Blue light-stimulated stomatal opening in detached epidermis of Vicia faba is reversed by green light. A 30 s green light pulse eliminated the transient opening stimulated by an immediately preceding blue light pulse. Opening was restored by a subsequent blue light pulse. An initial green light pulse did not alter the response to a subsequent blue light pulse. Reversal also occurred under continuous illumination, with or without a saturating red light background. The magnitude of the green light reversal depended on fluence rate, with full reversal observed at a green light fluence rate twice that of the blue light. Continuous green light given alone stimulated a slight stomatal opening, and had no effect on red light-stimulated opening. An action spectrum for the green light effect showed a maximum at 540 nm and minor peaks at 490 and 580 nm. This spectrum is similar to the action spectrum for blue light-stimulated stomatal opening, red-shifted by about 90 nm. The carotenoid zeaxanthin has been implicated as a photoreceptor for the stomatal blue light response. Blue/green reversibility might be explained by a pair of interconvertible zeaxanthin isomers, one absorbing in the blue and the other in the green, with the green absorbing form being the physiologically active one.

Relative molecular size distributions of pectic and hemicellulosic polysaccharides of pea (Pisum sativum cv Alaska) third intemode primary walls were determined by gel filtration chromatography. Pectic polyuronides have a peak molecular mass of about 1100 kilodaltons, relative to dextran standards. This peak may be partly an aggregate of smaller molecular units, because demonstrable aggregation occurred when samples were concentrated by evaporation. About 86% of the neutral sugars (mostly arabinose and galactose) in the pectin cofractionate with polyuronide in gel filtration chromatography and diethylaminoethyl-cellulose chromatography and appear to be attached covalently to polyuronide chains, probably as constituents of rhamnogalacturonans. However, at least 60% of the wall's arabinan/ galactan is not linked covalently to the bulk of its rhamnogalacturonan, either glycosidically or by ester links, but occurs in the hemicellulose fraction, accompanied by negligible uronic acid, and has a peak molecular mass of about 1000 kilodaltons. Xyloglucan, the other principal hemicellulosic polymer, has a peak molecular mass of about 30 kilodaltons (with a secondary, usually minor, peak of approximately 300 kilodaltons) and is mostly not linked glycosidically either to pectic polyuronides or to arabinogalactan. The relatively narrow molecular mass distributions of these polymers suggest mechanisms of co-or postsynthetic control of hemicellulose chain length by the cell. Although the macromolecular features of the mentioned polymers individually agree generally with those shown in the widely disseminated sycamore cell primary wall model, the matrix polymers seem to be associated mostly noncovalently rather than in the covalently interlinked meshwork postulated by that model. Xyloglucan and arabinan/galactan may form tightly and more loosely bound layers, respectively, around the cellulose microfibrils, the outer layer interacting with pectic rhamnogalacturonans that occupy interstices between the hemicellulose-coated microfibrils.The cell wall plays a critical role in plant cell enlargement. A wide range of growth-regulating agents, including both hormones and environmental factors, apparently act by modifying the ability of the primary wall to extend irreversibly.

Osmoregulation in guard cells of intact, attached Vicia faba leaves grown under growth chamber and greenhouse conditions was studied over a daily light cycle of stomatal movements. Under both growth conditions guard cells had two distinct osmoregulatory phases. In the first (morning) phase, opening was correlated with K+ uptake and, to a lesser extent, sucrose accumulation. In the second (afternoon) phase, in which apertures were maximal, K+ content declined and sucrose became the dominant osmoticum. Reopening of the stomata after a C0,-induced closure was accompanied by accumulation of either K+ or sucrose, depending on the time of day, indicating that a single environmental signal may use multiple osmoregulatory pathways. Malate accumulation, correlated with K+ uptake, was detected under growth chamber but not greenhouse conditions, whereas CI-was the main K+ counterion in the greenhouse. These results indicate that guard-cell osmoregulation in the intact leaf depends on at least two different osmoregulatory pathways, K+ transport and sucrose metabolism. Furthermore, the relative importance of the K+ counterions malate and CI-appears to be environment-dependent.

Changes in neutra1 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 peels were irradiated with 10 pmol m-' s-' of blue light, a fluence rate insufficient for the activation of guard cell photosynthesis, or 125 pmol m-'s-' of red light, in the presence of 1 mM KCI, 0

Effects of indoleacetic acid (IAA) and of turgor changes on the apparent molecular mass (Mr) distributions of cell wall matrix polysaccharides from etiolated pea (Pisum sativum L.) epicotyl segments were determined by gel filtration chromatography. IAA causes a two-to threefold decline in the peak M, of xyloglucan, relative to minus-auxin controls, to occur within 0.5 hour. IAA causes an even larger decrease in the peak Mr concurrently biosynthesized xyloglucan, as determined by [3H]fucose labeling, but this effect begins only after 1 hour. In contrast, IAA does not appreciably affect the M, distributions of pectic polyuronides or hemicellulosic arabinose/galactose polysaccharides within 1.5 hours. However, after epicotyl segments are cut, their peak polyuronide M, increases and later decreases, possibly as part of a wound response. Xyloglucan also undergoes IAA-independent changes in its M, distribution after cutting segments. In addition, the peak Mr of newly deposited xyloglucan increases from about 9 kilodaltons shortly after deposition to about 30 kilodaltons within 0.5 hour. This may represent a process of integration into the cell wall. A step increase in turgor causes the peak M, of previously deposited xyloglucan (but not of the other major polymers) to increase about 10-fold within 0.5 hour, retuming to its initial value by 1.5 hours. This upshift may comprise a feedback mechanism that decreases wall extensibility when the rate of wall extension suddenly increases. IAA-induced reduction of xyloglucan M, might cause wall loosening that leads to cell enlargement, as has been suggested previously, but the lack of a simple relation between xyloglucan M, and elongation rate indicates that loosening must also involve other wall factors, one of which might be the deposition of new xyloglucan of much smaller size. Although the M, shifts in polyuronides may represent changes in noncovalent association, and for xyloglucan this cannot be completely excluded, xyloglucan seems to participate in a dynamic process that can both decrease and increase its chain length, possible mechanisms for which are suggested. wall, which permits the wall to be deformed by the cell's turgor pressure. The nature of wall loosening, at the level of wall structure, continues to be debated. Many workers have expected, based on the popular sycamore primary wall structural model (2), that growing walls would be loosened by breaking covalent cross-links between wall polymers. However, our study of pea primary walls (50) revealed that its major polysaccharide components are mostly not covalently coupled. An alternative possible loosening mechanism, breakdown of polymer backbones, has received experimental support. Auxin-induced breakdown or turnover of hemicellulosic glucans has been reported in both monocots and dicots (18,23,33,35). Estimations of wall polysaccharide molecular mass by GFC3 have indicated that auxin can cause the Mr of XG to shift toward lower values (24,26,38,42,44,53). Auxin-induced release of soluble XG from the ...