Identification of the effects of low temperature on grain-setting rate of different types of late-season rice (Oryza sativa) during heading

The development of double-season rice cropping is advantageous for maximizing the utilization of radiation and temperature resources in rice production, while ensuring food security. However, late sowing often renders the late-season rice susceptible to cold stress during heading and flowering, leading to a low grain-setting rate and significant yield loss in subtropical zones. Despite this, the precise mechanisms through which low temperatures impact grain setting, along with the sensitivities of heading, flowering and pollination to cold stress, remain unclear across different types of rice. In this study, a field experiment involving multiple sowing dates and a pot experiment simulating low-temperature conditions during the booting and heading stages were conducted, using various late-season rice cultivars as materials. The heading degree, glume openness, anther dehiscence and pollen fertility were measured to determine their responses to ambient temperature. The results showed that late sowing significantly delayed the heading time of rice, exposing the plants to low temperatures during their heading and flowering in autumn. Under late sowing or simulated low-temperature conditions, the heading degree, anther dehiscence coefficient, and fertile pollen rate significantly decreased, resulting in lower grain-setting rates and grain yield across all rice types. However, glume opening remained unaffected in this study. The critical lowest daily temperatures for safe heading were identified as 18.5°C, 19.3°C and 22°C, and for safe anther dehiscence at heading, they were 19.9°C, 20°C and 22.3°C for japonica, indica-japonica hybrid and indica rice, respectively. Furthermore, the critical lowest daily temperatures for safe pollen fertility averaged 19.1°C, 20.2°C and 21.7°C in the 4–7 days prior to full heading for japonica, indica-japonica hybrid and indica rice, respectively. The cold tolerance in anther dehiscence of indica-japonica hybrid rice was similar to that of japonica rice but higher than that of indica rice, while the cold tolerance in heading and pollen fertility of indica-japonica hybrid rice was intermediate between japonica and indica rice. Path analysis revealed that low temperature decreased the grain-setting rate primarily by reducing anther dehiscence coefficient across all types of rice. Lower pollen fertility was another significant pathway through which low temperature decreased the grain-setting rate in indica rice. Planting japonica rice is recommended for the safe production of late-season rice in subtropical zones.