USE OF POTASSIUM AND SUCROSE BY ONION GUARD-CELLS DURING A DAILY CYCLE OF OSMOREGULATION

Solute content of stomata from intact onion cotyledons grown under eit her greenhouse or growth chamber conditions was followed over the cour se of a daily light cycle to determine patterns of osmoregulation. Ini tial opening of stomata was well correlated with guard cell potassium accumulation under both growth conditions. Subsequently, however, ther e was a consistent decrease in guard cell potassium content despite co nstant or increasing aperture. Although a secondary increase in potass ium was sometimes observed during the second half of the light cycle, guard cell potassium content was poorly correlated with aperture, Sucr ose levels in guard cells increased 60% during the period of declining potassium content, suggesting its use as an alternate osmoticum. Guar d cells are postulated to use multiple pathways for the production of osmotica over the course of a complete daily cycle of stomatal movemen ts.