Rice heterosis for panicle branching, spikelet number, and vascular bundle number [Philippines]

Heterosis for nine quantitative characters was assessed in crosses using seven high density (HD) grain parents (at least 30% of the grains having specific gravity greater than 1.20) and two low density (LD) grain parents. Estimates of overall degree and direction of heterosis were positive and direction significant for number of secondary branches, number of spikelets on secondary branches, and number of inner vascular bundle, indicating dominance of higher values. Heterosis was negative for number of spikelets on tertiary branches and number of outer vascular bundles where genes of lower count dominated. Low or non-significant-heterosis for other characters may be due to little genetic interactions or difference among parents. Values for individual F1 families differed from overall estimate for different characters, showing positive, negative, or no heterosis. The F1 means deviated conspicuously form parental and midparental values, indicating involvement of non-additive gene action in the expression of most characters. Substantial heterosis in the desirable direction was observed in number of primary branches, number of spikelets on primary branches, inner vascular bundles, and outer vascular bundles. Three crosses among HD-grain parents and one cross between HD- and LD grain parents exhibited positive heterosis for number of primary branches. The cross P2/P9 manifested positive heterosis for all other characters. Positive heterosis for total number of spikelets was observed in all crosses except P3/P8, through spikelet numbers on primary, secondary, and tertiary branches. Number of inner vascular bundles exhibited positive heterosis in most crosses, but the reverse was true for number of outer vascular bundles. A panicle with more primary branches accomodate more HD grains, substantial heterosis for that character may lead to higher yield. The three crosses with IR30 (P2) as a common parent in either direction (P5/P2, P1/P2, and P2/P9) had positive heterosis for panicles with more primary branches.