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 NO3-seen in barley and wheat relative to search support.I dedicate this paper to R.H. Hageman in honor of his pioneering research into the function of NR, and his insight into its importance in limiting final plant yields.