IMPROVEMENT FOR TOLERANCE TO LOW SOIL-NITROGEN IN TROPICAL MAIZE .2. GRAIN-YIELD, BIOMASS PRODUCTION, AND N ACCUMULATION

A program of full-sib recurrent selection to improve maize grain yield under conditions of low soil N, while maintaining grain yields under high soil N, was conducted for three cycles in the lowland tropical population, Across 8328. Superior families were identified from an index of traits comprising high grain yields under high (200 kg N ha-1) and low N (zero applied), and, under low N, high chlorophyll concentration per unit ear-leaf area, slow leaf senescence, and increased plant height. An attempt was made to keep plant height and time to flower under high N unchanged. The objective of this study was to evaluate changes which resulted from this selection program. Cycles 0, 1, 2, and 3 were evaluated under two N levels (0 and 200 kg N ha-1) in four seasons. The per cycle linear increase in grain yield under low N was 2.8% (0.075 Mg ha-1) (P < 0.10), and under high N was 2.3% (0.137 Mg ha-1) (P < 0.01), indicating that improved performance at low N is not incompatible with yield gains under high N. Increased grain yields were associated with significant linear increases per cycle, measured across N levels, in kernels ear-1 (4.6 kernels ear-1; 1.6%), plant height (8 cm; 4.4%), days to anthesis (0.3 d; 0.4%), aboveground biomass at silking (0.182 Mg ha-1; 3.5%) and at maturity (0.205 Mg ha-1; 1.9%), and N loss from vegetative parts during grain-filling (0.14 g m-2; 4.8%). Across N levels, there was a significant decrease in the number of florets formed per ear (- 8 florets ear-1; - 1.5%), but an increase in the proportion of florets that formed kernels (0.016 cycle-1; 3.1%). Leaf senescence rate decreased with selection. Selection cycles differed significantly in total N uptake and patterns of N and biomass accumulation with time, but no consistent trends were observed for those traits. Selection for performance under low N in elite maize germplasm appears to improve the efficiency with which N is utilized to produce biomass and grain.