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 polyethylene glycol (60 mM) to the hydroponic solution through inhibition of cell elongation. Measurement of the mechanical properties of the cell walls revealed that the cell wall of stressed hypocotyls was loosened as much as that of the unstressed hypocotyls, suggesting that the stressed hypocotyl could not elongate even though the cell wall loosened. Galactose and arabinose in the pectic fraction, which are probably attached to high mol wt rhamnogalacturonans, increased under stressed as well as under unstressed condition. Other polysaccharides including pectic low mol wt galacturonans, hemicellulosic xyloglucans, galactoglucomannans, arabinans, and glucuronoarabinoxylans increased more under unstressed condition. The mol wt of xyloglucans in the hemicellulosic fraction increased under unstressed but not under stressed condition. These results suggest that changes in wall structure, such as increases in high mol wt rhamnogaracturonans rich in arabinose and galactose residues, and the suppression of polymerization of xyloglucans are involved in the process of cell wall loosening.