SUGAR AND ORGANIC-ACID ACCUMULATION IN GUARD-CELLS OF VICIA-FABA IN RESPONSE TO RED AND BLUE-LIGHT

Changes in neutral sugar and organic acid content of guard cells were quantitated by high-performance liquid chromatography during stomatal opening in different light qualities. Sonicated Vicia faba epidermal p eels were irradiated with 10 mumol m-2 s-1 of blue light, a fluence ra te insufficient for the activation of guard cell photosynthesis, or 12 5 mumol m-2 s-1 of red light, in the presence of 1 mm KCl, 0.1 mm CaCl 2. The low-fluence-rate blue light stimulated an average net stomatal opening of 4.7 mum in 2 h, whereas the saturating fluence rate of red light stimulated an average net opening of 3.8 mum in 2 h. Under blue light, the malate content of guard cells increased to 173% of the init ial level during the first 30 min of opening and declined as opening c ontinued. Sucrose levels continuously rose throughout the blue light-s timulated opening, reaching 215% of the initial level after 2 h. The s tarch hydrolysis products maltose and maltotriose remained elevated at all times. Under red light, guard cells showed very little increase i n organic acid or maltose levels, whereas sucrose levels increased to 208% of the initial level after 2 h. Total measured organic metabolite concentrations were correlated with stomatal apertures in all cases e xcept where substantial malate increases occurred. These results suppo rt the hypothesis that light quality modulates alternative mechanisms of osmotic accumulation in guard cells, including potassium uptake, ph otosynthetic sugar production, and starch breakdown.