GENOTYPE BY ENVIRONMENT EFFECTS AND SELECTION FOR DROUGHT TOLERANCE IN TROPICAL MAIZE .1. 2 MODE-PATTERN ANALYSIS OF YIELD

Ten trials evaluated the performance of several late tropical maize po pulations (La Posta Sequia, Pool 26 Sequia and Tuxpeno Sequia) selecte d for tolerance to drought during flowering and grain filling and also for yield potential. Families (S-1 or full-sib) had been selected rec urrently for six to eight years on an index of traits. Pattern (cluste ring and ordination) analysis was used to analyse the relative perform ance of entries that included cycles of selection for drought toleranc e in the populations and non-drought tolerant checks. Mean environment (E) yields ranged from 1.0 to 10.4 t ha(-1). Analysis of variance sho wed that 97.9% of the total sums of squares was accounted for by E, an d that, of the remaining sums of squares the G x E (genotype by enviro nment interaction) was almost 3 times that of the contribution of G al one. Cluster analysis separated the checks, the earlier maturing droug ht tolerant entries and the later maturing drought tolerant entries. T his was verified by principal component (PC) analysis of the G x E mat rix. Grouping of the environments (i.e. based on entry performance), r esulted in the separation of different types of droughts, and of mediu m and high yielding well-watered environments. The patterns of discrim ination observed indicated that the yield gains under drought would ha ve been unlikely to occur if selection had been done only in well-wate red environments. Within each population, selection improved broad ada ptation (higher mean yield) to both drought and well-watered environme nts and cycles of selection 'jumped' from non-drought-tolerant to drou ght-tolerant groups as their specific adaptation to drought environmen ts increased.