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 guard cells under osmotic stress. Epidermal strips peeled from Vicia faba L. leaflets were sonicated and incubated in 0.4 M mannitol solution (osmotic stress) in either light or dark. Stomata were closed by osmotic stress. Under osmotic stress, malate accumulated light-dependently and sucrose accumulated light-independently in the guard cells. The level of K+ in guard cells increased slightly under osmotic stress in the light, although without statistical significance. The levels of all these solutes were reduced by 10 mu M ABA treatment. These results suggest that 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 osmotic stress. Therefore, the accumulation of malate and sucrose may be caused by reduced respiration under osmotic stress. Accumulation of solutes in guard cells by osmotic stress may result in increased osmotic pressure of guard cells and may play a role in protection of guard cells from osmotic stress.