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Effects of Diurnal Variation in Light and Temperature on the Acetylene Reduction Activity (Nitrogen Fixation) of Subterranean Clover¹

Singly, or in combination, both light intensity or temperature can limit symbiotic N fixation. Most reported studies have been done to investigate the effects of only one of these factors. The present study was done to determine N fixation response to variation in both light and temperature for an annual range legume which, during its normal growing season, may be subject to growth limiting levels of both environmental factors. Plants of subterranean clover (Trifolium subterraneum L., ‘Mt. Barker’) were grown from seed to the sixth trifoliolate leaf stage in sand with N-deficient nutrient solution. Plant containers were designed to permit sampling of the rooting atmosphere for acetylene reduction assay during treatment conditions, which were maintained with controlled-environment facilities. During the treatment phase light and temperature were investigated by: 1) varying both diurnally; 2) varying light diurnally under constant temperature; 3) varying temperature diurnally under constant light; and 4) maintaining both light and temperature constant. Apparent N, fixation (acetylene reduction) showed diurnal fluctuations under: 1) constant light; and 2) 12/12 hour light/dark regimes when root and air temperature fluctuated by 7 and 14 C, respectively. In constant temperature and either normal photoperiods or constant light, however, no significant diurnal fluctuation of acetylene reduction was measured. Under both constant temperature regimes a gradual increase in acetylene reduction activity, which may have been an artifact, was observed. These results show that diurnal changes in acetylene reduction by subterranean clover result more from fluctuations in temperature than from diurnal changes in light and suggest that N, fixation by root nodules of this species is buffered against shortterm changes in photosynthate supply.

Key Words: Annual legumes • Annual range • Symbiotic N fixation • Controlled environments • Nodule activity

² Graduate student and professor, Dep. of Agronomy and Range Science, UC-Davis.

Received for publication September 17, 1979.