The Basis of Osmotic Pressure Maintenance During Expansion Growth in Helianthus annuus Hypocotyls

Abstract

In etiolated sunflower (H. annuus) hypocotyls, the intracellular osmotic pressure was maintained in spite of the dilution caused by growth. The principal osmotic substances present were hexoses (44 ± 6 mol m^-3 glucose and 40 ± 5 mol m^-3 fructose) and organic potassium salts (30 mol m^-3). ,

Potassium fluxes in vivo (intact rooted seedlings) and in vitro (1 cm, peeled, cut sections in an aerated bathing solution) were similar, indicating that the sectioning had not affected cell fluxes. The uptake of 3-O-methyl glucose by cut sections from a bathing medium was lower than the uptake estimated using intact tissue (in vitro influx was 5 % of the net accumulation rate of intact tissue). This suggested that some pathway of hexose uptake other than from the extracellular spaces occurred in vivo.

To explain this difference in reducing sugar accumulation rates the following pathway is suggested:

(1) Transport of sucrose from the cotyledons via the phloem;

(2) Direct unloading of the sucrose into the symplast;

(3) Transport of the sucrose into the vacuole down a sucrose concentration gradient maintained by hydrolysis of sucrose to glucose and fructose;

(4) Low hexose efflux from the vacuole (5 % of in vivo influx), preventing loss from the vacuole.