Table 1. Morphological characteristics of Malus hupehensis during rhizogenesis of cuttings.

Rooting stage	Days after planting (d)	Description of appearance.	Function and significance	Critical Points
Root pre-emergence stage (S0)	0	Incision smooth, fresh.	Physiological induction and formation of root primordia.	P1’, P2’
	9	In the 1/4 to 1/2 of the edge of the incision, milk yellow callus formed with circular distribution.
	18	Milk yellow callus expanded to beyond 2/3 of the edge of the incision.
Early stage of root formation (S1)	27	Milky white adventitious roots emerged from the epidermis.	The formation and recovery of root absorption function is an important indicator of the survival rate of cuttings.	P3
	36	More milky white adventitious roots emerged from the epidermis and the length of the root increased.
Massive root formation (S2)	45	The differentiation of the main roots and the lateral root began to form; the number of main roots with length ≥ 1 cm was more than 3; the base of the root gradually took on a pale reddish brown color; level one lateral roots began to appear.	The root absorption function was significantly enhanced, and the ability of root regeneration was strong at transplanting stage, with certain stress resistance and adaptability.	P4
	54	The number of adventitious roots increased further; about 50% of the roots turned into pale brown red.
Root post-emergence stage (S3)	63	The structured dominant lateral root system has been formed, and about 80% of the roots changed to brownish red.	Stress resistance and adaptability were strong, but the ability of root regeneration decreased when transplanted decreased.
	72	A well-developed root system has developed with a dark brownish red color.

P₁’ represents the date of root primordia initial cell formation; P₂’ represents the date for formation of the root primordium; P₃ represents the date of adventitious root emergence through the stem epidermis, P₄ represents the date of massive root formation.