Nitrogen Fixing Activity of Water Stressed Siratro1

Abstract

AbstractSiratro [Macroptilium atropurpureum (DC.) Urb. cv. siratro] is grown in tropical and subtropical environments characterized by frequent periods of drought and is dependent on N2 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 following 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 following rewatering. Nodules were irreversibly inactivated by water stress more severe than −3.0 MPa but in approximately 12 d following rewatering new nodules developed. Regrowth of siratro following water stress may be delayed while the capacity for N2 fixation recovers.