Abstract: 【Objective】The responses of the rice grain qualities to the climate factors during the grain filling period were studied to facility the variety screening and the agronomic practices optimization for japonica rice during the late rice season in southern rice region. 【Method】 To evaluate the effects of different climate factors during grain filling period on the qualities of late japonica rice, a field experiment was conducted in Fuyang and Wenzhou, Zhejiang province, in 2018, using three the late indica rice as controls and 20 japonica rice (including 10 inbred japonica rice, 3 japonica hybrids and 7 indica-japonica hybrids) as evaluating varieties. 【Result】(1) Based on the results of clustering with grain qualities, the late indica rice varieties were classified as an unique category due to its highest aspect ratio (3.18) and high amylose content (19.40%). In compared to the late indica rice, the most of  hybrid varieties had greater brown rice rate (4.31%-5.28%,), milled rice rate (6.51%-9.33%), head rice rate (25.83%-28.34%), gel consistency (1.81%-4.27%,), alkali spreading value (11.62%-50.85%), and tasted value (2.31%-2.85%), while lower amylose content (20.98%-28.14%) and protein content (1.16%-14.85%), showing obvious improvement of rice quality. Whereas the rice quality performance response to late season were differentiated within inbred japonica varieties, and some inbred japonica rice originating from southern Jiangsu and Jiaxing (4 varieties) were similar to the those in the hybrid-japonica category; while the rest inbred japonica varieties from Jiangsu and Shanghai (6 varieties) was belonged to another category due to their relatively poor rice quality performance (high chalkiness, chalky grain rate and protein content) in the late season. (2) The rice grain qualities were closely related to climate factors during grain filling period of late rice season. The stage of 10-20 days after full heading was identified as the sensitive period of climate factors on rice qualities. The brown rice rate was was negatively correlated with daily solar radiation (R: -0.40--0.19, p<0.05), daily temperature difference (R: -0.45- -0.28, p<0.05), and positively correlated with daily minimum temperature (R: 0.24-0.53, p<0.05) and precipitation (R: 0.38-0.45, p<0.05). The chalkiness rate and chalkiness grain rate were significantly increased with the rainfall and night temperature (R: -0.37--0.16, p<0.05; R: -0.43--0.12, p<0.05), resulting in reducing rice appearance quality. Meanwhile, the rainfall and night temperature were negatively correlated with rice tasted value(R: -0.37--0.16, p<0.05; R: -0.43--0.12, p<0.05). The daily maximum temperature at grain filling stage was positively correlated with rice tasted value (R=0.37, p<0.05), while the daily minimum temperature was positively correlated with rice protein content (R=0.19, p<0.05), chalkiness (R=0.16, p<0.05) and chalkiness grain percentage (R=0.12, p<0.05). 【Conclusion】(1) In compared to the late indica rice, applying the japonica hybrids and some inbred japonica varieties in the late season could improve the milling and cooking qualities while maintained the grain taste value, but deteriorated the appearance qualities. Thereby, the improvement of rice qualities for late japonica varieties in southern rice region should focus on the integration of high-quality japonica genes with an indica gene of local adaptation, and the choice of hybrid rice (including indica-japonica hybrids and japonica hybrids) is more reliable and convenient than inbred japonica rice. The breeding of inbred japonica varieties should consider their ecological adaptability in combined with local climatic conditions. (2) The stage of 10-20 days after full heading was the key period for the effect of climate factors on rice grain quality. The great daily solar radiation and large day and night temperature differences led to a decrease in rice milling quality, but resulted in an improvement in rice appearance quality, cooking quality, and eating quality. In contrast, the increase in precipitation and the night temperature led to a decrease in eating quality. These results provided a theoretical basis for the selection of japonica rice for late season and the optimizations of agronomic practices in southern rice region.

Key words: double-cropping late rice, rice type, grain filling, rice quality, meteorological factors