Correlation between Long-Day Flowering and Root Elongation in <italic>Pharbitis nil</italic>, Sfrain Kidachi

Abstract

Long-day flowering of Pharbitis nil, dwarf strain Kidachi, at 20°C was greatly influenced by the size of the culture vessel and the number of plants per vessel. The smaller the vessel, the greater the flowering response. The volume of nutrient solution per plant was not decisive for long-day flowering. For instance, plants cultured singly in 200-ml beakers flowered, but those cultured in 5,000-ml vessels (33×26×11.5 cm, 48 plants per vessel) did not, even though there was only about 100 ml of nutrient solution per plant. Long-day flowering was always accompanied by the suppression of root elongation, but not by a decrease in the dry weight of roots or shoots, or in the rate at which the leaf primordia appeared (plastochrone). Aeration of the nutrient solution or culture in vermiculite promoted root elongation even in small vessels, thereby inhibiting long-day flowering. Thus the suppression of root elongation seems to be necessary for long-day flowering. Removal of the roots or cotyledons; however, suppressed long-day flowering even when root elongation was inhibited by culture in small vessels. When plants were cultured at 24°C, suppression of root elongation (culture in a small vessel) did not induce long-day flowering; but, short-day treatment induced flowering without suppressing root elongation.