On the Ogasawara Islands, located in the subtropical region, tomatoes are usually sown in late summer. Because fruit-set in the lower clusters are disturbed by plant-water-stress, studies were conducted to improve them; these experiments showed that plant-water-stress was relieved throughout the daytime by mist-spraying at 15:00 for nine days.
Two experiments were carried out to determine the effects of mist-spraying on root elongation. In the first experiment, seedlings of the cultivar 'kyoryoku-Sasuga' sown on August 1, 1987, were transplanted to the field on September 2. The four plots received the following treatment: Plot A was not mist-sprayed; Plot B was mist-sprayed at 9:00; Plot C was mistsprayed at 12:00; Plot D was mist-sprayed at 15:00. Mist-spraying was done continuously from September 3, daily except on September 8. Plant-water-stress in Plot B and D decreased immediately after the mist-spraying on September 10. In Plot D, plant-water-stress also decreased substantially in the forenoon. Diurnal variation of plant-water-absorption was measured on September 11. Plant-water-absorption decreased immediately after mist-spraying in Plot D than in Plot C.
In the second experiment, plants were cultivated in root containers. Seedlings and treatments were the same as those in the first experiment. The effect of the mist-spraying on daily variation in root growth was investigated on September 11. In Plot A and B, root growth was depressed before dawn and during the daytime. In Plot D, root growth was promoted immediately after the mist-spraying. Root growth over twenty-four hours on September 11 increased in the following order: Plot D>C>B>A.
Promotion of root growth by mist-spraying at 15:00 indicates that the soil mass occupied by the plant increases; i.e., the plant's ability to absorb water increases. Thus it is likely that the reduction in plant-water-stress in the forenoon is due to an increase in water absorbing ability.
