RESPONSE TO POPULATON DENSITY OF TROPICAL AND SUBTROPICAL MAIZE CROSSES ADAPTED TO HIGHLANDS

Abstract

In the High Valleys of México, local maize (Zea mays L.) varieties do not tolerate high population densities because of lodging problems. In trying to solve this problem, tropical and subtropical germplasm has been introduced. In this study we evaluated crosses among S3 maize lines derived from tropical and subtropical populations (LT x LT) adapted to highlands, as well as crosses between LT lines and a local line (LT x LL) derived from the race Cónico-Chalqueño. The populations were originated from crosses among Mexican races, tropical and subtropical, selected under high-density population at Montecillo, State of México. Fraternal crossings were made from generation F₂ to F₁₃, and thereafter they were selfed for three generations. From S₃ lines derived of these populations, 11 LT x LT crosses and six LT x LL crosses were made. The 17 F₁ crosses (LT x LT and LT x LL), two single crosses (‘CS’ and ‘H-CM’) and two three-way crosses (‘H-151’ and ‘H-40’) included as checks, were evaluated in an experiment at Montecillo in 1999, with plant densities of 60 (D₁), 70 (D₂) and 80 thousand ha^-1 (D₃). As plant density increased from D₁ to D₃ the number of days to flowering increased by 1.2 d, while ear length, 200 grains weight and ear number per plant decreased by 0.7 cm, 3.4 g and 0.2, respectively; however, there were not significant differences among densities for grain yield. The average yields for the control group, LT x LL and LT x LT were 9.4, 8.1 and 7.2 t ha^-1, respectively. The best LT x LT crosses had the same yield as ‘H-40’ (7.6 t ha^-1) and the best LT x LL cross yielded 9.3 t ha^-1. The LT x LT crosses had grain yield, earliness and plant height similar to the checks ‘H-40’ and ‘H-CM’. Therefore the selection for adaptation in highlands in tropical and subtropical germplasm was effective in obtaining crosses similar to the checks in yield, at commercial or higher population densities.