Eight cycles of selection for drought tolerance in lowland tropical maize. I. Responses in grain yield, biomass, and radiation utilization
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Drought is a major source of grain yield instability in maize (Zea mays L.) grown in the lowland tropics, and use of cultivars with improved drought tolerance may be the only affordable option for many small-scale farmers. Eight cycles of full-sib recurrent selection were carried out in the population 'Tuxpeno Sequia' during the rain-free winter season at Tlaltizapan, Mexico, under controlled moisture stress timed to coincide either with flowering or grain-filling. Selection was based on an index comprising gram yield and physiological and morphological traits with presumed adaptive value under drought. The objectives of this study were to evaluate direct and correlated responses to selection in grain yield and its components, total biomass, and radiation-use efficiency (RUE). Cycles 0, 2, 4, 6 and 8 of Tuxpeno Sequia, and a check cultivar representing the results of six cycles of selection based primarily on multilocation testing, were evaluated under three moisture regimes at Tlaltizapan during two consecutive winter seasons. Grain yield (GY) increased at 108 kg ha-1 cycle-1 across 12 yield environments ranging in yield potential from 1 to 8 Mg ha-1, with no significant interaction between pins and moisture environments. Yield gains resulted from an increase of 0.03 ears per plant (EPP) cycle-1 under drought, and small but significant increases in EPP, kernel number per ear and kernel weight in well-watered environments. Selection had no effect on biomass production, so yield increases were due to a gain in harvest index (HI) of 0.0058 to 0.0067 cycle-1 in either wet or dry environments. Seasonal radiation-use efficiency averaged 1.48 g MJ-1 PAR under well-watered conditions, a value lower than expected for maize. Although selection slightly reduced radiation interception and slightly increased RUE during the pre-anthesis phase, both changes were relatively unimportant. The regression of GY on biomass (B) (GY= -1.85+0.47B; R2 = 0.94**) predicted zero GY at biomass yields of less than 4 Mg ha-1. The check entry showed only limited progress in drought tolerance, EPP, HI, and GY. These results suggest that drought stress, when managed to coincide with flowering, can be an effective selection environment for increasing HI, yield stability, and GY of lowland tropical maize across a wide range of moisture environments.
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