Increased Leaf Water Potential Research Articles

Hydrogen peroxide pretreatment induces osmotic stress tolerance by influencing osmolyte and abscisic acid levels in maize leaves

Hydrogen peroxide (H2O2) functions as a signal molecule in plants under abiotic and biotic stresses. Leaves of detached maize (Zea mays L.) seedlings were used to study the function of H2O2 pretreatment in osmotic stress resistance. Low H2O2 concentration (10 mM) which did not cause a visual symptom of water deficit (leaf rolling) was applied to the seedlings. Exogenous H2O2 alone increased leaf water potential, endogenous H2O2 content, abscisic acid (ABA) concentration, and metabolite levels including soluble sugars, proline, and polyamines while it decreased lipid peroxidation and stomatal conductance. Osmotic stress induced by polyethylene glycol (PEG 6000) decreased leaf water potential and stomatal conductance but enhanced lipid peroxidation, endogenous H2O2 content, the metabolite levels, and ABA content. H2O2 pretreatment also induced the metabolite accumulation and improved water status, stomatal conductance, lipid peroxidation, ABA, and H2O2 levels under osmotic stress. These results indicated that H2O2 pretreatment may alleviate water loss and induce osmotic stress resistance by increasing the levels of soluble sugars, proline, and polyamines thus ABA and H2O2 production slightly decrease in maize seedlings under osmotic stress.

Augmenting drought tolerance in sorghum by silicon nutrition

Silicon (Si) distribution and accumulation in plant organs is widely reported as beneficial to overcome biotic and abiotic stresses. The investigation on distribution of Si in plant organs under water stress conditions was studied through experiments conducted at Arid Agriculture University, Pakistan during 2007–2009. Treatments (Si200: 200 ml l−1 of potassium silicate and Si0: control or absence of silicon) were replicated thrice with two sorghum cultivars; SPV462 (drought susceptible) and Johar-1 (drought tolerant) screened using osmotic media of PEG-6000 (−4.0, −6.0, −8.0 and −10.0 MPa). The results exhibited increased leaf water potential, leaf area index, SPAD chlorophyll with increased silicon concentration in leaves and roots of drought-tolerant genotype as compared to SPV462. Johar-1 exhibited maximum values for net assimilation and relative growth rate under silicon treatment in comparison to silicon absence. Similarly, maintenance of transpiration rate, because of accumulation of silicon in leaves resulted in optimum leaf water potential and optimum growth of crop.

Verification of a threshold concept of ecologically effective precipitation pulse: From plant individuals to ecosystem

In water-limited ecosystems, an ecologically significant rainfall pulse was defined as a rainfall event that altered both soil water status and plant physiological activity. We developed a new threshold concept of an ecologically effective precipitation pulse (EEPP) applicable to both plant individual and ecosystem scales. The concept was tested in a typical steppe on Inner Mongolia plateau. Two EEPPs, single 3-mm rainfall and 5-mm rainfall, were applied to investigate their effects on soil and plant water status, CO2 assimilation of five species (four C3 plants and one C4 plant), whole-plot soil respiration (Rs), and net ecosystem CO2 exchange (NEE) on 1 June and 28 July 2009, respectively. Both EEPPs increased leaf water potential (Ψl) of all the species, which peaked 1–3days after rainfall pulses. Soil water content (SWC) in two depths (5cm and 20cm) significantly increased after the two EEPPs for 1–3days. Soil water potential (Ψs) within 20‐cm soil layer in EEPP treatments significantly differed (p<0.05) from control. Net assimilation rates (Anet) of all C3 plants had a slight increase at the next day after two EEPPs, in contrast to the C4 species. Rs elevated and peaked 1–3days later after water supply. Ecosystem net CO2 absorption rate rose to maximum value 3days after the 5-mm pulse on 28 July, higher than the response to 3-mm pulse on June 1. The grassland turned to net emission of CO2 after 3-mm pulse on 28 July. The results supported that there was an ecosystem level threshold for EEPP, and the threshold was temporally variable. It also highlighted the necessity of considering the response threshold of EEPP in rainfall manipulative experiment. In addition, effective rainfall amount was more approriate than total rainfall amount in modeling ecosystem carbon balance.

Pot-in-pot Reduces Salinity, Chloride Uptake, and Maintains Aesthetic Value in Euonymus japonicus Thunb. Under Saline Irrigation

The appropriate management of crop conditions can reduce the salt damage suffered by ornamental species and produce high-quality plants even when saline irrigation water is used. The aim of this study was to determine whether the pot-in-pot (PIP) cultivation system can improve the saline irrigation tolerance of Euonymus japonicus compared with aboveground potting (AGP) in terms of growth and development, aesthetic quality, ion accumulation, and leaf potentials. A 5-month experiment started on 6 Mar., and the interaction between the cultivation system (PIP or AGP) and water quality (fresh water and saline water, with 1.76 and 9.04 dS·m−1, respectively) was assessed. The substrate used was a mixture of white peat, coconut fiber, and perlite (40/40/20, v/v/v). A soil moisture sensor-controlled system was used to irrigate all the treatments when the AGP treatment irrigated with fresh water reached a volume water content (θ) of 0.33–0.35 m3·m−3. An interaction effect reduced the salinity effects in PIP and saline irrigation (PIP-s) compared with AGP and saline irrigation (AGP-s) in terms of damaged leaf area, plant dry weight (DW), and the compactness index. The PIP-s plants showed a survival rate of 93% compared with 57% in AGP-s. The substrate temperatures were milder in PIP regardless of the irrigation water, and the pore water electrical conductivity (EC) was 36% lower in PIP-s than in AGP-s. PIP reduced the Cl− accumulated in leaves but did not influence Na+, Ca2+, Mg2+, or the K+/Na+ ratio. The lower amount of Cl− accumulated increased leaf water potential (Ψo) in PIP. Saline irrigation produced a general accumulation of Cl− and Na+ in leaves and decreased Ca2+, Mg2+, the K+/Na+ ratio, Ψo, the shoot to root ratio, and height. In general, PIP reduced the salinity damage to Euonymus japonicus, the main effect being the lower Cl− ion uptake, which improved its aesthetic value (less damage and greater compactness and growth).

CHARACTERISATION OF PLANT WATER STRESS OF GREENHOUSE TOMATOES (LYCOPERSICON ESCULENTUM) GROWN HYDROPONICALLY IN COIR

ISHS XXVIII International Horticultural Congress on Science and Horticulture for People (IHC2010): International Symposium on Greenhouse 2010 and Soilless Cultivation CHARACTERISATION OF PLANT WATER STRESS OF GREENHOUSE TOMATOES (LYCOPERSICON ESCULENTUM) GROWN HYDROPONICALLY IN COIR

Cavitation induced by a surfactant leads to a transient release of water stress and subsequent ‘run away’ embolism in Scots pine (Pinus sylvestris) seedlings

Cavitation decreases the hydraulic conductance of the xylem and has, therefore, detrimental effects on plant water balance. However, cavitation is also hypothesized to relieve water stress temporarily by releasing water from embolizing conduits to the transpiration stream. Stomatal closure in response to decreasing water potentials in order to avoid excessive cavitation has been well documented in numerous previous studies. However, it has remained unclear whether the stomata sense cavitation events themselves or whether they act in response to a decrease in leaf water potential to a level at which cavitation is initiated. The effects of massive cavitation on leaf water potential, transpiration, and stomatal behaviour were studied by feeding a surfactant into the transpiration stream of Scots pine (Pinus sylvestris) seedlings. The stomatal response to cavitation in connection with the capacitive effect was also studied. A major transient increase in leaf water potential was found due to cavitation in the seedlings. As cavitation was induced by lowering the surface tension, the two mechanisms could be uncoupled, as the usual relation between xylem water potential and the onset of cavitation did not hold. Our results indicate that the seedlings responded more to leaf water potential and less to cavitation itself, as stomatal closure was insufficient to prevent the seedlings from being driven to ‘run-away’ cavitation in a manner of hours.

Does silicon and irrigation have impact on drought tolerance mechanism of sorghum?

Abstract Silicon absorption by crops in the form of silicic acid confers efficient utilization of available irrigation water by minimizing transpiration losses. In present study, silicon and irrigation effects on sorghum growth dynamics and drought tolerance mechanism were evaluated during 2007–2008. Two sorghum cultivars: PARCSS2 and Johar1 were treated with two levels of silicon (Si0 = control and Si200 = 200 ml l−1 of potassium silicate per kg of soil) and irrigation (W0 = control, crop lower limit and W40 = 40 mm of water, crop upper limit). The results depicted that silicon absorption led to increased leaf water potential, growth, transpiration, net photosynthetic rate and decreased shoot to root ratio in sorghum cultivars compared to control treatment. It can be concluded that synergistic effect of silicon fertilization with ample irrigation may improve the crop stand under drought and biotic stresses.

Quantifying the effect of drought on carbon dioxide‐induced changes in competition between a C3 crop (tomato) and a C4 weed (Amaranthus retroflexus)

Valerio M, Tomecek MB, Lovelli S &amp; Ziska LH (2011). Quantifying the effect of drought on carbon dioxide‐induced changes in competition between a C3 crop (tomato) and a C4 weed (Amaranthus retroflexus). Weed Research51, 591–600.SummaryRecent and projected increases in atmospheric carbon dioxide concentration ([CO2]) and subsequent effects on climate are likely to alter competitive outcomes of weeds and crops. Rising [CO2] per se could increase the competitive ability of C3 crops relative to C4 weeds. However, such an outcome may depend on other climatic variables. In this study, tomato, a C3 crop species, was grown from emergence to anthesis using replacement series mixtures with Amaranthus retroflexus, a C4 weed species at three different [CO2], 400, 600 and 800 μmol mol−1, with and without water stress. Under well‐watered conditions, leaf photosynthetic rates and plant height, leaf area and biomass all increased with elevated [CO2] for tomato relative to A. retroflexus, consistent with the kinetics of C3 photosynthesis. However, if water was limiting, a significant positive effect of [CO2] was noted for plant height and biomass of A. retroflexus with increased competition. This result may be related to a greater increase in leaf water potential with rising [CO2] for A. retroflexus relative to tomato under water stress. Overall, these are the first data to suggest that increases in atmospheric CO2 could still exacerbate crop losses from a C4 weed, even with a C3 crop, if drought occurs.

Stomatal sensitivities to changes in leaf water potential, air humidity, CO2 concentration and light intensity, and the effect of abscisic acid on the sensitivities in six temperate deciduous tree species

To characterise the stomata of six temperate deciduous tree species, sets of stomatal sensitivities to all the most important environmental factors were measured. To compare the importance of abscisic acid (ABA) in the different stomatal responses, the effect of exogenous ABA on all the stomatal sensitivities was determined.Almost all the stomatal sensitivities: the sensitivity to a decrease in leaf water potential, air humidity, CO2 concentration ([CO2]) and light intensity, and to an increase in [CO2] and light intensity were the highest in the slow-growing species, and the lowest in the fast-growing species. Drought increased the sensitivity to the environmental changes that induce a decrease in the stomatal conductance, and decreased the sensitivity to the changes that induce an increase in this conductance. The sensitivities of the slow-growers were most strongly affected by drought and ABA. Therefore the success of the slow-growers in their ecological niches can be based on the highly sensitive and strictly regulated responses of their stomata. The fast-growers had the highest sensitivity to an increase in leaf water potential and this sensitivity was sharply reduced by drought and ABA. Thus, the dominance of the trees in riparian areas can be based on the ability of their stomata to quickly reach high conductance in well-watered conditions and to efficiently decrease this rate during drought.Stomatal sensitivities to the hydraulic environmental factors (water potentials in plant and air) had higher values in well-watered trees and a more pronounced response to drought than the sensitivities to the photosynthetic environmental factors ([CO2] and light intensity). Thus, the hydraulic factors most likely prevail over the photosynthetic factors in determining stomatal conductance in these species.In response to exogenous ABA, the rates of stomatal closure, following a decrease in air humidity and light intensity, and an increase in [CO2], were accelerated. Stomatal opening following an increase in air humidity and light intensity and a decrease in [CO2] was replaced by slow closing. The rate of stomatal opening following an increase in leaf water potential was reduced. As the sensitivities to changes in light were modified less by the ABA than the other stomatal sensitivities, the prediction of stomatal responses on the basis of the sensitivity to light alone should be excluded in stomatal models.

Impact of Kaolin Particle Film and Water Deficit on Wine Grape Water Use Efficiency and Plant Water Relations

Water use efficiency (WUE) and response of grape vines (Vitis vinifera L. cvs. ‘Cabernet Sauvignon’, ‘Merlot’, and ‘Viognier’) to a particle film treatment (PFT) under varying levels of applied water were evaluated in Victoria, Australia, and southwestern Idaho. Vines that received the least amount of water had the warmest canopy or leaf surface temperature and the lowest (more negative) leaf water potential, stomatal conductance (gS), transpiration (E), and photosynthesis (A). Vines with plus-PFT had cooler leaf and canopy temperature than non-PFT vines; however, temperature difference resulting from irrigation was greater than that resulting from PFT. In well-watered vines, particle film application increased leaf water potential and lowered gS. Point-in-time measurements of WUE (A/E) and gS did not consistently correspond with seasonal estimates of WUE based on carbon isotope discrimination of leaf or shoot tissue. The response of vines with particle film to undergo stomatal closure and increase leaf water potential conserved water and enhanced WUE under non-limiting soil moisture conditions and the magnitude of response differed according to cultivar.

Grazing winter wheat relieves plant water stress and transiently enhances photosynthesis

To model the impact of grazing on the growth of wheat (Triticum aestivum L.), we measured photosynthesis in the field. Grazing may affect photosynthesis as a consequence of changes to leaf water status, nitrogen content per unit leaf area (Na) or photosynthetic enzyme activity. While light-saturated CO2 assimilation rates (Asat) of field-grown wheat were unchanged during grazing, Asat transiently increased by 33–68% compared with ungrazed leaves over a 2- to 4-week period after grazing ended. Grazing reduced leaf mass per unit area, increased stomatal conductance and increased intercellular CO2 concentrations (Ci) by 36–38%, 88–169% and 17–20%, respectively. Grazing did not alter Na. Using a photosynthesis model, we demonstrated that the increase in Asat after grazing required an increase in Rubisco activity of up to 53%, whereas the increase in Ci could only increase Asat by up to 13%. Increased Rubisco activity was associated with a partial alleviation of leaf water stress. We observed a 68% increase in leaf water potential of grazed plants that could be attributed to reduced leaf area index and canopy evaporative demand, as well as to increased rainfall infiltration into soil. The grazing of rain-fed grain cereals may be tailored to relieve plant water stress and enhance leaf photosynthesis.

Sprinkler Irrigation Changes Maize Canopy Microclimate and Crop Water Status, Transpiration, and Temperature

During a sprinkler irrigation event some water is lost due to wind drift and evaporation (WDEL). After the irrigation event, plant‐intercepted water is lost due to evaporation. The water lost causes microclimatic changes which could result in positive or negative plant physiological changes. We studied the microclimatic and physiological changes on two fields grown with maize (Zea mays L.) irrigated with a solid‐set sprinkler system. The temperature and vapor pressure deficit (VPD) of the air were measured at the crop canopy level and above and below the canopy. Changes in maize canopy temperature, transpiration, and leaf water potential (LWP) were determined. Sprinkler irrigation during daytime strongly modified the microclimate where plants grow during the irrigation time and for a short period after the irrigation event finished. Daytime irrigation decreased air temperature by 3.3 to 4.4°C and VPD by 1.0 to 1.2 kPa at 0.5 m below the crop canopy height. The decrease was lower as the measurement height increased. Microclimatic changes during nighttime irrigation were minimal. Daytime irrigation reduced maize canopy temperature by 4 to 6°C and plant transpiration by 58%, and increased LWP from −1.2 and −1.4 MPa to −0.54 MPa. Transpiration reduction must be considered positive because it supposes a reduction of WDEL. The decrease in maize canopy temperature could be positive or negative, but the increase in LWP is a positive effect.

Fog interception by Sequoia sempervirens (D. Don) crowns decouples physiology from soil water deficit

Although crown wetting events can increase plant water status, leaf wetting is thought to negatively affect plant carbon balance by depressing photosynthesis and growth. We investigated the influence of crown fog interception on the water and carbon relations of juvenile and mature Sequoia sempervirens trees. Field observations of mature trees indicated that fog interception increased leaf water potential above that of leaves sheltered from fog. Furthermore, observed increases in leaf water potential exceeded the maximum water potential predicted if soil water was the only available water source. Because field observations were limited to two mature trees, we conducted a greenhouse experiment to investigate how fog interception influences plant water status and photosynthesis. Pre-dawn and midday branchlet water potential, leaf gas exchange and chlorophyll fluorescence were measured on S. sempervirens saplings exposed to increasing soil water deficit, with and without overnight canopy fog interception. Sapling fog interception increased leaf water potential and photosynthesis above the control and soil water deficit treatments despite similar dark-acclimated leaf chlorophyll fluorescence. The field observations and greenhouse experiment show that fog interception represents an overlooked flux into the soil-plant-atmosphere continuum that temporarily, but significantly, decouples leaf-level water and carbon relations from soil water availability.

INFLUENCE OF DEFICIT IRRIGATION AND PACLOBUTRAZOL ON PLANT GROWTH AND WATER STATUS IN LONICERA IMPLEXA SEEDLINGS

Mediterranean honeysuckle (Lonicera implexa) is the subject of great interest for xeroscaping and revegetation projects. Seedling pre-conditioning in the nursery is critical to achieve success in the establishment phase. The objective of this experiment was to evaluate the effect of irrigation regime [control (20% leaching) and deficit (43% of the control)] and paclobutrazol (PBZ) doses [control (0 mg/pot) and treated (80 mg/pot)] on growth parameters, water relations and gas exchange in honeysuckle plants during the nursery period. Seedlings were potted in 2.5 L pots and grown in a greenhouse located in the Campo of Cartagena (Spain). At the end of the experimental period, both deficit irrigation and PBZ significantly reduced shoot development. Significant interaction between the irrigation regime and PBZ doses was evident for plant height and stem diameter. The shoot/root ratio was not affected by the deficit irrigation and PBZ dose. Both the deficit irrigation and PBZ dose increased significantly the relative chlorophyll content in the leaves. Plants grown under deficit irrigation had lower values of water leaf potential, leaf osmotic potential and relative water content than the control plants. PBZ-treated plants had increased leaf water potential and turgor potential and had reduced leaf osmotic potential at full turgor. Significant interaction between the irrigation regime and PBZ dose was evident for the leaf water potential and leaf osmotic potential, demonstrating that PBZ attenuates the effects of deficit irrigation. The stomatal conductance decreased at the end of the hardening stage by 63% in plants grown under deficit irrigation and was not affected by PBZ dose. The results suggest that deficit irrigation and PBZ modify the plant growth and water status of honeysuckle plants improving the resistance mechanisms related to the plant adaptation to drought conditions. INTRODUCTION The transplantation and the establishment phase are critical to achieve success in revegetation and landscaping projects (Whitcomb, 1986). During establishment the plants are usually exposed to unfavorable environmental conditions, limiting the survival of those plants. Therefore, acclimatized plants that leave the nursery with a greater capacity to adapt to unfavorable conditions will provide greater possibilities for survival in the field. It is well known that deficit irrigation affects morphological and physiological aspects related to the hardening of the plants. (Liptay et al., 1998). Furthermore, the application of certain growth retardants, such as paclobutrazol, improves the resistance of the plants to environmental stresses (Fletcher et al., 2000; Banon et al., 2005). For that reason, the investigation of the mechanisms implicated in the hardening and acclimatization processes will allow us to improve our knowledge of the capacity of the species to grow in natural conditions. The objective of this study was to determine the effects of deficit irrigation and the application of paclobutrazol on the water relations, gas exchange and plant development in Lonicera implexa L. plants during the nursery phase. Proc. IV IS on Seed, Transplant and Stand Establishment

Short term stomatal responses to light intensity changes and osmotic stress in sorghum seedlings raised with and without silicon

Effects of silicon application on stomatal conductance in sorghum were investigated. Sorghum seedlings were grown in culture solution containing 0 or 1.67 mM silicon. On ninth day after giving silicon treatment, when there were no differences in shoot and root growth between the silicon-supplied and silicon-deficient treatments, seedlings were exposed to gradually increasing light intensity, combination of sudden increase and decrease in light intensity, and water stress induced by mixing sorbitol with the culture solution. There was increased stomatal conductance with increasing light intensity and no significant difference was observed between the two silicon treatments. Under water stress, the stomatal conductance of silicon-supplied seedlings was higher than that of silicon-deficient seedlings. The higher stomatal conductance in silicon-supplied seedlings under water stress was attributed to increased leaf water potential due to silicon-induced enhancement of hydraulic conductance. The results indicated that silicon application could affect stomatal conductance in sorghum seedlings through the modification of plant water relations.

The effect of root pressurization on water relations, shoot growth, and leaf gas exchange of peach (Prunus persica) trees on rootstocks with differing growth potential and hydraulic conductance

It is well known that rootstocks can have an effect on the vegetative growth and development of the tree; however, there has been no clear explanation about the physiological mechanism involved in this phenomenon. Evidence indicates that the rootstock effects on tree vegetative growth may be related to hydraulic limitations of the rootstock. The objective of these experiments was to investigate the shoot growth, water potential, and gas exchange of peach trees on different rootstocks in response to manipulations of water relations of trees on rootstocks that differ in root hydraulic conductance. Tree water relations were manipulated by applying different amounts of pneumatic pressure on the root system and then relative shoot extension growth rate, tree transpiration rate, leaf water potential, leaf conductance, leaf transpiration, and net CO(2) exchange rate responses were measured. Root pressurization increased leaf water potential, relative shoot extension growth rate, leaf conductance, leaf transpiration, and net CO(2) exchange rates of trees on both vigorous and dwarfing rootstocks. There was a significant positive linear correlation between applied pneumatic pressure and tree transpiration rate and leaf water potential. Leaf conductance, transpiration rate, and net CO(2) exchange rate as well as relative shoot extension growth rates were also positively correlated with the applied pneumatic pressure on the root system. These relationships were consistent across both vigorous and size-controlling rootstocks, indicating that rootstock hydraulic limitation may be directly involved in the vegetative growth control of peach trees.

Plant Responses to Water Deficit and Shade Stresses in Pigweed and Their Influence on Feeding and Oviposition by the Beet Armyworm (Lepidoptera: Noctuidae)

Water deficit and shade stress in weed-infested crops could alter plant growth and biochemistry and feeding and oviposition by the beet armyworm, Spodoptera exigua Hubner. Palmer amaranth pigweed, Amaranthus palmeri S. Wats., was grown under 25% of full watering (water deficit), 30% of full light (shade), or combined stress. All treatments decreased plant height and weight. Shade and combined stresses decreased leaf counts and increased leaf water content. Water deficit stress increased leaf water potential, soluble protein and carbohydrate contents, peroxidase activities, and accumulations of 10 individual free amino acids (FAAs), summed essential FAAs, and total FAAs. Combined stress increased water potential, soluble carbohydrates, 12 individual FAAs, summed essential FAAs, and total FAAs. Shade stress decreased water potential, soluble carbohydrates, seven individual FAAs, and essential FAAs. Beet armyworm larvae consumed similar leaf areas on water deficit–stressed and nonstressed plants and larger areas on plants grown under shade or combined stress. Larval survival was reduced, and time to pupation was higher on shade-stressed leaves. Adult females deposited more eggs on shade and combined stress plants and fewer eggs on water deficit-stressed plants compared with controls. Beet armyworm feeding and oviposition responded to variation in water content. Stress-induced changes in nutrients were not tied to insect preference but could have negatively influenced survival under shade stress. The results have implications for the plant stress hypothesis and for the use of pigweeds for beet armyworm detection.

The significance of episodic rains for reproductive phenology and productivity of trees in semiarid regions of northwestern Venezuela

The semiarid regions of northwestern Venezuela have extremely low and highly unpredictable precipitation, yet these conditions support species with contrasting phenology and leaf longevity. Episodic rains significantly increased leaf water potential (from −5 to −2.5 MPa) in several species and, in some cases, triggered flowering, leading us to hypothesize that the coexistence of species with contrasting phenology is due to differences in their ability to utilize small rainfall events. Irrigation treatments were used to simulate brief rainfall events, and the response of three species (Erythrina velutina [deciduous], Croton heliaster [semideciduous], and Capparis odoratissima [evergreen]) was monitored over a period of 14 months. To partition the effects of water reaching the canopy versus the soil, irrigation was supplied either in the form of mist to the canopy or by minisprinklers near the base of the trees. Nonirrigated trees were used as controls. Productivity (estimated as aboveground litter production) and water potential were enhanced by soil irrigation in two species. However, in the evergreen species canopy irrigation had a greater effect on water relations and productivity than soil irrigation, as indicated by higher predawn water potential, higher total annual flower (40 g m−2 year−1) and fruit (5 g m−2 year−1) production, and longer leaf longevity (410 days in control trees versus 520 days in canopy-irrigated trees). Canopy irrigation augmented flower and fruit production in all three species. Our findings suggest that reproductive phenology in these species is driven by episodic rains and that evergreen species may sustain productivity by their ability to make use of water deposited on leaf surfaces.

Responses to UV‐B radiation in Trifolium repens L. – physiological links to plant productivity and water availability

ABSTRACTThis study used comparisons across nine populations of Trifolium repens (white clover) in conjunction with drought to examine physiological responses to ultraviolet‐B radiation (UV‐B). Plants were exposed for 12 weeks to supplementation with 13.3 kJ m−2 d−1 UV‐B, accompanied by 4 weeks of drought under controlled environmental conditions. UV‐B increased the levels of UV‐B‐absorbing compounds and of flavonol glycosides and this effect was synergistically enhanced by water stress. These changes were more pronounced for the ortho‐dihydroxylated quercetin, rather than the monohydroxylated kaempferol glycosides. UV‐B increased leaf water potential (ψL) by 16% under drought and proline levels by 23% under well‐watered conditions. The intraspecific comparisons showed that higher UV‐B‐induced levels of UV‐B‐absorbing compounds, of quercetin glycosides and of ψL were linked to lower plant productivity and to higher UV‐B tolerance under well‐watered conditions. These findings suggest that: (1) slow‐growing T. repens ecotypes adapted to other stresses have higher capacity for physiological acclimation to UV‐B; and (2) that these attributes also contribute to decreased UV‐B sensitivity under drought.

Rate of stomatal opening, shoot hydraulic conductance and photosynthetic characteristics in relation to leaf abscisic acid concentration in six temperate deciduous trees.

Correlations between leaf abscisic acid concentration ([ABA]), stomatal conductance (gs), rate of stomatal opening in response to an increase in leaf water potential (si), shoot hydraulic conductance (L) and photosynthetic characteristics were examined in saplings of six temperate deciduous tree species: Acer platanoides L., Padus avium Mill., Populus tremula L., Quercus robur L., Salix caprea L. and Tilia cordata Mill. Species-specific values of foliar [ABA] were negatively related to the mean values of gs, si, L and light- and CO2- saturated net photosynthesis (P(max)), thus providing strong correlative evidence of a scaling of foliar gas exchange and hydraulic characteristics with leaf endogenous [ABA]. In addition, we suggest that mean gs, si, L and Pmax for mature leaves may partly be determined by the species-specific [ABA] during leaf growth. The most drought-intolerant species had the lowest [ABA] and the highest gs, suggesting that interspecific differences in [ABA] may be linked to differences in species-specific water-use efficiency. Application of high concentrations of exogenous ABA led to large decreases in gs, si and P(max), further underscoring the direct role of ABA in regulating stomatal opening and photosynthetic rate. Exogenous ABA also decreased L, but the decreases were considerably smaller than the decreases in gs, si and Pmax. Thus, exogenous ABA predominantly affected the stomata directly, but modification of L by ABA may also be an important mechanism of ABA action. We conclude that interspecific variability in endogenous [ABA] during foliage growth and in mature leaves provides an important factor explaining observed differences in L, gs, si and Pmax among temperate deciduous tree species.