Is stomatal conductance of plants in drying soil controlled by abscisic acid in the xylem stream?

Abstract

Soil drying usually leads to water deficit in the shoots, which will trigger ABA synthesis in large amount. Work in the last decade has also shown that well before this happens, stomata can respond directly to soil drying in the apparent absence of leaf water deficit (Bates and Hall, 1981; Blackman and Davies, 1985; Gollan et al. 1986). One explanation for this is that a chemical signal is produced in the roots in response to soil drying and transported through the xylem stream to regulate shoot physiology (Davies et al. 1987). Work in Lancaster has shown that root-sourced ABA, a result of partial drying of the root system, plays a major role as the root signal. For laboratory grown plants there is a good relationship between root ABA content and surrounding soil water content (Zhang and Davies, 1989a). There is also a good relationship between xylem ABA concentration and stomatal conductance (Zhang and Davies, 1989b, Zhang and Davies, 1990a). In contrast, there is no apparent relation between leaf conductance and leaf water potentials. An initial experiment showed that this behaviour could apply in the field (Tardieu et al. 1991a). These experiments were performed further to investigate the role played by ABA in the control of stomatal behaviour in the laboratory and in the field.