Contrasting Effects of GA3 Treatments on Tomato Plants Exposed to Increasing Salinity

The role of plant hormones under saline stress is critical in modulating physiological responses that will eventually lead to adaptation to an unfavorable environment. Nevertheless, the functional level of plant hormones, and their relative tissue concentration, may have a different impact on plant growth and stress tolerance at increasing salinity of the root environment. Vigorous plant growth may counteract the negative effects of salinization. In contrast, low gibberellin (GA) levels have been associated with reduced growth in response to salinity. Based on these facts and considering that the physiological basis of the cause-effect relationship between functional growth control and stress adaptation/survival is still a matter of debate, we hypothesized that exogenous applications of the plant hormone GA(3) may compensate for the salt-induced growth deficiency and consequently facilitate tomato plant adaptation to a saline environment. GA(3) application (0 or 100 mg GA(3) l(-1)) was compared under four salinity levels, obtained by adding equal increments of NaCl:CaCl2 (2:1 molar basis) (EC = 2.5, 6.8, 11.7, 16.7 dS m(-1)) to the nutrient solution. GA(3) treatment reduced stomatal resistance and enhanced plant water use at low salinity. These responses were associated with an increased number of fruit per plant at harvest. However, moderate and high salinity nullified these differences. The fruit carotenoid level was generally lower in GA(3)-treated plants, indicating either an inhibitory effect of GA(3) treatment on carotenoid biosynthesis or a reduced perception of the stress environment by GA(3)-treated tomato plants.