Average maize yields have increased steadily over the years in the USA and yet the variations in harvestable yield have also markedly increased. Much of the increase in yield variability can be attributed to (1) varying environmental stress conditions; (2) improved nitrogen inputs and better weed control; and (3) continuing sensitivity of different maize lines to the variation in input supply, especially rainfall. Drought stress alone can account for a significant percentage of average yield losses. Yet despite variable environments, new commercially available maize hybrids continue to be produced each year with ever-increasing harvestable yield. Since many factors contribute to high plant performance under water deficits, efforts are being made to elucidate the nature of water-stress tolerance in an attempt to improve maize hybrids further. Such factors include better partitioning of biomass to the developing ear resulting in faster spikelet growth and improved reproductive success. An emphasis on faster spikelet growth rate may result in a reduction in the number of spikelets formed on the ear that facilitates overall seed set by reducing water and carbon constraints per spikelet. To understand the molecular mechanisms for drought tolerance in improved maize lines better, a variety of genomic tools are being used. Newer molecular markers and comprehensive gene expression profiling methods provide opportunities to direct the continued breeding of genotypes that provide stable grain yield under widely varied environmental conditions.

Average maize yields have increased steadily over the years in the USA and yet the variations in harvestable yield have also markedly increased. Much of the increase in yield variability can be attributed to (1) varying environmental stress conditions; (2) improved nitrogen inputs and better weed control; and (3) continuing sensitivity of different maize lines to the variation in input supply, especially rainfall. Drought stress alone can account for a significant percentage of average yield losses. Yet despite variable environments, new commercially available maize hybrids continue to be produced each year with ever-increasing harvestable yield. Since many factors contribute to high plant performance under water deficits, efforts are being made to elucidate the nature of water-stress tolerance in an attempt to improve maize hybrids further. Such factors include better partitioning of biomass to the developing ear resulting in faster spikelet growth and improved reproductive success. An emphasis on faster spikelet growth rate may result in a reduction in the number of spikelets formed on the ear that facilitates overall seed set by reducing water and carbon constraints per spikelet. To understand the molecular mechanisms for drought tolerance in improved maize lines better, a variety of genomic tools are being used. Newer molecular markers and comprehensive gene expression profiling methods provide opportunities to direct the continued breeding of genotypes that provide stable grain yield under widely varied environmental conditions.

No abstract

A diallel study of spring hexaploid triticale (× Triticosecale Wittm.) lines was conducted to (i) determine the combining ability effects and heterotic patterns among eight complete spring hexaploid triticales, (ii) study the effect of stress on the stability of combining ability effects and heterotic patterns in two well‐defined moisture regimes, and (iii) determine the relative importance of additive vs. nonadditive genetic effects. Eight parents and 28 crosses were tested at two sites (full irrigation vs. under drought stress) in Mexico. Drought effects reduced yield from 8.3 to 5.7 Mg ha−1, test weight from 746 to 722 kg m−3, and height from 127.5 to 102.1 cm. Entry × site interaction was nonsignificant for yield, suggesting stability of combining ability and heterotic patterns. Entry × site interactions for test weight and height were due to one or a few parental lines. Tests for average heterosis (performance of parents vs. crosses) were highly significant for all traits in both environments. Yield showed mean percentage heterosis over high parent of 7.2% (19.7% maximum) under irrigation and 25.2% (68.2% maximum) under drought. Heterotic patterns for test weight and height were consistent across environments, but some combinations were positive and others negative relative to the high parent. Highly significant general combining ability (GCA) effects were found for all traits under both moisture regimes. Highly significant specific combining ability (SCA) effects were found only for yield, which was due to a single combination. The importance of GCA vs. SCA was clear for all traits in both environments, but GCA/ SCA sums of squares was lowest for yield, indicating more nonadditive effects for yield than for test or height among these Fl'S. This was further reflected by a large percentage high parent heterosis for yield. It seems that triticale has potential as a hybrid crop, particularly under droughtstressed growing conditions.