Effect of Temperature on Nitrogenase Functioning in Cowpea Nodules

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Abstract
Nitrogenase (EC 1.18.2.1) activity of a cowpea symbiosis formed with a Rhizobium strain (176A27) lacking uptake hydrogenase and maintained under conditions of a 12 h day at an air temperature of 30.degree. C (800-1000 microeinsteins [.mu.E]/m2 per s) and a 12 h night at an air temperature of 20.degree. C showed a marked diurnal variation in ratio of N fixed to H2 evolved. As little as 0.3 .mu.mol N was fixed per micromole hydrogen evolved in the photoperiod vs. up to 0.6 in the dark period. In plants maintained under the same diurnal illumination regime but at constant (day and night) air temperature (30.degree. C), this difference was abolished and a relatively constant ratio or N2 fixed to H2 evolved (around .mu.mol/.mu.mol) was observed day and night. Exposure of nodulated roots to a range of temperatures maintained for 2 h in a single photoperiod indicated that, whereas H2 evolution increased with increasing temperature from 15.degree. C to a maximum around 35.degree. C, N2 fixation was largely unaffected over this temperature range. Both functions of the enzyme declined sharply at temperatures above 38.degree. C. A similar general response of N2 fixation to root temperature was observed in glasshouse-grown, sand-cultured plants maintained under a range of temperatures (15-35.degree. C) for 14 day in mid vegetative growth. The effect of temperature on the proportion of electrons allocated to proton reduction compared with N reduction showed a linearly increasing relationship (correlation coefficient = 0.96) between 15 and 47.degree. C.

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