Effect of late-season N fertilization on photosynthesis and yield of transplanted and direct-seeded tropical flooded rice. I. Growth dynamics
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Wet direct-seeding is becoming an increasingly popular alternative to transplanting in Asian rice (Oryza sativa L.) culture. Modern tropical rice cultivars, when direct-seeded, because of vegetative overgrowth and foliar N dilution during reproductive growth, often fall short of their biological yield potential. A field study using 'IR72' rice during the 1989 dry season in the Philippines evaluated the effect of late-season N top-dressing on canopy CO2 exchange rate (P(n)), growth and yield of a transplanted (TP), hillwise dibbled (DS) and broadcast seeded (BS) crop. In BS, the higher seeding rate and absence of transplant shock led to a higher tiller number and leaf area index (L) than in TP. Foliar N concentration decreased faster in BS because of more rapid growth. Late-season N top-dressing, broadcast or applied as foliar spray, increased foliar N concentration and P(n), and led to greater spikelet number and grain-yield. Higher spikelet number was explained by reduced spikelet degeneration after top-dressing. Grain weight limited grain-yield when no N was applied, but was equal among N-fertilized treatments. Panicle number limited yield for TP, whereas grain number per panicle was more limiting for BS and DS. The P(n) during various reproductive growth stages was correlated with grain-yield, whereas dry-matter at maturity depended more on L. Harvest index (HI) gave best correlations with foliar N concentration. The P(n) depended on both L and foliar N concentration, but the latter became more limiting as plant development advanced. It is concluded that late-season foliar N application raised grain-yield by enhancing the assimilate source through increasing foliar N concentration at a stage when it was limiting, and by enhancing the sink through the reduction of spikelet degeneration. Considering that both foliar N concentration and filled spikelet number are commonly limiting factors for BS yields, late-season foliar top-dressing may be a means to specifically improve BS rice culture.
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