The efficiency of water use in water stressed plants is increased due to ABA induced stomatal closure

Gas exchange and abscisic acid content of Digitalis lanata EHRH. have been examined at different levels of plant water stress. Net photosynthesis, transpiration and conductance of attached leaves declined rapidly at first, then more slowly following the withholding of irrigation. The intercellular partial pressure of CO₂ decreased slightly. The concentration of 2-cis(S)ABA increased about eight-fold in the leaves of non-irrigated plants as compared with well-watered controls. A close linear correlation was found between the ABA content of the leaves and their conductance on a leaf area basis. In contrast, the plot of net assimilation versus ABA concentration was curvilinear, leading to an increased efficiency of water use during stress. After rewatering, photosynthesis reached control values earlier than transpiration, leaf conductance and ABA content. From these data it is concluded that transpiration through the stomata is directly controlled by the ABA content, whereas net photosynthesis is influenced additionally by other factors.Possible reasons for the responses of photosynthesis and water use efficiency to different stress and ABA levels are discussed.Abbreviations A net CO₂ assimilation - ABA abscisic acid - C_i intercellular CO₂ concentration - g stomatal conductance - T transpiration - WUE water use efficiency     

Abscisic Acid and stomatal regulation

The closure of stomata by abscisic acid was examined in several species of plants through measurements of CO₂ and H₂O exchange by the leaf. The onset of closure was very rapid, beginning at 3 minutes from the time of abscisic acid application to the cut base of the leaf of corn, or at 8 or 9 minutes for bean, Rumex and sugarbeet; rose leaves were relatively slow at 32 minutes. The timing and the concentration of abscisic acid needed to cause closure were related to the amounts of endogenous abscisic acid in the leaf. Closure was obtained in bean leaves with 8.9 picomoles/cm². (+)-Abscisic acid had approximately twice the activity of the racemic material. The methyl ester of abscisic acid was inactive, and trans-abscisic acid was likewise inactive. The effects of stress on levels of endogenous abscisic acid, and the ability of very small amounts of abscisic acid to cause rapid closure suggests that stomatal control is a regulatory function of this hormone.     

Very high CO2 partially restores photosynthesis in sunflower at low water potentials

We re-examined the question of whether the stomata limit photosynthesis in dehydrated sunflower (Helianthus annuus L.) plants having low leaf water potentials. A gas-exchange apparatus was modified to operate at external CO₂ partial pressures as high as 3000 Pa (3%), which were much higher than previously achieved. This allowed photosynthesis and stomatal behavior to be monitored simultaneously at very high CO₂ in the same leaf. The data were compared with those from leaves treated with abscisic acid (ABA) where effects on photosynthesis are entirely stomatal. Photosynthesis was inhibited at low water potential and was only slightly enhanced by increasing the external CO₂ partial pressure from 34 Pa (normal air) to 300 Pa. Photosynthesis in ABA-treated leaves was similarly inhibited but recovered fully at 300 Pa. In both cases, the stomata closed to the same extent as judged from the average conductance of the leaves. Because the ABA effect resulted from diffusion limitation for CO₂ caused by stomatal closure, the contrasting data show that most of the dehydration effect was nonstomatal at low water potentials. When CO₂ partial pressures were raised further to 3000 Pa, photosynthesis increased somewhat at low water potentials but not in ABA-treated leaves. This indicates that some nonstomatal component of photosynthesis responded differently in leaves at low water potential and leaves treated with ABA. Because this component was only partially restored by very high CO₂, it was likely to be metabolic and was an important source of photosynthetic inhibition.Abbreviations and Symbol ABA abscisic acid - Chl chlorophyll - p_a external partial pressure of CO₂ - P_i intercellular partial pressure of CO₂ - _w water potential This work was supported by grant DE-FG02-87ER13776 from the Department of Energy and a grant from E.I. DuPont de Nemours and Company.     

The Effects of N Nutrition on the Water Relations and Gas Exchange Characteristics of Wheat (Triticum aestivum L.)

The purpose of this study was to characterize leaf photosynthetic and stomatal responses of wheat (Triticum aestivum L.) plants grown under two N-nutritional regimes. High- and low-N regimes were imposed on growth-chamber-grown plants by fertilizing with nutrient solutions containing 12 or 1 millimolar nitrogen, respectively. Gas-exchange measurements indicated not only greater photosynthetic capacity of high-N plants under well-watered conditions, but also a greater sensitivity of CO₂ exchange rate and leaf conductance to CO₂ and leaf water potential compared to low-N plants. Increased sensitivity of high-N plants was associated with greater tissue elasticity, lower values of leaf osmotic pressure and greater aboveground biomass. These N-nutritional-related changes resulted in greater desiccation (lowered relative water content) of high-N plants as leaf water potential fell, and were implicated as being important in causing greater sensitivity of high-N leaf gas exchange to reductions in water potential. Water use efficiency of leaves, calculated as CO₂ exchange rate/transpiration, increased from 9.1 to 13 millimoles per mole and 7.9 to 9.1 millimoles per mole for high- and low-N plants as water became limiting. Stomatal oscillations were commonly observed in the low-N treatment at low leaf water potentials and ambient CO₂ concentrations, but disappeared as CO₂ was lowered and stomata opened.     

PHOTOSYNTHETIC SENSITIVITY TO TEMPERATURE IN POPULATIONS OF TWO C4 BOUTELOUA (POACEAE) SPECIES NATIVE TO DIFFERENT ALTITUDES

The relationships between photosynthesis, flowering, and growth temperatures were examined experimentally in four populations of the C₄ grass genus Bouteloua. Field-collected plants were grown under two temperature regimes, cool (20 C day/6 C night) and warm (30/16), representative of the extreme populations. Populations collected from the warm climates had significantly lower photosynthetic capacity when grown in the cool chamber relative to the warm chamber, while photosynthetic capacity in the cool climate populations did not differ between the growth conditions. Additionally, exposure to a 2-day cold temperature treatment (10/-2), representative of late-season frosts in high altitude sites, resulted in further reductions in photosynthesis in the warm climate plants, but not in the cool climate plants. This effect was greater for plants grown in the cool growth chamber. Flowering was reduced by 70% in the warm climate plants grown in the cool chamber, and was correlated with photosynthetic inhibition following the short-term cold temperature treatment. These results indicate that genetic differentiation for photosynthetic temperature sensitivity has occurred in the cool climate populations, and that long-term exposure to cool temperatures coupled with short-term relatively extreme low temperatures results in greater photosynthetic inhibition in nontolerant populations.     

Correlation of Stomatal Conductance with Photosynthetic Capacity of Cotton Only in a CO2-Enriched Atmosphere: Mediation by Abscisic Acid?

Some evidence indicates that photosynthetic rate (A) and stomatal conductance (g) of leaves are correlated across diverse environments. The correlation between A and g has led to the postulation of a “messenger” from the mesophyll that directs stomatal behavior. Because A is a function of intercellular CO₂ concentration (c_i), which is in turn a function of g, such a correlation may be partially mediated by c_i if g is to some degree an independent variable. Among individual sunlit leaves in a cotton (Gossypium hirsutum L.) canopy in the field, A was significantly correlated with g (r² = 0.41, n = 63). The relative photosynthetic capacity of each leaf was calculated as a measure of mesophyll properties independent of c_i. This approach revealed that, in the absence of c_i effects, mesophyll photosynthetic capacity was unrelated to g (r² = 0.06). When plants were grown in an atmosphere enriched to about 650 microliters per liter of CO₂, however, photosynthetic capacity remained strongly correlated with g even though the procedure discounted any effect of variable c_i. This “residual” correlation implies the existence of a messenger in CO₂-enriched plants. Enriched CO₂ also greatly increased stomatal response to abscisic acid (ABA) injected into intact leaves. The data provide no evidence for a messenger to coordinate g with A at ambient levels of CO₂. In a CO₂-enriched atmosphere, though, ABA may function as such a messenger because the sensitivity of the system to ABA is enhanced.     

Cloning and characterisation of a pepper aquaporin,CaAQP, which reduces chilling stress in transgenic tobacco plants

Ubiquitous cell membrane proteins called aquaporins are members of major intrinsic proteins (MIPs), which control the specific transport of water molecules across cell membranes. A pepper aquaporin gene (CaAQP), which exhibits the structural features of tonoplast intrinsic proteins of the MIP subfamily, was isolated from the leaves of chilling-treated seedlings of pepper (Capsicum annuum L.) cv. P70. Assays indicated high levels of expression in young seeds, green fruits and flower buds and low levels of expression in the stems, leaves and roots of pepper. The expression patterns were strongly and rapidly induced by HgCl₂, low temperature, abscisic acid, fluridone and osmotic stresses. The responsiveness of pepper seedlings pretreated with abscisic acid at low temperatures demonstrated up-regulation of CaAQP by chilling, which is potentially involved in ABA signalling. Our results indicated that overexpression of CaAQP decreased chilling stress in transgenic plants, likely by increasing the stomatal aperture under stress, increasing the rate of membrane damage during the recovery stage, thereby affecting the intercellular CO₂ concentration with lower stomatal conductance and transpiration rates. VIGS of CaAQP in pepper plants caused significant growth retardation. These results suggested that CaAQP played a crucial role in the plant response to abiotic stresses.     

Different relative humidity conditions combined with chloride and sulfate salinity treatments modify abscisic acid and salicylic acid levels in the halophyte Prosopis strombulifera

It has been shown that abscisic acid (ABA) and salicylic acid (SA) act as endogenous signal molecules responsible for inducing abiotic stress tolerance in plants. However, our knowledge on the role of both phytohormones in response to environmental conditions in halophytic plants is still limited. In this study endogenous ABA and SA levels, growth parameters and chlorophylls content were determined in leaves and roots of the halophyte Prosopis strombulifera cultivated under increasing NaCl and Na₂SO₄ concentrations, at 30 and 70 % relative humidity (RH) conditions. Endogenous ABA and SA content differed depending on the salt type and concentration, RH, plant age and the organ analyzed. Under low RH conditions P. strombulifera growth was strongly inhibited and chlorophyll a and b content were decreased. In leaves of Na₂SO₄-treated plants at 30 % RH, high ABA levels were correlated with protection against dehydration and ion toxicity. Instead, high SA levels were correlated with the damaging effect of sulfate anion and low RH on plant growth. NaCl-treated plants growth was also inhibited at 30 % RH although levels of both hormones were not significantly increased. Taken together, the salt toxic effects on growth parameters and photosynthetic pigments were accentuated by low RH conditions and these responses were reflected on ABA and SA content.     

Interaction of water supply and N in wheat

The purpose of this study was to investigate effects of N nutrition and water stress on stomatal behavior and CO₂ exchange rate in wheat (Triticum aestivum L. cv Olaf). Wheat plants were grown hydroponically with high (100 milligrams per liter) and low (10 milligrams per liter) N. When plants were 38 days old, a 24-day water stress cycle was begun. A gradual increase in nutrient solution osmotic pressure from 0.03 to 1.95 mega Pascals was achieved by incremental additions of PEG-6,000. Plants in both N treatments adjusted osmotically, although leaf water potential was consistently lower and relative water content greater for low N plants in the first half of the stress cycle. Leaf conductance of high N plants appeared greater than that of low N plants at high water potentials, but showed greater sensitivity to reductions in water potential as indicated by earlier stomatal closure during the stress cycle. The apparent greater stomatal sensitivity of high N plants was associated with a curvilinear relationship between leaf conductance and leaf water potential; low N plants exhibited more of a threshold response. Trends in [CO₂]_INT throughout the stress cycle indicated nonstomatal effects of water stress on CO₂ exchange rate were greater in high N plants. Although estimates of [CO₂]_INT were generally lower in high N plants, they were relatively insensitive to leaf water potential-induced changes in leaf conductance. In contrast, [CO₂]_INT of low N plants dropped concomitantly with leaf conductance at low leaf water potentials. Oxygen response of CO₂ exchange rate for both treatments was affected less by reductions in water potential than was CO₂ exchange rate at 2.5% O₂, suggesting that CO₂ assimilation capacity of the leaves was affected more by reductions in leaf water potential than were processes related to photorespiration.     

Gas Exchange, Stomatal Behavior, and deltaC Values of the flacca Tomato Mutant in Relation to Abscisic Acid

The relationship between stomatal conductance and capacity for assimilation was investigated in flacca, a mutant of tomato (Lycopersicon esculentum Mill.) that has abnormal stomatal behavior and low abscisic acid (ABA) content. The assimilation capacity, determined by measuring assimilation rate as a function of intercellular CO₂ pressure, did not differ in leaves of flacca and its parent variety, Rheinlands Ruhm (RR). On the other hand, stomatal conductance of flacca leaves was greater than that of RR, and could be phenotypically reverted by spraying with 30 micromolar ABA. Stomatal conductance of flacca leaves was also reduced by increasing CO₂ pressure, increasing leaf to air vapor pressure difference, and decreasing quantum flux, irrespective of ABA treatment.

The high conductance of flacca leaves resulted in a high intercellular CO₂ pressure. This allowed greater discrimination against ¹³CO₂, as evidenced by more negative δ ¹³C values for flacca as compared to RR. The δ ¹³C values of both flacca and RR plants as influenced by ABA treatment were consistent with predictions based on gas exchange measurements, using a recent model of discrimination.

     

Abscisic Acid in relation to mineral deprivation

Tobacco (Nicotiana rustica) plants growing in half-strength Hoagland solution were deprived of nutrients by being transferred to distilled water. The abscisic acid content of leaves in the mineral-deprived plants rose continuously throughout the 7 days of the experimental period. However, although the content of ABA rose within 24 hours, a decline in growth and leaf-chlorophyll were discernible only after the 4th day of mineral deprivation. As anticipated, mineral-deprived (stressed) plants exhibit “resistance” to lack of aeration in the root medium, similar to that shown in salt-stressed plants or plants that were pretreated with absiscic acid. When the mineral-deprived plants were returned to half-strength Hoagland, the content of leaf abscisic acid declined to the prestressed level and the “resistance” to lack of root aeration disappeared.     

Effect of two nutrient solution temperatures on nitrate uptake,nitrate reductase activity,NH4+ concentration and chlorophyll a fluorescence in rose plants

The effect of two nutrient solution temperatures (cold (10 °C) and warm (22 °C)) during two flowering events of rose plants (Rosa × hybrida cv. Grand Gala) were examined by measuring chlorophyll (Chl) a fluorescence, ammonium (NH₄⁺) content and nitrate reductase (NR) activity in four different leaf types, that is, external and internal leaves of bent shoots and lower and upper leaves of flowering stems. Besides, nitrate (NO₃^?) uptake and water absorption, total nitrogen (N) concentration in the plant, dry biomass, and the ratios of shoot/root and thin-white roots/suberized-brown roots were determined. Generally, cold solution increased NO₃^? uptake and thin-white roots production but decreased water uptake, so plants grown at cold solution had to improve their NO₃^? uptake mechanisms to obtain a higher amount of nutrient with less water absorption than plants grown at warm solution. The higher NO₃^? uptake can be related to an increase in NR activity, NH₄⁺ content and total N concentration at cold solution. Nutrient solution temperature also had an effect on the photosynthetic apparatus. In general terms, the effective quantum yield (?_PSII) and the fraction of open PSII reaction centres (q_L) were higher in rose plants grown at cold solution. These effects can be associated to a higher NO₃^? uptake and total N concentration in the plants and were modulated by irradiance throughout all the experiment. Plants could adapt to cold solution by enhancing their metabolism without a decrease in total dry biomass. Nevertheless, the effect of nutrient solution temperature is not simple and also affected by climatic factors.     

Water Relations of Cotton Plants under Nitrogen Deficiency: III. STOMATAL CONDUCTANCE, PHOTOSYNTHESIS, AND ABSCISIC ACID ACCUMULATION DURING DROUGHT

Nitrogen nutrition exerted a strong effect on stomatal sensitivity to water stress in cotton. In well-watered plants grown with 0.31 millimolar N in the nutrient solution, stomata closed at a water potential of -9 bars even though the wilting point was below -15 bars. For each doubling of nutrient N level, the water potential for stomatal closure decreased by about 2 bars. Elevated intercellular CO₂ concentrations caused only slight stomatal closure regardless of N nutrition. Exogenous abscisic acid (ABA) greatly increased stomatal sensitivity to elevated CO₂ concentrations.     

Lower incidence and severity of tomato virus in elevated CO2 is accompanied by modulated plant induced defence in tomato

Elevation in atmospheric CO₂ concentration broadly affects plant phenology and physiology, and these effects may alter the performance of plant viruses. The effects of elevated CO₂ on the susceptibility of tomato plants to Tomato yellow leaf curl virus (TYLCV) were examined for two successive years in open top chambers (OTC) in the field. We experimentally tested the hypothesis that elevated CO₂ would reduce the incidence and severity of TYLCV on tomato by altering plant defence strategies. Our results showed that elevated CO₂ decreased TYLCV disease incidence (by 14.6% in 2009 and 11.8% in 2010) and decreased disease severity (by 20.0% in 2009 and 10.4% in 2010). Elevated CO₂ also decreased the level of TYLCV coat protein in tomato leaves. Regardless of virus infection, elevated CO₂ increased plant height and aboveground biomass. Additionally, elevated CO₂ increased the leaf C:N ratio of tomato, but decreased soluble protein content in leaves. Notably, elevated CO₂ increased the salicylic acid (SA) level in uninfected and infected plants. In contrast, elevated CO₂ reduced jasmonic acid (JA) in uninfected plants while it increased JA and abscisic acid (ABA) in virus‐infected plants. Furthermore, combined exogenous SA and JA application enhanced resistance to TYLCV more than application of either SA or JA alone. Our results suggest that the modulated antagonistic relationship between SA and JA under elevated CO₂ makes a great contribution to increased tomato resistance to TYLCV, and the predicted increases in tomato productivity may be enhanced by reduced plant virus susceptibility under projected rising CO₂ conditions.     

Effect of drought stress on net CO2 uptake by Zea leaves

The net CO₂ assimilation by leaves of maize (Zea mays L. cv. Adonis) plants subjected to slow or rapid dehydration decreased without changes in the total extractable activities of phosphoenolpyruvate carboxylase (PEPC), malate dehydrogenase (MDH) and malic enzyme (ME). The phosphorylation state of PEPC extracted from leaves after 2–3 h of exposure to light was not affected by water deficit, either. Moreover, when plants which had been slowly dehydrated to a leaf relative water content of about 60% were rehydrated, the net CO₂ assimilation by leaves increased very rapidly without any changes in the activities of MDH, ME and PEPC or phosphorylation state of PEPC. The net CO₂-dependent O₂ evolution of a non-wilted leaf measured with an oxygen electrode decreased as CO₂ concentration increased and was totally inhibited when the CO₂ concentration was about 10%. Nevertheless, high CO₂ concentrations (5–10%) counteracted most of the inhibitory effect of water deficit that developed during a slow dehydration but only counteracted a little of the inhibitory effect that developed during a rapid dehydration. In contrast to what could be observed during a rapidly developing water deficit, inhibition of leaf photosynthesis by cis-abscisic acid could be alleviated by high CO₂ concentrations. These results indicate that the inhibition of leaf net CO₂ uptake brought about by water deficit is mainly due to stomatal closure when a maize plant is dehydrated slowly while it is mainly due to inhibition of non-stomatal processes when a plant is rapidly dehydrated. The photosynthetic apparatus of maize leaves appears to be as resistant to drought as that of C₃ plants. The non-stomatal inhibition observed in rapidly dehydrated leaves might be the result of either a down-regulation of the photosynthetic enzymes by changes in metabolite pool sizes or restricted plasmodesmatal transport between mesophyll and bundle-sheath cells.     

Daily Changes in CO(2) and Water Vapor Exchange, Chlorophyll Fluorescence, and Leaf Water Relations in the Halophyte Mesembryanthemum crystallinum during the Induction of Crassulacean Acid Metabolism in Response to High NaCl Salinity

Simultaneous measurements of net CO₂ exchange, water vapor exchange, and leaf water relations were performed in Mesembryanthemum crystallinum during the development of crassulacean acid metabolism (CAM) in response to high NaCl salinity in the rooting medium. Determinations of chlorophyll a fluorescence were used to estimate relative changes in electron transport rate. Alterations in leaf mass per unit area, which—on a short-term basis—largely reflect changes in water content, were recorded continuously with a beta-gauge. Turgor pressure of mesophyll cells was determined with a pressure probe. As reported previously (K Winter, DJ von Willert [1972] Z Pflanzenphysiol 67: 166-170), recently expanded leaves of plants grown under nonsaline conditions showed gas-exchange characteristics of a C₃ plant. Although these plants were not exposed to any particular stress treatment, water content and turgor pressure regularly decreased toward the end of the 12 hour light periods and recovered during the following 12 hours of darkness. When the NaCl concentration of the rooting medium was raised to 400 millimolar, in increments of 100 millimolar given at the onset of the photoperiods for 4 consecutive days, leaf water content and turgor pressure decreased by as much as 30 and 60%, respectively, during the course of the photoperiods. These transient decreases probably triggered the induction of the biochemical machinery which is required for CAM to operate. After several days at 400 millimolar NaCl, when leaves showed features typical of CAM, overall turgor pressure and leaf mass per unit area had increased above the levels before onset of the salt treatment, and diurnal alterations in leaf water content were reduced. Net carbon gain during photoperiods and average intercellular CO₂ partial pressures at which net CO₂ uptake occurred, progressively decreased upon salinization. Reversible diurnal depressions in leaf conductance and net CO₂ uptake, with minima recorded in the middle of the photoperiods, preceded the occurrence of nocturnal net CO₂ uptake. During these reductions, intercellular CO₂ partial pressure and rates of photosynthetic electron transport decreased. With advancing age, leaves of plants grown under nonsaline conditions exhibited progressively greater diurnal reductions in turgor pressure and developed a low degree of CAM activity.     

Involvement of abscisic acid in photosynthetic process in Hordeum vulgare L. during salinity stress

In Hordeum vulgare L. plants, NaCl stress imposed through the root medium for a period of 8 days decreased the rate of CO₂ assimilation, the chlorophyll and protein leaf content, and the activity of ribulose-1,5-bisphosphate carboxylase. The activity of phosphoenolpyruvate carboxylase was twofold over the control. Pretreatment with abscisic acid (ABA) for 3 days before salinization diminished the inhibitory effect of NaCl on the rate of CO₂ fixation. The leaf Na⁺ and Cl^– content decreased in ABA-pretreated plants. Both ABA and NaCl treatments led to an increase in the endogenous level of ABA in the plant leaves. Patterns of total proteins extracted from the leaves of control or ABA- and salt-treated plants were compared. Both ABA and NaCl induced marked quantitative and qualitative changes in the polypeptide profiles concerning mainly the proteins with approximately equal mobility. The results are discussed in terms of a possible role of ABA in increasing the salt tolerance when ABA is applied to the plants for a short period before exposure to salinity stress, thus improving the invulnerability to unfavorable conditions.Abbreviations RuBPC ribulose-1,5-bisphosphate carboxylase - PSII photosystem II - ABA abscisic acid - PEPC phosphoenolpyruvate carboxylase - DTTr dithiothreitol - BSA bovine serum albumin - ELISA enzyme-linked immunosorbent assay - SDS sodium dodecyl sulfate - PAGEr polyacrylamide gel electrophoresis     

A Transient Burst of CO(2) from Geranium Leaves during Illumination at Various Light Intensities as a Measure of Photorespiration

A transient CO₂ burst is exhibited by irradiated leaves of the C₃ plant geranium (Pelargonium X hortorum, Bailey) after the irradiance is quickly lowered. The light CO₂ burst appears to be related to photorespiration because of its irradiance dependency and its sensitivity to other environmental components such as CO₂ and O₂ concentration. The term post-lower-irradiance CO₂ burst or PLIB is used to describe the phenomenon. The PLIB appears to be a quantitative measurement of photorespiration with intact geranium leaves. The PLIB has been observed with intact leaves of other C₃ plants but not with C₄ leaves. Therefore, it is proposed that, after maximizing intact leaf photosynthetic rates and leaf chamber gas measuring conditions, photorespiration can be measured with intact C₃ leaves such as geranium as a transient post-lower-irradiance CO₂ burst.