Mobilization of Starch after Submergence of Air‐Grown Rice Coleoptiles. Implications for Growth and Gravitropism

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Abstract
Submergence of air‐grown rice seedlings (Oryza sativa L. var. Sasanishiki) induces elongation of the coleoptile. We investigated whether rapid underwater extension is associated with a loss of starch. After 1 d of submergence the starch content was reduced by 70%. This loss of reserve carbohydrate was accompanied by a 38% increase in the concentration of glucose in the cell sap of the coleoptiles. The submerged (starch‐depleted) coleoptiles had a slower negative gravitropism than the air‐grown controls, although the rate of elongation in the horizontal position was not impaired. We conclude that the submergence‐induced mobilization of starch provides substrates and osmotica for the rapidly growing cells. In addition, our results indicate that a full complement of starch is necessary for normal gravitropism in the rice coleoptile.