PHYSIOLOGICAL FACTORS IN TOMATO FRUIT‐SET

Abstract

THE CHANGE-OVER from the static condition of the flower ovary to the rapidly growing condition of a young fruit is a dramatic shift. This important phenomenon, commonly spoken of as fruit-set, is an essential step in the life cycle of all sexually reproducing higher plants. A major contribution to the understanding, of fruit-set was made by Gustafson (1936) who showed -that in some plants the simple application of auxins could brin, about fruit-set even without pollination. Subsequent work has demonstrated that fruit-set in general is at least in part a function of auxin in the ovary. Some genetic limitations to fruit-set?poken of as sterility and incompatability-are known (East, 1929), but the physiological basis for these limitations remains obscure. The requirement of moderately cool night temperatures for tomato fruit-set has been found by Went (1945). A decrease in fruit-set of tomatoes under low light invensiBies of winter has been noted by Hemphill and Murneek (1950). Other studies have indicated the existence of factors influencing fruit-set indirectly, such as flower abscission and pollen sterility (Howlett, 1936) associated with unbalanced carbohvdrate/nitrogen ratios in the plants. The present study was undertaken with the objective of identifying as far as possible the physiological factors involved in fruit-set of the tomato. METHODS.-The plants used in the present study were both fieldand greenhouse-grown, John Baer, self-sterile tomatoes. The stamens of the flowers of this self-sterile strain characteristically develop a brown rino of necrotic tissue at the anther tips which effectively prevents any pollen from reaching the stigma. By the use of these self-sterile tomato flowers complete control of fruit-set is made possible. In the experiments with excised flowers, the flower clusters were cut from the stems with a sharp blade and were immediately placed into separate vials of distilled water. They were then trimmed to 2-3 mature flowers per cluster and a fresh diagonal cut made at the base of the cluster. All stem material was cut away, leaving only the pedicels attached by approximately 0.5 cm. of peduncle. In order to force fruit-set, the flowers were sprayed individually with a 30 p.p.m. solution of PCA (parachlorophenoxyacetic acid). They were then placed in individual vials with 1 per cent agar media containing the materials to be tested. The flower cultures were maintained at a tempera-