This study aimed to evaluate the defoliation ability of 56 sugarcane varieties/lines, to identity the germplasm with easy defoliation, and to elucidate the cellular morphological and physiological basis of sugarcane defoliation, so as to provide a theoretical basis for exploring the regulatory mechanism of sugarcane defoliation.

The natural defoliation number, total number of nodes, natural defoliation rate, leaf sheath inclination angle (+8, +9, +10 leaf position) and defoliation force of 56 sugarcane varieties/lines were systematically investigated, and defoliation ability was comprehensively evaluated by entropy weight method and hierarchical cluster analysis. The base of the leaf sheath abscission zone at +2, +5 and +8 leaf positions of the easy-to-defoliate sugarcane variety XTT20 and the hard-to-defoliate sugarcane variety YAU11-818 were analyzed by combining paraffin sections, enzyme activities and endogenous hormone content the cell morphology and physiological basis of sugarcane defoliation.

Entropy weight analysis showed that the comprehensive score of defoliation of 56 sugarcane varieties/lines ranged from 0.001 to 0.978. Systematic cluster analysis divided the 56 sugarcane varieties/lines into 3 groups, including 3 easily-defoliation varieties (XTT20, XTT22, YAU01-106), 7 difficult-to-defoliation lines, and 46 medium defoliation varieties/lines, accounting for 5.36%, 12.50% and 82.14% of the total number of varieties/lines, respectively. Compared with the difficult-to-defoliation line YAU11-818, the easily-defoliation variety XTT20 had a higher natural defoliation rate, larger leaf sheath inclination angle and smaller defoliation force; The cells in the leaf sheath abscission zone of the +8 leaf position of the defoliant varieties were more loosely arranged, the cell density was relatively low, and the cell degradation was larger, the percentage of aerated tissue area was significantly higher than that of the difficult-to-defoliation line (14.54% increase), and the difference between different leaf positions was significant (+8 leaf position increased by 16.97% compared to +5 leaf position). In addition, the polygalacturonase activity, cellulase activity and abscisic acid (ABA) content in +8 leaf sheath of XTT20 were significantly higher than those of YAU11-818. On the contrary, the content of indole-3-acetic acid (IAA) was higher in the difficult-to-defoliation line YAU11-818.

The degree of abscission zone and defoliation ability of +8 leaf sheaths at the maturity stage of sugarcane may be related to the activity of polygalacturonase, cellulase activity, peroxidase activity, ethylene synthesis precursors, abscisic acid, auxin content, etc., which can provide a reference for the identification of defoliation of sugarcane varieties.