Variation of Endogenous Hormones during Flower and Leaf Buds Development in ‘Tianhong 2’ Apple

Hormones have an important role in apple flower bud differentiation; therefore, it is necessary to systematically explore the dynamic changes of endogenous hormones during flower and leaf bud development to elucidate the potential hormone regulation mechanism. In this study, we first observed the buds of ‘Tianhong 2’ apple during their differentiation stage using an anatomical method and divided them into physiologically differentiated stages of spur terminal buds, flower buds, and leaf buds. Then, we determined the contents of zeatin riboside (ZR), abscisic acid (ABA), auxin (IAA), and gibberellin (GA₃) in these various types of buds using an enzyme-linked immunosorbent assay. The results showed that the content of ZR and the ratio of ZR to IAA in spur terminal buds decreased significantly during physiological differentiation. The contents of ZR, IAA, and GA₃ in leaf buds culminated at the initial differentiation stage. The content of ZR in flower buds was significantly higher than that in leaf buds after formation of the inflorescence primordium and sepal primordium. Before the appearance of stamen primordium, the content of GA₃ in flower buds was remarkably lower than that in leaf buds. The ratios of ABA/IAA and ZR/IAA in flower buds were significantly higher than those in leaf buds before the appearance of flower organ primordium. Moreover, ABA content, ABA/ZR, and ABA/GA₃ in flower buds were higher than those in leaf buds throughout the whole flower bud morphological differentiation process. Therefore, the reduced ZR content was beneficial to floral induction. The low content of GA₃, and high ratios of ABA/IAA and ZR/IAA were conducive to early morphological differentiation. In addition, high ratios of ABA/GA₃ and ABA/ZR were beneficial to the morphological differentiation of flower buds. Moreover, the high ABA content was beneficial to floral induction and morphological differentiation of flower buds. Our results shed light on the mechanisms of hormonal regulation of apple flower bud differentiation and could potentially strengthen the theoretical basis for artificial regulation of apple flower bud development using exogenous plant hormones.