The Effects Of Nursery Managements And Plant Growth Regulators On Seedling Flexibility Of Late Season Rice And Their Mechanisms

In the double-cropping late season rice regions of middle and lower reaches of Yangtze, the sowing time of late season rice is in between early June and late June. The maturity time will be longer with delayed matured early rice or the reaping time of early rice will be postponed because of unfavorable weather conditions for harvest, which will lead to yield decreases due to longer seedling age. In order to identify the difference of seedling flexibility among different varieties and explore the effects of nursery managements and plant growth regulators (PGRs) on seedling flexibility and their mechanisms, the research was conducted at Dajin town of Wuxue city in Hubei province during 2007 and 2008 under dry bed nursery and wet bed nursery combined by different plant growth regulators treatments with round-grained rice and long-shaped rice. The results are as follow:1. Our study has demonstrated the variation of seedling flexibility among different varieties. Higher seedling flexibility was found for EJZ3 than the other two varieties (LYH6 and TY207). In wet bed nursery, the maximum time limit of seedling age for LYH6 was 35 days, while the gap was even longer for EJZ3 reaching almost 45 days. This increase in time flexibility for EJZ3 can be attributed to the relatively lower decrease of panicles m^-2, 1000-grain weight, spikelets per panicle and aboveground total biomass when seedlings with longer seedling age were transplanted to field.2. In case of nursery management, the seedling flexibility of varieties in wet bed nursery was higher than that in dry bed nursery. Under the high temperature of summer, the wet bed nursery was favorable for vigorous growth of seedlings mainly because of better performance of seedlings in terms of plant height, cauloid width, tillers per seedling, dry weight per seedling. Grain yield decreased with the increase in the seedling transplantation age irrespective of the nursery management method. Only 5%, -0.03%, 13.3% and 7.9% reductions in aboveground total biomass, spikelets m^-2, spikelets per panicle and grain filling percentage were observed when the seedling transplantation age was increased from 30 days after sowing (DAS) to 45 DAS in wet bed, against 21.1 %, 18.0%, 12.9% and 1.6% in dry bed, respectively.3. The effect of plant growth regulators on seedling flexibility also showed some relationship with seedling flexibility of varieties and nursery managements. In wet bed nursery, plant growth regulators have non significant effect on seedling flexibility of varieties with higher seedling flexibility. Spraying 100 mg/L PP₃₃₃ at 2-leaf stage was comparatively effective in improving the seedling flexibility. Nevertheless, for varieties with lower seedling flexibility, such as LYH6, plant growth regulators treatments showed some positive effects on improving the seedling flexibility. Spraying 200 mg/L of PP₃₃₃ at 2-leaf stage followed by an additional spray of 2.5 mg/L 6-BA at 3-leaf stage can be regarded as the best treatment combination due to less yield gap between older seedlings and younger seedlings.In dry bed nursery, treatments with appropriate concentration of PP₃₃₃ (200 mg/L PP₃₃₃) at 2-leaf stage and a supplementary spray of 6-BA with concentration of 2.5 mg/L were demonstrated to improve the seedling flexibility. Besides, seed soaking before sowing in 250 mg/L of PP₃₃₃ had showed positive effect on improving the seedling flexibility.4. The effectiveness of plant growth regulators in improving the seedling flexibility can be attributed to the increase in aboveground total biomass, specific leaf weight, leaf area index, breaking resistance of the lower node of N3 and N4 and dry weight per unit length of internodes and decrease in the lodging index of the lower node of N3 and N4 compared to the treatments without PGRs.