Abstract
Horticultural crops in Mediterranean environments are often exposed to drought stress which impairs gas exchange and growth. Stress conditions may stimulate the antioxidant metabolism protecting the photosynthetic apparatus exposed to stress. We measured reduction in crop yield, and changes in photosynthetic parameters and antioxidant contents in a rainfed chard crop (Beta vulgaris L., cv. Adriatica) as compared to well watered (100% ETc) and mild stressed (50% ETc) plants. The severity of water stress was estimated by monitoring the leaf water potentials with a pressure chamber. To assess whether water stress reduces photosynthesis, gas exchange was measured with an infra-red gas analysis system. In order to evaluate the effect of water stress on CO 2 internal concentrations, mesophyll conductance to CO2 was estimated by simultaneous measurements of chlorophyll fluorescence and leaf gas exchange under field conditions, with a portable system. Leaf-disk samples were collected to measure ascorbate and tocopherol contents. Aboveground dry mass accumulation was measured on 10 plants per treatment at each sampling date. Linear radiometers, connected to a data logger, measured the amount of daily incident radiation (PAR) intercepted by plants at half an hour intervals. A few days after the beginning of water stress, significant reductions in stomatal and mesophyll conductance, and photosynthetic rate were observed. Other physiological and morphological plant traits appeared to be affected later. Antioxidant contents were inversely related to photosynthetic rates, indicating they may be involved in plant defense mechanisms against water stress conditions