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Nitrogen Fixing Activity of Water Stressed Siratro¹

Siratro [Macroptilium atropurpureum (DC.) Urb. cv. siratro] is grown in tropical and subtropical environments characterized by frequent periods of drought and is dependent on Nj fixation for its N. These investigations were conducted to relate leaf and nodule water potentials to N₂ (C₂H₂) fixing activity and determine the capability of nodules to recover N₂ (C₂H₂) fixing activity following water stress. Plants were grown in pots containing vermiculite under controlled environment conditions. Forty-two days after planting four water treatments were imposed by withholding water for increasing periods of time. Water treatments created a decrease in leaf and nodule water potentials corresponding to the period of time water was withheld. However, when water potentials decreased below –1.5 MPa, nodule water potentials became more negative than leaf water potentials. A similar decrease in water potential suppressed growth of both nodules and shoots. In the longer duration, water withholding treatments nodule and shoot weights decreased as a result of nodule sloughing and leaf shedding. All water withholding treatments reduced the rate of N₂ (C₂H₂) fixing activity.

Nodules with water potentials of –1.5 MPa or greater were able to regain much of their N₂ (C₂H₂) fixing activity within 2 d foUowing rewatering. The specific activity of nodules surviving the most severe level of water stress (–1.9 MPa) were only about 60 % as active as the control 6 d foUowing rewatering. Nodules were irreversibly inactivated by water stress more severe than –3.0 MPa but in approximately 12 d foUowing rewatering new nodules developed. Regrowth of siratro following water stress may be delayed while the capacity for N₂ fixation recovers.

Key Words: Macroptilium atropurpureum (DC.) Urb. cv. siratro • Nodule water stress • Shoot water stress • Nodule recovery • Acetylene reduction

² Graduate research assistant and assistant professor, Range Science Dep.; professor, Soil and Crop Sci. Dep., respectively, Texas A&M Univ., College Station, TX 77843.

Received for publication August 2, 1982.