Sucrose Loading in Isolated Veins of Pisum sativum: Regulation by Abscisic Acid, Gibberellic Acid, and Cell Turgor

Enzymatically isolated vein networks from mature pea (Pisum sativum L. cv Alaska) leaves were employed to investigate the properties of sucrose loading and the effect of phytohormones and cell turgor on this process. The sucrose uptake showed two components: a saturable and a first-order kinetics system. The high affinity system (K_m, 3.3 millimolar) was located at the plasmalemma (p-chloromercuriphenylsulfonic acid and orthovanadate sensitivity). Further characterization of this system, including pH dependence and effects of energy metabolism inhibitors, supported the H⁺-sugar symport concept for sucrose loading. Within a physiological range (0.1-100 micromolar) and after 90 min, abscisic acid (ABA) inhibited and gibberellic acid (GA₃) promoted 1 millimolar sucrose uptake. These responses were partially (ABA) or totally (GA₃) turgor-dependent. In experiments of combined hormonal treatments, ABA counteracted the GA₃ positive effects on sucrose uptake. The abolishment of these responses by p-chloromercuriphenylsulfonic acid and experiments on proton flux suggest that both factors (cell turgor and hormones) are modulating the H⁺ ATPase plasmalemma activity. The results are discussed in terms of their physiological relevance.