Objective  Effects of planting density applied by the mechanical transplantation of pot seedlings on grain formation and yield of hybrid indica rice with different panicle types were studied.

  Method  A field experiment was carried out with 4 planting densities (i.e., D₁ at 33 cm×12 cm, D₂ at 33 cm×14 cm, D₃ at 33 cm×16 cm, and D₄ at 33 cm×18 cm) by mechanical seedling transplantation. Seedlings of two indica rice cultivars with different panicle types, i.e., the large panicle type Chaoyou 1000 and the medium panicle type Huiliangyou 630, were planted. Tiller growth and photosynthetic matter generation as well as the grain yield and yield traits of the rice plants were monitored for analysis.

  Result  (1) The number of stems and tillers at critical growth periods of the rice plants declined as the seedling density decreased. The productive tillers decreased significantly for the large panicle type cultivars but relative stable for the medium panicle type. (2) The leaf area index at key growth stages and the rate of change declined with decreasing planting density resulting in an increased spikelet per unit leaf area. (3) Reduced seedling density raised the grain yield initially followed by a decline. (4) Under D₂ or D₃, both cultivars showed better overall quality and grain yield with larger spikelets per unit leaf area, lower decreasing rate of leaf area, and the highest dry matter accumulation at late growth stage than the other treatments. (5) Varied planting density affected grain yield basically due to the altered number of effective panicles and spikelets per panicle of the rice plants. And (6) Lower seedling density significantly reduced the number of panicles but increased the spikelets per panicle. However, no significant differences on the 1000-grain weight were observed among the treatments.

  Conclusion  In applying mechanical means for pot seedlings transplantation, it would be advisable to properly reduce the density of the hybrid large panicles indica rice seedlings to increase the spikelets per panicle by allowing formation of ample panicles to maximize the grain yield. For the medium panicle type rice, on the other hand, combination of increasing panicles and spikelets per panicle by altering the planting density seemed necessary to achieve the full yield potential.