Photosynthetic responses of greenhouse tomato plants to high solution electrical conductivity and low soil water content

SUMMARY

Greenhouse tomato plants (Lycopersicon esculentum Mill. cv. Capello) were grown in a peal-moss based substrate and supplied with nutrient solutions of high (4.5 mS cm^-1) or low (2.3 mS cm^-1) electrical conductivity (EC) and under high (95 ± 5%) or low (55 ± 8% of capillary capacity) soil water content, to elucidate how EC and soil water status affect plant photosynthesis and related physiological processes. Two weeks after beginning the treatments, photosynthesis (Pn) was measured during changes of photo-synthetic photon flux (PPF) from 0 to 1200 u.mol m^-2 s^-1 using a gas exchange method. The rectangular hyperbolic model (Pn = P_max KI (1-KI)^-2 -r) provided a good fit for the photosynthetic light-response curve. High EC treatment changed the curve by increasing the initial slope (quantum yield) and decreasing photosynthetic capacity at high PPF. However, soil water deficit not only decreased the photosynthetic capacity, but also decreased quantum use efficiency. Depression of Pn was attributed to decreased stomatal (gs) and mesophyll (_gm) conductances, but g_s was depressed more than g_m. The ratio of _gm/(g_m + g_s), an indicator of water use efficiency and a measure of relative control of Pn by carboxylation and C02 supply, was higher for high-EC treated plants. Chlorophyll content was increased by high EC treatment, and was consistent with quantum yield. Leaf water 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 additive on photosynthesis and most related physiological processes.