Effects of heat stress and hypoxia on growth, water relations and ABA levels in bean (Phaseolus vulgaris L.) seedlings

In hydroponics, high root temperature is generally associated to hypoxia stress, since increasing temperature enhances the respiration and the oxygen requirements of the roots, while decreasing the oxygen content of the nutrient solution. This paper reports a study performed to separate the effects of heat stress and hypoxia on hydroponically-grown bean (Phaseolus vulgaris L.) seedlings. We showed also the first results of a series of experiments aimed to further investigate the role of ABA in regulating leaf stomatal behaviour and expansion in flooded plants grown at optimal root temperature. In bean hypoxia and high root temperature reduced strongly plant growth, but there were no additive effects at least when the plants were grown at optimal air temperature. The growth inhibition was caused by a reduction in both assimilation rate and leaf expansion, as indicated by decreased NAR and LAR. Root cooling can partially offset the inhibitory effects of high air temperature. Experiments on the effects of root hypoxia on bean plants grown at optimal temperature showed that oxygen deficiency in the nutrient solution elicited a fast inhibition of leaf growth, stomatal conductance, with no relevant changes in leaf hydration. Significant increase in the ABA concentration of bulk leaf tissues and a rise in alkalinity and in the ABA content of xylem sap were also observed. Non-hydraulic signals appeared therefore the main factors responsible for the inhibition of leaf expansion and stomatal conductance in plants with hypoxic roots.