Heterosis for vegetative stage cold tolerance in rice

Heterosis for low-temperature tolerance at the vegetative growth stage was studied in ninety F₁ rice hybrids. The F₁s were transplanted in 2.25-m long single row plots, replicated thrice and were subjected to cold water stress at 17℃ and 20℃, in separate blocks, starting from 20 days after transplanting until maturity. Leaf-discoloration was used as a measure of cold tolerance at the vegetative stage. Average mid-parent heterosis of - 40.7% and high-parent heterosis of - 61.9% was recorded in the japonica/indica group of crosses as compared to -18.6% and -26.0%, respectively, in the indica/indica group. Heterosis for cold hardiness was greater for the japonica/indica group of hybrids with 95.8% hybrids showing significant negative mid-parent heterosis and 100% high-parent heterosis (low leaf-discoloration scores mean high cold hardiness). In indica/indica group, only 6.3% had lower scores in japonica/indica group, suggesting some involvement of complimentary gene action in the inheritance of cold hardiness. Barkat/IR 29506, Shimokita/IR 29506 and Suweon 235 /IR 29506 from the japonica/indica group exhibited better cold tolerance than their respective cold tolerant parents. The correlation between mid-parent value and hybrid perfromance was of sufficient magnitude to indicate that prediction of hybrid performance from parental value should prove effective.