Search

The relation between bacteria numbers in vase water and vase life of gerbera cut flowers has recently been challenged because of reported negative effects of bactericidal compounds. This relation is investigated using two types of experiments that do not rely on antimicrobial compounds. The first type controls vase water temperature (4, 15, 22 or 28 °C) independently from air temperature (15, 22 or 28 °C) to investigate whether fresh weight behavior for two mini gerbera cultivars (‘Okidoki’ and ‘Kimsey’) is affected by bacterial growth and leaking of soluble sugars in the vase water, or by senescence of the flower head. Fresh weight loss, when compared at constant water temperatures, was higher at higher air temperatures. At higher water temperatures and constant air temperatures fresh weight loss was not higher, although bacterial levels were high enough to expect water uptake issues. Also sugar consumption in the vase water depended on water temperature. This indicates that senescence was the main reason for the decline in fresh weight for these flowers, not bacterial growth. The second type of experiments was based on adding predetermined levels of bacteria (0, 103 or 105 CFU mL−1) and sugars (0.1% glucose or 0.2% sucrose) into vase water of flowers of three large-bloomed (‘Carambole’, ‘Candela’ and ‘Iceberg’) cultivars harvested with closed stem-ends and had their scapes sterilized before the start of vase life. When bacteria were added varying types of responses were observed. ‘Carambole’ flowers showed lower water uptake and lower transpiration and, early scape bending. Petal wilting was observed for ‘Candela’ flowers. ‘Carambole’ flowers showed higher scape sugar leakage levels in the vase water while ‘Candela’ flowers had higher scape firmness. ‘Iceberg’ flowers were also affected by bacteria, resulting in early scape bending, although sugar levels in the vase water were low. Furthermore, adding sucrose and/or bacteria in the vase water of one ‘Iceberg’ and one ‘Carambole’ flower in the same flask resulted in later scape bending for ‘Iceberg’ flowers compared to having two ‘Carambole’ or two ‘Iceberg’ flowers. The results indicate that bacteria interactions with gerbera flowers depend strongly on genotype.

The role of nitric oxide (NO) in abscisic acid (ABA)-induced stomatal closure is a matter of debate. We conducted experiments in Vicia faba leaves using NO gas and sodium nitroprusside (SNP), a NO-donor compound, and compared their effects to those of ABA. In epidermal strips, stomatal closure was induced by ABA but not by NO, casting doubt on the role of NO in ABA-mediated stomatal closure. Leaf discs and intact leaves showed a dual dose response to NO: stomatal aperture widened at low dosage and narrowed at high dosage. Overcoming stomatal resistance by means of high CO2 concentration ([CO2]) restored photosynthesis in ABA-treated leaf discs but not in those exposed to NO. NO inhibited photosynthesis immediately, causing an instantaneous increase in intercellular [CO2] (Ci), followed by stomatal closure. However, lowering Ci by using low ambient [CO2] showed that it was not the main factor in NO-induced stomatal closure. In intact leaves, the rate of stomatal closure in response to NO was about one order of magnitude less than after ABA application. Because of the different kinetics of photosynthesis and stomatal closure that were observed, we conclude that NO is not likely to be the key factor in ABA-induced rapid stomatal closure, but that it fine-tunes stomatal aperture via different pathways., Stomatal closure dynamics in Vicia faba following application of NO gas to epidermal strips, leaf discs, and intact leaves suggest that NO is not essential in ABA-induced stomatal closure, but that it acts independently of ABA in the presence of mesophyll.

Although it is well known that, as a short-term response, stomata close at low relative humidity (RH) (high Vapour Pressure Deficit) and open at high RH (low Vapour Pressure Deficit), effects of long-term exposure to different Vapour Pressure Deficits (VPD’s) have only been studied in a few economically important horticultural crops, especially cut roses. To save energy costs, often low VPD conditions are present in greenhouses, due to low ventilation. We studied stomatal and desiccation responses of leaves of cut chrysanthemums after the plants had been exposed to different VPD conditions. While lower transpiration rate (E) was recorded for plants growing at low VPD (L-plants) in comparison with growth at moderate VPD (M-plants), higher stomatal conductance (g s ) was found in L-plants compared to M-plants. Bigger size of stomata and higher stomatal density were observed in L-plants. Besides differences in g s , E and stomatal characteristics during growth of the plants at different VPDs, stomatal closure response to desiccation (under the same VPD condition) was different when plants had been grown before at different VPD’s. Following desiccation, leaves of L-plants showed higher E compared with M-plants. Also E of leaves grown at moderate VPD but exposed to low VPD for only 5 days (M → L-plants) decreased less as response to desiccation than E of M-plants. After rapid desiccation, leaves of L and M → L plants had larger stomatal openings compared with leaves that were grown continuous at M condition, as indicated by their PSII efficiencies under non-photorespiratory conditions (Φ PSII ). In conclusion, control of VPD during growth will be important to prevent subsequent wilting of leaves of chrysanthemum cut flowers.

Bouvardia is an ornamental shrub, commercially cultivated as flowering stem. Occasionally, negative water balance, which leads to rapid leaf wilting, ends vase life immediately. This work studies the effect of preharvest and postharvest conditions on vase life, water uptake and transpiration. Stems grown at moderate RH show a significantly longer vase life, lower water uptake and transpiration rate compared to those grown at high RH. Postharvest treatments overcoming air emboli and wound response had a positive effect on vase life. One of the treatments to overcome air emboli, surfactant treatment, increased water uptake and transpiration rate compared to control. Higher water deficit was measured in stems that had reduced vase life. Stomata malfunction (not closing in response to closing stimuli) resulting from high air humidity during growth conditions is likely one cause for the negative water balance. A combination of high transpiration rate (due to high RH) and hindering of vase water uptake (due to air emboli and/or stem-wounding responses) is likely the main reason for early wilting of Bouvardia leaves and short vase life.

Breeding for postharvest performance in ornamentals is challenging, since many different deteriorative processes determine vase life. In order to improve postharvest performance by breeding, selection should take place on these processes separately. To define processes that are important for chrysanthemum postharvest performance, vase life was assessed after two weeks of cold storage in a set of 44 chrysanthemum cultivars. Since disk floret degreening was the most frequent reason for ending vase life, we further investigated this trait in a large biparental population (n = 381). To quantify disk floret degreening in this large number of genotypes, we developed a high-throughput phenotyping method. The method consists of the quantification of loss of green color as expressed by an increase of intensity of red divided by the intensity of green (R/G) in dark-held detached capitula. R/G increases when disk florets lose green color. The increase in R/G correlated significantly with the number of days until disk floret degreening occurred during vase life. This was the case for the 44-cultivar cultivar panel (Pearson’s correlation coefficient (ρ) of −0.70; p < 0.0001) as well as in a subset of the biparental pulation (n = 145; ρ = −0.67; p < 0.0001). R/G increase segregated in a quantitative manner in the full biparental population, and had a moderately high heritability of 0.73. Carbohydrate content after harvest was measured in a smaller subset of the biparental population (n = 55). The R/G increase correlated with carbohydrate content (ρ=-0.56; p < 0.0001). Since carbohydrate content did not explain all variation in degreening sensitivity, we discuss different possible mechanisms to cope with carbohydrate starvation and avoid degreening. In conclusion, disk floret degreening is an important postharvest trait in chrysanthemum, and it is related to carbohydrate starvation. The quantitative segregation suggests involvement of multiple alleles, probably at multiple loci. The moderately high heritability makes it a suitable trait for QTL mapping, which we will commence in the near future.

Key message We constructed the first integrated genetic linkage map in a polysomic hexaploid. This enabled us to estimate inheritance of parental haplotypes in the offspring and detect multi-allelic QTL. Abstract Construction and use of linkage maps are challenging in hexaploids with polysomic inheritance. Full map integration requires calculations of recombination frequency between markers with complex segregation types. In addition, detection of QTL in hexaploids requires information on all six alleles at one locus for each individual. We describe a method that we used to construct a fully integrated linkage map for chrysanthemum (Chrysanthemum × morifolium, 2n = 6x = 54). A bi-parental F1 population of 406 individuals was genotyped with an 183,000 SNP genotyping array. The resulting linkage map consisted of 30,312 segregating SNP markers of all possible marker dosage types, representing nine chromosomal linkage groups and 107 out of 108 expected homologues. Synteny with lettuce (Lactuca sativa) showed local colinearity. Overall, it was high enough to number the chrysanthemum chromosomal linkage groups according to those in lettuce. We used the integrated and phased linkage map to reconstruct inheritance of parental haplotypes in the F1 population. Estimated probabilities for the parental haplotypes were used for multi-allelic QTL analyses on four traits with different underlying genetic architectures. This resulted in the identification of major QTL that were affected by multiple alleles having a differential effect on the phenotype. The presented linkage map sets a standard for future genetic mapping analyses in chrysanthemum and closely related species. Moreover, the described methods are a major step forward for linkage mapping and QTL analysis in hexaploids. Electronic supplementary material The online version of this article (doi:10.1007/s00122-017-2974-5) contains supplementary material, which is available to authorized users.

Chlorophyll fluorescence imaging was used to measure stomatal closure in response to desiccation of Tradescantia virginiana leaves grown under high (90%) and moderate (55%) relative humidities (RHs), or transferred between these humidities. Stomata in leaves grown at high RH were less responsive to desiccation than those of leaves grown at moderate RH. Stomata of plants transferred from moderate RH conditions to high RH showed the same diminished closure in response to desiccation as did stomata that developed at high RH. This response was found both when the leaves were fully expanded and when still actively expanding during the moderate RH pre-treatment. Four days of exposure to high RH was the minimal exposure time to induce the diminished closure response. When leaves were grown in high RH prior to a 10 d moderate RH treatment, the reduced stomatal closure response to desiccation was only reversed in leaves (regions) which were actively expanding during moderate RH treatment. This indicates that with respect to stomatal responses to desiccation, high RH leaf regions have a limited capacity to adapt to moderate RH conditions. The decrease in responsiveness to desiccation of the stomata, induced by long-term exposure to high RH, was not due to osmotic adjustment in the leaves. Within 1 d after transferring moderate RH-grown plants to a high RH, the abscisic acid (ABA) concentration of their leaves decreased to the low level of ABA found in high RH-grown leaves. The closure response in leaves exposed to high RH for 5 d, however, could not be fully restored by the application of ABA. Transferring plants from high to moderate RH resulted in increased ABA levels within 2 d without a recovery of the stomatal closing response. It is discussed that the diminished stomatal closure in plants exposed to high RH could be due to changes in the signalling pathway for ABA-related closure of stomata or to an increased sequestration of ABA by mesophyll tissue or the symplast in the epidermis, induced by a longer period (several days) of a low ABA level.

The spatial heterogeneity of stomatal closure in response to rapid desiccation of excised well-watered Tradescantia virginiana leaves grown at moderate (55%) or high (90%) relative air humidity (RH) was studied using a chlorophyll fluorescence imaging system under non-photorespiratory conditions. Following rapid desiccation, excised leaves grown at high RH had both a greater heterogeneity and a higher average value of PSII efficiency (Phi(PSII)) compared with leaves grown at moderate RH. Larger decreases in relative water content resulted in smaller decreases in water potential and Phi(PSII) of high RH-grown leaves compared with moderate RH-grown leaves. Moreover, the Phi(PSII) of excised high RH-grown leaves decreased less with decreasing water potential, implying that the stomata of high RH-grown leaves are less sensitive to decreases in leaf water potential compared with moderate RH-grown leaves. After desiccation, some non-closing stomata were distributed around the main vein in high RH-grown leaves. Direct measurements of stomatal aperture showed 77% stomatal closure in the margins after 2 h desiccation compared with 40% closure of stomata in the main-vein areas in high RH-grown leaves. Faster closure of stomata in leaf margins compared with main-vein areas of leaves grown at high RH was related to substantially lower relative water content in these areas of the leaves.

Plants produced at high relative air humidity (RH) show poor control of water loss after transferring to low RH, a phenomenon which is thought to be due to their stomatal behaviour. The stomatal anatomy and responses of moderate (55%) and high (90%) RH grown Tradescantia virginiana plants to treatments that normally induce stomatal closure, i.e. desiccation, abscisic acid (ABA) application and exposure to darkness were studied using attached or detached young, fully expanded leaves. Compared with plants grown at moderate RH the transpiration rate, stomatal conductance and aperture of high RH grown plants measured at the same condition (40% RH) were, respectively, 112, 139 and 132% in light and 141, 188 and 370% in darkness. Besides the differences in stomatal size (guard cell length was 56.7 and 73.3 µm for moderate and high RH grown plants, respectively), there was a clear difference in stomatal behaviour. The stomata responded to desiccation, ABA and darkness in both moderate and high RH grown plants, but the high variability of stomatal closure in high RH grown plants was striking. Some stomata developed at high RH closed in response to darkness or to a decrease in relative water content to the same extent as did stomata from moderate RH grown plants, whereas others closed only partly or did not close at all. Evidently, some as yet unidentified physiological or anatomical changes during development disrupt the normal functioning of some stomata in leaves grown at high RH. The failure of some stomata to close fully in response to ABA suggests that ABA deficiency was not responsible for the lack of stomatal closure in response to desiccation.

Exposing plants to low VPD reduces leaf capacity to maintain adequate water status thereafter. To find the impact of VPD on functioning of stomata, stomatal morphology and leaf anatomy, fava bean plants were grown at low (L, 0.23 kPa) or moderate (M, 1.17 kPa) VPDs and some plants that developed their leaves at moderate VPD were then transferred for 4 days to low VPD (M[RIGHTWARDS ARROW]L). Part of the M[RIGHTWARDS ARROW]L-plants were sprayed with ABA (abscisic acid) during exposure to L. L-plants showed bigger stomata, larger pore area, thinner leaves and less spongy cells compared with M-plants. Stomatal morphology (except aperture) and leaf anatomy of the M[RIGHTWARDS ARROW]L-plants were almost similar to the M-plants, while their transpiration rate and stomatal conductance were identical to that of L-plants. The stomatal response to ABA was lost in L-plants, but also after 1-day exposure of M-plants to low VPD. The level of foliar ABA sharply decreased within 1-day exposure to L, while the level of ABA-GE (ABA-glucose ester) was not affected. Spraying ABA during the exposure to L prevented loss of stomatal closing response thereafter. The effect of low VPD was largely depending on exposure time: the stomatal responsiveness to ABA was lost after 1-day exposure to low VPD, while the responsiveness to desiccation was gradually lost during 4-day exposure to low VPD. Leaf anatomical and stomatal morphological alterations due to low VPD were not the main cause of loss of stomatal closure response to closing stimuli.

The response of stomata to many environmental factors is well documented. Multiple signalling pathways for abscisic acid (ABA)-induced stomatal closure have been proposed over the last decades. However, it seems that exposure of a leaf for a long time (several days) to some environmental conditions generates a sort of memory in the guard cells that results in the loss of suitable responses of the stomata to closing stimuli, such as desiccation and ABA. In this review paper we discuss changes in the normal pattern of signal transduction that could account for disruption of guard cell signalling after long-term exposure to some environmental conditions, with special emphasis on long-term low vapour pressure deficit (VPD).

Soil-bound intensive greenhouse production has been scrutinized for its sustainability due to contamination of ground water by over-fertilization resulting in leaching of nutrients. As environmental guidelines are becoming more restrictive worldwide, and especially in Europe, many greenhouse growers have converted to more sustainable production systems including rockwool culture with recycled water and organic cropping systems in soil. The increase in popularity of organic production systems has amplified the debate whether organically grown produce is healthier than conventional produce. So far, little is known about the variations in fruit quality associated with production systems for greenhouse grown tomatoes. Thus, two organic (organic fertilization with and without straw amendment) and three conventional tomato cropping systems (regular and increased nutrient solution in rockwool and regular fertilization in soil) were compared in order to evaluate differences in nutrient availability and effects on fruit quality over a three-year period. Three modern medium-sized round tomato cultivars and one old cultivar were compared. There were no significant interactions between cropping systems and cultivars, so that main effects of systems and cultivars could be evaluated. Fruit yields in the organic systems were similar to those obtained in the conventional soil-bound system, but 15% lower than in the regular rockwool system, even though nitrogen concentrations in soil were not limiting in any of the production systems. Frequent organic amendments resulted in higher soil contents in the organic system without straw than in the other soil-bound systems, indicating that the organic systems were not yet stable in terms of nutrient availability after three years. A fruit quality index, based on the contents of compounds such as lycopene, ß-carotene and vitamin C, was similar in all cropping systems. The old cultivar had a significantly higher quality index, but a lower yield than the other cultivars. According to this study, high quality tomatoes can be obtained through proper adjustment of the quantity and the source of nitrogen fertilizers in organic and conventional cropping systems and the use of selected cultivars with a high nutrient use efficiency for organic systems. zijn vergeleken gedurende een periode van drie jaar. De beschikbaarheid van nutriënten en de effecten op de vruchtkwaliteit in twee biologische en drie gangbare teeltsystemen van tomaat zijn vergeleken gedurende een periode van drie jaar. Drie moderne middelgrote ronde cultivars en één oude cultivar zijn vergeleken. De opbrengst is de biologische systemen waren vergelijkbaar met die van de gangbare grondgebonden teelt, maar 15% lager dan die van teelt op steenwol, ondanks dat de stikstofconcentraties in de grond niet beperkend waren. Een vruchtkwaliteitindex, gebaseerd op op concentraties van stoffe zoals lycopeen, β-caroteen en vitamine C, was vergelijkbaar voor alle teeltsystemen. De oude cultivar had een hogere vruchtkwaliteitindex, maar een lagere opbrengst dan de andere cultivars. Volgens deze studie kan een hoge kwaliteit tomaten verkregen worden door de juiste hoeveelheid stikstofmeststoffen toe te passen in biologische en gangbare teelten en het gebruik van cultivars met een efficiënte stikstofbenutting voor biologische teelten.

In this study, the role of abscisic acid (ABA) in altered stomatal responses of Tradescantia virginiana leaves grown at high relative air humidity (RH) was investigated. A lower ABA concentration was found in leaves grown at high RH compared with leaves grown at moderate RH. As a result of a daily application of 20 microM ABA to leaves for 3 weeks during growth at high RH, the stomata of ABA-treated leaves grown at high RH showed the same behaviour as did the stomata of leaves grown at moderate RH. For example, they closed rapidly when exposed to desiccation. Providing a high RH around a single leaf of a plant during growth at moderate RH changed the stomatal responses of this leaf. The stomata in this leaf grown at high RH did not close completely in response to desiccation in contrast to the stomata of the other leaves from the same plant. The ABA concentration on a fresh weight basis, though not on a dry weight basis, of this leaf was significantly lower than that of the others. Moreover, less closure of stomata was found in the older leaves of plants grown at high RH in response to desiccation compared with younger leaves. This was correlated with a lower ABA concentration in these leaves on a fresh weight basis, though not on a dry weight basis. Stomata of leaves grown at moderate RH closed in response to short-term application of ABA or sodium nitroprusside (SNP), while for leaves grown at high RH there was a clear difference in stomatal responses between the leaf margins and main-vein areas. The stomatal aperture in response to short-term application of ABA or SNP at the leaf margins of leaves grown at high RH remained significantly wider than in the main-vein areas. It was concluded that: (i) a long-term low ABA concentration in well-watered plants during growth at high RH could be a reason for less or no stomatal closure under conditions of drought stress; and (ii) the long-term ABA concentration on a fresh weight basis rather than on a dry weight basis is likely to be responsible for structural or physiological changes in stomata during leaf growth.

Flower opening and closure are traits of a reproductive syndrome, as it allows pollen removal and/or pollination. Various types of opening can be distinguished such as nocturnal and diurnal and single or repetitive. Opening is generally due to cell expansion. Osmotic solute levels increase by the conversion of polysaccharides (starch or fructan) to monosaccharides, and/or the uptake of sugars from the apoplast. Repeated opening and closure movements are often brought about by differential elongation. In tulip petals, for example, the upper and lower sides of the mesophyll exhibit a 10 degrees C difference in optimum temperature for elongation growth, resulting in opening in the morning and closure in the evening. Opening and closure in several other species is regulated by changes in light intensity and, in some species with nocturnal opening, by an increase in relative humidity. A minimum duration of darkness and light are usually required for opening and closure, respectively, in flowers that open during the day. Both phytochrome and a blue light receptor seem involved in light perception. In some species, opening and closure are regulated by an endogenous rhythm, which, in all cases investigated, can be reset by changes from dark to light and/or light to dark. So far, Arabidopsis mutants have not been used to investigate the timing of flower opening and closure. As its flowers open and close in a circadian fashion, several mutants that are involved in the circadian clock and its light input may help to provide an insight into this type of flower opening. The co-ordination of processes culminating in synchronized flower opening is, in many species, highly intricate. The complex control by endogenous and exogenous factors sets flower opening and closure apart from most other growth processes.

Keywords: Abscisic acid, Arabidopsis thaliana, calcium, CYP707As, desiccation, environmental factors, guard cells’ signalling pathway, hydrogen peroxide, natural variation, nitric oxide, photosystem II efficiency, RD29A, relative water content, secondary messengers, stomata, vapour pressure deficit, Vicia faba In short-term, guard cells close stomata in response to an increase in vapour pressure deficit (VPD) and they open the stomata after exposure to low VPDs. However, in long-term responses to low VPD, adaptation processes occur which make stomata less sensitive to stimuli which usually induce stomatal closure (stomatal malfunctioning). Cellular mechanism(s) leading to occurrence of stomatal malfunctioning is (are) still unknown. The aim of this project was to elucidate the processes that are involved in the malfunctioning of stomata after long-term exposure to low VPD. To elucidate whether the problem of stomatal malfunctioning is due to alterations in stomatal morphology and leaf anatomy or in the ABA signalling pathway, fava bean plants were grown at low or moderate VPDs and some plants that developed their leaves at moderate VPD were then transferred for four days to low VPD. Leaf anatomical and stomatal morphological alterations due to low VPD were not the main reason of stomatal malfunctioning in response to ABA and desiccation. Within one day exposure to low VPD, the level of foliar ABA decreased to the same level as in low VPD-grown plants, while the level of ABA-glucose ester was not affected. Spraying ABA during a 4-day exposure to low VPD maintained closing ability of the stomata after 4-day low VPD-exposure. Therefore, alteration in the signalling pathways due to low foliar ABA level was recognized as the main reason for stomatal malfunctioning after long-term low VPD-exposure. Coincidence in changes of Ca2+, ABA receptors, and positive and negative regulators of ABA signalling are proposed as early steps for stomatal malfunctioning induced by low VPD-exposure. Transcriptional activators, transcriptional repressors as well as E3 ligases are proposed for long-term adaptation of cellular processes which consequently cause decreased stomatal response to closing stimuli afterwards. In order to find the molecular mechanism(s) of stomatal malfunctioning, possible variation in stomatal response to closing stimuli was studied among Arabidopsis thaliana accessions after a 4-day low VPD-exposure. Accessions could be grouped to very sensitive, moderately sensitive and less sensitive to closing stimuli using principle component analysis. A positive correlation was found between foliar ABA level (before desiccation) and stomatal closure response to ABA (but not to desiccation) after exposure to different VPDs. Stomatal response to desiccation was positively correlated with the foliar ABA level after desiccation. In order to elucidate the molecular network underlying stomatal malfunctioning in response to ABA due to long-term low VPD-exposure, two groups of Arabidopsis accessions were used as accessions that maintained responsiveness to ABA after low VPD-exposure and accessions with low VPD induced non-ABA-responsive stomata. The foliar ABA content in all accessions correlated with the stomatal response to ABA: only when the ABA level was above a threshold value, stomata responded to ABA. After low VPD-exposure, mainly due to catabolism of ABA, the foliar ABA content decreased. This decrease in ABA level resulted in down regulation of RD29A, which caused decreased stomatal responsiveness to ABA.

As a result of contemporary horticultural practices relative air humidities (RH) in greenhouses are often very high. In particular in cut flowers, this results in quality problems after harvest when flowers are transferred to low RH conditions at the consumers. The quality problems are related to excessive water loss caused by a disturbance in normal functioning of stomata. The general aim of this study was to investigate the effects of high RH during growth on the stomatal response characteristics of Tradescantia virginiana L. To reach this aim, the quantitative effects of moderate (55%) and high (90%) RH during growth on the stomatal anatomy and responses of well-watered plants in response to desiccation, abscisic acid (ABA) application and light/dark transition were studied. Higher variability of stomatal closure and the presence of some non-closing stomata in high RH grown plants were striking. The dynamics of spatial heterogeneity of stomatal closure within a leaf altered by growth at high RH were studied using a chlorophyll fluorescence imaging system under non-photorespiratory conditions, and the estimates of stomatal closure obtained by means of Φ PSII measurements was correlated with direct measurements of stomatal closure and water relation parameters in plants subjected to water stress. It was shown that following desiccation, leaves grown at high RH had both a greater heterogeneity and a higher PSII efficiency compared to leaves grown at moderate RH. The stomata of high RH grown leaves were less sensitive to decreases in leaf relative water content and water potential. Moreover, the role ofABAin the disturbed stomatal responses of plants during growth at high RH was studied. The concentration ofABAin leaves grown at moderate and high RH was compared and stomatal responses to short-termABAapplication, and stomatal responses to long-termABAapplication was analysed.The results reinforce the proposal that a lowABAconcentration in well-watered plants during growth at high RH could be a reason for the structural or physiological changes of stomata. In addition, the consequences for stomatal responses of maintaining a high or low RH around only one leaf are described. The rate and the reversibility of adaptation of stomatal behaviour to moderate or high RH was also investigated and correlated with leaf ABA concentration and leaf hydraulic properties of the adapted plants. The main achievements and practical implications of this study are discussed, and suggestions for further research are presented.