Heterosis for photosynthesis and dry matter accumulation in F₁ hybrid rice (Oryza sativa L.) produced from thermo-sensitive male sterile line under drought stress at heading stage

Heterosis is defined as increased vigour of hybrids in comparison to their parents. In this study, seven cultivars of upland inbred rice (male parents) and their F, hybrids generated by crossing with a thermo-sensitive genie male sterile (TGMS) 103 line (female parent). The cultivars were examined for characters of photosynthesis and dry matter production under drought stress at flowering stage and recovery at ripening stage. The results showed that under drought, all F₁, hybrids exhibited very low negative heterosis for CO₂ exchange rate, stomatal conductance, mesophyll conductance and transpiration rate, but high positive heterosis for intercellular CO₂ concentration. The heterosis value increased much more in the F₁, hybrids under drought recovery than under well-water conditions. During drought stress, heterosis value decreased slightly for dry matter accumulation compared to well-irrigated conditions. Although CO₂ exchange rate significantly decreased, dry matter accumulation was maintained in all F₁, hybrids after drought recovery, which suggests the potential for using upland rice as male parent to produce F, hybrids from TGMS lines for drought tolerance.