Interaction of cell turgor and hormones on sucrose uptake in isolated Phloem of celery

Phloem tissue isolated from celery (Apium graveolens L.) was used to investigate the regulation of sucrose uptake by turgor (manipulated by 50-400 milliosomolal solutions of polyethylene glycol) and hormones indoleacetic acid (IAA) and gibberillic acid (GA₃). Sucrose uptake was enhanced under low cellular turgor (increase in the V_max). Furthermore, enhancement of sucrose uptake was due to a net increase in influx rates since sucrose efflux was not affected by cell turgor. Manipulations of cell turgor had no effect on 3-O-methyl glucose uptake. When 20 millimolar buffer was present in uptake solutions, low turgor-induced effects were observed only at low pH range (4.5-5.5). However, the effect was extended to higher external pH (up to 7.5) when buffer was omitted from uptake solutions. A novel interaction between cellular turgor and hormone treatments was observed, in that GA₃ (10 micromolar) and IAA (0.1-100 micromolar) enhanced sucrose uptake only at moderate turgor levels. The hormones elicited little or no response on sucrose uptake under conditions of low or high cell turgor. Low cell turgor, IAA, GA₃, and fusicoccin caused acidification by isolated phloem segments in a buffer-free solution. It is suggested that enhanced sucrose uptake in response to low turgor and/or hormones was mediated through the plasmalemma H⁺-ATPase and most likely occurred at the site of loading.