Correlation and Regressive Model Between Spikelet Fertilized Rate and Temperature in Inter-Subspecific Hybrid Rice

To study the sensitivity of inter-subspecific hybrid rice to climatic conditions, the spikelet fertilized rate (SFR) of four types of rice including indica-japonica hybrid, intermediate hybrid, indica and japonica were analyzed during 2000–2004. The inter-subspecific hybrids showed lower SFR, and much higher fluctuation under various climatic conditions than indica and japonica rice, showing the inter-subspecific hybrids were sensitive to ecological conditions. Among 12 climatic factors, the key factor affecting rice SFR was temperature, with the most significant factor being the average temperature of the seven days around panicle flowering (T₇). A regressive equation of SFR-temperature by T₇, and a comprehensive synthetic model by four important temperature indices were put forward. The optimum temperature for inter-subspecific hybrids was estimated to be 26.1–26.6°C, and lower limit of safe temperature to be 22.5–23.3°C for panicle flowering, showing higher by averagely 0.5°C and 1.7°C, respectively, to be compared with indica and japonica rice. This suggested that inter-subspecific hybrids require proper climatic conditions. During panicle flowering, the suitable daily average temperature was 23.3–29.0°C, with the fittest one at 26.1–26.6°C. For an application example, optimum heading season for inter-subspecific hybrids in key rice growing areas in China was as same as common pure lines, while inferior limit for safe date of heading was about a ten-day period earlier than those of common pure lines.