Relative importance of general combining ability and specific combining ability among tropical maize (Zea mays L.) inbreds under contrasting nitrogen environments

Abstract

Low-N stress is among the major abiotic stresses causing yield reductions in maize grown in the mid-altitude tropical environments of Africa. This study estimates the relative importance of general combining ability (GCA) and specific combining ability (SCA) in CIMMYT's tropical mid-altitude inbred lines under contrasting N environments. Six hundred and thirty five lines (S2-S7) were evaluated in different crossing designs (Diallels, North Carolina Design II and Line x Tester crosses). Results of experiments conducted under low and high N at the same site in adjacent fields with the same soil type within the same year and season from 1999-2003 were compared. The contribution of GCA to total genetic variation was higher than SCA for anthesis date, ear height and plant height under both high and low N levels. However, contribution of GCA was higher for grain yield only under high-N conditions. The average relative contribution of SCA, indicative of non-additive gene effects, to total genetic variation for grain yield under low-N accounted for 51% (average across all trials) but only for 36% under high-N. Pair-wise t-test for diallels and Design IIs showed significant difference (P<0.05) between the proportion of SCA sum of squares for grain yield under high and low N conditions. This implies that breeding strategies that increase grain yield under optimal (high N) conditions will not address the needs of a resource poor farmer producing maize under low N conditions, especially in sub-Saharan Africa. Targeted strategies that increase yield under low N conditions are thus required.