Tropentag, October 7 - 9, 2008 in Hohenheim "Competition for Resources in a Changing World: New Drive for Rural Development"

Physiological Adjustment Mechanism of Medicago truncatula N₂ Fixation under Environmental Perturbations

¹Georg-August-Universität Göttingen, Institute of Plant Nutrition, Germany
²University of Khartoum, Department of Agronomy, Faculty of Agriculture, Sudan

Abstract

To regulate the uptake of nutrients that may be available in excess, plants may use feedback systems. These systems are envisaged to involve the cycling of nutrients within an organ or a plant and the regulation of further nutrient uptake by the products of assimilation. The concept of feedback regulation of N₂ fixation has been developed in the last decade of the previous century as a general mechanism governing regulation of N₂ fixation by environmental factors. Currently, we are testing the validity of such hypothesis on forage legumes under various environmental perturbations using the model plant Medicago truncatula as a test crop.
Two experiments were carried in a growth chamber in a nutrient solution and inoculated with Sinorhizobium meliloti 102F51. In the 1^st experiment, 60% of the leaves (lower part) were individually darkened (against control) while keeping the percentage of treated leaves constant by further darkening appropriate to new leaf expansion for 2 week. In the 2^nd experiment, an open-flow gas measurement system was used to measure H₂ & CO₂ evolution and to calculate N₂ fixation and electron allocation from apparent nitrogenase activity (ANA) [80% N₂/20% O₂] and total nitrogenase activity (TNA) [80% Ar/20% O₂] before and after high KNO₃^- application.
Darkened leaves were senescened and %N in the whole plant was highly increased versus untreated controls thus leading towards the tendency of reduced C/N ratios. Surprisingly, the growth rate of treated plants exceeded untreated controls. The gas measurement trial revealed that H₂ evolution was decreased slightly while electron allocation coefficient (EAC = 1 – ANA/TNA) was increased (11%) and the amount of N-fixed per day remained stable.
According to the N-feedback hypothesis, excess and high soluble N levels in the shoots reduce N₂ fixation rates after sensing and sending certain signal(s) and the repetition of this messenger(s) can induce nodule senescence. To avoid such detrimental effect the legumes must ‘lock up' excess-N in proteins and retain it in this form. This could explain why N₂ fixing legumes are normally protein-rich plants. Additionally, perennial pasture legumes posses other physiological adaptation for such purpose (i.e. tannins and/or polyphenol oxidase).

Keywords: Combined-N, leaf darkening, legume, N-feedback, N₂ fixation, regulation

Contact Address: Saad Abdel Rahman Sulieman, Georg-August-Universität Göttingen, Institute of Plant Nutrition, Carl-Sprengel-Weg 1, 37075 Göttingen, Germany, e-mail: ssuliemgwdg.de