Effects of Fruit Load on Fruit Growth, Mesocarp Starch Grain Appearance and Sucrose-catalysing Enzyme Activity in a Gynoecious Cucumber Fruit

Abstract

To examine the relationship between fruit growth and sugar metabolism, gynoecious parthenocarpic cucumbers (Cucumis sativus L.) ‘Freedom House No. 3’ were grown in a glasshouse with different fruit load conditions, and starch grains in the mesocarp were observed microscopically and activities of sucrose-catalysing enzymes were measured in the fruit at node 15. Flower buds below node 9 and all lateral shoots were removed, but the plant was not pinched. Fruit load was changed as follows: (1) all flowers below node 15 were removed on the day of the anthesis of the flower at node 15; (2) all fruits below node 15 were removed 4 days after anthesis (DAA) of the fruit at node 15; (3) all fruits below node 15 were removed 8 DAA; and (4) all fruits on nodes 9–15 were allowed to grow. The growth of fruit at node 15 was suppressed by fruit load and resumed 4 days after the removal of fruits at the lower nodes. Starch grains disappeared 4 DAA in fruit load treatment and reappeared 4 days after the removal of fruits at the lower nodes. The activities of sucrose-catalysing enzymes, especially acid invertase, were suppressed by heavy fruit load and increased 4 days after the removal of the fruit load. These results suggested that the existence of starch grains and activities of sucrose-catalysing enzymes reflect the level of the photoassimilate imported into fruits.