White Clover N2-Fixation in Response to Root Temperature and Nitrate

Abstract

Established, nodulated white clover plants were transferred to eight tanks of a flowing culture apparatus with solution temperatures of 5, 11, 17, and 25 °C (two tanks per temperature). Shoot temperature and light environment were common to all plants. After 7 d, NO3- (10 mmol m−3) was continuously supplied to one tank at each temperature while in the remaining four tanks (one at each temperature) the plants were completely dependent on nodule N2-fixation. Plants were randomly selected at intervals during the following 14 d period in order to measure root and nodule respiration and acetylene reduction activity (ARA) in a flow-through system set at the adapted root temperature. Additional plants were assayed for in vitro nitrate reductase activity in leaves, roots, and nodules. Apparent nitrogenase activity (ARA) and respiration associated with it were each markedly affected by temperature in two ways; (1) Activity per unit weight of nodule was reduced at lower temperatures; (2) Development of the plant, and thus also nodule mass, was restricted at lower temperatures which, in turn, restricted total nodule activity per plant. The presence of nitrate significantly reduced ARA of nodules, particularly at higher temperatures. However, significant discrepancies were found when N2-fixation rates, estimated from the acetylene reduction assay, were compared with N2-fixation rates calculated from curves fitted to N accumulation data (minus the rate of NO3- uptake in the case of nitrate-treated plants). Carbon use efficiency (CO2 respired per C2H4 produced) was not significantly affected by temperature or the presence of nitrate. Nitrate reductase activity (NRA) developed in all plant parts at the three highest temperatures, but not at 5 °C. We calculate that leaf NRA may account for 82, 75, and 68% of total nitrate reduction at 11, 17, and 25 °C respectively.