Temperature Response of Caryopsis Elongation in Relation to Grain Notching of Rice

[摘要] Many researchers have reported that unsuitable ripening conditions such as low temperature, drought, shading, or fertilizer deficiency lead to grain notching of rice. In this study the mechanism of grain notching has been re-investigated from a view point of temperature effect on the caryopsis elongation. As the caryopsis of rice plant develops encased in floral glumes, the size and shape of the caryopsis are restricted by the floral glumes. However, it has been clarified that the caryopsis is capable of elongating longer than the floral glumes in general, and that the varieties in which the potential volume of the caryopsis far exceeds the capacity of floral glumes tend to produce such mulformed grains as notched or hull-cracked ones. In this study, for allowing full growth of the potential length of caryopsis (T1), upper parts of the floral glumes on two or three panicles of each plant were clipped off three days after anthesis, and these panicles were protected by parchment bags. The rest of panicles were kept untreated to determine the natural length of caryopsis (C1) which developed within the floral glumes. Seven rice cultivars (Table 1) were grown in pots and kept outdoors until heading time. Thereafter, they were divided into three lots and moved to phytotrons in which the temperature was maintained high (33/23°C), medium (28/18°C) or low (23/13°C), and grown under 30, 000 lux artificial light for 14 hours a day. Humidity was regulated to 70-80% and 85-95% during the light and dark period, respectively. As shown in Table 2, actual temperature inside the floral glumes and parchment bag was a little higher than ambient air temperature under illumination. Twenty-30 caryopses were sampled every, other day and measured to give growth curves for T1 and C1. Several growth parameters were estimated according to Robertson's equation. At maturity, rest of materials were harvested to examine seed fertility, notched grain percentage, and the size and shape of the caryopses. The result of present investigation has indicated followings : Although the growth late of C1 is lowered under low temperature, the maximum value of C1 is not different with temperature conditions, because C1 is restricted by the scope of floral glumes. As shown in Fig. 2, C1 reaches to its ceiling at 7 days (at high temp.), 9 days (at medium temp.) and 11 days (at low temp.) after anthesis. Under low temperature condition the growth rate of T1 is reduced, but because the growth duration is extended, the final length of T1 is the longest. Thus, the unbalance between the potential length of caryopsis and the capacity of floral glumes, or T1/C1 is higher under low temperature condition. Notched grain percentage increases under low temperature condition, and as shown in Fig. 3, it correlates with T1/C1 ratio. From these observations it may be concluded that under low temperature condition T1 shows excessive elongation, which leads to stronger unbalance between T1 and C1. When T1 is much longer than C1, the longitudinal development of caryopsis is restricted by floral glumes resulting in grain notching.

[学科分类] 农业科学（综合）