Direct action of the biocide carbendazim on phenolic metabolism in tobacco plants.

In view of the essential role of phenolic compounds in the development of pathogen resistance in plants, and given the influence that fungicides exert over phenolic metabolism, the aim of the present study was to determine the effect of the application of different rates of fungicide on the metabolism of phenolic compounds in tobacco plants (Nicotiana tabacum L. cv. Tennessee 86). The fungicide applied was carbendazim, with a purity of 100%, at three different rates: 1.3 mM (carb(1)), 2.6 mM (this being the recommended concentration, carb(2)), and 5.2 mM (carb(3)). The control treatment was without carbendazim. The results in relation to control plants indicate that the application of carb(1) in tobacco plants not afflicted by damaging biotic and abiotic agents boosts phenolic accumulation. Therefore, in the case of carbendazim, the application of 50% less (carb(1), 1.3 mM) than the recommended dosage (carb(2), 2.6 mM) of this fungicide could be more effective, because the foliar accumulation of phenolics presented at carb(1) may imply an increased resistance of plants to pathogen infection. On the other hand, we found an inhibition of the phenolic oxidation by the application of carbendazim, principally at carb(3). These results suggest that the excessive application of carbendazim (5.2 mM) could be harmful for healthy plants, because, on inhibiting phenolic metabolism (biosynthesis and oxidation), such treatment would also sharply reduce the capacity of these plants to respond against pathogen attack.

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 plants, Citrullus lanatus [Thomb.] Mansf. cv. Dulce maravilla, were grown for 30 days at different temperatures (15, 25 and 35 degrees C). We analysed soluble phenolics, enzymatic activities (phenylalanine ammonia-lyase, polyphenol oxidase and peroxidase), and dry weight. The impact of the three temperatures was different in tomato and watermelon. Our results indicate that heat stress in tomato plants occurred at 35 degrees C, while chilling stress occurred in watermelon plants at 15 degrees C. Thermal stress in both plants caused: (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 biosynthesis as well as inhibiting their oxidation. This could be considered an acclimation mechanism of the plant against thermal stress.