PHYSIOLOGICAL STUDIES OF FRUIT SET IN STRAWBERRY (FRAGARIA X ANANASSA DUCH.) (CARBOHYDRATE, ASSIMILATE TRANSPORT, AUXIN).

摘要

Knowledge of the mechanism of fruit set would be helpful in order to consistently control and manipulate cropping. Although hormones have been implicated as a controlling factor in fruit set, previous work indicates there is no consistent correlation between endogenous hormone levels and fruit set. However, hormones have been shown to affect the transport and accumulation of nutrients and photoassimilates during plant development. The present study was initiated to test the hypothesis that hormones control fruit set by controlling the amount of assimilate transport and accumulation in fruit tissues. The objectives were to compare assimilate mobilization and accumulation in two treatments which induced fruit set; i.e. pollination and exogenous hormone application, versus a non-pollinated treatment which did not induce fruit set. Initial experiments revealed that several auxins which induce parthenocarpic set in June-bearing strawberries did not induce set in the day-neutral cultivar used in this study. The only auxin found which induced set with a single application was indoleacetic acid ethyl ester (Et-IAA). Subsequently, ('14)C-Et-IAA was synthesized and the translocation and metabolism of this auxin was examined and correlated with fruit set and initial growth. Et-IAA transport out of the receptacle was not required for induction of set and growth. Receptacle growth followed the rapid metabolism of Et-IAA to highly polar compounds. Anatomical evaluation of the phloem transport system within the receptacle revealed no differences in either the rate or amount of sieve tube file differentiation among the three treatments. This study indicated there was no correlation between the degree of assimilate transport pathway development and fruit set/initial growth. Examination of ('14)C-photoassimilate transport among the three treatments during the fruit-setting period revealed that fruit set/initial growth was not limited by the ability of receptacles to mobilize source leaf assimilates. Furthermore, carbohydrate levels (soluble sugars plus starch) in receptacles were not different among the three treatments, indicating that fruit set was not limited by the ability to accumulate carbohydrates. These results indicate that fruit set/initial growth in strawberry cannot be explained on the basis of photoassimilate or carbohydrate limitations within the developing receptacle.