How the roots contribute to the ability of Phaseolus vulgaris L. to cope with chilling-induced water stress

Intact plants and stem-girdled plants of Phaseolus vulgaris grown hydroponi cally were exposed to 5 degreesC for up to 4 d; stem girdling was used to i nhibit the phloem transport from the leaves to the roots. After initial wat er stress, stomatal closure and an amelioration of root water transport pro perties allowed the plants to rehydrate and regain turgor. Chilling augment ed the concentration of abscisic acid (ABA) content in leaves, roots and xy lem sap. In intact plants stomatal closure and leaf ABA accumulation were p receded by a slight alkalinization of xylem sap, but they occurred earlier than any increase in xylem ABA concentration could be detected. Stem girdli ng did not affect the influence of chilling on plant water relations and le af ABA content, but it reduced slightly the alkalinization of xylem sap and , principally, prevented the massive ABA accumulation in root tissues and t he associated transport in the xylem that was observed in non-girdled plant s. When the plants were defoliated just prior to chilling or after 10 h at 5 degreesC, root and xylem sap ABA concentration remained unchanged through out the whole stress period. When the plants were chilled under conditions preventing the occurrence of leaf water deficit (i.e. at 100% relative humi dity), there were no significant variations in endogenous ABA levels. The i ncrease in root hydraulic conductance in chilled plants was a response neit her to root ABA accretion, nor to some leaf-borne chemical signal transport ed downwards in the phloem, nor to low temperature per se, as indicated by the results of the experiments with defoliated or girdled plants and with p lants chilled at 100% relative humidity. It was concluded that the root sys tem contributed substantially to the bean's ability to cope with chilling-i nduced water stress, but not in an ABA-dependent manner.