Dinitrogen fixation in a water-stressed Alnus clone is limited by host xerotolerance

Abstract

The osmotolerance, rather than the halotolerance, of the endosymbiont predicted the xerotolerance of acetylene reduction by Alnus nodulated withFrankia ARgP5AG. Cloned plants ofAlnus glutinosa (L.) Gaertn. AG8022-16 were subjected to water stress under controlled conditions in an environmental growth chamber. Transpiration, stomatal conductance, and leaf water potential had decreased after successive 10 day periods of moderate (75% of water demand) and severe (50% of water demand) water stress. After severe stress had wilted the plants, reducing leaf water potential to −2.10 MPa, nitrogenase activity had fallen to 2.51 μM per plant per hour. The reported rapid turnover of nitrogenase implies thatFrankia mycelium was metabolically active at this low water potential, a water potential at which no Alnus-derivedFrankia has been reported active. Although ARgP5AG was similar to other such strains in halotolerance (lower limitca.−1.25 MPa), the low water potential limit for growth with glucose (a non-assimilated osmoticum) wasca.−2.53 MPa. Nitrogenase activity was apparently more limited by host xerotolerance than by endophyte xerotolerance.