Efficiency of Nitrogen Utilization in C3 and C4 Cereals.

Addition of nitrogen leads to increased dry matter accumulation in vegetative plant parts and to increased final yields in cereal crops (Hageman and Lambert, 1988). The efficiency with which nitrogen is used varies with plant species and with environmental conditions. For example, plants that possess a C4 pattem of photosynthesis have, in addition to a superior method for trapping COz from the atmosphere, a greater nitrogen use efficiency (g dry matter gain per mg nitrogen utilized) than do C3 plants (Brown, 1978). Although there are many differences in the metabolism of C3 and C4 plants, the major difference between these two pattems of photosynthesis is the contribution of photorespiration to both carbon and nitrogen metabolism. When photorespiration is reduced in C3 plants either by increasing ambient levels of COz or reducing levels of 02, both the yield (vegetative dry matter) and nitrogen use efficiency are enhanced (Evans, 1989). As indicated in Table I, this effect is apparent in wheat, a C3 cereal, but not in maize, a C4 cereal (Hocking and Meyer, 1991). Factors that could be altered by reducing the contribution of photorespiration are the carbon supply necessary to drive the net increase in carbohydrate and protein and the availability of reductant and ATP. In this article I demonstrate that high levels of NO3seen in barley and wheat relative to maize and sorghum (Martin et al., 1983) are related to a carbon deficiency caused by the inhibition of the mitochondrial PDC by monovalent cations, in particular by the NH4+ produced by photorespiration in C3 plants (Schuller and Randall, 1989; Gemel and Randall, 1992). NH4+ production is lower in C4 than in C3 cereals (Martin et al., 1983). In addition, since NH4+ production is localized in bundle sheath cells in C4 plants, whereas NO3assimilation is found in mesophyll cells (Edwards, 1986; Becker et al., 1993), its impact on the carbon flow required for NO3assimilation should be negligible in C4 plants.