Uptake and Assimilation of Nitrate by Corn Roots During and After Induction of the Nitrate Uptake System

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Abstract
Dark-grown, decapitated corn (Zea mays L.) seedlings that had been grown without nitrogen were used to characterize relationships among uptake, translocation and in vivo reduction of [15N] nitrate during induction of the nitrate uptake process and throughout the subsequent steady-state period. During induction, cumulative nitrate reduction increased from less than 20%of cumulative nitrate uptake to about 30%Concurrently, translocation of nitrate increased from less than 30%to over 50%of that absorbed. During the following steady-state period, partitioning of incoming nitrate between reduction and translocation remained relatively constant. Initially, removal of the endosperm had little effect on nitrate uptake, but by 6 h cumulative uptake had been depressed 30%relative to control plants. In contrast, endosperm removal limited nitrate reduction within 1 h, and as a consequence nitrate reduction during the 6 h exposure period was 60% less in endosperm-free tissues. Collectively these observations indicate that nitrate uptake and reduction are independent processes, since they develop at dissimilar rates upon initial exposure to nitrate, and since they differ markedly in response to endosperm removal. However, the constancy of nitrate reduction during steady-state uptake (30% of incoming nitrate), does reflect an association between the two processes.