Use of Potassium and Sucrose by Onion Guard Cells during a Daily Cycle of Osmoregulation

Abstract

Solute content of stomata from intact onion cotyledons grown under either greenhouse or growth chamber conditions was followed over the course of a daily light cycle to determine patterns of osmoregulation. Initial opening of stomata was well correlated with guard cell potassium accumulation under both growth conditions. Subsequently, however, there was a consistent decrease in guard cell potassium content despite constant or increasing aperture. Although a secondary increase in potassium was sometimes observed during the second half of the light cycle, guard cell potassium content was poorly correlated with aperture. Sucrose levels in guard cells increased 60% during the period of declining potassium content, suggesting its use as an alternate osmoticum. Guard cells are postulated to use multiple pathways for the production of osmotica over the course of a complete daily cycle of stomatal movements.