Red and blue light both stimulate growth and ion accumulation in bean (Phaseolus vulgaris L.) leaves, and previous studies showed that the growth response is mediated by phytochrome and a bluelight receptor. Results of this study confirm that there is an additional photosynthetic contribution from the growing cells that supports ion uptake and growth. Disc expansion in the light was enhanced by exogenous K' and Rb', but was not specific for anions. Light increased K' accumulation and the rate of 'Rb' uptake by discs, over darkness, with no effect of light quality. The photosynthetic inhibitor, 3-(3,4-dichlorophenyl)-1,1-dimethylurea, inhibited light-driven "6Rb+ uptake by 75%. Light quality caused differences in short-term kinetics of growth and acidification of the leaf surface. At comparable fluence rates (50 ,umol m-2 s-'), continuous exposure to blue light increased the growth rate 3-fold after a 2-min lag, whereas red light caused a smaller growth response after a lag of 12 min. In contrast, the acidification of the leaf surface normally associated with growth was stimulated 3-fold by red light but only slightly (1.3-fold) by blue light. This result shows that, in addition to acidification caused by red light, a second mechanism specifically stimulated by blue light is normally functioning in lightdriven leaf growth.The expansion of dicotyledonous leaves is stimulated by light acting photomorphogenically through at least two photoreceptors, phytochrome and a blue-light receptor (for review, see refs. 9 and 14). For example, in primary leaves of bean (Phaseolus vulgaris L.), red and blue light both stimulate leaf expansion, but simultaneous exposure to far-red light significantly reduces growth only in red, but not blue, light (53). Photosynthesis is not necessary for light-stimulated growth (10, 52) but may enhance growth (10, 23, 52) and is known to be necessary for some light-driven transport processes (11,27,41,46). The stimulatory effect of light on growing leaves includes cell wall loosening and solute uptake for turgor maintenance (9). Cell expansion depends on acidification of the apoplast (50) and on acid-induced wall loosening (50, 54). Acidification may also be associated with the uptake of sugars and salts that is necessary for osmotic regulation by growing cells (8,15,32,45,51 of the cells, setting up a proton motive force that is used to drive uptake of sugars and ions (38).The mechanisms underlying light-driven leaf growth were addressed in this study. The photosynthetic dependence of K+ uptake associated with growth was investigated by measuring the effect of DCMU, an inhibitor of PSII, on accumulation of K+ and uptake of 86Rb+ by leaf discs. The stimulation of K+ and Rb+ uptake by light has been observed in Nitella (28), Vallisneria (39), corn (Zea mays) (40), green bean (Phaseolus vulgaris) (19), pea (Pisum sativum) (34), and broad bean (Vicia faba) (18). Methods for studying ion uptake have included using whole cells, leaf slices and fragments, submerged leaves, and protoplasts. A ...

Proton excretion from bean (Phaseolus vulgaris L.) leaf cells is increased by bright white light. To test whether this could be due, at least in part, to an increase in plasma membrane (PM) ATPase activity, PM vesicles were isolated from primary leaves by phase partitioning and used to characterize PM ATPase activity and changes in response to light. ATPase activity was characterized as magnesium ion dependent, vanadate sensitive, and slightly stimulated by potassium chloride. The pH optimum was 6.5, the Km was approximately 0.30 millimolar ATP, and the activity was about 60% latent. PM vesicles were prepared from leaves of plants grown for 11 days in dim red light (growing slowly) or grown for 10 days in dim red light and then transferred to bright white-light for I day (growing rapidly). For both light treatments, ATPase specific activity was approximately 600 to 700 nanomoles per milligram protein per minute, and the latency, Km, and sensitivity to potassium chloride were also similar. PM vesicles from plants grown in complete darkness, however, exhibited a twofold greater specific activity. We conclude that the promotion of leaf growth and proton excretion by bright white light is not due to an increase in ATPase specific activity. Light does influence ATPase activity, however, both dim red light and bright white light decreased the ATPase specific activity by nearly 50% as compared with dark-grown leaves. environment and hyperpolarizing the membrane potential (19). The resulting electrochemical gradient facilitates the uptake of ions, amino acids, and sugars (19). The lightinduced proton excretion from bean leaf cells is presumed to be mediated by the PM H+-ATPase. Light might stimulate H+ excretion in at least four ways: (a) by increasing the supply of substrate (ATP); (b) by altering the cytoplasmic pH; (c) by increasing the activity of the existing enzyme (e.g. by phosphorylation of the enzyme); or (d) by increasing the amount of H+-ATPase in the PM. Both of the latter two mechanisms should result in an enhanced activity of the PM ATPase in isolated PM vesicles.Although the PM ATPase from roots and stems has been extensively studied (19), the enzyme in leaves has received only limited attention. In the study reported here, we have used the aqueous phase partitioning method (7) to isolate highly purified PM vesicles from the primary leaves of Phaseolus vulgaris L. and have characterized the ATPase in these vesicles. We have used this system to show that while light promotes the ATPase-mediated proton excretion (24), a lightinduced change in ATPase activity cannot explain this effect. MATERIALS AND METHODS