Fruit Set Improvement of Tomato Plants by Reducing Plant Water Stress

On the Ogasawara islands, tomatoes are usually sown in late summer and seedlings are then transplanted to the fields in the fall. Early yields are very low, however, because of poor fruit-set in lower clusters. Ozawa (1986) reported that fruit-set was largely influenced by plant water status. Several experiments were carried out in the present study to discover effective ways to reduce plant water stress. In the first experiment, the effects on fruit-set of leaf water spray were investigated. Cultivar ‘Azuma’ sown on August 25, 1982, were transplanted to the field on October 27, 1982. Each of six plots received a different treatment: Plot A was untreated, Plot B was mist-sprayed at 9:00, Plot C was mist-sprayed at 12:00, Plot D was mist-sprayed at 15:00, Plot E was irrigated, and Plot F was irrigated and was mist-sprayed at 12:00. Mist-spraying was done continuously for nine days from October 29, during the period of first cluster anthesis. Irrigation was done intermittently for 22 days from October 27. First-cluster fruit-setting was improved in Plot F and D. Second-cluster fruit-setting increased in the following order: Plot D, Plot C, Plot B, Plot F, Plot E. Total yields of the first and second clusters were high in Plots D and F. In the second experiment, diurnal variations in water saturation deficit (W.S.D.) were measured on November 2. In Plots C and D, W.S.D. was decreased for two hours after spraying. Irrigation also markedly decreased W.S.D. throughout the daytime. W.S.D. was lower throughout the daytime in Plot D than in Plots A, B, and C. The first two experiments showed that fruit-setting was improved by rapidly decreasing plant water stress in the evening. Two interesting phenomena were also observed: plant water stress fell at midday after the evening decrease had continued for four days, and fruit-setting of the inflorescence that bloomed after treatment were also improved. These findings suggest that plant physiological changes can lessen plant water stress. In the third experiment, plants were cultivated in pots of different sizes in a greenhouse. Leaf water potentials increased with increase in soil mass. In the fourth experiment, practical measures to promote root development were examined. Early transplanting to the field and cutting of three folioles in the upper part of each leaf at the time of transplanting effectively promoted root development. In the fifth experiment, yields from plots that had been given different treatments were compared with those from the irrigated plot. Plots were divided according to treatment into control, soil surface irrigation, sprinkler irrigation, early transplanting and early transplanting plus leaf cutting. Total yields of first and second clusters increased in the following order: early transplanting plus leaf cutting, early transplanting, soil surface irrigation and sprinkler irrigation. Early transplanting and leaf cutting at the time of transplanting appears to be more valuable than irrigation for improving fruit-setting on the Ogasawara Islands.