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Environmental Sensitivity of Acetylene Reduction Activity in Prediction of N Fixation in Soybeans¹

The rate of acetylene reduction by nodules of legumes often is assumed to be related directly to the actual rate of N₂ fixation. This study was conducted to evaluate the relationship between effects of water stress and temperature on acetylene reduction activity and actual N accumulation in soybean [Glycine max (L.) Merrill ‘Hill’]. Nodulated plants were grown in a phytotron at day/night temperatures of 22/18, 26/18, and 30/18 C. The plants, contained in individual pots filled with a peat moss-vermiculite and gravel media, were watered in the morning with a minus-N nutrient solution and in the afternoon with deionized water. At the beginning pod stage, a single cycle of leaf moisture stress to –24 bars and recovery was applied to half of the plants at each temperature by withholding water and nutrient solution for four days. Plants were sampled over an 18-day period beginning on the first day that water was withheld. The relation between rate of N accumulation in the plant and rate of acetylene reduction varied with temperature but was not greatly affected by leaf moisture stress within a temperature. Thus, if acetylene reduction is to be used as an indicator of amount of N₂ fixed during growth, the effects of temperature during growth must be considered. However, a change in acetylene reduction may be an adequate indicator of response in N₂ fixation to such severe, abrupt stress conditions as the single cycle of moisture stress if temperature is constant.

Key Words: Glycine max (L.) Merr. • Temperature • Water stress

² Associate professor, Dep. of Soil Science, and professor, Dep. of Crop Science, North Carolina State Univ., Raleigh, NC 27650.

Received for publication September 1, 1979.