The responses of the rice grain qualities to the climate factors during the grain filling period were studied to facilitate the variety screening and the agronomic practices optimization for japonica rice during the late rice season in southern rice region in China.

To evaluate the effects of different climate factors during the grain filling period on the qualities of late japonica rice, the field experiment was conducted in Fuyang and Wenzhou, Zhejiang province, in 2018, using three late indica rice as control varieties and 20 japonica rice (including 10 inbred japonica rice, 3 japonica hybrids and 7 indica-japonica hybrids) as evaluating varieties.

(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%). Compared with 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%), with 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 varieties 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) belonged to another category due to their relatively poor rice quality performances (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 negatively correlated with daily solar radiation (R: -0.40−-0.19, P<0.05) and 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).

The stage of 10-20 days after full heading was the key period for affecting rice grain quality by climate factor, and 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) was 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.