Effect of seed presoaking in acetone containing ABA on plumule elongation of rice seedlings.

Abstract

When rice plants of japonica type were grown on the agar medium containing ABA in darkness, the elongation of mesocotyl was strikingly simulated but that of coleoptile and seminal root was inhibited. In this experiment, the rice seeds presoaked in acetone containing ABA were used for ascertaining the direct effect of ABA on the elongation of mesocotyl and coleoptile. A japonica type variety "Koshijiwase" and an indica type variety "Peta" were uscd. After presoaking seeds in acetone containing ABA at 25°C, they were dried for two hours in a vacuum desiccator and stored for two days at room temperature. In all the experiments, each lot consisted of 60-80 plants. The results obtained were as follows: 1. In the plants grown from the presoaked seeds in acetone contatining 10-2M or 2×10-3M of ABA, not only the elongation of mesocotyl but also that of coleoptile was sitmulated, reaching the plateau at the soaking of 24 hours. Thus the soaking for about 48 hours may be enough to examine the effect of ABA (Fig. 1). 2. By the 48 hours soaking in ABA, the stimulation of mesocotyl elongation occurred between the concentration of 10-3M to 2×10-2M. The length of mesocotyl in ABA treated plants was from 3 to 16 times longer than that of control treated with pure acetone, while the coleoptile length was from 1.0 to 1.5 times longer than that of the control. Accordingly, the morphology of plumules was almost similar those from the high temperature pre-treated seed and those of indica type rice. On the other hand, elongation of seminal root was somewhat inhibited even at 10-3M of ABA, and the inhibitory effect increased with increasing ABA concentration (Fig. 2). 3. When the seeds presoaked with acetone containing ABA were again soaked in pure acetone, the stimulatory effect of ABA on the elongation of mesocotyl and coleoptile and inhibitory effect on that of seminal root disappeared. When the seeds of high temperature pre-treated japonica rice and those of nontreated indica rice were soaked in pure acetone, whereas, no effect was detected in the elongation of either mesocotyl or coleoptile (Fig. 3). 4. Parenchymatous cell length and number in a longitudinal section of mesocotyl with about 23 mm long in each experimental lot were measured. The plumules grown from the presoaked seeds in acetone containing 2×10-2M of ABA were almost similar to those from the high temperature pre-treated seeds, 40°C for 10 days. On the other hand, the plumules grown on agar medium containing 2×10-6M ABA were almost similar to those from immature seeds, harvested 15 days after anthesis (Table 1). 5. Considering the similarities of plumules between those originated from seeds presoaked with 2×10-2M ABA and those originated from seeds pre-treated with 40°C for 10 days, about 1.6 μg of ABA was estimated to be synthesized in a seedling originated from the latter, if the assumption that the mesocotyl elongation is based only on ABA is adopted (Fig. 4).