Effect of late-season N fertilization on photosynthesis and yield of transplanted and direct-seeded tropical flooded rice. II. A canopy stratification study

Abstract

Grain-yields of broadcast-seeded tropical semidwarf lowland rices ( Oryza sativa L.) frequently fall short of potential yield because of excessive vegetative growth and subsequent N deficiency in the foliage during the reproductive phase. A field experiment was conducted during the 1989 dry season in the Philippines to determine relationships between canopy structure and CO 2 assimilation rates at anthesis with yield and yield components at maturity. Canopies of broadcast-seeded flooded rice had greater leaf area index ( L), dry matter, N uptake, and tiller number than had canopies of transplanted rice. Grain-yields were equal for both planting methods. Early and midseason N fertilization increased dry-matter, L and tiller number, whereas late-season foliar N application increased foliar N concentration and delayed senescence. The N status of leaves affected stomatal conductance, CO 2 assimilation and leaf width. Despite N content being greater in the uppermost than in lower leaf positions, N limited photosynthesis for all leaves. Potential canopy CO 2 assimilation rates ( P n) depended linearly on L, whereas actual P n was little affected by L. High tiller number was detrimental to P n, possibly because of shading by panicles, whereas high N uptake and flag-leaf N concentration promoted P n. The P n at anthesis was linearly correlated with grain-yield, whereas the harvest index ( hi) was high at a combination of high P n, low L, and high flag-leaf N content. Late-season foliar N application favored these canopy characteristics, thus improving grain-yields and hi. It is concluded that late-season foliar N application can improve yields of tropical semidwarf rices, particularly for direct-seeded culture. Future breeding efforts for rices adapted to direct seeding should reduce L and tillering activity while increasing the N concentration of the uppermost leaves during the reproductive growth phase.