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Temperature and Chilling Sensitivity of Nodule Nitrogenase Activity of Unhardened Alfalfa¹

The adaptability of alfalfa (Medicago sativa L.) to a broad range of environments suggests that alfalfa nodules fix nitrogen effectively under diverse temperature conditions. The objectives of this research were to investigate the sensitivity of the nitrogenase activity of attached alfalfa nodules to changes in root temperature, and to determine the effect of previous temperature exposure on subsequent nitrogenase activity of nodules. Experiments were conducted on the excised, nodulated roots of plants transferred from the field and acclimated to greenhouse or growth chamber conditions. Nodule activity responded significantly (P < 0.05) to a 5 C temperature change within 16 rain. Nitrogenase activity of plants grown at a mean root temperature of 20 C responded linearly with a Q₁₀ of 1.7 from 5 to 25 C and was irreversibly inactivated at 45 C. The apparent activation energies for nodule activity were 40 KJ/mole from 5 to 25 C and 9 KJ/mole from 25 to 35 C. This relatively low activation energy for nitrogenase of alfalfa nodules may be related to the high activity of this enzyme in alfalfa nodules compared to those of other species.

Chilling attached nodules from 20 to 10 C for either brief (ca. 1 rain) or long (16 to 32 rain) exposures significantly (P < 0.05) reduced nitrogenase activity. The effect of long chilling was more severe than that of brief chilling, with reductions in nitrogenase activity following long (16 to 32 rain) exposures to 10 C never fully reversible upon root rewarming to 20 C. The results suggested that chilling of nodules during root washing or plant culture before acetylene reduction assays may cause significant bias in the analysis. From a practical perspective, chilling injury of alfalfa nodules may be an important limitation to the onset of nitrogen fixation activity in the spring in overwintering alfalfa nodules.

Key Words: Nitrogen fixation • Nodule physiology • Cold adaptation • Acetylene reduction assay • Medicago satlva L.

² Research assistant, Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, and plant physiologist, USDA-SEA-AR, Univ. of Minnesota, St. Paul. 55108.

Received for publication February 9, 1981.