Decline In Leaf Water Potential Research Articles

Mucilage facilitates root water uptake under edaphic stress: first evidence at the plant scale.

Mucilage has been hypothesized to soften the gradients in matric potential at the root-soil interface, hereby facilitating root water uptake in dry soils and maintaining transpiration with a moderate decline in leaf water potential. So far, this hypothesis has been tested only through simplified experiments and numerical simulations. However, the impact of mucilage on the relationship between transpiration rate (E) and leaf water potential (ψleaf) at the plant scale remains speculative. We utilized an automated root pressure chamber to measure the E(ψleaf) relationship in two cowpea genotypes with contrasting mucilage production. We then leveraged a soil-plant hydraulic model to reproduce the experimental observations and inferred the matric potential at the root-soil interface for both genotypes. In wet soil, the relationship between the leaf water potential and transpiration rate (E) was linear for both genotypes. However, as the soil progressively dried, the E(ψleaf) relationship exhibited nonlinearity. Genotype with low mucilage production exhibited nonlinearity earlier during soil drying, i.e. in wetter soil conditions, (soil water content < 0.36cm3 cm-3) compared to Genotype with high mucilage production (soil water content < 0.30cm3 cm-3). The incidence of nonlinearity was concomitant with the decline in matric potential across the rhizosphere. High mucilage production attenuated water potential diminution at the root-soil interface with increased E. This shows, for the first time at the plant scale, that root mucilage softened the gradients in matric potential and maintained transpiration in drying soils. The model simulations indicate that a plausible explanation for this effect is an enhanced hydraulic conductivity of the rhizosphere in genotype with higher mucilage production. Mucilage exudation maintains the hydraulic continuity between soil and roots and decelerates the drop in matric potential near the root surface, hereby postponing the hydraulic limitations to transpiration during soil drying.

Stomatal regulation prevents plants from critical water potentials during drought: Result of a model linking soil–plant hydraulics to abscisic acid dynamics

AbstractUnderstanding stomatal regulation during drought is essential to correctly predict vegetation‐atmosphere fluxes. Stomatal optimization models posit that stomata maximize the carbon gain relative to a penalty caused by water loss, such as xylem cavitation. However, a mechanism that allows the stomata to behave optimally is unknown. Here, we introduce a model of stomatal regulation that results in similar stomatal behaviour without presupposing an optimality principle. By contrast, the proposed model explains stomatal closure based on a well‐known component of stomatal regulation: abscisic acid (ABA). The ABA level depends on its production rate, which is assumed to increase with declining leaf water potential, and on its degradation rate, which is assumed to increase with assimilation rate. Our model predicts that stomata open until the ratio of leaf water potential to assimilation rate, proportional to ABA level, is at a minimum. As a prerequisite, the model simulates soil–plant hydraulics and leaf photosynthesis under varying environmental conditions. The model predicts that in wet soils and at low vapour pressure deficit (VPD), when there is no water limitation, stomatal closure is controlled by the relationship between photosynthesis and stomatal conductance. In dry soils or at high VPD, when the soil hydraulic conductivity limits the water supply, stomatal closure is triggered by the sharp decline in leaf water potential as transpiration rate increases. Being adaptive to changing soil and atmospheric conditions, the proposed model can explain how plants are enabled to avoid critical water potentials during drought for varying soil properties and atmospheric conditions.

Drought resistance increases from the individual to the ecosystem level in highly diverse Neotropical rainforest: a meta-analysis of leaf, tree and ecosystem responses to drought

Abstract. The effects of future warming and drying on tropical forest functioning remain largely unresolved. Here, we conduct a meta-analysis of observed drought responses in Neotropical humid forests, focusing on carbon and water exchange. Measures of leaf-, tree- and ecosystem-scale performance were retrieved from 145 published studies conducted across 232 sites in Neotropical forests. Differentiating between seasonal and episodic drought, we find that (1) during seasonal drought the increase in atmospheric evaporative demand and a decrease in soil matric potential result in a decline in leaf water potential, stomatal conductance, leaf photosynthesis and stem diameter growth while leaf litterfall and leaf flushing increase. (2) During episodic drought, we observe a further decline in stomatal conductance, photosynthesis, stem growth and, in contrast to seasonal drought, a decline also in daily tree transpiration. Responses of ecosystem-scale processes, productivity and evapotranspiration are of a smaller magnitude and often not significant. Furthermore, we find that the magnitude and direction of a drought-induced change in photosynthesis, stomatal conductance and transpiration reported in a study is correlated to study-averaged wood density. Although wood density is often not functionally related to plant hydraulic properties, we find that it is a good proxy for hydraulic behaviour and can be used to predict leaf- and tree-scale responses to drought. We present new insights into the functioning of tropical forest in response to drought and present novel relationships between wood density and tropical-tree responses to drought.

The importance of cuticular permeance in assessing plant water–use strategies

Accurate understanding of plant responses to water stress is increasingly important for quantification of ecosystem carbon and water cycling under future climates. Plant water use strategies can be characterized across a spectrum of water stress responses, from tight stomatal control (isohydric) to distinctly less stomatal control (anisohydric). A recent and popular classification method of plant water use strategies utilizes the regression slope of predawn and midday leaf water potentials, σ, to reflect the coupling of soil water availability (predawn leaf water potential) and stomatal dynamics (daily decline in leaf water potential). This type of classification is important in predicting ecosystem drought response and resiliency. However, it fails to explain the relative stomatal responses to drought of Acer sacharrum and Quercus alba, improperly ranking them on the spectrum of isohydricity. We argue this inconsistency may be in part due to the cuticular conductance of different species. We used empirical and modeling evidence to show that plants with more permeable cuticles are more often classified as anisohydric; the σ values of those species were very well correlated with measured cuticular permeance. Furthermore, we found that midday leaf water potential in species with more permeable cuticles would continue to decrease as soils become drier, but not in those with less permeable cuticles. We devised a diagnostic parameter, Γ, to identify circumstances where the impact of cuticular conductance could cause species misclassification. The results suggest that cuticular conductance needs to be considered to better understand plant water use strategies and to accurately predict forest responses to water stress under future climate scenarios.

The Effect of Calcium on Photosynthetic Rate due to ABA and Proline Behaviour of Oil Palm (Elaeis guineensis Jacq.) Seedlings under Drought Conditions

&lt;p&gt;Planting drought-resistance plants in terms of agronomy, such as induction of plant tolerance using calcium is assumed to be able to solve the climate anomaly problem. Calcium is known as an element that plays an essential role in determining the response of plant resistance to drought through biochemical activity. This study aimed to determine the role of calcium in changing photosynthesis activity in order to increase the resistance to drought stress. The treatment was arranged in factorial of 3 x 4 in a split plot Randomized Complete Block Design replicated three times. The first factor was the dose of calcium application consisted of 0 (control/without calcium), 0.04, 0.08 and 0.12 g. The second factor was the intensity of drought stress, which referred to the Fraction of Transpirable Soil Water method consisted of 1 (control/field capacity), 0.35 (moderate drought) and 0.15 (severe drought). The measurement data of stomatal aperture, Abscisic Acid (ABA) content, chlorophyll content, carotenoid content, proline content, nitrate reductase activity and photosynthesis rate that fulfill the assumption of homogeneity and normality were analyzed using variance at 95% accuracy and continued using DMRT. Moreover, regression analysis were determined of relationship between the treatment and parameters. The results revealed that drought resulted in a decline in leaf water potential and stomatal aperture. The effects of calcium on chlorophyll and carotenoid under drought stress could not be explained in this study. However, the application of calcium has a significant effect on decreased ABA, increased proline and nitrate reductase activity resulting in an increase in the photosynthetic rate of oil palm seeds in drought stress.&lt;/p&gt;

Drought stress adaptation modulates plant secondary metabolite production in Salvia dolomitica Codd

Sage is an important medicinal and aromatic plant. While Salvia officinalis and S. miltiorrhiza have been widely studied, little information regarding S. dolomitica exists, although it has recently attracted attention due to its anti-plasmodial and anti-inflammatory properties. This study investigated the performance and metabolic profile of this species in response to two drought treatments (moderate or severe) relative to well-watered control plants. Changes in growth and ecophysiological traits, as well as in bioactive and volatile compounds and essential oil production were determined. Given that terpenoids are the most representative class of secondary metabolites, the gene expression of key enzymes of terpenoid biosynthesis was also investigated. Moderate drought stimulated a decline in leaf water potential, growth and stomatal conductance, as well as an increase in deyhdrin expression. Serious stress symptoms occurred only in severe drought-stressed plants, where a decline in net photosynthesis and transpiration and an increase in endogenous abscisic acid was observed. Both drought stress conditions led to modulate the expression of some genes involved in biogenic volatile organic compound and essential oil biosynthesis and metabolic profile. In particular, drought induced an increase in sesquiterpene production, a class of terpenoids that is important in the food, cosmetics, and pharmaceutical industries. Thus, controlled drought, in addition to water savings during cultivation, can be applied to improve the production of secondary metabolites in S. dolomitica.

Stomatal and growth responses to hydraulic and chemical changes induced by progressive soil drying.

A better understanding of physiological responses of crops to drought stress is important for ensuring sustained crop productivity under climate change. Here, we studied the effect on 15-day-old maize (Zea mays L.) plants of a 6 d non-lethal period of soil drying [soil water potential (SWP) decreased from -0.20 MPa to -0.81 MPa]. Root growth was initially stimulated during drying (when SWP decreased from -0.31 MPa to -0.38 MPa, compared with -0.29 MPa in well-watered pots), followed by inhibition during Days 5-6 (SWP from -0.63 MPa to -0.81 MPa). Abscisic acid (ABA) in the root began to accumulate as the root water potential declined during Days 2-3. Leaf elongation was inhibited from Day 4 (SWP less than -0.51 MPa), just after leaf ABA content began to increase, but coinciding with a decline in leaf water potential. The stomatal conductance was restricted earlier in the younger leaf (fourth) (on Day 3) than in the older leaf (third). The ethylene content of leaves and roots decreased during drying, but after the respective increase in ABA contents. This work identified critical timing of hydraulic and chemical changes at the onset of soil drying, which can be important in initiating early stomatal and growth responses to drought.

Potassium improves photosynthetic tolerance to and recovery from episodic drought stress in functional leaves of cotton (Gossypium hirsutum L.)

To investigate whether potassium (K) application enhances the potential of cotton (Gossypium hirsutum L.) plants to maintain physiological functions during drought and recovery, low K-sensitive (Siza 3) and -tolerant (Simian 3) cotton cultivars were exposed to three K rates (0, 150, and 300 K2O kg ha−1) and either well-watered conditions or severe drought stress followed by a recovery period. Under drought stress, cotton plants showed a substantial decline in leaf water potential, stomatal conductance, photosynthetic rate, and the maximum and actual quantum yield of PSII, resulting in greater non-photochemical quenching and lipid peroxidation as compared to well-watered plants. However, plants under K application not only showed less of a decline in these traits but also displayed greater potential to recover after rewatering as compared to the plants without K application. Plants receiving K application showed lower lipid peroxidation, higher antioxidant enzyme activities, and increased proline accumulation as compared to plants without K application. Significant relationships between rates of photosynthetic recovery and K application were observed. The cultivar Siza 3 exhibited a more positive response to K application than Simian 3. The results suggest that K application enhances the cotton plant's potential to maintain functionality under drought and facilitates recovery after rewatering.

Diurnal depression in leaf hydraulic conductance at ambient and elevated [CO2] reveals anisohydric water management in field-grown soybean and possible involvement of aquaporins

Diurnal cycles of photosynthesis and water use in field-grown soybean (Glycine max) are tied to light intensity and vapor pressure deficit (VPD). At high mid-day VPD, transpiration rates can lead to a decline in leaf water potential (Ψleaf) if leaf hydraulic conductance (Kleaf) is insufficient to supply water to intercellular airspaces in pace with demand. Kleaf is determined by leaf xylem conductivity to water, as well as extra-xylem pathways that are likely mediated by aquaporin water transport proteins. When transpiration demand exceeds the maximum capacity of Kleaf to supply water, high tension in the water column can cause cavitation in xylem, and these emboli-blocked xylem vessels reduce water transport and thus lower Kleaf. Stomatal conductance typically remains high at mid-day for soybean, suggesting either a mid-day increase in Kleaf or that photosynthesis may be maintained at the cost of leaf water status, indicative of an anisohydric water management strategy in soybean. This study examined diurnal fluctuations in Kleaf and Ψleaf, showing a mid-day depression in Kleaf in a pattern closely reflecting that of Ψleaf, indicating that Kleaf depression is the result of cavitation in leaf xylem. The diurnal depression of Kleaf was not prevented by growth at elevated [CO2], which lowered stomatal conductance. Diurnal transcription patterns of aquaporin genes showed that a total of 34 genes belonging to 4 aquaporin families were expressed in soybean leaves, of which 22 were differentially expressed between at least two time points. These data suggest that mid-day Kleaf depression was driven primarily by cavitation at increasing xylem water tensions, but that aquaporins are also likely involved in diurnal regulation of soybean leaf water status. It is further concluded that because soybean photosynthesis is typically sustained at mid-day, Kleaf even at the depressed level was in excess of that needed to sustain a stomatal conductance sufficient to prevent depression of photosynthesis in soybean.

Sensitivity of Two Quinoa (ChenopodiumquinoaWilld.) Varieties to Progressive Drought Stress

AbstractQuinoa (ChenopodiumquinoaWilld.) is a highly nutritious Andean seed crop which shows great potential to grow under a range of hostile environments. The objective of this study was to investigate the differences of drought tolerance of a Bolivian (Achachino) and a Danish (Titicaca) variety, and especially drought‐related adaption strategies. Soil water status was expressed as the fraction of transpirable soil water (FTSW). Relative stomatal conductance (RSC), relative transpiration (RT) and relative leaf water potential (RLW) were calculated by determining stomatal conductance, transpiration rate and leaf water potential of the drought‐treated plants relative to those of fully irrigated plants. The responses of RSC, RT and RLW to decreasing FTSW were described by a linear‐plateau model. The critical value of FTSW was the threshold of FTSW where the parameters studied decreased. The thresholds increased CS for stomatal conductance, CT for transpiration and CLfor leaf water potential. Achachino showed significantly lower CT and CL when compared with Titicaca, implying that transpiration and leaf water potential were less affected under mild drought conditions in the Bolivian variety. CS in Achachino was significantly higher than CL and CT, which indicated that stomatal conductance declined before transpiration and leaf water potential were reduced. Such difference was found in Titicaca where reduction of leaf area had more effect on transpiration than stomatal closure. Slower growth rate and smaller leaf area in combination with a lower stomatal conductance was found to contribute to drought resistance in Achachino. ABA concentration in the xylem sap tended to increase in both varieties after 2 days onset of drought, prior to decline in leaf water potential. Titicaca showed significantly (P &lt; 0.05) higher ABA concentration when compared with Achachino under both fully irrigated and drought conditions. Titicaca had higher xylem nutrient concentration in comparison with Achachino in both fully‐watered and drought plants at day 2 after onset of soil drying. It was concluded that Titicaca was more sensitive to progressive drought than Achachino which avoided water loss by means of lower growth rate and smaller leaf area.

Compatible Solute Engineering in Plants for Abiotic Stress Tolerance - Role of Glycine Betaine

Abiotic stresses collectively are responsible for crop losses worldwide. Among these, drought and salinity are the most destructive. Different strategies have been proposed for management of these stresses. Being a complex trait, conventional breeding approaches have resulted in less success. Biotechnology has emerged as an additional and novel tool for deciphering the mechanism behind these stresses. The role of compatible solutes in abiotic stress tolerance has been studied extensively. Osmotic adjustment, at the physiological level, is an adaptive mechanism involved in drought or salinity tolerance, which permits the maintenance of turgor under conditions of water deficit, as it can counteract the effects of a rapid decline in leaf water potential. Increasing evidence from a series of in vivo and in vitro studies of the physiology, biochemistry, genetics, and molecular biology of plants suggest strongly that Glycine Betaine (GB) performs an important function in plants subjected to environmental stresses. It plays an adaptive role in mediating osmotic adjustment and protecting the sub-cellular structures in stressed plants, protection of the transcriptional and translational machineries and intervention as a molecular chaperone in the refolding of enzymes. Many important crops like rice do not accumulate glycinebetaine under stress conditions. Both the exogenous application of GB and the genetically engineered biosynthesis of GB in such crops is a promising strategy to increase stress tolerance. In this review we will discuss the importance of GB for abiotic stress tolerance in plants. Further, strategies like exogenic application and transgenic development of plants accumulating GB will be also be discussed. Work done on exogenic application and genetically engineered biosynthesis of GB will be listed and its advantages and limitations will be described.

Limits to the height growth of Caragana korshinskii resprouts

Predawn leaf water potential (LWP), the LWP between 09:00 and 10:30 h (termed minimum LWP), stem xylem hydraulic conductivity, foliar nitrogen, leaf gas exchange and leaf traits were measured on the same days in adults and 1-year-old to 7-year-old resprouts that had regrown after removing all the aboveground shoots. Height growth and accumulation of aboveground biomass quickly decreased with resprout age and there was no difference between 7-year-old resprouts and the uncut adults. Predawn LWP showed no significant difference between resprouts and adults, but the minimum LWP decreased gradually from -2.0 MPa in 1-year-old resprouts to -3.0 MPa in 7-year-old resprouts. The decrease in minimum LWP was associated with increased hydraulic resistance, as indicated by the gradual decrease in leaf area-specific hydraulic conductivity (KL) and sapwood area-specific hydraulic conductivity (KS) and the associated increase in stem native percentage loss of hydraulic conductivity in older than 2-year-old resprouts. The leaf nitrogen content per unit area (Narea) also decreased steadily from 3.6 g m(-2) in 1-year-old resprouts to 1.7 g m(-2) in 7-year-old resprouts. With the decline in LWP and Narea, the rate of leaf photosynthesis per unit area (Aarea) decreased from 20 μ mol m(-2) s(-1) in 1-year-old resprouts to 11 μ mol m(-2) s(-1) in 7-year-old resprouts. In adults, although KS decreased further compared with 7-year-old resprouts, the minimum LWP, KL, Narea and the rate of photosynthesis increased by 0.3 MPa, 29, 34 and 23%, respectively. The results show that a progressive loss of stem hydraulic conductivity and a steady decrease in foliar nitrogen with age were associated with a decrease in the photosynthetic rate of Caragana korshinskii Kom. resprouts, possibly changing the allocation of photosynthetic assimilates and slowing resprout height growth.

Low O 2 supply is involved in the poor growth in root-restricted plants of tomato ( Lycopersicon esculentum Mill.)

To investigate whether O 2 deficit is involved in the depressed shoot growth under root restriction condition, tomato ( Lycopersicon esculentum Mill.) plants were grown hydroponically and subjected to root restriction with or without supplemental aeration. Plant biomass, leaf nutrient concentration, dissolved O 2 concentration in nutrient solution, root respiration, root hydrolytic ATPase activities, xylem sap abscisic acid (ABA) concentration, water relations, leaf gas exchange, and root cell viability were investigated throughout the experiment. Root restriction significantly depressed root and shoot growth as early as 15 days after treatment and this depressive effect was alleviated by vigorous aeration around the restricted root zone. Growth suppression by root restriction occurred concomitantly with sharp decreases in dissolved O 2 concentration in solution together with significant decreases in root total respiration, cytochrome pathway capacity, hydrolytic ATPase activities, and root cell viability. However, no such decreases were found in well aerated root restriction plants. Root restriction-induced growth suppression was independent of nutrients level in leaves and was not primarily related to the decline of leaf water potential or the gas exchange. Root restriction resulted in an increase in xylem sap ABA concentration from day 15 to the end of the experiment but no such effect was observed in well aerated plants. It is likely therefore, that O 2 was one of the main limiting factors to the reduced shoot growth under root restriction condition.

Seasonal Variations in Soil and Plant Water Status in a Quercus suber L. Stand: Roots as Determinants of Tree Productivity and Survival in the Mediterranean-type Ecosystem

Studies were conducted to examine changes in soil (Ψs) and plant water status during summer in a 16-year old Quercus suber plantation in southern Portugal. Continuous measurements were conducted between May 2003 and August 2004, while discontinuous measurements were conducted on a monthly basis between May and September 2003 and repeated between March and September 2004. Intensive measurements were conducted on five trees with mean height and DBH of 5.3 m and 11.6 cm, respectively, growing at close proximity to each other. Weather conditions and soil water potential (Ψs) at the rhizosphere of each of the trees measured at 0.3 and 1 m soil depth were continuously monitored. Predawn (Ψpd) and midday (Ψmd) leaf water potentials were determined every month. Soil and plant samples were also collected in June and September from different locations within the study site for δ18O isotope composition analysis. Pressure–volume (p–v) curves were constructed from plant shoots at different times during the vegetative period to determine osmotic potential at full saturation (Π100), water potential at turgor loss point (Ψtlp), relative water content at turgor loss point (R*tlp) and bulk modulus of elasticity (e). Significant P < 0.05 decline in Ψs occurred between May and September, the lowest value recorded being –2.0 MPa. Decline in soil moisture affected tree water status, but decline in leaf water potential varied significantly (P < 0.05) among the trees. At the end of summer drought, lowest Ψpd measured was –1.7 MPa while the highest measured during this time was –0.8 MPa. Differences among trees were attributed to differences in rooting depth, as shown by regression analysis of 18O isotopes. Radial stem growth ceased when Ψs within the upper 0.3 m depth approached –1.5 MPa. The upper soil layers contributed approximately 33% of the total tree water requirement, between spring and mid summer when drought was experienced by trees. Deep soil layers however, supplied most of the water required during drought and no growth was recorded during this time. Stressed trees increased solute concentration of their tissues by a Magnitude of 0.7 MPa while bulk tissue elastic modulus increased by about 17 MPa. The study emphasizes the significance of roots as determinants of tree productivity and survival in the Mediterranean ecosystems.

Leaf physiology does not predict leaf habit; examples from tropical dry forest

Leaf structure and physiology are thought to be closely linked to leaf longevity and leaf habit. Here we compare the seasonal variation in leaf hydraulic conductance (kleaf) and water potential of two evergreen tree species with contrasting leaf life spans, and two species with similar leaf longevity but contrasting leaf habit, one being deciduous and the other evergreen. One of the evergreen species, Simarouba glauca, produced relatively short-lived leaves that maintained high hydraulic conductance year round by periodic flushing. The other evergreen species, Quercus oleoides, produced longer-lived leaves with lower kleaf and as a result minimum leaf water potential was much lower than in S. glauca (−2.8 MPa vs −1.6 MPa). Associated with exposure to lower water potentials, Q. oleoides leaves were harder, had a higher modulus of elasticity, and were less vulnerable to cavitation than S. glauca leaves. Both species operate at water potentials capable of inducing 20 (S. glauca) to 50% (Q. oleoides) loss of kleaf during the dry season although no evidence of cumulative losses in kleaf were observed in either species suggesting regular repair of embolisms. Leaf longevity in the deciduous species Rhedera trinervis is similar to that of S. glauca, although maximum kleaf was lower. Furthermore, a decline in leaf water potential at the onset of the dry season led to cumulative losses in kleaf in R. trinervis that culminated in leaf shedding.

Dehydrin Gene Expression and Leaf Water Potential Differs between Spring and Winter Cereals During Cold Acclimation

Summary Spring and winter cultivars of wheat (Triticum aestivum L.) and rye (Secale cereale L.) were cold acclimated using controlled environment and natural conditions. With respect to freezing tolerance, winter cereal seedlings could be distinguished from their spring counterparts by their initiation of acclimation at a warmer temperature, increasing in freezing tolerance sooner, and by achieving greater freezing tolerance at the end of the acclimation period. The timing and extent of expression of a family of dehydrin genes correlated with the increase in measured freezing tolerance in both spring and winter genotypes. The expression of these genes was detected sooner in the winter types, and dehydrin mRNA accumulated to higher levels in the winter cereals. Dehydrin transcripts could be detected throughout the acclimation period in winter cereals, but were only moderately expressed in spring cereals in response to acclimation. Similar results were obtained using western blot analysis with a dehydrin carboxy terminal antibody. Crown moisture content (CMC), crown osmotic potential (COP) and leaf water potential decreased in spring and winter cereals in response to acclimating conditions in both controlled environment and field conditions, but were lowest in fully acclimated winter cereals. However, the onset and rate of decrease in CMC and COP did not differ between the spring and winter genotypes, suggesting that neither CMC nor COP were involved in the initial regulation of dehydrin gene expression. Leaf water potential (LWP) also declined at similar rates in the spring and winter cereals in the field between September and November. However, a difference in LWP was observed between spring and winter wheat subjected to a cold shock treatment. The winter genotype LWP decreased within 10 h of exposure to 2 °C, reached significantly lower levels than prior to the cold shock, but returned to pre cold-shock level after 7 days at 2 °C. In contrast, the decline in leaf water potential in spring wheat was slower and less pronounced than in winter wheat. These results correlate well with those observed with dehydrin gene expression and suggest a relationship between water potential and cold-induced gene expression.

High vapour pressure deficit results in a rapid decline of leaf water potential and photosynthesis of carrots grown on free-draining, sandy soils

Two carrot (Daucus carota L.) genotypes (Nantes and Imperator) were grown in the field on a coarse-textured, sandy soil. Experiments were conducted over 2 consecutive seasons, one providing cool growing conditions and the other much warmer growing conditions during which the vapour pressure deficit was up to 2-fold higher than in the first season. Changes in growth, soil water content, and environmental conditions were monitored for both seasons, and diurnal measurements of leaf water potential and leaf photosynthesis were taken near maturity. Frequent irrigation maintained bulk soil water content above, or near, field capacity, with the sum of rainfall and irrigation exceeding potential evaporation by 1.4- and 1.3-fold during the cool and warm seasons, respectively. Even under such well-watered conditions, a large diurnal variation in leaf water potential (1200 and 1800 kPa for the cool- and warm-season crops, respectively) was recorded. During the cool season, withholding irrigation for up to 60 h resulted in further reductions in midday leaf water potential. However, there was no effect of withholding irrigation on leaf water potential during the warm season. During both seasons, leaf photosynthetic rate of well-watered plants peaked at around 20 µmol/m2.s in the early morning (0900 hours) and then decreased throughout the day, with the magnitude of the decline associated with the prevailing vapour pressure deficit. Under well-watered conditions, leaf water potential and photosynthesis were both negatively correlated with vapour pressure deficit, for both genotypes. Leaf water potential and photosynthesis were positively correlated with each other and we conclude that a high hydraulic resistance in the plant or soil results in a vapour pressure deficit-induced reduction in leaf water potential, which in turn reduces the rate of leaf photosynthesis.

Changes in net photosynthetic rate and chlorophyll fluorescence in potato leaves induced by water stress

Net rates of photosynthesis (PN) saturated by irradiance of >500 μmol m-2 s-1 (PAR) significantly decreased in water-stressed potato (Solanum tuberosum L. cv. Kufri Sindhuri) plants. The quantum yield of photochemical energy conversion (Fv/Fm), relative electron transport rate (ETR), and photochemical quenching (Qp) exhibited a parallel decline at high irradiance. A slight decrease in relative water content (RWC) was accompanied by a drastic decline in leaf water potential (Ψw) from -0.2 to -1.0 MPa. Dehydrated leaves showed an increase in the amount of total soluble sugars per unit leaf area which inhibited the photosynthesis in a feedback manner. After rewatering, PN and Fv/Fm were restored to the values of control plants within 24 h, and the restoration was accompanied by a proportionate lowering of content of total soluble sugars in the leaves.

Elevated CO2 mitigates chilling‐induced water stress and photosynthetic reduction during chilling

ABSTRACTBean, cucumber and corn plants were grown in controlled‐environment chambers at 25/18 °C day/night temperature and either ambient (350 μmol mol−1) or elevated (700 μmol mol−1) CO2 concentration, and at 20–30 d after emergence they were exposed to a 24 h chilling treatment (6.5 ± 1.5 °C) at their growth CO2 concentration. Whole‐plant transpiration rates (per unit leaf area basis) during the first 3 h of chilling were about 26,28 and 13% lower at elevated than at ambient CO2 for bean, cucumber and corn, respectively. The decline in leaf water potential (ψL) and visible wilting of bean and cucumber during chilling were significantly less at elevated than at ambient CO2. Corn ψL was not significantly affected by chilling, and corn did not exhibit any other symptoms of chilling‐induced water stress. Leaf osmotic potentials (measured before chilling only) of bean and cucumber were more negative at elevated than at ambient CO2, and the corresponding calculated leaf turgor potentials were significantly higher at elevated than at ambient CO2. Leaf relative water content (RWC) during chilling at ambient CO2fell to 62 and 48% for bean and cucumber, respectively. RWC during chilling at elevated CO2 was never below 79% for bean or 63% for cucumber. Corn RWC was not measured. After 24 h of chilling at ambient CO2, net photosynthetic rate (PN) reductions were 83, 89 and 24% for bean, cucumber and corn, respectively. PN reductions during chilling were less at elevated CO2: 53, 40 and 4% for bean, cucumber and corn, respectively. At ambient CO2, none of the species fully recovered to pre‐chilling PN, but at elevated CO2 both bean and corn recovered fully. The average percentage leaf area with visible leaf damage due to chilling was 20.6 and 9.6% at ambient and elevated CO2, respectively, for bean, and 32.4 and 23.6% at ambient and elevated CO2, respectively, for cucumber. Corn showed no significant permanent leaf damage from chilling at either CO2 concentration. These results indicate that cucumber was most sensitive to chilling as imposed in this study, followed by bean and corn. The results support the hypothesis that, at least in young plants under controlled‐environment conditions, elevated CO2 improves plant water relations during chilling and can mitigate photosynthetic depression and chilling damage. The implications for long‐term growth and reproductive success in managed and natural ecosystems will require testing of this hypothesis under field conditions.

Water Relations and Drought Tolerance of Four Turfgrasses

Greenhouse studies were conducted on three warm-season turfgrasses, `Midlawn' bermudagrass [Cynodon dactylon (L.) Pers. × C. transvaalensis Burtt-Davy], `Prairie' buffalograss [Buchloe dactyloides (Nutt.) Engelm.], and `Meyer' zoysiagrass (Zoysia japonica Steud.), and a cool-season turfgrass, `Mustang' tall fescue (Festuca arundinacea Schreb.) to determine 1) water relations and drought tolerance characteristics by subjecting container-grown grasses to drought and 2) potential relationships between osmotic adjustment (OA) and turf recovery after severe drought. Tall fescue was clipped at 6.3 cm once weekly, whereas warm-season grasses were clipped at 4.5 cm twice weekly. The threshold volumetric soil water content (SWC) at which a sharp decline in leaf water potential (ψL) occurred was higher for tall fescue than for warm-season grasses. Buffalograss exhibited the lowest and tall fescue exhibited the highest reduction in leaf pressure potential (ψP) per unit decline in ψL during dry down. Ranking of grasses for magnitude of OA was buffalograss (0.84 MPa) = zoysiagrass (0.77 MPa) &gt; bermudagrass (0.60 MPa) &gt; tall fescue (0.34 MPa). Grass coverage 2 weeks after irrigation was resumed was correlated positively with magnitude of OA (r = 0.66, P &lt; 0.05).