ABA Levels and Effects in Chilled and HardenedPhaseolus vulgaris

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Abstract
Leaf abscisic acid (ABA) levels of chilled P. vulgaris were measured after 18 h chilling at 5°C, at a saturation deficit of 1.24 g m−3 (SD), and after chilling in a water-saturated atmosphere. Changes were also followed during a chill hardening period of 4 d at 12°C, 2.1 g m−3 SD. It was found that hardening resulted in an almost 5. fold increase in ABA levels after 3 d at 12°C, and this decreased to approximately control levels on the fourth day. Subsequent chilling of hardened plants produced no change in ABA levels from that of control plants (22° C). In contrast, non-hardened plants chilled at 1.24 g m−3 SD had ABA levels almost 3 times the level of control plants. However, chilling in a water-saturated atmosphere resulted in a decrease in ABA levels. In addition, the response of leaf diffusion resistance (LDR) to exogenous ABA fed via the transpiration stream was measured at 5 ° C and 22° C in hardened and non-hardened plants. Use of tritium-labelled ABA was made to calculate the stomatal sensitivity to ABA. It was found that exogenous ABA caused an increased in LDR at 22°C in both hardened and non-hardened plants. However, the sensitivity of the hardened plants to ABA was greater in terms of rate of closure and amount of ABA required to close the stomata. At 5°C, however, ABA caused stomatal opening and the maintainance of open stomata in non-hardened plants. In hardened plants, ABA caused stomatal closure at 5°C. These results are discussed in relation to the locking-open response of chilled P. vulgaris stomata.