Carbon Dioxide Exchange Rates, Ribulose Bisphosphate Carboxylase/Oxygenase and Phosphoenolpyruvate Carboxylase Activities, and Kernel Growth Characteristics of Maize

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Abstract
Four high-yield-potential maize hybrids (FS854, CB596 .times. LH38, B73 .times. LH38, and B73 .times. Mo17) and four inbred lines (LH38, CB59G, Mo17, and B73) were grown in the field to study traits associated with leaf area duration (LAD) and the relationship between LAD and kernel growth characters. Based on decline in chlorophyll, leaf N concentration, CO2 exchange rate, and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPCase) activities, the hybrid B73 .times. Mo17 had a significantly shorter LAD than the other three hybrids. The shorter LAD was not due to maturity because B73 .times. Mo17 is in a maturity class similar to the other hybrids except CB59G .times. LH38, which is approximately 1 week earlier. At the time of grain maturity, leaves of B73 .times. Mo17 had lost all chlorophyll and CO2 exchange and carboxylase activities. The other three hybrids, however, retained green leaves which still had 20% of the maximum CO2 exchange rate. In addition, B73 .times. Mo17 remobilized leaf N more extensively. For all hybrids, declines in CO2 exchange were closely correlated with declines in PEPCase activity, whereas the relationship between CO2 exchange and Rubisco activity was weak. Responses of the inbred lines predicted, to some extent, physiological characteristics of the hybrids. CB59G and LH38 both had a longer LAD than either B73 or Mo17 as judged by decline in chlorophyll, leaf N, CO2 exchange rate, and Rubisco and PEPCase activities. With the exception of B73 .times. LH38, kernel growth characteristics of the hybrids were related to LAD. Effective filling period (EFP) measured in days was 32.9, 31.5, 30.8, and 30.4 for FS854, CB59G .times. LH38, B73 .times. LH38, and B73 .times. Mo17, respectively. For FS854 and CB59G .times. LH38, the longer EFP was associated with a larger kernel weight. These data suggested that late season photoassimilate resulting from longer LAD could be utilized by the kernels of these two hybrids. For B73 .times. Mo17, the shorter LAD and EFP was associated with a kernel dry matter accumulation rate (10.1 milligrams per kernel per day) which was significantly higher than for the other three hybrids. Thus, the more rapid leaf senescence of B73 .times. Mo17 appeared to be coordinated with efficient leaf N remobilization and a relatively short grain-filling period characterized by rapid kernel dry matter accumulation.