XYLEM-TRANSPORTED CHEMICAL SIGNALS AND THE REGULATION OF PLANT-GROWTHAND PHYSIOLOGY

There is now a substantial body of evidence that shoot growth and phys iology of plants rooted in drying soil may be regulated by chemical si gnals moving from the root to the shoot in the xylem stream. Although some evidence suggests that soil drying can reduce the supply of promo ters of leaf growth and stomatal opening, there is now compelling evid ence for an enhanced flux of inhibitors in the xylem stream of drought ed plants. Some of this inhibitory activity is still to be identified but at least in some plants the bulk of activity can be explained by t he enhanced concentration of the plant hormone abscisic acid (ABA). A series of field experiments has now shown that ABA, moving as a signal from the roots to the leaves in the transpiration stream, can provide a measure of the access that the plant has to water in the soil in th e rooting zone. We show here how this signal may be a variation in the concentration of ABA arriving at the sites of action in the leaf. The response to such a signal apparently varies as a function of the phys iological state of the leaf. The basis of such variation in the sensit ivity of response is also discussed. One other interpretation of the f ield data is that leaves respond to the amount of ABA arriving in the leaf, rather than the concentration. We show some evidence for this co ntention.