Apical development, varietal differences and their relation to input timing in lowland rice
Apical development of rice was studied in glasshouse experiments at the International Rice Research Institute, Los Banos, Philippines. Thirteen morphologically different panicle development stages (from vegetative growth up to the complete development of floral organs) were identified in IR50 and IR42. The duration of development stages varied between varieties. The total period from necknode differentiation (physiological panicle initiation) to flowering varied from 20-54 days in contrast to earlier findings that duration from VPI to flowering was 23-25 days preceded by 7-10 days from physiological PI to VPI. The standard practice of final top dressing at 5-7 days before VPI did not coincide with physiological PI in all varieties. Visual panicle initiation coincided with spikelet differentiation in all varieties, which suggests that N applications at this stage might be too late to affect the number of primary and secondary branches, the major yield components. N timing studies at different growth stages indicated a response to extra N application at GS 1.9 and at GS 1.13, 3.7. When final N top dressing was at GS 1.90 within a recommended total N level of 120 kg N/ha, an application split 3 ways between GS 1.7, GS 1.9 and GS 1.13 gave a yield advantage over standard N application through an increased percentage of high density grain. In all experiments leaf dry weight was highest at maturity, except for the long duration variety FB12. Kinetin application at GS 5.5 increased the spikelet survival at maturity. Results suggest that early panicle senescence is one factor limiting the increase of grain yield under the fertiliser management system implemented in these experiments.