Resistance to cold and heat stress: accumulation of phenolic compounds in tomato and watermelon plants

Tomato plants, Lycopersicon esculentum L. cv. Tmknvf(2), and watermelon pla nts, Citrullus lanatus [Thomb.] Mansf. cv. Dulce maravilla, were grown for 30 days at different temperatures (15, 25 and 35 degreesC). We analysed sol uble phenolics, enzymatic activities (phenylalanine ammonia-lyase, polyphen ol oxidase and peroxidase), and dry weight. The impact of the three tempera tures was different in tomato and watermelon. Our results indicate that hea t stress in tomato plants occurred at 35 degreesC, while chilling stress oc curred in watermelon plants at 15 degreesC. Thermal stress in both plants c aused: (1) decreased shoot weight; (2) accumulation of soluble phenolics; ( 3) highest phenylalanine ammonia-lyase activity; and (4) lowest peroxidase and polyphenol oxidase activity. These results indicate that thermal stress induces the accumulation of phenolics in the plant by activating their bio synthesis as well as inhibiting their oxidation. This could be considered a n acclimation mechanism of the plant against thermal stress. (C) 2001 Elsev ier Science Ireland Ltd. All rights reserved.