ABSTRACT

Genetic increases in cereal grain yield are, for the most part, parallelled by decreases in grain protein concentration. The grain protein concentration reflects the ratio of grain protein yield to grain yield. If the grain protein concentration is to remain constant, then genetic increments in grain protein yield must keep pace with those in grain yield. Efforts to overcome the inverse yield-protein relationship must, therefore, concentrate on improving the grain protein yield. The increased nitrogen requirements of high protein cultivars may be met by improving the nitrogen partitioning efficiency, enhancing the uptake of nitrogen from the soil, and reducing the loss of nitrogen from the plants. Depending on climate, cereal species, cultivar, and husbandry techniques, 40 to 90 % of total shoot nitrogen is stored in the grains, indicating that the scope for genetically improving the partitioning efficiency is extremely variable. Breeding for a higher nitrogen uptake rate could reduce the loss of mineral nitrogen from the soil, because plant-available nitrogen is removed from the soil before it disappears as a consequence of biological immobilization, denitrification, leaching, and fixation in the interlayers of clay minerals. Higher rates of nitrogen uptake are also benefical when the uptake of nitrogen is limited by the length of the vegetation cycle. Potential bottlenecks in locating nitrogen in the soil and in the absorption, assimilation, mobilization, translocation, and storage of nitrogen are addressed. Genetic manipulation of the quantity and quality of materials released from the roots into the rhizosphere may affect the mineralization, immobilization, and denitrification of nitrogen as well as the dinitrogen fixation by diazotrophic associative bacteria. Future cultivars may be better at producing grain protein because of their ability to use nitrogen that present-day cultivars lose from the tops and roots. It is concluded that the nature and size of the nitrogen pools and, thus, the scope for genetically improving the grain protein yield, depends strongly on environmental conditions and cultural practices. Breeding for elevated protein may lead to a negative nitrogen balance and may, therefore, jeopardize long-term soil fertility if additional nitrogen fertilizer is not applied.