Effects of drought tolerant quantitative trait loci on flowering traits, panicle exsertion rate, spikelet sterility and grain yield of rice under rainfed lowland conditions

Drought hinders rice production and yield stability in rainfed lowland ecosystems. Quantitative trait loci (QTL) for root traits on chromosomes 1, 2, 7 and 9 in backcross introgression lines (BIL) of elite rice variety IR64, derived from Azucena/IR64, were designated as Targets 1, 2, 7 and 9 in the present study. Severe stress developed during the period of optimum flowering as indicated by 98-100% yield reduction. Significant phenotypic variations in BIL were observed on days to flowering (DF), delay in flowering (DeF), panicle exsertion rate (PEX), percent spikelet sterility (PSS) and grain yield (GY). Variations among target QTL were also found in the traits studied, particularly under drought stress, indicating that BIL containing Targets 1 and 2 had early DF, higher PEX, lower PSS and higher GY under stress than those with Targets 7 and 9. Targets 7 and 9 lacked QTL for root thickness, indicating that it plays a role in drought tolerance under rainfed lowland conditions. Early flowering BIL had an advantage over later flowering ones because water can be extracted at the early drought stage, which can support the PEX and result in low PSS. In a very severe drought at the flowering stage, PSS directly affected GY. Further study of root morphology and early flowering under stress are necessary to improve rice yields under rainfed lowland conditions.