An understanding of the mechanisms that determine plant response to reduced water availability is essential to improve water-use efficiency (WUE) of stone fruit crops. The physiological, biochemical and molecular drought responses of four Prunus rootstocks (GF 677, Cadaman, ROOTPAC 20 and ROOTPAC(®) R) budded with 'Catherina' peach cultivar were studied. Trees were grown in 15-l containers and subjected to a progressive water stress for 26 days, monitoring soil moisture content by time domain reflectometry. Photosynthetic and gas exchange parameters were determined. Root and leaf soluble sugars and proline content were also measured. At the end of the experiment, stressed plants showed lower net photosynthesis rate, stomatal conductance and transpiration rate, and higher intrinsic leaf WUE (AN/gs). Soluble sugars and proline concentration changes were observed, in both root and leaf tissues, especially in an advanced state of stress. The accumulation of proline in roots and leaves with drought stress was related to the decrease in osmotic potential and increase in WUE, whereas the accumulation of sorbitol in leaves, raffinose in roots and proline in both tissues was related only to the increase in the WUE. Owing to the putative role of raffinose and proline as antioxidants and their low concentration, they could be ameliorating deleterious effects of drought-induced oxidative stress by protecting membranes and enzymes rather than acting as active osmolytes. Higher expression of P5SC gene in roots was also consistent with proline accumulation in the tolerant genotype GF 677. These results indicate that accumulation of sorbitol, raffinose and proline in different tissues and/or the increase in P5SC expression could be used as markers of drought tolerance in peach cultivars grafted on Prunus rootstocks.

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A study was conducted over 2 years to determine whether growth under elevated CO(2) (700 μmol mol(-1) ) and temperature (ambient + 4 °C) conditions modifies photochemical efficiency or only the use of electron transport products in spring wheat grown in field chambers. Elevated atmospheric CO(2) concentrations increased crop dry matter at maturity by 12-17%, while above-ambient temperatures did not significantly affect dry matter yield. In measurements with ambient CO(2) at ear emergence and after anthesis, growth at elevated CO(2) concentrations decreased flag leaf light-saturated carbon assimilation. The quantum yield of electron transport (Φ(PSII) ) measured at ambient CO(2) and higher irradiances increased at ear emergence and decreased after anthesis in plants grown at elevated CO(2) . At higher light intensities, but not in low light, photochemical quenching (qP) decreased after growth in elevated CO(2) conditions. Growth under CO(2) enrichment increased dark- (Fv:Fm) and light-adapted (Fv':Fm') photochemical efficiencies, and decreased the chlorophyll a:b ratio, suggesting an increase in light-harvesting complexes relative to PSII reaction centres. A relatively higher decrease in carbon assimilation than the decrease in Φ(PSII) pointed to a sink other than CO(2) assimilation for electron transport products at defined growth stages. With higher light intensities, warmer temperatures increased Φ(PSII) and Fv':Fm' at ear emergence and decreased Φ(PSII) after anthesis; in ambient-but not elevated-CO(2) , warmer temperatures also decreased qP after anthesis. CO(2) fixation increased or did not change with temperature, depending on the growth stage and year. We conclude that elevated CO(2) decreases the carbon assimilation capacity, but increases photochemistry and resource allocation to light harvesting, and that elevated levels of CO(2) can mitigate photochemistry inhibition as a result of warm temperatures.

Acclimation of photosynthetic capacity to elevated CO₂ involves a decrease of the leaf Rubisco content. In the present study, it was hypothesized that nitrogen uptake and partitioning within the leaf and among different aboveground organs affects the down-regulation of Rubisco. Given the interdependence of nitrogen and cytokinin signals at the whole plant level, it is also proposed that cytokinins affect the nitrogen economy of plants under elevated CO₂, and therefore the acclimatory responses. Spring wheat received varying levels of nitrogen and cytokinin in field chambers with ambient (370 μmol mol⁻¹) or elevated (700 μmol mol⁻¹) atmospheric CO₂. Gas exchange, Rubisco, soluble protein and nitrogen contents were determined in the top three leaves in the canopy, together with total nitrogen contents per shoot. Growth in elevated CO₂ induced decreases in photosynthetic capacity only when nitrogen supply was low. However, the leaf contents of Rubisco, soluble protein and total nitrogen on an area basis declined in elevated CO₂ regardless of nitrogen supply. Total nitrogen in the shoot was no lower in elevated than ambient CO₂, but the fraction of this nitrogen located in flag and penultimate leaves was lower in elevated CO₂. Decreased Rubisco: chlorophyll ratios accompanied losses of leaf Rubisco with CO₂ enrichment. Cytokinin applications increased nitrogen content in all leaves and nitrogen allocation to senescing leaves, but decreased Rubisco contents in flag leaves at anthesis and in all leaves 20 days later, together with the amount of Rubisco relative to soluble protein in all leaves at both growth stages. The results suggest that down regulation of Rubisco in leaves at elevated CO₂ is linked with decreased allocation of nitrogen to the younger leaves and that cytokinins cause a fractional decrease of Rubisco and therefore do not alleviate acclimation to elevated CO₂.