Factors comprising tolerance to high temperature induce floret sterility in rice (Oryza sativa L.) cultivars in paddy field under common agricultural practices in Cagayan, Philippines

Global warming may increase the occurrence of heat-induced floret sterility in rice. Previous studies have analyzed paths from meteorological and morphological factors to pollination and floret sterility (FS) in well-managed high-temperature experimental paddy fields in China and evaluated the impact of the factors on FS. However, there are no studies on the paths to pollination and FS in paddy fields under real farmers' agricultural practices. Suppressing heat-induced FS under farmers’ management will require the identification of the major factors determining occurrence of FS in those conditions. This study aimed to understand the major paths from environment and morphology to the occurrence of sterility in paddy fields under common agricultural practices and to identify the factors comprising the tolerance to FS. Experiments were conducted in farmers’ paddy field in the hot region in the Philippines in dry summers. Twelve cultivars, including three cultivars identified as having wide variation in tolerance to heat-induced FS were used with eight sowing times in two years. Effects of meteorology and rice morphology on pollination and sterility were examined. The heat-tolerant reference cultivar, Nagina 22, showed stable pollination, but failed to show much lower FS than the other cultivars and the range of FS(%) was narrow (23.4–31.7 %) across the 12 cultivars. Although the path analysis detected the significant paths from meteorological factors and anther traits to FS directly and through pollen germination rate (GR) and floret rate with fewer than 20 pollen grains on the stigma after flowering (FT20TP), coefficient of determination for logit transformed FS was low (R 2 =0.356). The floret rate with fewer than 10 germinated pollen grains on the stigma (FT10GP) well correlated with FS across days but did not correlate across cultivars. Length of dehiscence formed at basal part of the anther (BDL) of Nagina 22 was moderate and the anthers were short. Low correlation between FT10GP and FS across cultivars suggested that number of germinated pollen grains required for fertilization varied among cultivars. Differences in high temperature tolerance across cultivars therefore consisted of two steps: pollination stability and the number of germinated pollen grains required for fertilization. Stable pollination in Nagina 22 suggested the advantage of short anthers for stable pollination. This study emphasized a concept of the number of germinating pollen grains required for fertilization. The question of what factors generate the variation in the number of germinated pollen grains required for fertilization in farmers' fields under their management is a further issue. How the short anthers of N22, which have been considered a high temperature tolerant reference, stabilize pollination is also an important question from this study. • Cultivar regarded as heat tolerant was not good in seed set in the farmer’s field. • Poor pollen germination was a factor of heat induced floret sterility in farmers’ field. • Number of germinated pollen grains needed for seed set was a key for heat tolerance. • Long basal dehiscence of anther contributes to stable pollination. • Short anther may be advantageous for stable pollination under high temperatures.