The Effect of Osmotic Stress on the Solutes in Guard Cells of Vicia faba L.

We investigated the changes in the levels of solutes in guardcells under osmotic stress. Epidermal strips peeled from Viciafaba L. leaflets were sonicated and incubated in 0.4 M mannitolsolution (osmotic stress) in either light or dark. Stomata wereclosed by osmotic stress. Under osmotic stress, malate accumulatedlight-dependently and sucrose accumulated light-independentlyin the guard cells. The level of K⁺ in guard cells increasedslightly under osmotic stress in the light, although withoutstatistical significance. The levels of all these solutes werereduced by 10 µM ABA treatment. These results suggestthat osmotic stress affects carbon metabolism in guard cells;this metabolic change is different from that caused by ABA alone.Respiratory activity of guard cells decreased under osmoticstress. Therefore, the accumulation of malate and sucrose maybe caused by reduced respiration under osmotic stress. Accumulationof solutes in guard cells by osmotic stress may result in increasedosmotic pressure of guard cells and may play a role in protectionof guard cells from osmotic stress. (Received December 17, 1998; Accepted May 28, 1999)