Résumé
•Large diversity exists in indica rices for chilling induced spikelet sterility.•Phenotypic clusters were related to genetic subgroups.•Thermal sensitivity was different for altitude vs. season effects.•A genetic indica group from Madagascar had outstanding cold tolerance.
Rice spikelet sterility caused by chilling during microspore stage of panicle development is a major cause of the rareness of the indica sub-species in cool environments. A diversity panel of 200 indica accessions including traditional and improved accessions forming 4 genetic sub-groups (I1 to I4), along with 22 accessions representing other genetic groups of Oryza sativa L., was field phenotyped for spikelet sterility under flooded management. Environments were six seasonal climatic situations (sowing dates) in Senegal, ranging from cool to hot; and at two altitudes (857 and 1497m asl) and two years in Madagascar. The apex temperature during the critical phenological stage was estimated from weather records and an empirical model estimating the daily minimum water temperature. Sterility incidence was regressed for each accession against apex temperature across all environments after discarding observations potentially involving heat induced sterility. From the correlations, the slope of the temperature response and sterility at 20°C (no stress) and 15°C (chilling stress) were derived and used to evaluate the accessions and establish clusters. Indicas were on average more sensitive than temperate and tropical japonicas. Among indicas, large diversity in cold sterility was observed that formed 6 clusters, which represented the different genetic sub-groups at a distinct frequency. Sterility was greater in Senegal than in Madagascar for all clusters. Sub-group I3 indigenous to Madagascar (traditional varieties) was nearly exclusively present in the most cold tolerant cluster C1. Some improved indicas were also in C1 but belonged to sub-group I2. The phenomics resource will be used in a genome wide association (GWA) study to identify putative candidate genes.