Carbon dioxide assimilation efficiency of maize leaves under nitrogen stress at different stages of plant development

Sub-optimal nitrogen (N) affects the N-rich carbon dioxide (CO2) assimilation enzymes which can limit maize (Zea mays) production. The status of the carboxylation system is closely correlated to the Assimilation Efficiency Index (AEI) which is the initial slope of the CO2 assimilation rate versus intercellular leaf CO2 concentrations. Experiments were undertaken to ascertain the effect of soil N nutrition on the AEI, determine genotypic variability for AEI under N-deficiency, determine how leaf and plant development affect treatment differences, and examine correlations between the AEI and plant development. Studies were conducted in the field and greenhouse on five maize genotypes on leaves of different ages at three stages of plant development. Field studies were conducted on a fine, silty mixed, mesic cumulic Hapludoll (1.2 g N kg-1), and high and low N treatments were imposed in the greenhouse. Quantum yield of emerging and mature leaves was determined. Results indicated that emerging and fully mature leaves had the greatest AEI values compared to other expanding leaves. Low N as availability reduced the AEI of younger leaves but increased the AEI on the oldest leaf. The AEI increased until tasseling and then declined. Correlations were established between the AEI and leaf N concentrations and with CO2 assimilation. Grain yield was correlated to the AEI during grain fill. Quantum yield of the mature leaf was greater with low N than with high N availability indicating that the energy capture or transfer mechanism was less affected by N levels than was the CO2 trapping mechanism. There were pronounced genotypic differences in the AEI at tassel emergence but not in leaf N concentrations intimating differences in the distribution of N to enzymes and other compounds important for CO2 assimilation. Internal N distribution was also dependent upon available N. The study demonstrated that the ability of a plant to maintain high carboxylation activity under N stress mav be a valuable selection criteria for obtaining tolerance of corn to low soil N.