IMPROVEMENT FOR TOLERANCE TO LOW SOIL-NITROGEN IN TROPICAL MAIZE .1. SELECTION CRITERIA

Inadequate nitrogen supply limits maize production in much of the trop ics because inorganic fertilizer is unavailable or is often costly rel ative to the expected returns. The objective of this study was to eval uate potential selection criteria for improving the tolerance of maize cultivars to low soil N supply. Relationships among primary (grain yi eld) and secondary traits were examined at two N levels among full-sib families forming part of two selection cycles (C0 and C2) of a recurr ent selection scheme in the tropical maize population Across 8328 BN. This population was undergoing improvement for grain yield under low s oil N while maintaining yield potential under fertile conditions. The phenotypic correlations (r(p)) between grain yield at +N and -N, among full-sib progenies, were weak (r(p)) = 0.11 to 0.38, with 224 to 251 df), though the genetic correlation (r(g)) was stronger (r(g) = 0.51). Significant values of r(p) between grain yield under low N and ear-le af chlorophyll concentration, ear-leaf area, plant height, the anthesi s-silking interval and senescence rate were detected under low soil N (- N). These associations were less strong when traits were measured u nder high soil N (+ N). Genetic and phenotypic correlations were gener ally similar in sign and magnitude, except in the case of ear-leaf chl orophyll concentration, which showed no genetic correlation with grain yield - N, even though the value of r(p) ranged from 0.45 to 0.74. Di vergent full-sib selection was performed for grain yield and for corre lated secondary traits in C0 and C2 of Across 8328 BN. When divergent selections from both cycles were evaluated under two N levels, the lar gest grain yields under - N were obtained from direct selection for th at trait. Simultaneous selection for yield and secondary traits in C0 resulted in increased biomass production at both N levels. The realize d heritability (h2) for grain yield at - N was: 0.32 to 0.58; grain yi eld at + N: 0.20 to 0.46; grain yield across N levels: 0.16 to 0.27; e ar-leaf area at - N: 0.18 to 0.74; ear-leaf chlorophyll concentration at - N: 0 to 0.21; and green leaf number below the ear at - N: 0.13 to 0.23. These results indicate that selection for yield and correlated traits under low N should result in improved maize performance in the low-N target environment, with modest increases in yield potential in fertile environments.