Early-morning flowering and heat tolerance QTLs on floret characteristics and spikelet fertility in Oryza sativa L. var. IR64

The effects of QTLs for heat tolerance (qHTSF4.1) and early-morning flowering (qEMF3) during flowering stage of rice (Oryza sativa L.) was validated by subjecting three IR64 near isogenic lines (NILs: IR64-HT, IR64-HT, IR64-EMF, IR64-HT-EMF) to four temperature settings with different times of onset of high temperature (38 deg C). N22 and IR64 served as check varieties. Mean spikelet fertility, flower opening time, agronomic measurements, climate data, pollen count and anther size were analyzed. Field evaluation was performed in Cagayan State University in Piat. Diverse flower opening times were observed among genotypes. Under field conditions, qEMF3 was able to advance BFOT approx 0.5 to approx 1.3 h, while PFOT was advanced by 0.85 h to 1.3 h. Under control conditions, BFOT and PFOT were advanced by approx 3.5 h and approx 3.0 h respectively. Significant differences in mean spikelet fertility were found among genotypes, treatments, and their interactions (P 0.01). In all high temperature treatments, IR64 NILs had higher mean spikelet fertility than IR64, IR64-HT was significantly more fertile than IR64 in 0700H, while IR64-EMF was significantly more fertile than IR64 in 1000H. This suggests that qHTSF4.1 increased spikelet of IR64 even when high temperature began early in the morning, while qEMF3 increased spikelet fertility of IR64 when high temperature was experienced in the later part of the morning. IR64-HT-EMF outperformed these two NILS by having significantly higher mean spikelet fertility than IR64 in the 0700H, 0900H, and 1000H treatments. The qHTSF4.1 enabled heat tolerance during flowering in the first two treatments, while the qEMF3 enabled heat escape mechanism, making the genotype heat tolerant regardless of high temperature onset. Under field conditions, IR62-HT-EMF increased spikelet fertility of IR64 by 21.03%. However this did not translate to grain yield which may be attributed to high temperature effect on grain-filling and production practices. Correlation between floret characters and mean spikelet fertility was not observed.