PHYTOALEXINS, WATER‐STRESS AND STOMATA

Abstract

SummaryThe effects of the phytoalexins, rishitin, pisatin, wyerone acid and phaseollin, (all at 0.1 mol m−3) were tested on stomatal responses in epidermal strips of Commelina communis L. and Vicia faba L. Additionally, the effects of the phytoalexins on guard cell protoplasts of C. communis were examined and compared with those of several‘water‐stress' compounds namely abscisic acid (ABA), farnesol, nonanoic and undecanoic acids, and α‐linolenic acid. Phytoalexins may be loosely defined as‘microbial stress' compounds while‘water‐stress' compounds are denned as those which accumulate in leaf tissue under water‐stress and close stomata or act as osmotic adjusters.Wyerone acid and phaseollin inhibited stomatal opening in C. communis to the same extent as ABA, while rishitin had no significant effect and pisatin had an intermediate effect. When stomata were open, treatments with phaseollin and ABA brought about a marked closing response, while those with pisatin and rishitin were not statistically significant from the controls; wyerone acid initially opened stomata further but after 5 h exposure stomatal apertures were similar to those of the controls.Stomatal opening in epidermis of V. faba was greatly inhibited by ABA, while all phytoalexin treatments resulted in intermediate inhibitory effects between those obtained with the control and ABA treatments, wyerone acid being the most inhibitory.Pisatin and rishitin (at 0.1 mol m−3) had no discernible effects on guard cell protoplasts, while phaseollin and wyerone acid (at 0.1 mol m−3) caused loss of protoplast viability. The‘water‐stress' compounds, nonanoic, undecanoic and α‐linolenic acids, also caused gradual loss of protoplast viability and farnesol caused lysis of protoplasts. Only ABA caused protoplasts to contract without loss of vitality. Thus phaseollin and, to a lesser extent, wyerone acid have similar properties to ABA in that they inhibit stomatal opening and promote closure. However, unlike ABA but like farnesol, nonanoic, undecanoic and α‐linolenic acids, their effects are likely to be associated with phytotoxicity, their phytotoxicity depending on duration of exposure and on concentration.