Defoliation and Plant Density Effects on Maize Selected for Reduced Plant Height
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Selection for reduced plant height in maize (Zea mays L.) his been accompanied by a reduction in stem volume and in stem nonstructural carbohydrate (SNSC) reserves, which may result in increased instability of grain yield when plants are exposed to stress during grain filling. This possibility was examined when cycles 0, 6, 12, and 18 of a tropical maize population selected recurrently for reduced height were grown in four environments at 3.5 and 6.7 plants m-2 and defoliated by removing all but five, all but three, or no leaves 7 d after 50% silking. Per-cycle changes were observed in undefoliated plots for stem biomass (-0.116 Mg ha-1), grain yield (0.052 Mg ha-1), optimum plant density for grain yield (0.14 plants m-2), and harvest index (0.009). Lodging declined from 39 to 10% with 18 cycles (C18) of selection. The reduction in SNSC in C18, compared with C0, was 0.66 Mg ha-1. Defoliation increased the per-cycle reduction in final stem biomass to - 0.146 Mg ha-1, but reduced grain yield and harvest index equally in all cycles of selection. At 7 d after silking, SNSC was correlated with final plant height (r = - 0.66**, 16 df) and with final kernel number per plant (r = 0.48*, 16 df). Reduction in plant height increased stability of grain yield by reducing lodging and by increasing tolerance to stresses occurring near flowering.
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