Manuel Sánchez-Díaz

Involvement of abscisic acid in leaf and root of maize (Zea mays L.) in avoiding chilling-induced water stress

In the present study, we investigated the role of abscisic acid (ABA) on chilling tolerance of maize. Two maize genotypes differing in chilling sensitivity (Z7 tolerant and Penjalinan sensitive) were subjected to chilling (5 8C, 12 h photoperiod, 150 mmol m � 2 s � 1 PPFD) for 3 days under two relative humidity (RH) regimes (60 or 100% RH). Some plants were exogenously treated 24 h before chilling with ABA (100 mM). As expected, high humidity (100% RH) or ABA pre-treatment prevented the leaf water deficit induced by chilling at 60% RH in chilling sensitive Penjalinan plants. ABA pre-treatment improved chilling tolerance of Penjalinan plants, mainly by decreasing leaf conductance and by increasing root water flow. At the leaf level, we found a relationship between ABA content and chilling tolerance in both maize genotypes. No relationship between ABA content and leaf conductance was found. Moreover, during chilling, no differences on leaf conductance between the two genotypes were observed, probably indicating that the different water stress suffered by the two genotypes could be linked to differences in the root water uptake. The rise in leaf ABA content during chilling was independent of the leaf water status, so it must be induced by the low temperature per se, and after a longer cold exposure also by the vapour pressure deficit (VPD) (a higher VPD allows more ABA accumulation). At the root level, we did not observe a relationship between the root hydraulic acclimation to chilling and the root ABA content. Z7 plants chilled at 60% RH had the same root ABA content as those which were chilled at 100% RH and as Penjalinan plants; however, the former showed a higher root hydraulic conductance. The rise in the root ABA content in Z7 plants followed the same pattern as observed in the leaves. In Penjalinan plants, the rise in root ABA content was linked only to low temperatures per se, since it increased in the same way in plants chilled under 60 or 100% RH. # 2003 Elsevier Ireland Ltd. All rights reserved.

Biomass partitioning, morphology and water status of four alfalfa genotypes submitted to progressive drought and subsequent recovery

The predicted worldwide increase of arid areas and water stress episodes will strongly affect crop production. Numerous plants have developed specific morphological and physiological mechanisms as a means to increase their tolerance to drought. Water stress modifies dry matter partitioning and morphological components such as leaf area ratio (LAR), specific leaf area (SLA) and leaf weight ratio (LWR). Alfalfa has a wide-ranging distribution and is thus expected to show differing levels of drought tolerance. The aim of our study was to determine the effect of progressive drought and subsequent recovery in four alfalfa genotypes differing in drought sensitivity: three cultivars adapted to a Mediterranean climate, Tafilalet (TA), Tierra de Campos (TC) and Moapa (MO), and another representative of an oceanic climate, Europe (EU). Mild drought did not affect biomass production or water status in the studied varieties. Under moderate drought conditions, TA and MO showed decreased leaf production, which may help them to maintain relative water content (RWC). Despite observations that water stress did not affect root growth, after the recovery period, TA increased its root biomass, making higher water soil prospecting possible. Mediterranean cultivars modified LAR and SLA depending on water availability, whereas EU alters LWR. At the end of the experiment, TC was the most productive cultivar, but severe drought did not predict differences among cultivars. Severe water stress increased the root/shoot ratio in order to diminish water consumption and increase absorption of water. In spite of all cultivars showing a decreased LWR, TA also decreased SLA, which may suggest higher drought resistance. Morphological traits from Mediterranean cultivars, including the ability to alter SLA or LAR may be used for drought-tolerant cultivar improvement.

Effect of water stress on photosynthetic activity in the Medicago-Rhizobium-Glomus symbiosis

Abstract CO 2 exchange rate (CER), internal CO 2 concentration (Ci), photosynthetical phosphorus use efficiency (PPUE), nodule activity (ARA) and growth parameters were compared in mycorrhizal nodulated (VAM) and P-compensated nodulated alfalfa plants, under both well watered and drought acclimated conditions. Leaf area ratio significantly decreased in P-fertilized plants in response to drought treatment whereas mycorrhizal plants maintained rather constant values. As stress progressed, VAM plants developed lower root-shoot and higher leaf area ratios than non-VAM. Total dry weight decreased for both treatments in response to drought, but reduction was more pronounced in P-fertilized plants. Under watered conditions, CER and PPUE were approximately 30% and 55% higher respectively in VAM than in non-VAM plants while specific nodule activity was similar in both groups of plants. Throughout drought, the three parameters (CER, PPUE and ARA) maintained significantly higher values in myocorrhizal plants. Internal CO 2 concentration was always higher in P-compensated than in VAM plants. Results show that under water stress the tripartite symbiosis maintains higher values of photosynthetic and nodule activities than non-myocorrhizal plants suggesting enhanced drought tolerance by mycorrhizal condition.

Influence of arbuscular mycorrhizae and Rhizobium on nutrient content and water relations in drought stressed alfalfa

The objective of this research was to study the effect of drought on nutrient content and leaf water status in alfalfa (Medicago sativa L. cv Aragón) plants inoculated with a mycorrhizal fungus and/or Rhizobium compared with noninoculated ones. The four treatments were: a) plants inoculated with Glomus fasciculatum and Rhizobium meliloti 102 F51 strain, (MR); b) plants inoculated with R. meliloti only (R); c) plants with G. fasciculatum only (M); and d) noninoculated plants (N). Nonmycorrhizal plants were supplemented with phosphorus and nonnodulated ones with nitrogen to achieve similar size and nutrient content in all treatments. Plants were drought stressed using two cycles of moisture stress and recovery. The components of total leaf water potential (osmotic and pressure potentials at full turgor), percentage of apoplastic water volume and the bulk modulus of elasticity of leaf tissue were determined. Macronutrient (N, P, K, Ca, S and Mg) and micronutrient (Co, Mo, Zn, Mn, Cu, Na, Fe and B) content per plant were also measured. Leaves of N and R plants had decreased osmotic potentials and increased pressure potentials at full turgor, with no changes either in the bulk modulus of elasticity or the percentage of apoplastic water upon drought conditions. By contrast, M and MR leaves did not vary in osmotic and turgor potentials under drought stress but had increased apoplastic water volume and cell elasticity (lowering bulk modulus). Drought stress decreased nutrient content of leaves and roots of noninoculated plants. R plants showed a decrease in nutrient content of leaves but maintained some micronutrients in roots. Leaves of M plants were similar in content of nutrients to N plants. However, roots of M and MR plants had significantly lower nutrient content. Results indicate an enhancement of nutrient content in mycorrhizal alfalfa plants during drought that affected leaf water relations during drought stress.

Thermotolerance responses in ripening berries of Vitis vinifera L. cv Muscat Hamburg

Berry organoleptic properties are highly influenced by ripening environmental conditions. In this study, we used grapevine fruiting cuttings to follow berry ripening under different controlled conditions of temperature and irradiation intensity. Berries ripened at higher temperatures showed reduced anthocyanin accumulation and hastened ripening, leading to a characteristic drop in malic acid and total acidity. The GrapeGen GeneChip® combined with a newly developed GrapeGen 12Xv1 MapMan version were utilized for the functional analysis of berry transcriptomic differences after 2 week treatments from veraison onset. These analyses revealed the establishment of a thermotolerance response in berries under high temperatures marked by the induction of heat shock protein (HSP) chaperones and the repression of transmembrane transporter-encoding transcripts. The thermotolerance response was coincident with up-regulation of ERF subfamily transcription factors and increased ABA levels, suggesting their participation in the maintenance of the acclimation response. Lower expression of amino acid transporter-encoding transcripts at high temperature correlated with balanced amino acid content, suggesting a transcriptional compensation of temperature effects on protein and membrane stability to allow for completion of berry ripening. In contrast, the lower accumulation of anthocyanins and higher malate metabolization measured under high temperature might partly result from imbalance in the expression and function of their specific transmembrane transporters and expression changes in genes involved in their metabolic pathways. These results open up new views to improve our understanding of berry ripening under high temperatures.

Influence of arbuscular mycorrhizae and Rhizobium on free polyamines and proline levels in water-stressed alfalfa

Summary The objective of this research was to study the effect of drought on polyamine and proline levels in alfalfa ( Medicago sativa L. cv. Aragon) plants inoculated with a mycorrhizal fungus and/or Rhizobium compared with non-inoculated ones. The four treatments were: a) plants inoculated with Glomus fasciculatum (Taxter sensu Gerd.) Gerdemann and Trappe and Rhizobium meliloti 102 F51 strain (MR), b) plants inoculated with Rhizobium only (R), c) plants inoculated with Glomus only (M), and d) non-inoculated plants (N). Plants were drought stressed during two cycles of moisture stress and recovery. Although proline concentrations increased and free polyamine (spermidine and spermine) contents decreased in leaves and roots of alfalfa under water stress, symbiotic R, M and MR plants maintained higher free polyamine concentrations than non-symbiotic N ones. Results suggest that symbiotic alfalfa plants are better adapted than non-symbiotic ones to cope with water deficit.

Harvest index, a parameter conditioning responsiveness of wheat plants to elevated CO2

The expansion of the world’s population requires the development of high production agriculture. For this purpose, it is essential to identify target points conditioning crop responsiveness to predicted [CO2]. The aim of this study was to determine the relevance of ear sink strength in leaf protein and metabolomic profiles and its implications in photosynthetic activity and yield of durum wheat plants exposed to elevated [CO2]. For this purpose, a genotype with high harvest index (HI) (Triticum durum var. Sula) and another with low HI (Triticum durum var. Blanqueta) were exposed to elevated [CO2] (700 µmol mol–1 versus 400 µmol mol–1 CO2) in CO2 greenhouses. The obtained data highlighted that elevated [CO2] only increased plant growth in the genotype with the largest HI; Sula. Gas exchange analyses revealed that although exposure to 700 µmol mol–1 depleted Rubisco content, Sula was capable of increasing the light-saturated rate of CO2 assimilation (Asat) whereas, in Blanqueta, the carbohydrate imbalance induced the down-regulation of Asat. The specific depletion of Rubisco in both genotypes under elevated [CO2], together with the enhancement of other proteins in the Calvin cycle, revealed that there was a redistribution of N from Rubisco towards RuBP regeneration. Moreover, the down-regulation of N, NO3 –, amino acid, and organic acid content, together with the depletion of proteins involved in amino acid synthesis that was detected in Blanqueta grown at 700 µmol mol–1 CO2, revealed that inhibition of N assimilation was involved in the carbohydrate imbalance and consequently with the down-regulation of photosynthesis and growth in these plants.

Plant availability of heavy metals in a soil amended with a high dose of sewage sludge under drought conditions

The objective of this research was to study the effect of water deficit on soil heavy metal availability and metal uptake by ryegrass (Lolium multiflorum Lam.) plants grown in a soil amended with a high dose of rural sewage sludge. Three fertility treatments were applied: sewage sludge (SS), mineral fertilizer (M), and control (C); unamended). The levels of irrigation were: well-watered (W) and water deficit (D). Microbial respiration decreased the total organic C (TOC) in sludge-treated soils, but this did not enhance soil DTPA-extractable heavy metal concentrations. Indeed, Zn, Cu, Mn and Ni availability decreased during the experiment. C- and M-treated soils showed either no changes or increases of some trace element concentrations during the incubation. In the plant experiment, ryegrass dry matter (DM) yield, relative water content (RWC) and leaf water potential (Ψw) decreased in drought conditions. Sludge addition increased metal concentrations in plants. However, in some instances, SS-treated plants showed either similar or lower transfer coefficient (Tc) values than did plants in the C and M treatments. Water deficit decreased the concentration and the Tc of some metals in roots of M and SS plants. Results indicate that sludge-borne heavy metals were maintained in chemical forms of low availability. The lower metal uptake by SS and M plants under dry conditions cannot be attributed to a lower availability of these elements in soil.

Effects of temporary drought on nitrate-fed and nitrogen-fixing alfalfa plants

Abstract The effect of temporary drought on growth, carbon exchange and solute accumulation has been examined in alfalfa plants dependent on either N2 or nitrate. Plants were subjected to cyclical moderate or severe drought (drought/recovery). Growth parameters, photosynthetic rate (Pn), leaf conductance to water vapour (gw), chlorophyll content, and solute accumulation were determined. Growth decreased markedly under water deficit, but no significant differences between either groups of plants were found. Nitrogen-fixing plants developed higher root/shoot ratios maintaining larger leaves with increased specific leaf area and greater chlorophyll content than nitrate-fed ones. Leaf conductance and net photosynthetic rate declined simultaneously with the drought treatments in both groups of plants; however, N2 fixing plants retained higher Pn and gw values than nitrate-fed plants at lower RWC. Upon rewatering, a considerable stomatal closure remained in nitrate-fed plants. Drought treatment induced an increase in solute concentrations, mainly potassium, especially important in nitrate-fed plants. The interactions between the type of N nutrition and drought tolerance in alfalfa plants during temporary drought are discussed.

Photosynthetic down-regulation under elevated CO2 exposure can be prevented by nitrogen supply in nodulated alfalfa.

Increasing atmospheric CO₂ concentrations are expected to enhance plant photosynthesis and yield. Nevertheless, after long-term exposure, plants acclimate and show a reduction in photosynthetic activity (called down-regulation), which may cause a reduction in potential yield. Some authors suggest that down-regulation is related to nutrient availability, and more specifically, to an insufficient plant C sink strength caused by limited N supply. In this paper, we tested whether or not N availability prevents down-regulation of photosynthesis in nodulated alfalfa plants (Medicago sativa L.). To do so, we examined the effect of the addition of different levels of NH₄NO₃ (0, 10, and 15 mM) to 30-day-old nodulated alfalfa plants exposed to ambient (approximately 400 μmol mol⁻¹) or elevated CO₂ (700 μmol mol⁻¹) during a period of 1 month in growth chambers. After 2 weeks of exposure to elevated CO₂, no significant differences were observed in plant growth or photosynthesis rates. After 4 weeks of treatment, exclusively N₂ fixing alfalfa plants (0 mM NH₄NO₃) showed significant decreases in photosynthesis and Vc(max). Photosynthetic down-regulation of these plants was caused by the C/N imbalance as reflected by the carbohydrate and N data. On the other hand, plants supplied with 15 mM NH₄NO₃ grown under elevated CO₂ maintained high photosynthetic rates owing to their superior C/N adjustment. The intermediate N treatment, 10 mM NH₄NO₃, also showed photosynthetic down-regulation, but to a lesser degree than with 0 mM treatment. The present study clearly shows that external N supply can reduce or even avoid acclimation of photosynthesis to elevated CO₂ as a consequence of the increase in C sink strength associated with N availability.