Effect of Temperature on Nitrate Uptake, Translocation and Metabolism in Pennisetum americanum

Abstract

Daily cycles of net nitrate uptake, in vitro and in vivo nitrate reduction, and nitrogen transport from roots to tops were studied in two cultivars of Pennisetum americanum growing at day/night temperatures of 18/13°C and 30/25°C or 33/28°C. Rates of net nitrate uptake, xylem exudation and loading of amino acids into the xylem usually peaked once in the light period, whereas rates of in vitro nitrate reductase activity and in vivo reduction of NO₃^- to NO₂^- showed two peaks, one in the light and one in the dark period. Mean hourly rates per gram dry weight root of net nitrate uptake, xylem exudation and loading of amino acids into the xylem were 20, 44 and 40% respectively lower at 18/13°C than at 30/25°C. A whole-plant budget showed that both genotypes reduced the same amount of nitrate per day when they were grown at the same temperature. The proportion of nitrate which was reduced in roots was higher at 30/25°C (50%) than at 18/13°C (20%). At 30/25°C there was net export of reduced nitrogen from roots to tops whereas at 18/13°C there was import of reduced nitrogen into roots. Temperature-induced changes in daily cycles of nitrate uptake, rates of nitrate reduction and transport are interpreted as adaptations toward better carbon economy at low temperature.