Changes in Wall Polysaccharides of Squash (Cucurbita maxima Duch.) Hypocotyls under Water Stress Condition

Hypocotyl growth of dark-grown squash (Cucurbita maxima Duch.) seedlings was greatly reduced by the addition of 60 mM polyethylene glycol (PEG) to hydroponic solution (water stress). When PEG was removed after one day, growth promptly recovered. The contents of hemicelluloses and cellulose in the wall increased under unstressed condition as hypocotyls grew but these increases were substantially reduced by water stress. The increases in wall polysaccharide contents recovered when the water stress was relieved. The amounts per hypocotyl of cellulose and that of uronic acid in pectin changed in parallel with the growth (r=0.95 and 0.98, respectively). The amounts of most of the sugar components of hemicelluloses also changed in parallel with hypocotyl growth. Pectic and hemicellulosic galactose content of unstressed hypocotyls increased to day 2 when the hypocotyl grew at a maximum growth rate, then decreased. In contrast, galactose content of stressed hypocotyls progressively increased to the end of the experiment. The results indicated that water stress substantially reduced net increases in most of the polysaccharides of the hypocotyl cell walls when it reduced the growth, but it did not affect syntheses of some galactosic polysaccharides in pectin and hemicellulose B. We assume that the syntheses of non-galactosic wall polysaccharides are associated with hypocotyl growth and the synthesis of galactose-containing polysaccharides with preservation of the potential of the cell wall to be loosened, since hypocotyl growth promptly and completely recovers when water stress is relieved.