Hybrid vigor for leaf CO2‚ exchange rate and dry matter accumulation throughout the life cycle in maize
Heterosis has been studied and utilized in corn (' Zea mays' L.) production, but the physiological basis of hybrid vigor remains unclear. It is recognized that grain yield is closely associated with dry matter (DM) accumulation that is determined by canopy light interception and leaf photosynthesis throughout the life cycle. Hybrid vigor for grain yield has been associated with the genetic distance of the parental lines. Hence, the first objective of this study was to quantify hybrid vigor for yield component processes throughout the life cycle of maize. The second objective of this study was to quantify leaf CO2 exchange rate (CER) throughout the life cycle of maize hybrids and their parental inbred lines. It was postulated that hybrid vigor confers stress tolerance. Thus, the third objective of this study was to examine the effect of stress on leaf CER. Sister line (SL) hybrids, unrelated cross (UC) hybrids and their inbred parents were grown in a modified hydroponic system in the field in the summer season of 2002 and 2003. Hybrid vigor for total DM, shoot DM, root DM, grain yield and harvest index were all significantly greater for UC hybrids than SL hybrids at maturity. Mean leaf CER of hybrids was not different from that of their parental inbred lines at silking. The reduction in leaf CER after either a cold stress or drought stress was similar in hybrids and their parental inbred lines.