J.J. Alarcón

Variations in water status, gas exchange, and growth in Rosmarinus officinalis plants infected with Glomus deserticola under drought conditions

The influence of the arbuscular mycorrhizal fungus Glomus deserticola on the water relations, gas exchange parameters, and vegetative growth of Rosmarinus officinalis plants under water stress was studied. Plants were grown with and without the mycorrhizal fungus under glasshouse conditions and subjected to water stress by withholding irrigation water for 14 days. Along the experimental period, a significant effect of the fungus on the plant growth was observed, and under water stress, mycorrhizal plants showed an increase in aerial and root biomass compared to non-mycorrhizal plants. The decrease in the soil water potential generated a decrease in leaf water potential (psi(l)) and stem water potential (psi(x)) of mycorrhizal and non-mycorrhizal plants, with this decrease being lower in mycorrhizal water-stressed plants. Mycorrhization also had positive effects on the root hydraulic conductivity (Lp) of water stressed plants. Furthermore, mycorrhizal-stressed plants showed a more important decrease in osmotic potential at full turgor (psi(os)) than did non-mycorrhizal-stressed plants, indicating the capacity of osmotic adjustment. Mycorrhizal infection also improved photosynthetic activity (Pn) and stomatal conductance (g(s)) in plants under water stress compared to the non-mycorrhizal-stressed plants. A similar behaviour was observed in the photochemical efficiency of PSII (Fv/Fm) with this parameter being lower in non-mycorrhizal plants than in mycorrhizal plants under water stress conditions. In the same way, under water restriction, mycorrhizal plants showed higher values of chlorophyll content than did non-mycorrhizal plants. Thus, the results obtained indicated that the mycorrhizal symbiosis had a beneficial effect on the water status and growth of Rosmarinus officinalis plants under water-stress conditions.

Effect of water and salt stresses on the growth, gas exchange and water relations in Argyranthemum coronopifolium plants

Plants of Argyranthemum coronopifolium were submitted to water stress (preconditioned by watering every 3 days, two dry–wet cycles were imposed) and salt stress (15 days of exposure to 140 mm NaCl followed by a recovery period of 11 days), independently. Effects of water and salt stresses on gas exchange, water relations and growth parameters were investigated in order to know the resistance of A. coronopifoliumto these kinds of stress. Water and salt stress promoted reductions in leaf biomass due to both senescence and death of leaves, what has been considered an avoidance mechanism that allows minimising water losses. The degree of osmotic adjustment reached by the plants was very similar in both stresses studied, however; the maintenance of turgor did not maintain growth. Probably, the osmotic adjustment reached via salinity treatment induced some damage to the photosynthetic apparatus that was not induced by the water stress. In this sense, reductions in the photosynthetic rate and chlorophyll content were observed even when the salts were removed. It suggested that there was a toxic effect of salt concentration that could also explain the greater effect on the growth of the salinized plants.

Growth and osmotic adjustment of two tomato cultivars during and after saline stress

The effect of a short period of saline stress was studied in two phenotypically different cultivars, one of normal fruit-size (L. esculentum cv. New Yorker) and one of cherry fruit-size (L. esculentum var.cerasiforme cv. PE-62). In both cultivars the relative growth rate (RGR) and the leaf area ratio (LAR) decreased following salinisation. The leaf turgor potential (ψp) and the osmotic potential at full turgor (ψos) decreased to the same extent in both cultivars. However, the contributions of organic and inorganic solutes to the osmotic adjustment was different between cultivars. New Yorker achieved the osmotic adjustment by means of the Cl− and Na+ uptake from the substrate, and by synthesis of organic solutes. In the cherry cultivar organic solutes did not contribute to the osmotic adjustment, instead, their contribution decreased after salinisation. After the salt stress was removed, the water stress disappeared, the content of organic solutes decreased in plants of both cultivars and, therefore, their growth was not retarded by the diversion of resources for the synthesis of organic solutes. However, the toxic effects of the Cl− and Na+ did not disappear after removal of the salt stress, and the net assimilation rate (NAR) and the rate of growth (RGR) did not recover.

Changes in photosynthetic parameters and antioxidant activities following heat-shock treatment in tomato plants

Seedlings of two tomato genotypes, Lycopersicon esculentum Mill. var. Amalia and the wild thermotolerant type Nagcarlang, were grown under a photoperiod of 16 h light at 25°C and 8 h dark at 20°C. At the fourth true leaf stage, a group of plants were exposed to a heat-shock temperature of 45°C for 3 h, and measurements of chlorophyll fluorescence, gas-exchange characteristics, dark respiration and oxidative and antioxidative parameters were made after releasing the stress. The heat shock induced severe alterations in the photosynthesis of Amalia that seem to mitigate the damaging impact of high temperatures by lowering the leaf temperature and maintaining stomatal conductance and more efficient maintenance of antioxidant capacity, including ascorbate and glutathione levels. These effects were not evident in Nagcarlang. In Amalia plants, a larger increase in dark respiration also occurred in response to heat shock and the rates of the oxidative processes were higher than in Nagcarlang. This suggests that heat injury in Amalia may involve chlorophyll photooxidation mediated by activated oxygen species (AOS) and more severe alterations in the photosynthetic apparatus. All these changes could be related to the more dramatic effect of heat shock seen in Amalia than in Nagcarlang plants.

Strategies for drought resistance in leaves of two almond cultivars

Abstract Potted plants of two cultivars of almond (Amygdalus communis L.), Ramillete and Garrigues, growing under field conditions were subjected to drought by withholding irrigation for 28 days. Stressed plants were reirrigated and the recovery was studied for 8 days. Control plants were drip irrigated daily maintaining the soil matric potential at about −30 kPa. Predawn leaf water potential declined progressively due to water stress effect, reaching −0.80 MPa in Garrigues and −0.98 MPa in Ramillete at the end of the stress period. The reduction in midday leaf conductance in plants under water stress could be related to the reduction in midday leaf water potential. The reductions in leaf conductance values were about 62% and 79% for Garrigues and Ramillete, respectively. Garrigues exhibited osmotic adjustment during the greater part of the stress period and this was sufficient to maintain the leaf turgor. The relationship between leaf conductance and turgor pressure at midday appeared to be nearly linear over the range of turgor pressure observed. Ramillete presented lower leaf water potential at turgor loss point than did Garrigues. Both cultivars presented high relative apoplastic water contents. The increase in these parameter values by water stress effect observed in Ramillete could indicate changes in cell wall structure. After rewatering, most of the parameters studied recovered quickly, although leaf conductance was delayed.

Effect of Regulated Deficit Irrigation and Crop Load on the Antioxidant Compounds of Peaches

The use of regulated deficit irrigation (RDI) strategies is becoming a common practice in areas with low water availability. Little information is available about the effects of RDI on the antioxidant content of fruits. In this study, the influence of RDI on the content of vitamin C, phenolic compounds and carotenoids was investigated. Two irrigation strategies, fully irrigated (FI) and RDI, were compared at two levels of thinning, commercial and half of the commercial crop load. RDI strategies affected the content of vitamin C, phenolics and carotenoids of Flordastar peaches. RDI caused fruit peel stress lowering the content of vitamin C and carotenoids, while increasing the phenolic content, mainly anthocyanins and procyanidins. Fruit weight was the only quality index influenced by the crop load as it increased in FI fruits at low crop load. In general, fruits from commercial crop load had slightly higher content of antioxidants to fruits from low crop load, although these influences were only observed in the peel. Additionally, the influence of irrigation controlled by two sensors related to plant water level, maximum daily trunk shrinkage (MDS) and sap flow (SF) on the antioxidant constituents of peaches was evaluated. The response of the fruits to SF sensor was similar to that observed for RDI strategy. According to the tested water sensors, SF did not act as a good plant-based water indicator for use in irrigation scheduling, as it caused an increase in the content of phenolics, similar to that observed for fruits subjected to RDI. Therefore, selection of RDI strategies and plant water indicators should be taken into account as they affect the content of antioxidants of peaches.

Sap flow as an indicator of transpiration and the water status of young apricot trees

The relationship between water loss via transpiration and stem sap flow in young apricot trees was studied under different environmental conditions and different levels of soil water status. The experiment was carried out in a greenhouse over a 2-week period (November 2–14, 1997) using three-year-old apricot trees (Prunus armeniaca cv. Búlida) growing in pots. Diurnal courses of leaf water potential, leaf conductance and leaf turgor potential also were recorded throughout the experiment. Data from four days of different enviromental conditions and soil water availability have been selected for analysis. On each of the selected days the leaf water potential and the mean transpiration rates were well correlated. The slope of the linear regression of this correlation, taken to indicate the total hydraulic resistance of the tree, confirmed an increasing hydraulic resistance under drought conditions. When the trees were not drought stressed the diurnal courses of sap flow and transpiration were very similar. However, when the trees were droughted, measured of sap flow slightly underestimated actual transpiration. Our heat-pulse measurements suggest the amount of readily available water stored in the stem and leaf tissues of young apricot trees is sufficient to sustain the peak transpiration rates for about 1 hour.

Diurnal and seasonal osmotic potential changes in Lotus creticus creticus plants grown under saline stress

Lotus creticus creticus plants growing in a greenhouse were exposed to 0. 70 and 140 mM NaCl for 4 months (September-December). Salinity caused a reduction in total dry weight of Lotus plants treated with 140 mM NaCl, whereas no significant effects on growth were observed with 70 mM NaCl. Predawn leaf water potential and predawn leaf osmotic potential showed constant values in control plants during all the experiment, whereas a decrease of both parameters was observed between September and October for the saline treatments. The relative contribution of passive (dehydration) versus active mechanisms (osmotic adjustment) involved in seasonal leaf osmotic potential changes were determined. Seasonal decreases of the osmotic potential at full turgor in the treated plants showed the capacity for osmotic adjustment by accumulation of Na + and Cl -- , because the accumulation of organic solutes due to salts was not consistent. In plants treated with 70 mM NaCl. the seasonal changes of the osmotic potential were produced by net solute accumulation, because the dehydration contribution was negligible. In plants treated with 140 mM NaCl, the seasonal changes of the osmotic potential were originated by ions accumulation, but also by tissues dehydration. At the end of the salinization period (December), a possible diurnal adaptation in water relations was also considered. The dehydration was the major mechanism involved in diurnal changes of leaf osmotic potential, and only at the highest salinity level some diurnal osmotic adjustment could be observed. In conclusion. the osmotic adjustment in Lotus might be a beneficial trait when the plants are treated with moderate levels of salinity (70 mM NaCl). At higher salinity (140 mM NaCl), the high absorption and accumulation of ions caused important toxic effects and induced leaf tissue dehydration. 0 1998 Published by Elsevier Science Ireland Ltd. All rights reserved.

Composition of xylem and phloem exudates in relation to the salt-tolerance of domestic and wild tomato species.

Summary Plants of Lycopersicon esculentum cv. Volgogradskij ( Le ) and L. pennellii accession PE-47 ( Lp ) were cultivated for 22 days under control and saline (100 mmol/L NaCl) conditions. Besides growth parameters and leaf water content, toxic ions (Na + , Cl − ), major nutrients (K + , Ca 2+ , Mg 2+ and sPO 4 3− ) and assimilates (carbohydrates and amino acids) were determined weekly in xylem and phloem exudates. Salinity reduced plant dry weight by 55 % and 29 % in Le and Lp , respectively, while a 46 % increase in the root biomass was found in the latter. An earlier and higher Na + and Cl − accumulation in the xylem and a greater Na + retranslocation through phloem was found in Lp . The better nutritional status found in the xylem of the salinized plants of the wild species at the end of the salinization period seems also to be linked to phloem-xylem retranslocation. Thereafter, a more rapid and higher transport of carbohydrates and amino acids from roots to shoot through xylem was found in Lp than in Le under salinity. The implication of these parameters in the salt-tolerance of tomato has been discussed.

Comparison of continuously recorded plant-based water stress indicators for young lemon trees

Continuously recorded plant-based water stress indicators (sap flow and parameters derived from trunk diameter fluctuations) were compared in potted young lemon trees (Citrus limon (L.) Burm. fil, cv. Verna) grafted on sour orange (C. aurantium L.) rootstock submitted to deficit irrigation. Daily maximum (MXTD) and minimum (MNTD) trunk diameters and daily trunk diameter shrinkage (MDS) were directly influenced by the water supply to the trees from the soil. The continuously recorded plant-based water stress indicators presented different degrees of sensitivity when used to estimate the water status of the plants. Sap flow (SF) and MDS were more immediate and sensitive than MXTD and MNTD. However, the higher signal intensity: noise for SF and the fact that its signal intensities remained clearly above unity during the stress period, indicating that the soil water was depleted, point to the greater reliability of this indicator. Also, the possibility of developing further baseline relationships between SF and air vapour pressure deficit in fully irrigated trees in field conditions increases the feasibility of using this parameter in automatic irrigation systems.