Proline as a measure of stress in tomato plants

Tomato (Lycopersicon esculentum Mill. cv. Counter) plants were grown in 12 L polyethylene containers in aerated nutrient solutions containing equal nutrient ratios at increasing concentrations (X1 = standard concentration, and X3, X5.5, X8 or X11 = 3, 5.5, 8 or 11 times the standard concentration). Fruit yield decreased significantly from moderate (X3) to high (X11) nutrient concentration during summer, but remained nearly unaffected by the strength of the nutrient solution under low-radiation conditions in autumn. This stress-induced difference in yield was reflected by higher proline concentrations in leaves of plants grown during the summer compared to those grown during the late season. The decrease in fruit fresh and dry weight observed in summer was due to reduced availability of water and the distribution of dry matter towards the vegetative plant parts at the expense of reproductive growth. The proline content of tomato leaves fluctuated according to nutrient concentration and total radiation, and was closely related to the relative water content of leaves. It was concluded that proline is a reliable indicator of the environmental stress imposed on plants, thus allowing us to establish stress thresholds for fruit yield and product quality of hydroponically grown tomato.