María Jesús Sánchez-Blanco

Influence of Brassinosteroids on Antioxidant Enzymes Activity in Tomato Under Different Temperatures

The effect of brassinosteroids (BRs) on catalase (EC 1.11.1.6), peroxidase (EC 1.11.1.7) and superoxide dismutase (SOD, EC 1.15.1.1) activity in tomato leaf discs was analyzed at 25 and 40 °C. Tomato leaf discs were preincubated for 24 h in Petri dishes with 24-epibrassinolide (EBR) or a polyhydroxylated spirostanic analogue of brassinosteroids (MH5). Both concentrations (10.60 and 2.12 nM) of EBR and MH5 stimulated the activity of SOD at 25 and 40 °C, the MH5-stimulated increase of this enzyme activity was greater. Peroxidase activity was unaffected at 25 °C, while at 40 °C this activity was enhanced by both compounds. The changes in catalase activity markedly depended on the structure BRs, doses and temperature. The results suggest a possible role of EBR and MH5 in the reduction of cell damage produced by heat stress due to induction of enzymatic antioxidants.

Water relations, growth and yield of Fino lemon trees under regulated deficit irrigation

Fino lemon trees (Citrus limon L. Burm. fil.) on sour orange (Citrus aurantium L.), growing on a low water retention capacity soil, were submitted to three different irrigation treatments over four years: 100% ETc all year (T-0), 25% ETc all year except during the rapid fruit growth period when 100% ETc was applied (T-1) and 100% ETc all year, except during the rapid fruit growth period when 70% ETc was applied (T-2). A water saving of 30 and 20% was achieved in the T-1 and T-2 treatments, respectively. The plant responses to irrigation treatments were similar in all the years studied. Leaf water potential decreased during deficit irrigation periods in T-1 and T-2 treatments. Larger differences were found in values taken at predawn (ψpd) than at midday (ψmd), indicating thatψpd is a more useful indicator of plant water status. There was neither osmotic nor elastic adjustment in response to deficit irrigation treatment. A clear separation between the main periods of shoot and fruit growth was found, which can be considered an advantageous characteristic in applying regulated deficit irrigation strategies. Onset of the critical period of rapid fruit growth could be determined precisely by considering the decrease in relative fruit growth rate values. T-2 treatment did not induce a significant reduction in total yield, but it caused a delay in reaching marketable lemon fruit size. T-1 treatment did not affect total yield, with a reduction in yield on the first pick occurring in only one year. Chemical characteristics of lemon fruit were not significantly modified by irrigation treatment.

High-temperature preconditioning and thermal shock imposition affects water relations, gas exchange and root hydraulic conductivity in tomato

Potted tomato plants (Lycopersicon esculentum Mill. cv. Amalia) were submitted to three different treatments: control (C) plants were maintained at day/night temperature of 25/18 °C; preconditioned plants (PS) were submitted to two consecutive periods of 4 d each, of 30/23 and 35/28 °C before being exposed to a heat stress (40/33 °C lasting 4 d) and non-preconditioned (S) plants were maintained in the same conditions as the C plants and exposed to the heat stress. The inhibition of plant growth was observed only in PS plants. Heat stress decreased chlorophyll content, net photosynthetic rate and stomatal conductance in both PS and S plants. However, PS plants showed good osmotic adjustment, which enabled them to maintain leaf pressure potential higher than in S plants. Furthermore, at the end of the recovery period PS plants had higher pressure potential and stomatal conductance than in S plants.

Comparative growth and water relations of Cistus albidus and Cistus monspeliensis plants during water deficit conditions and recovery

Four month old Cistus albidus and Cistus monspeliensis plants growing in a growth chamber were submitted to two consecutive cycles, during which irrigation water was withheld for 7 days before being reconnected and the plants rewatered to run off (WD treatment). Control treatment plants were irrigated daily. At the end of the second dry period the treated plants were submitted to control plant conditions for 15 days. C. monspeliensis plants withstood the water deficit better than C. albidus, mainly due to its capacity to recover a water deficit situation. C. monspeliensis plants show net assimilation rate (NAR) levels above those of the control plants during the recovery period. In C. albidus the main growth limiting factor was the cellular expansion, whereas in C. monspeliensis photosynthesis was the limiting factor. Both species developed complementary avoidance mechanisms based on stomatal closure, a reduction in leaf area and root hydraulic conductivity, and epinasty. C. monspeliensis plants showed good osmotic adjustment in response to water deficit, which enabled it to avoid decreased leaf turgor potential and partial dehydration. After the water deficit, the mobilisation of accumulated organic solutes may also have contributed to the increased relative growth rate and NAR during the recovery period.

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.

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.

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.

Growth and Water Relations in Mycorrhizal and Nonmycorrhizal Pinus Halepensis Plants in Response to Drought

Mycorrhizal and nonmycorrhizal Pinus halepensis plants were subjected to water stress by withholding irrigation for four months and then rehydrated for 30 d. Water stress affected plants growth and mycorrhizal association was unable to avoid the effects of drought on plant growth. However, when irrigation was re-established the increase in height, number of shoots, total dry mass, and chlorophyll content in the mycorrhizal plants were greater than in non-mycorrhizal plants. The decrease in soil water content decreased the leaf water potential, leaf pressure potential and stomatal conductance. These decreases were higher for nonmycorrhizal than for mycorrhizal plants, indicating that the mycorrhizal fungi permit a higher water uptake from the dry soils. The total content of inorganic solutes was not changed by presence of mycorrhizae.