Quantifying rice spikelet sterility in potential heat-vulnerable regions: Field surveys in Laos and southern India

•Field surveys on rice spikelet sterility were conducted in heat vulnerable regions.•Significant increase in sterility at flowering stage was observed in local varieties.•Heat stress during the daytime is the primary cause of spikelet sterility.•Effective protocols for assessing spikelet sterility at flowering stage were developed. Global warming is projected to cause yield losses due to heat-induced spikelet sterility (HISS) in hot, vulnerable rice-growing regions, but reports documenting HISS at a regional level in the tropics are very limited. As a case study, HISS at flowering stage was surveyed under field conditions using local popular varieties in Savannakhet, Laos, and Tamil Nadu, India, during the dry season. At the field site in Savannakhet, sterility in variety Thadokkham1 (TDK1) was 10.8% when the maximum average temperature at heading was 32.9°C, which increased to 23.3% at 37.3°C. When plots were covered with a shading net, solar radiation was reduced by 60% at heading, which significantly reduced sterility to 10.0%. Panicle temperature estimates produced with a micrometeorology model revealed that shading during the hours when flowering occurs decreased panicle temperature by 2.2–2.6°C. At the field sites in Tamil Nadu, sterility of variety Coimbatore 51 (CO51) was highly correlated with the maximum temperature at heading; sterility was around 5–10% when maximum average temperature was below 36.5°C, which increased to 33.4% at 38.0°C. Marking of opened spikelets at hourly intervals allowed us to detect increases in sterility as temperature increased from early morning until noon. We conclude that HISS at flowering stage can occur in local popular varieties grown in Savannakhet and Tamil Nadu during the hot, dry season. Effective protocols such as using a shading net and hourly marking of open spikelets are useful approaches to quantifying HISS during flowering stage across hot and potential heat-vulnerable rice-growing regions.