Cotyledon expansion in response to blue light was compared for wild-type Arabidopsis thaliana (1.) Heynh. and the mutants blu3 and hy4, which show reduced inhibition of hypocotyl growth in blue light. White, blue, and red light stimulated cotyledon expansion in both intact and excised cotyledons of wild-type seedlings (ecotypes No-O, WS, Co-O, La-er). Cotyledons on intact blu3 and hy4 seedlings did not grow as well as those on the wild type in response to blue light, but pretreatment of blu3 seedlings with low fluence rates of red light increased their responsiveness to blue light. Excision of cotyledons alleviated the mutant phenotype so that both mutant and wild-type cotyledons grew equally well in blue light. The loss of the mutant cotyledon phenotype upon excision indicates that the blu3 and hy4 lesions affect cotyledon expansion indirectly via a whole-plant response to light. Furthermore, the ability of excised, mutant cotyledons to grow normally in blue light shows that this growth response to blue light is mediated by a photosystem other than the ones impaired by the blu3 and hy4 lesions.Blue light-induced photomorphogenic responses in angiosperms include phototropism, inhibition of hypocotyl and epicotyl elongation, hook unfolding, and cotyledon and leaf expansion. The latter is a complicated process that can be driven through several photoreceptors acting at different sites. For example, in Phaseolus vulgaris (L.), full expansion of the primary leaves can be driven by both blue and red light acting photosynthetically and photomorphogenically (Van Volkenburgh and Cleland, 1990; Van Volkenburgh et al., 1990; Blum et al., 1992). Perception of the red light signal is at two sites, the hook and the primary leaves (De Greef et al., 1978).Cellular pathways mediating responses to specific stimuli can be elucidated by analyzing mutants deficient in these responses. Severa1 Arabidopsis thaliana mutants, blul, blu2, blu3 (Liscum and Hangarter, 1991), and hy4 (Koomneef et al., 1980), have been identified by a screen for altered hypocotyl growth inhibition. These mutants do not show wildtype inhibition of hypocotyl growth in response to blue light, but they display normal response to white and far-red light in the case of the blu mutants (Liscum and Hangarter, 1991) and to far-red light in hy4 (Koomneef et al., 1980). Addi- tionally blul, 61242, and blu3 do not exhibit full cotyledon expansion in blue light when compared to white light (Liscum and Hangarter, 1991). Of the mutants found by Liscum and Hangarter, blu3 showed the greatest difference between white-and blue-light-induced cotyledon expansion.To understand the cellular mechanism of blue-light-driven cell growth in cotyledons, their growth response in the mutants blu3 and hy4 has been investigated. Realizing that several sites of perception may be involved (Black and Shuttleworth, 1974; De Greef et al., 1978; Oelze-Karow and Mohr, 1988) and that excision sometimes changes responses (Cosgrove, 1981), we also examined whether the mutant phenotype woul...

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 ...