Endogenous hormone levels regulate the leaf abscission process of Cyclocarya paliurus stem segments in vitro

During the in vitro development of stem segments of Cyclocarya paliurus, leaf abscission is extremely severe and pronounced, which hinders its breeding. Endogenous hormones play important roles in regulating plant growth and development and are among the main effectors of plant organ abscission. To explore how endogenous hormones regulate leaf abscission in C. paliurus in vitro, the emerged leaves of C. paliurus stem segments were cultured for 22 days (T0) in vitro; leaves at 27 days (T1) and leaves that had dropped after ≥ 32 days (T2) were used as materials, the changes in the internal tissue of the abscission zone (AZ) cells were observed during the leaf abscission process, and the contents of eight endogenous hormones, namely, auxin (AUX), cytokinins (CKs), gibberellins (GAs), abscisic acid (ABA), ethylene, jasmonic acid (JA), salicylic acid (SA) and strigolactones (SL), were determined. During the leaf abscission process in C. paliurus, the following was observed: (1) The strong metabolism of AZ cells and the increase in the number of chloroplasts and mitochondria led to the excessive accumulation of reactive oxygen species (ROS), which stimulated leaf abscission. (2) The IAA content gradually decreased, whereas the contents of 1-aminocyclopropane-1-carboxylic acid (ACC), GAs, and active jasmonic acid (JA-Phe, JA-Val-and JA-Ile) gradually increased, the SA content first increased and then decreased, and the contents of the active cytokinins IP, tZ and BAP were significantly greater at T2 than at T0 and T1; there was no significant difference in ABA and SL contents. (3) The ABA/GA3 ratio was the lowest at T2, and ABA/CK was the highest at T0, while the CK/ACC, CK/IAA, ACC/IAA and GA3/IAA ratios gradually increased with leaf abscission. (4) Correlation analysis showed that ABA and JA were not related to the other indicators and that IAA and SA inhibited leaf abscission, while CKs, GA3 and ACC promoted leaf abscission. It was speculated that the increase in the number of chloroplasts and mitochondria may lead to a dynamic imbalance of ROS in AZ cells, which is the cytological basis of leaf abscission in C. paliurus. IAA and SA negatively regulate the leaf abscission process in C. paliurus. In contrast, CKs, GA3 and ACC positively regulate abscission.