Nitrogen Nutrition and Grain Protein in Two Spring Wheat Genotypes Differing in Nitrate Reductase Activity1

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Abstract
Two spring wheat (Triticum aestivum L.) genotypes were studied to identify and isolate factors involved in grain protein production, with the aim of establishing physiological criteria for use in breeding and selecting wheat lines efficient in N utilization. ‘Anza’ and ‘UC44‐111’, which differed greatly from each other in tissue nitrate concentration in a field experiment, were tested in a growth chamber experiment to determine any differences in nitrate absorption and nitrate reduction capacities and their effects on straw and grain protein content. Anza, with higher tissue nitrate concentration than UC44‐111, had lower nitrate absorption capacity in short‐term experiments (up to 6 hours). UC44‐111 had a 70% higher in vitro nitrate reductase activity than Anza but differed little in in vivo nitrate reduction. This indicates that UC44‐111 has other factors limiting its potential for nitrate reduction.This study reveals no single identifiable factor that can be used as a physiological criterion in selecting wheat genotypes for efficient N utilization. Selection must consider two or more factors simultaneously, including long‐term capacity for absorption and in vivo reduction of nitrate and efficiency of translocation of vegetative N to the developing grain.