The role of paclobutrazol on accumulation of antioxidant in tomato plants (lycopersicum esculentom L.) under cold stress.

Low temperature stress (LTS), change many biochemical and physiological processes in plants. Cold stress cause alteration in membrane structure and metabolic modifications, changes in protein content and reduction of photosynthesis rates by inhibiting the light and dark reactions of photosynthesis. Moreover, chilling sensitive plants is unable to control photosynthesis reactions at low temperature; excitation energy may accumulate leading to a formation of destructive activated oxygen species (AOS). Triazoles, the most potent group of growth retardants, have pleotropic effects. It was found that triazoles compounds are able to protect plants from the environmental stress condition, e.g. low temperature stress. The aim of the present study was to determine whether the plant growth regulators, such as paclobutrazol are able to improved tolerance to oxidative stress and increase tolerance to cold stress in tomato seedlings. Two different concentration (30, 60 mgl-1) of PBZ were sprayed on 5-week-old plants. After 3 days, plants were exposed to LTS by placing them in cold incubator at 1±0/2 °C for 6 h daily and returned to their original place. Treatment was applied daily for 5 days. After treatment of plants, some biochemical and chemical parameters such as leaf ascorbate and dehydroascorbate content, phenolic compounds, polyphenol oxidase, and peroxidase and ascorbate peroxidase activity were measured. Data of all parameters were analyzed with SPSS software version 9.0. The results showed that LTS increased ascorbate content of leaf, instead LTS decreased dehydroascorbate content. Although paclobutrazol caused accumulation dehydroascorbate in the leaves of tomato in cold stress condition but ascorbate in leaves decreased. Low temperature stress cause decrease in polyphenol oxidise and ascorbate peroxidase activity. In addition, our results showed that PBZ treated plants when exposed to the cold stress condition enhance ascorbate peroxidase activity. Results obtained from this study showed improvement in cold induced injuries in seedlings which were pretreated with PBZ. We suggest that PBZ treatment promotes antioxidant compounds and increase resistance to oxidative stress in tomato plants.