INCREASED ETHYLENE SYNTHESIS ENHANCES CHILLING TOLERANCE IN TOMATO

Exposure to chilling temperatures (0-10 degrees C) increases ethylene synthesis in several species, but it is not clear whether this increas e in ethylene synthesis is involved in the development of chilling tol erance. To determine this relationship, chilling tolerance development of the ethylene-insensitive Never-ripe (Nr) tomato (Lycopersicon escu lentum Mill. cv. Pearson) mutant was compared to that of a normal isog enic line. Plants were grown for 3 weeks at 25/20 degrees C day/night, and then half the plants of each genotype were chill-hardened at 25/5 degrees C day/night for 10 days, while the other half remained at 25/ 20 degrees C. All plants were then exposed to a severe chill (5/5 degr ees C day/night) for 7 days, and a recovery period (25/20 degrees C da y/night) of 4 days. Chill hardening increased ethylene synthesis in bo th genotypes, but increased the rate of leaf development and dry weigh t accumulation during the recovery period only for the normal plants. Hardening also caused a greater decrease in lesion development in the normal plants following the severe chill. Therefore, chill hardening w as more effective in the ethylene-sensitive normal plants than in the ethylene-insensitive mutants, indicating that a response to ethylene i s involved in chilling tolerance development. However, chill hardening was effective for both genotypes in maintaining chlorophyll a fluores cence levels (F-v/F-m), suggesting that ethylene does not control all aspects of chilling tolerance. In a separate experiment, application o f a 500 ppm ethephon solution to normal 'Pearson' plants led to higher rates of ethylene synthesis and a higher rate of leaf development fol lowing a 6 day severe chill (5/5 degrees C day/night). These results f urther indicate that ethylene is involved in the development of chilli ng tolerance.