Phytoalexins, Water‐stress and Stomata

Abstract

SUMMARYThe effects of the phytoalexins, rishitin, pisatin, wyerone acid and phaseollin, (all at 0-1 mol m~^) were tested on stomatal responses in epidermal strips of Commelina communis L. and Viciafaba 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 ahscisic acid (ABA), farnesol, nonanoic and undecanoic acids, and a-linolenic acid. Phytoalexins may be loosely defined as 'microbial stress' compounds while 'water-stress' compounds are defined 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"^) had no discernible effects on guard cell protoplasts, while phaseollin and wyerone acid (at 0-1 mol m~^) caused loss of protoplast viability. The 'water-stress' compounds, nonanoic, undecanoic and a-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 a-linolenic acids, their effects are likely to be associated with phytotoxicity, their phytotoxicity depending on duration of exposure and on concentration.

Phytoalexins, Water‐stress and Stomata

“…As was observed with phytoalexins (Plumbe & Willmer, 1986), the phenolics and the ' water-stress' compounds were generally more effective in preventing opening than bringing about closure of stomata. Possible reasons for this were discussed by Plumbe & Willmer (1986) and are believed to be related to the relatively high KCl concentration in the medium.…”

“…Possible reasons for this were discussed by Plumbe & Willmer (1986) and are believed to be related to the relatively high KCl concentration in the medium. Few studies have been made of the effects of phenolics on stomatal behaviour, but published data have generally agreed with those presented here.…”

“…The methods of growing plants of C. communis and the choice of leaf material were as described by Plumbe & Willmer (1986). Preparation and incubation of epidermal strips and measurement of stomatal apertures were also as described by Plumbe & Willmer (1986) except that 75 mol m'^ KCl at pH 6-0 was used in the incubation medium instead of 100 mol m"^ KCl at pH 5-5.…”

“…Preparation and incubation of epidermal strips and measurement of stomatal apertures were also as described by Plumbe & Willmer (1986) except that 75 mol m'^ KCl at pH 6-0 was used in the incubation medium instead of 100 mol m"^ KCl at pH 5-5.…”

“…In previous papers (Plumbe & Willmer, 1985, 1986 it was demonstrated that certain phytoalexins accumulate in severely water-stressed k-if tissue of Vicia faba L. and Pisum sativum L. and that some of the phytoalexins inhibited stomatal opening and brought about stomatal closure. The tissues of some plants also accumulate phenolics in response to attack by micro-organisms, although they are not generally considered to be phytoalexins because they are present at low levels within the tissues before infection (Link, Dickson & Walker, 1929;Harborne, 1980;Kuc, 1982).…”