CARBON-DIOXIDE ASSIMILATION EFFICIENCY OF MAIZE LEAVES UNDER NITROGENSTRESS AT DIFFERENT STAGES OF PLANT DEVELOPMENT

Sub-optimal nitrogen (N) affects the N-rich carbon dioxide (CO2) assim ilation enzymes which can limit maize (Zea mays) production. The statu s of the carboxylation system is closely correlated to the Assimilatio n Efficiency Index (AEI) which is the initial slope of the CO2 assimil ation rate versus intercellular leaf CO(2 )concentrations. Experiments were undertaken to ascertain the effect of soil N nutrition on the AE I, determine genotypic variability for AEI under N-deficiency, determi ne how leaf and plant development affect treatment differences, and ex amine correlations between the AEI and plant development. Studies were conducted in the field and greenhouse on five maize genotypes on leav es of different ages at three stages of plant development. Field studi es 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 greenh ouse. Quantum yield of emerging and mature leaves was determined. Resu lts indicated that emerging and fully mature leaves had the greatest A EI values compared to other expanding leaves. Low N availability reduc ed the AEI of younger leaves but increased the AEI on the oldest leaf. The AEI increased until tasseling and then declined. Correlations wer e established between the AEI and leaf N concentrations and with CO 2 assimilation. Grain yield was correlated to the AEI during grain fill. Quantum yield of the mature leaf was greater with low N than with hig h N availability indicating that the energy capture or transfer mechan ism was less affected by N levels than was the CO( 2 )trapping mechani sm. There were pronounced genotypic differences in the AEI at tassel e mergence but not in leaf N concentrations intimating differences in th e distribution of N to enzymes and other compounds important for CO 2 assimilation. Internal N distribution was also dependent upon availabl e N. The study demonstrated that the ability of a plant to maintain hi gh carboxylation activity under N stress may be a valuable selection c riteria for obtaining tolerance of corn to low soil N.