Gas exchanges and leaf water potential (Ψw) of six-years-old trees of fourteen Prunus amygdalus cultivars, grafted on GF-677, were studied in May, when fruits were in active growing period, and in October, after harvesting. The trees were grown in the field under rain fed conditions. Predawn Ψw showed lower water availability in October compared with May. The lowest Ψw values at midday in May increased gradually afterwards, while in October they decreased progressively until night, suggesting a higher difficulty to compensate the water lost by transpiration. However, relative water content (RWC) measured in the morning was similar in both periods, most likely due to some rainfall that occurred in September and first days of October that could be enough to re-hydrate canopy without significantly increasing soil water availability. The highest net photosynthetic rate (PN) was found in both periods early in the morning (08:00-11:00). Reductions in PN from May to October occurred in most cultivars except in José Dias and Ferrastar. In all cultivars a decrease in stomatal conductance (gs) was observed. Photosynthetic capacity (Pmax) did not significantly change from spring to autumn in nine cultivars, revealing a high resistance of photosynthetic machinery of this species to environmental stresses, namely high temperature and drought. Osmotic adjustment was observed in some cultivars, which showed reductions of ca. 23 % (Duro d' Estrada, José Dias) and 15 % (Tuono) in leaf osmotic potential (Ψπ). Such decreases were accompanied by soluble sugars accumulation. The Portuguese cultivar José Dias had a higher photosynthetic performance than the remaining genotypes. and M. C. Matos ... [et al.].
Pachyrhizus ahipa (Wedd.) Parodi, originally from Latin America, is an agronomy interesting legume crop due to high seed protein content and saccharides-rich tuber root. Its capacity of adaptation to Mediterranean climate, where heat and water stress are frequently associated, is being tested. Two accessions of P. ahipa (AC 102 and AC 524) differing in field production were compared as concerns the effects of water stress and high temperature on photosynthetic performance. Membrane integrity was also evaluated through electrolyte leakage (injury index, I%), lipid composition, and ultrastructure observations. Short-term heat stress (40 °C) did not affect net photosynthetic rate (PN), stomatal conductance (gs), and most of fluorescence parameters in both accessions, what was consistent with low electrolyte leakage. However, photosynthetic capacity (Pmax) showed a significant reduction, AC 524 being more affected than AC 102. Relative water content (RWC) below 70 % caused a drastic decrease in PN and gs. Fluorescence parameters, Pmax, and I% were affected in the two accessions, which also presented a strong reduction (42 %) in total fatty acids (TFA). Contents of galactolipids were drastically reduced, and changes in their saturation also occurred, namely a decrease in linolenic acid (C18:3) percentage of monogalactosyl-diacylglycerol (MGDG) in both accessions. Thylakoid ultrastructure in AC 524 submitted to drought showed disorganisation of grana stacking. Mitochondria presented signs of injured cristae. When water-stressed plants were subjected to high temperature, photosynthesis and fluorescence parameters did not show significant additional changes in both accessions. The exposure of drought stressed plants to 40 °C further increased electrolyte leakage in AC 524, but not in AC 102. Chloroplasts, mitochondria, and plasmalemma showed an increased disorganisation. Vesicles appeared in the cytoplasm, which became electron-transparent, reflecting a strong reduction in the number of ribosomes. Hence AC 102 was less affected than AC 524 as regards some components of photosynthetic process, namely Pmax and membrane integrity. This could account for its better yield production previously observed in field grown plants. and M. C. Matos ... [et al.].