Physiological basis of heterosis for grain yield in maize

Although heterosis in maize (Zea mays L.) has been studied since the early 1900s, very little is known about how heterosis affects the physiological components of grain yield. The objective of this study was to quantify the physiological basis of heterosis for grain yield in maize by examining maize hybrids and their parental inbred lines in terms of grain yield and its component processes, dry matter accumulation (DMA) at maturity, and the partitioning of DMA to the grain (i.e., harvest index), as well as in terms of the physiological processes underlying those two components. The genetic material consisted of 12 maize hybrids and seven parental inbred lines. Experiments were conducted from 2000 to 2002 at the Elora Research Station, ON, Canada. Data were recorded on grain yield, DMA at four stages of development, harvest index, leaf area index (LAI), final leaf number, leaf width and length, rate of leaf appearance, stay green, ear number, kernel number and weight, and number of days to silking and physiological maturity. Mean heterosis across the 3 yr was 167% for grain yield and 85 and 53% for its two component processes, DMA at maturity and harvest index, respectively. Results show that heterosis for grain yield in maize can be attributed to (i) heterosis for DMA before silking, which results mainly from greater light interception due to increased leaf size; (ii) heterosis for DMA during the grain-filling period, which results from greater light interception due to greater maximum LAI and increased stay green, and (iii) heterosis for harvest index.