THE ROLE OF SUCROSE IN GUARD-CELL OSMOREGULATION

Stomatal apertures are regulated by changes in the solute content of g uard cells. The identity of the solutes involved in guard cell osmoreg ulation has been the subject of much study. Early consensus in the fie ld held that carbohydrates derived from starch constitute the principa l osmoticum. This starch-sugar hypothesis has been replaced by the pre sent paradigm of guard cell osmoregulation by K+ and its counterions. Recent studies, however, show that both K+ and sucrose are primary gua rd cell osmotica, and that the use of these two solutes is separated i nto two distinct phases in which one or the other constitutes the domi nant osmoticum. In the intact leaf, opening at the beginning of a dail y cycle is supported by K+ and its counterions, malate(2-) and Cl-. Ma late(2-) is the dominant counterion in growth chamber-grown leaves, wh ereas Cl- predominates in a greenhouse environment. In the second half of the daily cycle, K+ content in guard cells decreases drastically a nd sucrose becomes the dominant solute. Manipulation of stomatal apert ures by altering ambient CO2 concentration shows that either K+ or suc rose accumulation can sustain rapid opening. The functional implicatio ns of two distinct osmoregulatory phases of stomatal movements remains to be elucidated. The guard cell content of K+, its counterions, and sucrose can be modulated by at least three osmoregulatory pathways in guard cells. Experimental conditions favouring three distinct pathways have been established, but major uncertainties remain about the contr ol of guard cell solute content in the intact leaf.