Effect of Temperature on Nitrogenase Activity in White Clover

Abstract

Single, clonal plants of white clover were grown without inorganic nitrogen in four contrasting day/night temperature regimes, with a 12 h photoperiod, in controlled environments. Root and nodule respiration and acetylene reduction activity were measured in a flow-through system during both day and night for plants acclimated to day/night regimes of 23/18, 15/10 and 10/5 °C. Similar measurements were made on plants acclimated to 20/15 °C and stepwise at temperatures from 4 to 33 °C. Peak rate of ethylene production, nitrogenase-linked respiration and basal root + nodule respiration increased approximately linearly from 5 to 23 °C both in temperature-acclimated plants and in plants exposed to varying measurement temperatures. The measured attributes did not vary significantly between day and night. Temperatures above 23-25 °C did not further enhance the rate of ethylene production, which remained essentially the same up to the maximum measured temperature of 33 °C. The measurements of nitrogenase-linked respiration between 5 and 23 °C, during both day and night, demonstrated a constant 'energetic cost' of acetylene reduction of 2-9 fimol C02 ^mol C2H4-1. Over the same temperature range, the approximate activation energy of acetylene reduction was 60 kJ mol ~1. The integrated day plus night nitrogenase-linked respiration accounted for 13-4—16% of the plant's net shoot photosynthesis in a single diurnal period: there was no significant effect of temperature between 5 and 23 °C.