ABSTRACT
Restriction of water supply from the soil can result in shoot water deficit and this will limit shoot growth and functioning. Nevertheless, shoot water deficits do not always result from soil drying; indeed in some circumstances, drought-induced stomatal closure and growth limitation can even result in an increase in shoot water potential relative to that of a well watered plant (Jones et al., 1983). It is necessary to ask what is limiting the reopening of stomata and the growth of the shoots under such circumstances. Some ingenious experimental manipulations such as the root pressure vessel (Munns and Passioura, 1985) and the split-root technique (Blackman and Davies, 1985; Gowing et al., 1990) have allowed several groups to conclude that chemical signaling between roots and shoots can limit shoot growth and functioning even when the water relations of shoots of plants that grow in drying soil are comparable to those of plants in well-watered soil. Of course in nature, chemical and hydraulic regulation of growth occur together, with the relative importance of the two mechanisms varying between genotypes and with different soil conditions and with the extent of the drought conditions. The literature has concentrated on hydraulic limitations on plant functioning (e.g., Nonami et al., 1997) in drying soil, so it is important to put the case for chemical regulation. We should not forget, however, that the two systems depend upon each other for effective functioning and that often distinctions between them are arbitrary and unrealistic.
