On the Growth Process of Rice Mesocotyl : I. Effect of abscisic acid on the growth correlation between mesocotyl and shoot

Abstract

Germinated rice seeds (Oryza sativa L. var. Sasanishiki, a Japonica rice) were planted on the agar medium contained various levels of ABA (abscisic acid; 0, 0.1, 0.5, 2.0, 4.0 ppm) and GA₃ (gibberellic acid; 0, 1, 10, 100, 1000 ppm) in the test tubes, being incubated at 30°C in a dark chamber for two weeks. Single application of ABA induced a marked elongation of the mesocotyls, producing more growth synergistically when ABA was applied with GA₃, e.g. the mesocotyl reached about 45 mm length with 2 ppm ABA and 1000 ppm GA₃ mixture. On the contrary, the elongation of the shoot above coleoptilar node was much retarded by ABA and stimulated by GA₃. A similar tendency was found when growth was compared in terms of dry matter production. The dry weight of mesocotyl increased and that of shoot decreased by ABA treatment, with a concomitant decrease in the consumption of endosperm. Under dark condition with pure water, every 200-500 plants were sampled and the length of the mesocotyl and shoot was measured at 4, 5, 6, 7, 9, and 12 days after seed immersion. At the 4th and 5th day after immersion, the plants with longer shoot had the longer mesocotyl, but at the 6th day the almost all shoot reached about 23 mm in length regardless of the variation of mesocotyl length, when the mesocotyls seemed to have reached the final length. After the 7th day, the shoot grew rapidly in most plants the mesocotyl length of which was less than 4 mm, but in some plants with elongated mesocotyls more than 5 mm the following shoot growth was much retarded. At the 12th day, the seedlings were grouped into two; the plants with tall shoot and short mesocotyl, and the plants with short shoot and long mesocotyl. The percentage of the former was about 90% and the latter about 10%, respectively. The growth pattern of the latter plants was very similar to that of the plants treated with ABA. From these results, it may be suggested that endogenous ABA plays a role in the elongation of rice mesocotyl.