Changes in respiration,dry-matter production and its partition in rice (Oryza sativa L.) in response to water deficits during the grain-filling period

A simple, physiologically based model was devised and used for estimating the respiration rate and the overall conversion efficiency of photosynthate into the grain dry-matter in dehydrated plants relative to well watered controls. The model described mathematically the partitioning of assimilate produced by current photosynthesis and of assimilate stored previously between the grain and those plants parts other than grain (the “straw”). Using data obtained from the dry-matter analysis and CO₂ gas exchange measurements, the model gave us two independent estimates of the respiration rate and the overall conversion efficiency; one for the plants given a prolonged dark period and the other for those grown in a normal light and dark cycle. The rate of dark respiration increased with mild water stress: 4.3 mg g⁻¹ day⁻¹ in control plants with leaf water potential of around −0.4 MPa and 11.3 mg g⁻¹ day⁻¹ in dehydrated plants with leaf water potential of around −1 MPa, when both the control and dehydrated plants were left in the dark for ten days. Similar values were obtained for plants in a normal light and dark cycle: 5.6 in well watered and 8.1 mg g⁻¹ day⁻¹ in the stressed plants. Accordingly, the overall conversion efficiency (the ratio of grain dry-matter against the gross carbohydrate input to the construction and maintenance processes) was 0.7 to 0.8 for the well hydrated control and 0.4 to 0.5 for moderately dehydrated plants. With increasing water deficits, however, the respiration rate decreased: 4.8 mg g⁻¹ day⁻¹ when plants were severely stressed (below −3 MPa in midday leaf water potential). The decrease in straw dry weight alone overestimates dry-matter partition of the stored assimilate in the straw into grain by 20 to 30% in well watered plants and the error increases to 50 to 60% in more dehydrated plants.     

Hydraulic resistance of two sorghums varying in drought resistance

Genotypes of sorghum [Sorghum bicolor (L.) Moench] vary in drought resistance. Yet it is not known if their hydraulic resistances vary. The objective of this study was to determine if the hydraulic resistance of a drought-resistant sorghum was the same as that of a drought-sensitive sorghum. Leaf water and osmotic potentials were measured daily, during a 14-d period, in leaves of a drought-resistant (‘KS9’) and a drought-sensitive (‘IA25’) sorghum, which had the roots in pots with a commercial potting soil that was either well watered or allowed to dry. Soil water potential, adaxial stomatal resistance, and transpiration rate were determined daily. Hydraulic resistance of the plants was calculated from the slope of the line relating soil water potential minus leaf water potential versus transpiration rate. When the soil was not watered, the drought-sensitive sorghum had a water potential that averaged −0.50 MPa lower and an osmotic potential that averaged −0.57 MPa lower, but a similar adaxial stomatal resistance (1.19 s mm⁻¹), compared with the drought-resistant sorghum. Seven days after the beginning of the experiment, the water potential of the soil with the drought-sensitive sorghum was −0.25 MPa lower than that of the soil with the drought-resistant sorghum. With the water-limited conditions, the drought-sensitive sorghum depleted the soil-water reserve more quickly and died 2 d before the drought-resistant sorghum. Under well watered conditions, the two sorghums had similar water potentials (−1.64 MPa), osmotic potentials (−2.83 MPa), and adaxial stomatal resistances (0.78 s mm⁻¹). The calculated hydraulic resistance of the two sorghums did not differ and averaged 3.4 × 10⁷ MPa s m⁻¹. The results suggested that the variation in susceptibility to drought between the two genotypes was due to differences in rate of soil-water extraction. Contribution No. 86-249-J from the Kansas Agricultural Experiment Station. The paper is dedicated to the memory of Dr Dan M Rodgers.     

Enhancement of superoxide dismutase activity in the leaves of white clover (Trifolium repens L.) in response to polyethylene glycol-induced water stress

Effects of polyethylene glycol (PEG)-induced water stress on the activities of total leaf superoxide dismutase (SOD) and chloroplast SOD (including thylakoid-bound SOD and stroma SOD) are described in white clover (Trifolium repens L.) grown in solution culture from rooted cuttings. Both leaf SOD and chloroplast SOD activities were markedly enhanced with increasing concentration of PEG stress, generating osmotic potentials around the roots 0, −0.5, −1.0, −1.5 MPa. The effects increased with time up to 72 h. Chloroplast Fe-containing SOD represented about 30% of the total leaf SOD activity in the control plants and a significant increase in chloroplast SOD activity was found during the stress period. This accounted for about 35.5–71.1% of the total leaf SOD activity. The proportion of chloroplast SOD in total leaf SOD not only increased with the decreasing of osmotic potential, but also increased with incubation time. Furthermore, the increase in thylakoid-bound SOD activity was much higher than that of stroma SOD in chloroplast of plants under water stress. The enhanced chloroplastic SOD activity, especially thylakoid-bound SOD activity, demonstrated in Trifolium repens suggests that Fe-SOD located in chloroplasts play a more important role than cytosolic Cu/Zn-containing SODs in scavenging O₂ ⁻.     

Leaf water relations characteristics of differently potassium fertilized and watered field grown barley plants

Seasonal leaf water relations characteristics were studied in fully irrigated spring barley (Hordeum distichum L. cv. Gunnar) fertilized at low (50 kg K ha⁻¹) or high (200 kg K ha⁻¹) levels of potassium applied as KCl. The investigation was undertaken from about 14 days before anthesis until the milk ripe stage in leaves of different position and age. Additionally, the effects of severe water stress on leaf water relations were studied in the middle of the grain filling period in spring barley (cv. Alis). The leaf water relations characteristics were determined by the pressure volume (PV) technique. Water relations of fully irrigated plants were compared in leaf No 7 with the water relations of slowly droughted plants (cv. Alis). Leaf osmotic potential at full turgor (ψ _π ¹⁰⁰ ) decreased 0.1 to 0.3 MPa in droughted leaves indicating a limited osmotic adjustment due to solute accumulation. The leaf osmotic potential at zero turgor (ψ _π ⁰ ) was about −2.2 MPa in fully irrigated plants and −2.6 MPa in droughted plants. The relative water content at zero turgor (R⁰) decreased 0.1 unit in severely droughted leaves. The ratio of turgid leaf weight to dry weight (TW/DW) tended to be increased by drought. The tissue modulus of elasticity (ε) decreased in droughted plants and together with osmotic adjustment mediated turgor maintenance during drought. A similar response to drought was found in low and high K plants except that the R⁰ and ε values tended to be higher in the high K plants. Conclusively, during drought limited osmotic adjustment and increase in elasticity of the leaf tissue mediated turgor maintenance. These effects were only slightly modified by high potassium application. The seasonal analysis in fully irrigated plants (cv. Gunnar) showed that within about 14 days from leaf emergence ψ _π ¹⁰⁰ decreased from about −0.9 to −1.6 MPa in leaf No 7 (counting the first leaf to emerge as number one) and from about −1.1 to −1.9 MPa in leaf No 8 (the flag leaf) due to solute accumulation. A similar decrease took place in ψ _π ⁰ except that the level of ψ _π ⁰ was displaced to a lower level of about 0.2 to 0.3 MPa. Both ψ _π ¹⁰⁰ and ψ _π ⁰ tended to be 0.05 to 0.10 MPa lower in high K than in low K plants. R⁰ was about 0.8 to 0.9 and was independent of leaf position and age, but tended to be highest in high K plants. The TW/DW ratio decreased from about 5.5 in leaf No 6 to 4.5 in leaf No 7 and 3.8 in leaf No 8. The TW/DW ratio was 4 to 10% higher in high K than in low K plants indicating larger leaf cell size in the former. The apoplastic water content (V_a) at full turgor constituted about 15% in leaf No 7. ε was maximum at full turgor and varied from about 11 to 34 MPa. ε tended to be higher in high K plants. Conclusively, in fully watered plants an ontogenetically determined accumulation of solutes (probably organic as discussed) occurred in the leaves independent of K application. The main effect of high K application on water relations was an increase in leaf water content and a slight decrease in leaf ψ_π. The effect of K status on growth and drought resistance is discussed.     

Can water responses in Stipa tenacissima L. during the summer season be promoted by non-rainfall water gains in soil?

This article reports on quantified soil water gains and their possible effects on summer water relationships in a semiarid Stipa tenacissima L. grasslands located in SE Spain. We believe that the net soil water gains detected using minilysimeters could be from soil water vapour adsorption (WVA). Our study of high water-stress showed stomatal conductance (21.8–43.1 mmol H₂O m⁻² s⁻¹) in S. tenacissima leaves unusual for the summer season, and the evapotranspiration from S. tenacissima grassland, estimated by a multi-source sparse evapotranspiration model, closely corresponding to total WVA. This highlights the importance of summer soil WVA to stomatal conductance and vital transpiration in S. tenacissima. This study measured pre-dawn leaf water potential (ψ) response to sporadic light rainfall, finding that a light summer rainfall (1.59 mm day⁻¹) was sufficient to vary ψ in S. tenacissima from −3.8 (close to the turgour loss point) to −2.7 MPa. We hypothesize that soil WVA can supply vegetation with water vital to its survival in seasons with a severe water deficit, giving rise to a close relationship between soil water dynamics and plant water response.     

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.     

Evaluating the effects of gross nitrogen mineralization,immobilization, and nitrification on nitrogen fertilizer availability in soil experimentally contaminated with diesel

Sandy clay loam soil was contaminated with 5000 mg kg⁻¹ diesel, and amended with nitrogen (15.98 atom% ¹⁵N) at 0, 250, 500, and 1000 mg kg⁻¹ to determine gross rates of nitrogen transformations during diesel biodegradation at varying soil water potentials. The observed water potential values were −0.20, −0.47, −0.85, and −1.50 MPa in the 0, 250, 500, and 1000 mg kg⁻¹ nitrogen treatments respectively. Highest microbial respiration occurred in the lowest nitrogen treatment suggesting an inhibitory osmotic effect from higher rates of nitrogen application. Microbial respiration rates of 185, 169, 131, and 116 mg O₂ kg⁻¹ soil day⁻¹ were observed in the 250, 500, control and 1000 mg kg⁻¹ nitrogen treatments, respectively. Gross nitrification was inversely related to water potential with rates of 0.2, 0.04, and 0.004 mg N kg⁻¹ soil day⁻¹ in the 250, 500, and 1000 mg kg⁻¹ nitrogen treatments, respectively. Reduction in water potential did not inhibit gross nitrogen immobilization or mineralization, with respective immobilization rates of 2.2, 1.8, and 1.8 mg N kg⁻¹ soil day⁻¹, and mineralization rates of 0.5, 0.3, and 0.3 mg N kg⁻¹ soil day⁻¹ in the 1000, 500, and 250 mg kg⁻¹ nitrogen treatments, respectively. Based on nitrogen transformation rates, the duration of fertilizer contribution to the inorganic nitrogen pool was estimated at 0.9, 1.9, and 3.2 years in the 250, 500, and 1000 mg kg⁻¹ nitrogen treatments, respectively. The estimation was conservative as ammonium fixation, gross nitrogen immobilization, and nitrification were considered losses of fertilizer with only gross mineralization of organic nitrogen contributing to the most active portion of the nitrogen pool.     

Implication of ABA and proline on cell membrane injury of water deficit stressed barley seedlings

The aim of this work was to examine the ability of ABA and proline to counteract the deleterious effect of water deficit stress on cell membrane injuries. Six-day-old seedlings of two barley genotypes (cv. Aramir, line R567) were treated with ABA (2·10⁻⁴ M) or proline (0.1 M) for 24 h, and then subjected to osmotic stress for 24h, by immersing their roots in polyethylene glycol (PEG 6000) solution of osmotic potential of −1.0 MPa and −1.5 MPa or by submerging the leaf pieces in PEG solution of osmotic potential of −1.6 MPa. Pretreatment of plants with ABA and proline caused an increase of free proline level in the leaves. Plants treated with ABA exhibited a lower membrane injury index under water stress conditions than those untreated even when no effect of this hormone on RWC in the leaves of stressed plants was observed. Pretreatment of plants with proline prevented to some extent membrane damage in leaves of the stressed seedlings, but only in the case when stress was imposed to roots. Improvement in water status of leaves was also observed in seedlings pretreatment with proline. The protective effect of both ABA and proline was more pronounced in line R567 that exhibited higher membrane injury under water deficit stress conditions.     

Water stress and photoinhibition in acclimatization ofRosa hybrida plantlets

Summary MicropropagatedRosa hybrida plantlets were simultaneously rooted and acclimatized under 100 and 200 μmol m⁻² s⁻¹ light for 2 wk. At the end of the first week of acclimatization, the plantlets were transferred onto a low water potential medium (from −0.06 MPa to −0.3 MPa). Dry weight was decreased by increased hight and low water potential. Photoinhibition of photosynthesis, expressed as a decrease in Fv/Fm ratio and ΦPSII and an increase in 1 −qp, occurred in plants grown under 200 μmol m⁻² s⁻¹. When high light (200 μmol m⁻² s⁻¹) and water stress were applied simultaneously, their effects on chlorophyll fluorescence parameters depended on stress duration; after 1 d of water stress, photoinhibition was more pronounced; after 7 d of stress, Fv/Fm ratio and ΦPSII were higher than after 1 d of stress; photoinhibition was reduced. This suggests that after a 1-d stress, the effect of water stress alone included a superimposed effect of photoinhibition to which the water-stressed plants were sensitized; after 7 d, plantlets had adapted to water stress. The photoprotective effects under high light might result in energy dissipative mechanisms linked to photochemical and nonphotochemical quenching other than CO₂ fixation.     

Differential Snowpack Accumulation and Water Dynamics in Aspen and Conifer Communities: Implications for Water Yield and Ecosystem Function

Early succession aspen and late succession conifer forests have different architecture and physiology affecting hydrologic transfer processes. An evaluation of water pools and fluxes was used to determine differences in the hydrologic dynamics between stands of quaking aspen (Populus tremuloides) and associated stands of mixed conifer consisting of white fir (Abies concolor), Douglas-fir (Pseudotsuga menziesii), and Engelmann spruce (Picea engelmannii). In 2005 and 2006, measurements of snow water accumulation, snow ablation (melt), soil water content, snowpack sublimation, and evapotranspiration (ET) were measured in adjacent aspen and conifer stands. Peak snow water equivalent (SWE) averaged 34–44% higher in aspen in 2005 (average snow fall) and 2006 (above average snow fall), respectively, whereas snow ablation rates were greater in aspen stands (21 mm day⁻¹) compared to conifer stands (11 mm day⁻¹). When changes in soil water content (due to over-winter snowmelt) were combined with peak snow accumulation in 2006, aspen had greater potential (42–83%) water yield for runoff and groundwater recharge. Snowpack sublimation during the ablation period was not significantly different between meadow, aspen, and conifer sites and comprised less than 5% of the winter precipitation. Extended conifer transpiration in spring and fall did not contribute to large differences in water yield (<28 mm y⁻¹). Summertime ET rates were higher in aspen plots (3.6 mm day⁻¹) than in conifer plots (2.7 mm day⁻¹), and differences in net ET largely reflected soil column porosity. This study shows that the largest differences in annual water yield between aspen and conifer stands result from differences in SWE and net summertime ET. Although SWE and accumulation of water in soil was greater in aspen, it was partly offset by greater net annual ET losses in aspen.     

Effects of nutrient availability on water hyacinth standing crop and detritus deposition

Nutrient-enriched water hyacinths were stocked in outdoor tanks and cultured under both high nutrient (HN) and low nutrient (LN) regimes for 10 months. Seasonal changes in standing crop biomass and morphology of LN water hyacinths were similar to those of HN water hyacinths, despite a ten-fold between-treatment difference in N availability and a two-fold difference in average plant N concentrations (1.0 and 2.0% for LN and HN plants, respectively). Tissue N accumulated by the LN plants prior to stocking helped support standing crop development during the 10 month study. In both HN and LN treatments, the rate of detritus deposition, or the sloughing of dead plant tissues from the mat, was lower than the actual detritus production rate because of the retention of dead ‘aerial’ tissues (laminae and petioles) in the floating mat. The retention of laminae and petioles may serve as a nutrient conservation mechanism, since nutrients released from decomposing tissues in the mat-water environment may be assimilated by adjacent plants. The average rate of detritus deposition (both dry matter and N) by LN water hyacinths (1.2 g dry wt. m⁻² day⁻¹ and 0.017 g N m⁻² day⁻¹) was lower than that of HN plants (3.0 g dry wt. m⁻² day⁻¹ and 0.075 g N m⁻² day⁻¹) during the study. Low detrital N losses by the water hyacinth probably enhance the survival of this species in aquatic systems which receive nutrient inputs intermittently.     

Antioxidative system in maize roots as affected by osmotic stress and different nitrogen sources

The activities of antioxidative enzymes and contents of proline and total phenolics were assayed in roots of two maize (Zea mays L.) genotypes grown in a medium containing nitrate (NO₃ ⁻) or both nitrogen forms, nitrate and ammonium (NH₄ ⁺/NO₃ ⁻). An increase in the activities of class III peroxidases (POD), superoxide dismutase (SOD), ascorbate peroxidase (APX), ascorbate oxidase (AO) and proline content, and decrease in phenolic content were observed in NH₄ ⁺/NO₃ ⁻ in comparison with NO₃ ⁻ grown plants. When polyethylene glycol (PEG) was added to both nitrogen treatments, the content of total phenolics and proline was increased, especially in NH₄ ⁺/NO₃ ⁻ treatment. The PEG treatment decreased enzyme activities in NH₄ ⁺/NO₃ ⁻ grown plants, but in NO₃ ⁻ grown plants activities of POD and SOD were increased, opposite to decreased APX and AO. Isoelectric focusing demonstrated increased activities of acidic POD isoforms in PEG treated NO₃ ⁻ grown plants, and lower activities of both, acidic and basic isoforms in NH₄ ⁺/NO₃ ⁻grown plants.     

Ammonium nutrition increases photosynthesis rate under water stress at early development stage of rice (Oryza sativa L.)

An hydroponic experiment with a simulated water stress induced by PEG (6000) was conducted in a greenhouse to study the effects of nitrate (NO₃ ⁻), ammonium (NH₄ ⁺) and the mixture of NO₃ ⁻ and NH₄ ⁺, on water stress tolerance of rice seedlings. Rice (Shanyou 63) was grown under non- or simulated water stress condition (10% (w/v) PEG, MW6000) with the 3 different N forms during 4 weeks. Under non-stressed condition no difference was observed among the N treatments. Under simulated water stress, seedlings grown on N-NO₃ ⁻ were stunted. Addition of PEG did not affect rice seedling growth in the treatment of only NH₄ ⁺ supply but slightly inhibited the rice seedling growth in the treatment of mixed supply of NO₃ ⁻ and NH₄ ⁺. Simulated water stress, when only N-NH₄ ⁺ was present, did not affect leaf area and photosynthesis rate, however, both parameters decreased significantly in the NO₃ ⁻ containing solutions. Under water stress, Rubisco content in newly expanded leaves significantly increased in the sole NH₄ ⁺ supplied plants as compared to that in plants of the other two N treatments. Under water stress, the ratio of carboxylation efficiency to Rubisco content was, respectively, decreased by 13 and 23% in NH₄ ⁺ and NO₃ ⁻ treatments, respectively. It is concluded that, water stress influenced the Rubisco activity than stomatal limitation, and this effects could be regulated by N forms. Responsible Editor: Herbert Johannes Kronzucker. Shiwei Guo and Gui Chen contributed equally to this paper.     

What determines interspecific variation in relative growth rate of <Emphasis Type="Italic">Eucalyptus</Emphasis> seedlings?

The present study examines relative growth rate (RGR) and its determinants in seedlings of nine Eucalyptus species. Species were selected from mesic (1,800 mm a⁻¹ rainfall) through to semi-arid habitats (300 mm a⁻¹), and thus, notionally vary in “stress” tolerance. Seedlings were grown in a glasshouse during early summer and received between 33 mol and 41 mol PAR m⁻² day⁻¹ . The mean RGR varied among species—from a minimum of 66 mg g⁻¹ day⁻¹ in E. hypochlamydea to a maximum of 106 mg g⁻¹ day⁻¹ in E. delegatensis. RGR was positively related to rainfall at the sites of seed collection. Neither specific leaf area (SLA) nor net assimilation rate was related to rainfall or RGR. While the absence of relationships with SLA and net assimilation rate contrasts with other studies and species, we cannot rule out the effects of sample size (n=9 species) and modest ranges in SLA and RGR. The ratio of leaf mass to total mass (LMR) varied from 0.49±0.07 g g⁻¹ in E. socialis to 0.74±0.04 g g⁻¹ in E. delegatensis and was strongly positively related with rainfall (r ²=0.77). Interspecific differences in RGR were strongly related to LMR (positive relationship, r ²=0.50) and the rate of dry matter production per mol of leaf nitrogen (positive relationship, r ²=0.64). Hence, the slow RGR of low-rainfall species was functionally related to a lower growth rate per mol of leaf nitrogen than high-rainfall species. Furthermore, slow RGR of low-rainfall species was related to greater allocation to roots at the expense of leaves. Increasing allocation to roots versus leaves is likely an adaptation to soil and atmospheric water deficits, but one that comes at the expense of a slow RGR.     

Water relations, gas exchange characteristics, and the level of some metabolites in two cultivars of spring wheat under different N regimes

Twenty eight-day old plants of two spring wheat cultivars differing in salinity tolerance were subjected to varying levels of nitrogen (56, 112, and 224 mg N·kg⁻¹ soil) for 42 days. Both cultivars performed differently under varying soil N levels in terms of growth, and grain yield and yield components. Nitrogen levels, 112 and 224 mg·kg⁻¹ soil, caused maximal growth in Sarsabz and Barani-83, respectively. Cv Sarsabz maintained higher leaf water and turgor potentials, but lower leaf osmotic potential than those of Barani-83 at all external N regimes. Sarsabz had higher Chl a, Chl b and carotenoids contents in leaves than those in Barani-83 at 56 and 112 mg N·kg⁻¹ soil. Sarsabz had higher contents of leaf soluble proteins, soluble sugars, and free amino acids than those in Barani-83 at all external N levels. In Barani-83 net CO₂ assimilation rate remained almost unchanged, whereas in Sarsabz it decreased consistently with increase in external N level. The better growth performance of Sarsabaz as compared to Barani-83 under varying soil N levels except 224 mg N·kg⁻¹ soil was associated with maintenance of high leaf turgor potential but not with net CO₂ assimilation rate.     

Hydraulic Lift in Cork Oak Trees in a Savannah-Type Mediterranean Ecosystem and its Contribution to the Local Water Balance

The aim of this study was to identify the sources and depth of water uptake by 15-years old Quercus suber L. trees in southern Portugal under a Mediterranean climate, measuring δ¹⁸O and δD in the soil–plant-atmosphere continuum. Evidence for hydraulic lift was substantiated by the daily fluctuations observed in Ψ_s at 0.4 and 1 m depth and supported by similar δ¹⁸O values found in tree xylem sap, soil water in the rhizosphere and groundwater. From 0.25 m down to a depth of 1 m, δD trends differed according to vegetation type, showing a more depleted value in soil water collected under the evergreen trees (−47‰) than under dead grasses (−35‰). The hypothesis of a fractionation process occurring in the soil due to diffusion of water vapour in the dry soil is proposed to explain the more depleted soil δD signature observed under trees. Hydraulically lifted water was estimated to account for 17–81% of the water used during the following day by tree transpiration at the peak of the drought season, i.e., 0.1–14 L tree⁻¹ day⁻¹. Significant relationships found between xylem sap isotopic composition and leaf water potential in early September emphasized the positive impact of the redistribution of groundwater in the rhizosphere on tree water status.     

Partition of photosynthates between shoot and root in spring wheat (Triticum aestivum L.) as a function of soil water potential and root temperature

Low soil water potential and low or high root temperatures are important stresses affecting carbon allocation in plants. This study examines the effects of these stresses on carbon allocation from the perspective of whole plant mass balance. Sixteen-day old spring wheat seedlings were placed in a growth room under precisely controlled root temperatures and soil water potentials. Five soil water potential treatments, from −0.03 MPa to −0.25 MPa, and six root temperature treatments, from 12 to 32°C were used. A mathematical model based on mass balance considerations was used, in combination with experimental measurements of rate of net photosynthesis, leaf area, and shoot/root dry masses to determine photosynthate allocation between shoot and root. Partitioning of photosynthates to roots was the lowest at 22–27°C root temperature regardless soil water potential, and increased at both lower and higher root temperatures. Partitioning of photosynthates to the roots increased with decreasing soil water potential. Under the most favourable conditions, i.e. at −0.03 MPa soil water potential and 27°C root temperature, the largest fraction, 57%, of photosynthates was allocated to the shoots. Under the most stressed conditions, i.e. at −0.25 MPa soil water potential and 32°C root temperature, the largest fraction, more than 80%, of photosynthates was allocated to roots.     

Salinity effect on plant growth and leaf demography of the mangrove, Avicennia germinans L.

We assessed the effect of salinity on plant growth and leaf expansion rates, as well as the leaf life span and the dynamics of leaf production and mortality in seedlings of Avicennia germinans L. grown at 0, 170, 430, 680, and 940 mol m⁻³ NaCl. The relative growth rates (RGR) after 27 weeks reached a maximum (10.4 mg g⁻¹ d⁻¹) in 170 mol m⁻³ NaCl and decreased by 47 and 44% in plants grown at 680 and 940 mol m⁻³ NaCl. The relative leaf expansion rate (RLER) was maximal at 170 mol m⁻³ NaCl (120 cm m⁻² d⁻¹) and decreased by 57 and 52% in plants grown at 680 and 940 mol m⁻³ NaCl, respectively. In the same manner as RGR and RLER, the leaf production (P) and leaf death (D) decreased in 81 and 67% when salinity increased from 170 to 940 mol m⁻³ NaCl, respectively. Since the decrease in P with salinity was more pronounced than the decrease in D, the net accumulation of leaves per plant decreased with salinity. Additionally, an evident increase in annual mortality rates (λ) and death probability was observed with salinity. Leaf half-life (t _0.5) was 425 days in plants grown at 0 mol m⁻³ NaCl, and decreased to 75 days at 940 mol m⁻³ NaCl. Thus, increasing salinity caused an increase in mortality rate whereas production of new leaves and leaf longevity decreased and, finally, the leaf area was reduced.     

Monovalent cation transporters; establishing a link between bioinformatics and physiology

Toxic aluminum (Al) ion is a major constraint to plant growth in acid soils. Aluminum tolerance in wheat (Triticum aestivum L.) is strongly related to the Al-triggered efflux of malate from root apices. A role of the secreted malate has been postulated to be in chelating Al and thus excluding it from root apices (malate hypothesis), but the actual process has yet to be fully elucidated. We measured Al content and root growth during and after Al exposure using seedlings of near-isogenic lines [ET8 (Al tolerant) and ES8 (Al sensitive)] differing in the capacity to induce Al-triggered malate efflux. Aluminum doses that caused 50% root growth inhibition during 24-h exposure to Al in calcium (Ca) solution (0.5 mM CaCl₂, pH 4.5) were 50 μM in ET8 and 5 μM in ES8. Under such conditions, the amount of Al accumulated in root apices was approximately 2-fold higher in ET8 than ES8. Al-treated seedlings were then transferred to the Al-free Ca solution for 24 h. Compared to control roots (no Al pretreatment), root regrowth of Al-treated roots was about 100% in ET8 and about 25% in ES8. The impaired regrowth in ES8 was observed even after 24-h exposure to 2.5 μM Al which had caused only 20% root growth inhibition. The addition of malate (100 μM) during exposure to 50 μM Al in ES8 enhanced root growth 1.6 times and regrowth in Al-free solution 7 times, resulting in similar root growth and regrowth as in ET8. Short-term Al treatments of ES8 for up to 5 h indicated that the Al-caused inhibition of root regrowth started after 1-h exposure to Al. The stimulating effect of malate on root regrowth was observed when malate was present during Al exposure, but not when roots previously exposed to Al were rinsed with malate, although Al accumulation in root apices was similar under these malate treatments. We conclude that the malate secreted from root apices under Al exposure is essential for the apices to commence regrowth in Al-free medium, the trait that is not related to the exclusion of Al from the apices.     

Gibberellic acid improves water deficit tolerance in maize plants

The combination effects of water stress and gibberellic acid (GA₃) on physiological attributes and nutritional status of maize (Zea mays L. cv., DK 647 F1) were studied in a pot experiment. Maize plants were grown in the control (well watered WW) and water stress subjected to treated both water stress and two concentrations of gibberellic acid (GA₃ 25 mg L⁻¹, 50 mg L⁻¹). WS was imposed by maintaining the moisture level equivalent to 50 % pot capacity whereas the WW pots were maintained at full pot capacity. Water stress reduced the total dry weight, chlorophyll concentration, and leaf relative water content (RWC), but it increased proline accumulation and electrolyte leakage in maize plants and appears to affect shoots more than roots. Both concentrations of GA₃ (25 and 50 mg L⁻¹) largely enhanced the above physiological parameters to levels similar to control. WS reduced leaf Ca²⁺ and K⁺ concentrations, but exogenous application of GA₃ increased those nutrient levels similar or close to control. Exogenous application of GA₃ improved the water stress tolerance in maize plants by maintaining membrane permeability, enhancing chlorophyll concentration, leaf relative water content (LRWC) and some macro-nutrient concentrations in leaves.