PHOTOSYNTHETIC RESPONSES OF GREENHOUSE TOMATO PLANTS TO HIGH SOLUTIONELECTRICAL-CONDUCTIVITY AND LOW SOIL-WATER CONTENT

Greenhouse tomato plants (Lycopersicon esculentum Mill. cv. Capello) w ere grown in a peat-moss based substrate and supplied with nutrient so lutions of high (4.5 mS cm-1) or low (2.3 mS cm-1) electrical conducti vity (EC) and under high (95 +/- 5%) or low (55 +/- 8% of capillary ca pacity) soil water content, to elucidate how EC and soil water status affect plant photosynthesis and related physiological processes. Two w eeks after beginning the treatments, photosynthesis (Pn) was measured during changes of photosynthetic photon flux (PPF) from 0 to 1200 mumo l m-2 s-1 using a gas exchange method. The rectangular hyperbolic mode l (Pn = P(max) KI (1-KI)-1 -r) provided a good fit for the photosynthe tic light-response curve. High EC treatment changed the curve by incre asing the initial slope (quantum yield) and decreasing photosynthetic capacity at high PPF. However, soil water deficit not only decreased t he photosynthetic capacity, but also decreased quantum use efficiency. Depression of Pn was attributed to decreased stomatal (g(s)) and meso phyll (g(m)) conductances, but g, was depressed more than g(m). The ra tio of g(m)/(g(m) + g(s)), an indicator of water use efficiency and a measure of relative control of Pn by carboxylation and CO2 supply, was higher for high-EC treated plants. Chlorophyll content was increased by high EC treatment, and was consistent with quantum yield. Leaf wate r potential was decreased by high EC and/or low soil water content and the decreases in leaf water potential ultimately accounted for the Pn depressions. The effects of high EC and soil water deficit were addit ive on photosynthesis and most related physiological processes.