Effect of symbiotic N2 fixation on leaf protein contents, protein degradation and nitrogen resorption during leaf senescence in temperate deciduous woody species

Nitrogen (N) resorption from senescing leaves enables plants to reuse N, making them less dependent on current N uptake from the environment, leading to higher fitness, particularly under low N supply. Species that form a symbiotic association with N-2-fixing bacteria have not evolved proficient N resorption, i.e., they retain more N in the senesced leaves than non-N-2-fixing species. However, the physiological mechanism underlying the difference is still unknown. Metabolic and structural protein contents in green and senesced leaves, as well as protein degradation during leaf senescence-a critical initial process for subsequent N resorption-were determined in four N-2-fixing legumes and in four non-N-2-fixers. The metabolic proteins were highly degraded in legumes and to a lesser extent in nonlegumes. Nonetheless, legumes retained more metabolic proteins in their senesced leaves than nonlegumes, because symbiotic N-2 fixation improved the metabolic protein content in green leaves. Symbiotic N-2 fixation did not change the structural protein content in green leaves. The structural proteins were moderately degraded in nonlegumes, and almost undegraded in legumes, and more structural proteins remained in the senesced leaves of legumes than in those of nonlegumes. The higher metabolic and structural protein contents in the senesced leaves of N-2-fixing legumes properly explained the less proficient N resorption. This is an important step in unraveling molecular mechanisms of different N conservation strategies among plant functional types.