Physiological changes in rice seedlings induced by extracellular polysaccharide produced by Xanthomonas campestris pv. oryzae

The extracellular polysaccharide (EPS) produced by Xanthomonas campestris pv. oryzae induced wilting of rice seedlings even at a concentration as low as 1 μg ml−1. At this concentration too, it interfered with the water movement from veinlets to the leaf surface, stomatal resistance to transpiration increased and the drying rate of the leaf tissues decreased. The EPS also reduced the rate of water uptake by rice seedlings, increased water potential of the stem tissues, and, at higher concentrations, retarded water movement in the xylem vessels. The EPS increased the permeability of cell membranes of the leaf tissues. Blockage of dye movement inside the plant tissues induced by the EPS was partially reversed by sodium chloride (0·1 m) treatment; but the reversal did not prevent the wilting of plants. Sodium chloride did not reverse the action of EPS on transpiration and drying rate of the leaves. Electrolyte leakage induced by the EPS also was not altered by sodium chloride treatment. The results suggested that the EPS would have passed through large vessels of the stem and petiole, accumulated in tiny veinlets and blocked water transfer from veins to the leaf surface. The EPS might have increased cell membrane leakage and both blockage and leakage would have resulted in wilting of rice plants.