nummularia, suggesting that this species is capable of fully exploiting brief periods where conditions are limited for growth. These attributes, in combination with its inherent tolerance to drought, may make Z. nummularia a suitable wood crop species for rain-fed plantations in drought-prone areas. © 2015 Friends Science Publishers [ABSTRACT FROM AUTHOR]

Abstract: Acclimatory and direct responses of photosynthesis (A), stomatal (g s) and mesophyll (gm ) conductance, light (R L) and dark (R D) respiration, and isoprene emission, measured at different temperatures, were compared in one-year-old Populus × euramericana saplings grown at 25°C, 35°C, and in the saplings grown initially at 35°C and then exposed for two weeks at 25°C. Results show that A, g s and gm were significantly lower at 35°C, than at the other growth temperatures. This resulted in a downward acclimation of these parameters over the range of measuring temperatures. Both R L and R D also showed a considerable downward acclimation. However, the respiration to photosynthesis ratio increased with high temperatures, for R L and R D were more responsive to increasing growth temperatures than A at all measuring temperatures. This type of acclimation would lead to a shift in the carbon balance between photosynthesis and respiration under changing climatic conditions. Isoprene emission was greatly affected by temperature treatments and showed a downward acclimation to higher temperatures. Respiration and isoprene emission rates were directly related, independently of growth and measuring temperatures. These findings may be likely relevant to predict the emissions of isoprenoid in globally changing environmental conditions. [ABSTRACT FROM AUTHOR]