Response of yield heterosis to increasing plant density in maize
Genetic yield improvement in maize (Zea mays L.) has been associated with heterosis and increased tolerance to high plant densities, but the physiological processes underlying heterosis for tolerance to plant density stress have not been identified. The objective of this study was to quantify the response of heterosis for grain yield to increasing plant density, and to examine the processes underlying this response. Field experiments were conducted in Ontario, Canada, during the 2005 and 2006 growing seasons in which the hybrid CG60 x MBS1236 and its parental inbred lines were grown at a low plant density (4 plants m–2), a high plant density (12 plants m–2), and a plant density resulting in an approximately equal leaf area index (LAI) for all three genotypes. Increasing plant density from 4 to 12 plants m–2 resulted in an increase in heterosis for grain yield and harvest index (HI), but did not affect heterosis for dry matter at maturity. Heterosis for dry matter accumulation did not differ between the low and the high plant density treatment, increased from the presilking to the postsilking period, and was only to a small extent attributable to a higher maximum LAI and light interception by the hybrid as compared to its parental inbred lines. Increased heterosis for HI was associated with a greater plant-to-plant variability and a higher threshold plant dry matter for HI at maturity in the inbred lines as compared to the hybrid.