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 [Sorg hum bicolor (L.) Moench] is dependent on the capacity of the crop to a ssimilate CO2. Carbon dioxide assimilation can be measured directly at the leaf level (C-A) and indirectly at the canopy levels by determini ng radiation use efficiency (RUE). Since leaf N content per unit leaf area (LN) was expected to influence both of these measures of CO2 assi milation, 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 amo unt 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 h yperbolic relationship between C-A and LN. Similarly, RUE increased wi th LN, but maximum RUE and LN values were higher in maize than in sorg hum. In maize maximum RUE was 1.7 g MJ(-1) and maximum canopy LN was a bout 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 l ess than expected from published theoretical analyses. It was hypothes ized 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.