NITROGEN RESPONSE OF LEAF PHOTOSYNTHESIS AND CANOPY RADIATION USE EFFICIENCY IN FIELD-GROWN MAIZE AND SORGHUM

The grain yielding capability of maize (Zea mays L.) and sorghum [Sorghum bicolor (L.) Moench] is dependent on the capacity of the crop to assimilate CO2. Carbon dioxide assimilation can be measured directly at the leaf level (C-A) and indirectly at the canopy levels by determining radiation use efficiency (RUE). Since leaf N content per unit leaf area (LN) was expected to influence both of these measures of CO2 assimilation, the primary objective of this study was to obtain field data on C-A and RUE in response to differing LN induced by varying the amount of applied N fertilizer. The C-A was most responsive to LN at low values of LN, but there was no difference between the species in the hyperbolic relationship between C-A and LN. Similarly, RUE increased with LN, but maximum RUE and LN values were higher in maize than in sorghum. In maize maximum RUE was 1.7 g MJ(-1) and maximum canopy LN was about 1.8 g N m(-2), while in sorghum the maximum RUE values were less than 1.3 g MJ(-1) and the maximum canopy LN was less than 1.3 g N m(-2). The experimental increase in RUE with increasing LN, however, was less than expected from published theoretical analyses. It was hypothesized that leaf quantum efficiency per unit incident radiation may also depend on LN such that at low LN, quantum efficiency is decreased and RUE is decreased to less than expected levels.