Leaf gas exchange and water potential responses to drought in nine poplar (Populus spp.) clones with contrasting drought tolerance

The dynamic responses of stomatal conductance (g _s) net photosynthesis (A) and leaf water potential (Ψ_leaf) to a progressive drought were examined in nine poplar clones (Populus spp.) with contrasting drought tolerance from the Canadian Prairies, a region prone to frequent droughts. Plants were grown in a greenhouse and either well-watered or drought preconditioned (5–6 cycles of drought) for 8 weeks. At the end of the last cycle, plants were watered to saturation then progressively dried-down (−1.25 MPa Ψ_soil) during which A, g _s and Ψ_leaf were measured. Drought tolerant Okanese reached the lowest combined Ψ_leaf while sensitive clones (Assiniboine and Imperial) had the highest (−1.6 vs. −1.1 MPa). Steady state g _s (measured under well watered conditions) was lower in tolerant (Okanese and Tristis SBC#1) than in sensitive clones. Preconditioning reduced steady state g _s in all clones, lowered the threshold Ψ_leaf for stomatal closure and the minimum Ψ_leaf in most clones but did not affect the steady state A. Tolerant and some moderately tolerant clones maintained higher A at lower Ψ_leaf than the other clones. Stomatal closure was gradual in tolerant clones and in moderately tolerant Northwest but rapid in the other clones. Stomata in the sensitive clones closed at the highest Ψ_leaf, Okanese closed at the lowest. The substantial range in gas exchange and Ψ_leaf responses observed here represented both drought tolerance and taxonomic (Aegiros or Tacamahaca sections) traits which could play a role in the survival and productivity in environments with limited water or during periods of drought.     

Hydraulic conductance and water potential gradients in squash leaves showing mycorrhiza-induced increases in stomatal conductance

Stomatal conductance (g _s) and transpiration rates vary widely across plant species. Leaf hydraulic conductance (k _leaf) tends to change with g _s, to maintain hydraulic homeostasis and prevent wide and potentially harmful fluctuations in transpiration-induced water potential gradients across the leaf (ΔΨ _leaf). Because arbuscular mycorrhizal (AM) symbiosis often increases g _s in the plant host, we tested whether the symbiosis affects leaf hydraulic homeostasis. Specifically, we tested whether k _leaf changes with g _s to maintain ΔΨ _leaf or whether ΔΨ _leaf differs when g _s differs in AM and non-AM plants. Colonization of squash plants with Glomus intraradices resulted in increased g _s relative to non-AM controls, by an average of 27% under amply watered, unstressed conditions. Stomatal conductance was similar in AM and non-AM plants with exposure to NaCl stress. Across all AM and NaCl treatments, k _leaf did change in synchrony with g _s (positive correlation of g _s and k _leaf), corroborating leaf tendency toward hydraulic homeostasis under varying rates of transpirational water loss. However, k _leaf did not increase in AM plants to compensate for the higher g _s of unstressed AM plants relative to non-AM plants. Consequently, ΔΨ _leaf did tend to be higher in AM leaves. A trend toward slightly higher ΔΨ _leaf has been observed recently in more highly evolved plant taxa having higher productivity. Higher ΔΨ _leaf in leaves of mycorrhizal plants would therefore be consistent with the higher rates of gas exchange that often accompany mycorrhizal symbiosis and that are presumed to be necessary to supply the carbon needs of the fungal symbiont.     

Changes of leaf water potential and gas exchange during and after drought in triticale and maize genotypes differing in drought tolerance

Influence of drought (D) on changes of leaf water potential (Ψ) and parameters of gas exchange in D-resistant and D-sensitive genotypes of triticale and maize was compared. Soil D (from −0.01 to −2.45 MPa) was simulated by mannitol solutions. At −0.013 MPa significant differences in Ψ, net photosynthetic rate (P _N), transpiration rate (E), stomatal conductance (g _s), and internal CO₂ concentration (C _i) of D-resistant and D-sensitive triticale and maize genotypes were not found. Together with the increase in concentration of the mannitol solution the impact of D on E and g _s for D-sensitive genotypes (CHD-12, Ankora) became lower than for the D-resistant ones (CHD-247, Tina). Inversely, impact of D on Ψ was higher in D-sensitive than D-resistant genotypes. From 1 to 3 d of D, a higher decrease in P _N was observed in D-resistant genotypes than in the D-sensitive ones. Under prolonged D (5–14 d) and simultaneous more severe D the decrease in P _N was lower in D-resistant than in D-sensitive genotypes. Changes in Ψ, P _N, E, and g _s caused by D in genotypes differing in the drought susceptibility were similar for triticale and maize. Compared to control plants, increase of C _i was different for triticale and maize genotypes. Hence one of the physiological reasons of different susceptibility to D between sensitive and resistant genotypes is more efficient protection of tissue water status in resistant genotypes reflected in higher decrease in g _s and limiting E compared to the sensitive ones. Other reason, observed in D-resistant genotypes during the recovery from D-stress, was more efficient removal of detrimental effects of D.     

Seasonal variations on water relations ofAmygdalus communis L. under drip irrigated and non irrigated conditions

Almond plants (Amygdalus communis L.) of the Garrigues variety were grown in the field drip irrigated and rainfed. Leaf water potential (Ψ) and leaf conductance (g₁) were determined throughout one growing season. Pre-dawn measurement for Ψ in the irrigated treatment was consistent through the growing season, whereas in the rainfed treatment it decreased gradually. Ψ values at midday (Ψ minimum) was closely dependent on atmospheric evaporative demand, and their recovery was quicker in the wet treatment than in the dry. The g₁ values were higher in the wet than dry treatments, decreasing in both cases by leaf ageing. Maximum values for g₁ were reached when evaporative demand was highest in the day. The relationship between Ψ and g₁ revealed a decrease in the hysteresis throughout the growing season, being most marked in the dry treatment. The results highlight the close dependence of Ψ and g₁ on evaporative demand, leaf ageing and irrigtion treatment during the growing season.     

Differences in abscisic acid concentration in roots and leaves of two young Olive (<Emphasis Type="Italic">Olea europaea</Emphasis> L.) cultivars in response to water deficit

Differences in abscisic acid (ABA) accumulation between two olive cultivars were studied by enzyme-linked immunosorbent assay in roots and leaves, leaf water potential (Ψ_l), stomatal conductance (g _s) as well as photosynthetic rate (A) were also determined in well-watered (WW) and water-stressed (WS) plants of two olive cultivars ‘Chemlali’ and ‘Chetoui’. ‘Chemlali’ was able to maintain higher leaf CO₂ assimilation rate and leaf stomatal conductance throughout the drought cycle when compared with ‘Chetoui’. Furthermore, leaf water potential of ‘Chemlali’ decreased in lower extent than in Chetoui in response to water deficit. Interestingly, significant differences in water-stress-induced ABA accumulation were observed between the two olive cultivars and reflect the degree of stress experienced. Chemlali, a drought tolerant cultivar, accumulated lower levels of ABA in their leaves to regulate stomatal control in response to water stress compared to the drought sensitive olive cultivar ‘Chetoui’ which accumulated ABA in large amount.     

Does habitat of <Emphasis Type="Italic">Atriplex halimus</Emphasis> L. affect plant strategy for osmotic adjustment?

The impact of water stress was analysed in the xero-halophyte Mediterranean shrub Atriplex halimus using two Tunisian populations originating from a sub-humid coastal site (Monastir) or from a semi-arid area (Kairouan). Seedlings were exposed for 10 days to nutrient solution containing either 0 or 15% polyethylene glycol. Water potential (Ψ_w), osmotic potential (Ψ_s), osmotic potential at full turgor [Ψ_s(100)], relative water content (RWC), shoot dry weight (DW) and changes in solute concentrations were quantified every 2 days throughout the stress period and inorganic solutes contents were determined at the end of the treatment. The water deficit induced a decrease in Ψ_w, Ψ_s and RWC in both populations, recorded changes being higher in plants of Monastir than those of Kairouan while the shoot dry weight was reduced in a similar extent in stressed plants from both populations. Water deficit induced an increase in proline, glycinebetaine and sugar concentrations. Proline accumulated as early as after the 24-h stress treatment while, glycinebetaine required more than 6 days of stress to accumulate. At the end of the stress period, the plants of Kairouan population accumulated higher amounts of proline than those of Monastir, while an opposite trend was reported for glycinebetaine. Both populations specifically accumulated Na⁺ in response to drought stress, suggesting that this element could play a physiological role in the stress response of this xero-halophyte species. Presented results suggest that the non-recyclable osmotic solute glycinebetaine does not necessarily preferentially accumulates in population facing permanent water stress and that other strategy than osmotic adjustment might be involved in drought tolerance of A. halimus.     

Partitioning of photosynthetic electron flow between CO<Subscript>2</Subscript> assimilation and O<Subscript>2</Subscript> reduction in sunflower plants under water deficit

In sunflower (Helianthus annuus L.) grown under controlled conditions and subjected to drought by withholding watering, net photosynthetic rate (P _N) and stomatal conductance (g _s) of attached leaves decreased as leaf water potential (Ψ_w) declined from −0.3 to −2.9 MPa. Although g _s decreased over the whole range of Ψ_w, nearly constant values in the intercellular CO₂ concentrations (C _i) were observed as Ψ_w decreased to −1.8 MPa, but C _i increased as Ψ_w decreased further. Relative quantum yield, photochemical quenching, and the apparent quantum yield of photosynthesis decreased with water deficit, whereas non-photochemical quenching (q_NP) increased progressively. A highly significant negative relationship between q_NP and ATP content was observed. Water deficit did not alter the pyridine nucleotide concentration but decreased ATP content suggesting metabolic impairment. At a photon flux density of 550 μmol m⁻² s⁻¹, the allocation of electrons from photosystem (PS) 2 to O₂ reduction was increased by 51 %, while the allocation to CO₂ assimilation was diminished by 32 %, as Ψ_w declined from −0.3 to −2.9 MPa. A significant linear relationship between mean P _N and the rate of total linear electron transport was observed in well watered plants, the correlation becoming curvilinear when water deficit increased. The maximum quantum yield of PS2 was not affected by water deficit, whereas q_P declined only at very severe stress and the excess photon energy was dissipated by increasing q_NP indicating that a greater proportion of the energy was thermally dissipated. This accounted for the apparent down-regulation of PS2 and supported the protective role of q_NP against photoinhibition in sunflower.     

Leaf water characteristics and drought acclimation in sunflower genotypes

The responses of leaf water parameters to drought were examined using three sunflower (Helianthus annuus L.) genotypes. Osmotic potential at full water saturation (π¹⁰⁰), apoplastic water fraction (AWF) and bulk elastic modulus (BEM) were determined by pressure-volume curve analysis on well watered or on water-stressed plants (−1.0 MPa Ψ₁ < −1.5 MPa) previously drought-pretreated or not. The drought-pretreated plants were subjected to a 7-day drought period (predawn leaf water potential reached −0.9 MPa) followed by 8 days of rewatering. In well watered plants, all genotypes in response to drought acclimation displayed a significantly decreased π¹⁰⁰ associated with a decrease in the leaf water potential at the turgor-loss point (decrease in Ψ_tlp was between 0.15 and 0.21 MPa, depending on the genotype). In two genotypes, drought acclimation affected the partitioning of water between the apoplastic and symplastic fractions without any effect on the total amount of water in the leaves. As a third genotype displayed no modification of AWF and BEM after drought acclimation, the decreased π¹⁰⁰ was only due to the net accumulation of solutes and was consistent with the adjustment of the photochemical efficiency observed previously in this genotype in response to drought acclimation. In water-stressed plants, the osmotic adjustment (OA) can increase further beyond that observed in response to the drought pretreatment. However, the maintenance of photosynthetic rate and stomatal conductance at low leaf water potentials not only depends on the extent of osmotic adjustment, but also on the interaction between OA and AWF or BEM. Adaptative responses of leaf water parameters to drought are thus quite contrasted in sunflower genotypes. This revised version was published online in June 2006 with corrections to the Cover Date.     

Aquaporin expression in response to different water stress intensities and recovery in Richter-110 (<Emphasis Type="Italic">Vitis</Emphasis> sp.): relationship with ecophysiological status

Aquaporins seem essential for the regulation of plant water status and expenses. Richter-110 is a Vitis hybrid (Vitis berlandieri × rupestris) reputed to be strongly drought-tolerant. Three irrigation treatments were established in Richter-110 plants growing outdoors defined by the resulting maximum stomatal conductance (g _s), and ensuring water stress situations not severe enough as to stop photosynthesis and growth: well-watered plants (g _s about 250 mmol H₂O m⁻² s⁻¹), moderate water stress (g _s about 150 mmol H₂O m⁻² s⁻¹) and severe water stress (g _s about 50 mmol H₂O m⁻² s⁻¹). Plants under water stress were kept at constant water availability for 7 days to check for possible acclimation. Finally, plants were re-watered, and allowed to recover, for 3 days. Stomatal conductance, leaf water potential, xylem abscisic acid (ABA) content and root and stem hydraulic conductivity were determined. The relative amounts of expression of mRNA encoding seven putative aquaporins were determined in roots and leaves by RT-PCR. The decrease in stomatal conductance with moderate and severe water stress was associated with increasing ABA contents, but not with the leaf water potential and hydraulic conductivities, which remained unchanged during the entire experiment. Aquaporin gene expression varied depending on which aquaporin, water stress level and the plant organ. We suggest that aquaporin expression was responsive to water stress as part of the homeostasis, which resulted in constant leaf water potential and hydraulic conductivity.     

Shade does not ameliorate drought effects on the tree fern species Dicksonia antarctica and Cyathea australis

We examined the responses of two tree fern species (Dicksonia antarctica and Cyathea australis) growing under moderate and high light regimes to short-term water deficit followed by rewatering. Under adequate water supply, morphological and photosynthetic characteristics differed between species. D. antarctica, although putatively the more shade and less drought adapted species, had greater chlorophyll a/b ratio, and greater water use efficiency and less negative δ¹³C. Both species were susceptible to water deficit regardless of the light regime showing significant decreases in photosynthetic parameters (A _max, V _cmax, J _max) and stomatal conductance (g _s ) in conjunction with decreased relative frond water content (RWC) and predawn frond water potential (Ψ_predawn). During the water deficit period, decreases in g _s in both species started one day later, and were at lower soil water content, under moderate light compared with high light. D. antarctica under moderate light was more vulnerable to drought than all other plants as was indicated by greater decreases in Ψ_predawn, lowest stomatal conductance, and photosynthetic rates. Both tree fern species were able to recover after a short but severe water stress.     

Effects of Drought Stress Induced by Polyethylene Glycol on Pigment Content and Photosynthetic Gas Exchange of Pistacia Khinjuk and P. Mutica

The effects of drought stress induced by polyethylene glycol, PEG (molecular mass 6000) on some ecophysiological characteristics of two wild pistachio species, Mastic and Khinjuk (P. mutica and P. khinjuk) selected as root stocks for production of edible pistachio trees (P. vera) in Iran and Turkey, were studied. Net photosynthetic rate (P _N), stomatal conductance (g _s), chlorophyll (Chl) fluorescence parameters, leaf water potential (Ψ₁), leaf osmotic potential (Ψ_π), leaf osmotic adjustment (ΔΨ_π), and Chl a and b were measured. All parameters were influenced by increase in concentra-tion of PEG in the nutrient solutions. P _N, g _s, and Chl a were significantly higher in P. mutica than in P. khinjuk but, compared to the control treatment, P. khinjuk showed a higher resistance to drought stress than P. mutica. This revised version was published online in August 2006 with corrections to the Cover Date.     

Leaf gas exchange in a clonal eucalypt plantation as related to soil moisture, leaf water potential and microclimate variables

In order to determine how environmental and physiological factors affect leaf gas exchange in a 9-year-old clonal eucalypt plantation (Eucalyptus grandis Hill ex. Maiden hybrids) in the State of Espirito Santo, Brazil, the diurnal patterns of predawn leaf water potential (Ψ_pd), and leaf gas exchange were monitored from November 1995 to August 1996. Soil water content (Θ) and microclimatic variables were also recorded. Most of the rainfall during the experimental period occurred from October to December 1995 and from March to April 1996, causing a significant variation in Θ and Ψ_pd. A high positive correlation (r ²=0.92) was observed between Ψ_pd and Θ measured at 0.3 m depth from the soil surface. During conditions of high soil water availability, the maximum values of stomatal conductance for water vapor (g _s) and net photosynthetic rate (A) were over 0.4 mol m^–2 s^–2 and l5 μmol m^–2 s^–1, respectively. The results showed that Ψ_pd and leaf gas exchange of the examined trees were susceptible to changes in the water content of the upper soil layers, where the major concentration of active roots occur. Multiple linear regression analysis indicated that photosynthetic active radiation (Q), vapor pressure deficit (VPD), atmospheric CO₂ molar fraction (C _a), and Ψ_pd were the most important factors controlling g _s whereas Q and VPD were the main microclimatic variables controlling A. Received: 5 November 1998 / Accepted: 10 November 1999     

Leaf water relations characteristics of Lupinus angustifolius and L. cosentinii

Summary Lupins (Lupinus angustifolius and L. cosentinii) growing in 321 containers in a glasshouse were exposed to drought by withholding water. Leaf water potential (₁), and leaf osmotic potential (_s) were measured daily as soil water became depleted. Leaf water relations were further assessed by a pressure-volume technique and by measuring _s and relative water content of leaves after rehydration. Analysis by pressure-volume or cryoscopic techniques showed that leaf osmotic potential at saturation (_s100) decreased from -0.6 MPa in well watered to -0.9 MPa in severely droughted leaves, and leaf water potential at zero turgor (_zt) decreased from about -0.7 to -1.1 MPa in well watered and droughted plants, respectively. Relative water content at zero turgor (RWC_zt) was high (88%) and tended to be decreased by drought. The ratio of turgid leaf weight to dry weight was not influenced by drought and was high at about 8.0. The bulk elastic modulus () was approximately halved by drought when related to leaf turgor potential (_p) and probably mediated turgor maintenance during drought. The latter was found to be negatively influenced by rate of drought. Supplying the plants with high levels of K salts did not promote adjustment or turgor maintenance.     

Effects of soil and air drought on growth,plant water status and leaf gas exchange in three Mediterranean cedar species: <Emphasis Type="Italic">Cedrus atlantica</Emphasis>, <Emphasis Type="Italic">C. brevifolia</Emphasis> and <Emphasis Type="Italic">C. libani</Emphasis>

Three- and four-year-old potted, greenhouse-grown cedar seedlings were subjected to two different watering regimes: half received full water supply and the other half was submitted to moderate drought (50% of the full water supply). Height growth was the greatest for C. atlantica and the most-limited for C. brevifolia in the well-watered set. However, in the dry set, height growth was less affected by drought conditions for C. brevifolia than for C. atlantica. Cedrus libani gave intermediate results for both watering regimes. Moderate drought provoked a decrease in osmotic potential at full leaf turgor and a long-lasting osmotic adjustment. When irrigation was withheld completely to induce severe soil drying, gas exchange decreased and then stopped at predawn water potentials of −3.0 MPa for C. brevifolia, between −2.6 and −2.8 MPa for C. libani, and at −2.4 MPa for C. atlantica, irrespective of watering regime. For all species, the dry set showed lower net photosynthesis (A) and stomatal conductance (g _s) than the plants in the well-watered set. A and g _s responded to variations in atmospheric water-vapour pressure deficit (VPD). As VPD increased, A and g _s decreased, and this trend was proportionate to initial values at low VPD, but remained independent of previous watering treatments, plant water status or species. To conclude, C. brevifolia appears to be a species with limited growth potential but strong soil drought tolerance whereas C. atlantica has strong growth potential when an adequate water supply is available but is more sensitive to soil drought. C. libani shows an intermediate behaviour for growth and drought tolerance.     

Leaf–water relations and ion concentrations of the halophyte <Emphasis Type="Italic">Atriplex hortensis</Emphasis> in response to salinity and water stress

The impact of salinity and water stress was analyzed in the xero-halophyte Atriplex hortensis using two varieties: green orach (A. hortensis var. purpurea) and red orach (A. hortensis var. rubra). A. hortensis L. is a C₃ species well adapted to salt and drought conditions. To collect information on the physiological impact of different salt and water deficit levels on their water stress resistance, plants were exposed for 3 months to solution containing four levels of NaCl or to water stress regimes including four levels of field capacity. Osmotic potential at zero turgor Ψ_s⁰, osmotic potential at full turgor (Ψ_s¹⁰⁰), relative water content (RWC), ion concentration (Na⁺, K⁺, Ca²⁺, Mg²⁺, and Cl⁻), and malondialdehyde (MDA) were determined at the end of the treatment. The salinity and water stress induced a decrease in Ψ_s¹⁰⁰, Ψ_s⁰, and RWC in both varieties, recorded changes being higher in plants of red variety than those of green variety. Both varieties specifically accumulated Na⁺ in response to drought and salt stress, suggesting that this element could play a physiological role in the stress response of this xero-halophyte species. In contrast, the presence of NaCl and water stress induced a decrease in K⁺, Ca²⁺, and Mg²⁺ concentration in both varieties. Salinity clearly induced an increase in Cl^– concentration in all tissues, but water stress had no impact on this parameter. MDA concentration increased in response to water stress and exogenous NaCl. Based on these findings the more drought-tolerant red orach may be grown in water-limiting soils.     

Stomatal conductance and leaf water potential responses to hydraulic conductance variation in <Emphasis Type="Italic">Pinus pinaster</Emphasis> seedlings

In this study, tree hydraulic conductance (K _tree) was experimentally manipulated to study effects on short-term regulation of stomatal conductance (g _s), net photosynthesis (A) and bulk leaf water potential (Ψ_leaf) in well watered 5–6 years old and 1.2 m tall maritime pine seedlings (Pinus pinaster Ait.). K _tree was decreased by notching the stem and increased by progressively excising the root system and stem. Gas exchange was measured in a chamber at constant irradiance, vapour pressure deficit, leaf temperature and ambient CO₂ concentration. As expected, we found a strong and positive relationship between g _s and K _tree (r = 0.92, P = 0.0001) and between A and K _tree (r = 0.9, P = 0.0001). In contrast, however, we found that the response of Ψ_leaf to K _tree depended on the direction of change in K _tree: increases in K _tree caused Ψ_leaf to decrease from around −1.0 to −0.6 MPa, but reductions in K _tree were accompanied by homeostasis in Ψ_leaf (at −1 MPa). Both of these observations could be explained by an adaptative feedback loop between g _s and Ψ_leaf, with Ψ_leaf prevented from declining below the cavitation threshold by stomatal closure. Our results are consistent with the hypothesis that the observed stomatal responses were mediated by leaf water status, but they also suggest that the stomatal sensitivity to water status increased dramatically as Ψ_leaf approached −1 MPa.     

Gas exchange inHardwickia binata after water stress and rewatering

The net photosynthetic rate (P_N), stomatal conductance (g_s) and transpiration (E) ofHardwickia binata Roxb. leaves were reduced due to decrease in the leaf water potential (ψ_w) from -2.0 to - 5.7 MPa. P_N partially recovered in the treated plants upon rewatering. Decrease in g_s due to water stress may be the main factor for reduction of P_N. This work was supported by a financial grant from the MNES, India to KP.     

The effect of drought stress on chlorophyll fluorescence in Lolium-Festuca hybrids

The effects of drought on photochemical efficiency of PSII in leaves of 22 hybrids of Festuca pratensis × Lolium multiflorum and Festuca pratensis × Lolium perenne and of Festuca pratensis cv. Skra were investigated. A significant decrease of electron transport efficiency (about 25%) in PSII (Φ_PSII) was not found before 9 days of seedling growth in hydroponics with water potential (Ψ_w) equal to −0.8 MPa (simulated “soil drought”). The decrease of Φ_PSII was similarly related to that of excitation energy capture by open PSII reaction centre (Fv’/Fm’) and also to the decrease of the proportion of oxidized to reduced Q_A (photochemical fluorescence quenching, q_p). According to the drought prolongation, variation of all parameters of fluorescence between genotypes significantly increased. The seedlings of some genotypes were able to recover electron transport efficiency in PSII after increasing water potential in nutrient solution (removing the “soil drought”). When plants grew in containers with soil and 4 genotypes with the highest sensitivity of electron transport to drought (S) as well as 4 genotypes with the highest tolerance (T) were compared 17 days after watering ceased, Ψ_w in leaves considerably decreased, but the differences between S and T genotypes were often not significant in this respect. The differences between S and T genotypes, as values of Fv/Fm were concerned, also appeared small (about 5%), similarly as that of Fv’/Fm’ (5%), q_p (12%) and Φ_PSII (about 15%). Drought stress increased non-photochemical quenching of chlorophyll fluorescence (NPQ) 15 to 47% and this could protect the PSII reaction centres from damages because of energy excess. The increase of NPQ was not closely connected with drought resistance of plants because it was similar in some genotypes tolerant to dehydration as well as in sensitive ones. The results of the experiments suggest that resources of genetic variability in Festulolium may be sufficient for revealing differences between genotypes on the basis of measurement of chlorophyll a fluorescence, as far as their tolerance to soil drought is concerned. As the tolerance of PSII against drought is high, the determinations of fluorescence should be performed rather under severe stress. Such methods seem to be useful for selection of genotypes with high drought tolerance as well as with the ability to at least partial repairing of PSII after drought.     

Interactive effects of elevated CO2 and drought stress on leaf water potential and growth in Caragana intermedia

We studied the responses of leaf water potential (Ψ_w), morphology, biomass accumulation and allocation, and canopy productivity index (CPI) to the combined effects of elevated CO₂ and drought stress in Caragana intermedia seedlings. Seedlings were grown at two CO₂ concentrations (350 and 700 μmol mol⁻¹) interacted with three water regimes (60–70%, 45–55%, and 30–40% of field capacity of soil). Elevated CO₂ significantly increased Ψ_w, decreased specific leaf area (SLA) and leaf area ratio (LAR) of drought-stressed seedlings, and increased tree height, basal diameter, shoot biomass, root biomass as well as total biomass under the all the three water regimes. Growth responses to elevated CO₂ were greater in well-watered seedlings than in drought-stressed seedlings. CPI was significantly increased by elevated CO₂, and the increase in CPI became stronger as the level of drought stress increased. There were significant interactions between elevated CO₂ and drought stress on leaf water potential, basal diameter, leaf area, and biomass accumulation. Our results suggest that elevated CO₂ may enhance drought avoidance and improved water relations, thus weakening the effect of drought stress on growth of C. intermedia seedings.     

Predawn disequilibrium between plant and soil water potentials in two cold-desert shrubs

Classical water relations theory predicts that predawn plant water potential should be in equilibrium with soil water potential (soil Ψ_w) around roots, and many interpretations of plant water status in natural populations are based on this expectation. We examined this expectation for two salt-tolerant, cold-desert shrub species in glasshouse experiments where frequent watering assured homogeneity in soil Ψ_w and soil-root hydraulic continuity and where NaCl controlled soil Ψ_w. Plant water potentials were measured with a pressure chamber (xylem Ψ_p) and thermocouple psychrometers (leaf Ψ_w). Soil Ψ_w was measured with in situ thermocouple psychrometers. Predawn leaf Ψ_w and xylem Ψ_p were significantly more negative than soil Ψ_w, for many treatments, indicating large predawn soil-plant Ψ_w disequilibria: up to 1.2 MPa for Chrysothamnus nauseosus (0 and 100 mm NaCl) and 1.8 MPa for Sarcobatus vermiculatus (0, 100, 300, and 600 mm NaCl). Significant nighttime canopy water loss was one mechanism contributing to predawn disequilibrium, assessed by comparison of xylem Ψ_p for bagged (to minimize transpiration) and unbagged canopies, and by gas exchange measurements. However, nighttime transpiration accounted for only part of the predawn disequilibrium. Other mechanisms that could act with nighttime transpiration to generate large predawn disequilibria are described and include a model of how leaf apoplastic solutes could contribute to the phenomenon. This study is among the first to conclusively document such large departures from the expectation of predawn soil-plant equilibrium for C₃ shrubs, and provides a general framework for considering relative contributions of nighttime transpiration and other plant-related mechanisms to predawn disequilibrium. Received: 12 November 1998 / Accepted: 5 May 1999