Table 1.

The morphology, physiological and biochemical changes of legume nodule senescence and the corresponding detection methods.

Characteristics of Nodule Senescence	Corresponding Detection Methods	Reference
Morphology changes
The color of nodules is converted to green; loss of turgidity in nodules	Calculating the ratio of senescent nodules to the total nodules	[15]
Less electron dense; appearance of numerous vesicles and peroxisomes;	Nodule paraffin sections micromorphology analysis; electron microscope scanning	[14,19,20]
The formation of complex elongated structures by mitochondria
Symbiotic membrane disintegration; damaged cell wall and lysis of bacteroids	Nodule paraffin sections micromorphology analysis; electron microscope scanning;TUNEL staining assay; in situ live/dead staining assay	[14,19,20,24,25]
Physiological and Biochemical changes
Decreased nitrogenase activity	Acetylene reduction assay (ARA)	[26,27]
Decreased leghemoglobin content	The cyanmethemoglobin method, qPCR and Western blotting analysis	[17,18]
Decreased lifestyle and growth capacity of Rhizobium	Testing the lifestyle and growth capacity of Rhizobium	[28,29]
Increased the concentration of nitric oxide (NO), ethylene, ABA and so on	Measuring the nodule senescence-related metabolites and hormone signals	[5,30]
Increased in the expression of nodule senescence-related marker genes	Laser microdissection; ACC immunolocalization; qPCR	[31]