Apple (Malus domestica Borkh.) is one of the most important fruit trees in the world. However, due to the infrequent formation of flower buds during its growth, apple fruit production is significantly affected; therefore, the mechanism of apple flower bud differentiation has been widely studied. There are four stages of flowering: flower induction, flower initiation, flower differentiation, and blooming (Hanke et al., 2007), which is a highly complex physiological, biochemical, and morphogenetic process.
Endogenous hormones have important regulatory roles in all stages of flower development (Chandler, 2011). Kondo et al. (1999) suggested that the content of IAA might be the key factor influencing apple flower bud differentiation. However, IAA has been shown to have both promotional (Li et al., 1996) and inhibitory (Cao et al., 2000) effects on flower initiation, depending on the content of IAA. Cytokinin promotes the induction of flower formation (Hoad, 1984). Spraying 6-BA could induce a higher expression of MdTFL1, which promotes apple flower formation (Li et al., 2016, 2017). Sanyal and Bangerth (1998) showed that the decreasing content of ZR increased the proportion of flower buds. In addition, GA3 inhibited the flowering of woody plants (Pharis and King, 1985), and a low level of GA3 was beneficial to flower induction (Xing et al., 2014, 2016); however, spraying GA3 could break the ZR/GA ratio to suppress the expression of MdSPL genes and inhibit flower bud formation (Zhang et al., 2016). Chang and Huang (2018) found that GA3 could promote flowering. Moreover, a high content of ABA promoted apple flower formation (Cao et al., 2000). Spraying uniconazole increased the ABA content in spur terminal buds and accelerated flower bud differentiation (Cao et al., 2003). Furthermore, the balance of endogenous hormones was proposed as the key factor for flower bud differentiation (Wang, 2010; Yang, 2010). Therefore, there are different views on the effect of endogenous hormones on apple flower formation.
‘Tianhong 2’ apple, a dwarf mutant of the ‘Nagano Fuji 2’ apple in China, is characterized by its spur type and higher rate of flower formation (Shao et al., 2008). In apple, the flower buds usually grew at the top of the annual spur (short branches) and the lateral ends of the branches (Buban and Faust, 1982). Both of the vegetative organs and floral structures can be found in the flower buds, which are called mixed-type buds. Several studies have reported the role of plant hormones in flower bud differentiation in apple, but hormone regulating studies of apple flower buds and leaf buds remain limited.
Zhou et al. (1988) studied the changes in endogenous hormone contents in the leaves of flower and leaf buds under normal growth conditions during apple flower bud differentiation stages. However, the changes in endogenous hormones and their potential mechanisms in flower and leaf buds in apple are still unclear. Therefore, in the current study, we investigated the spur terminal buds by anatomical observation. They were distinguished into flower buds with different differentiation stages and leaf buds. The dynamic contents of endogenous hormones during physiological differentiation that spur terminal buds, flower buds, and leaf buds during flower bud differentiation were also determined. The results could provide a theoretical basis for how hormones regulate apple flower bud differentiation.
Bertelsen, M.G., Tustin, D.S. & Waagepetersen, R.P. 2002 Effects of GA3 and GA4+7 on early bud development of apple J. Hort. Sci. Biotechnol. 77 83 90
Cai, K.X., Niu, X.Q., Wang, L.P., Yang, J.J. & Chen, Z.D. 2017 Dynamic changes on endogenous hormones in leaves of Phyllagathis fordii during flower bud differentiation Fujian J. Agr. Sci. 32 278 281
Cao, S.Y. 2000 Studies on the differentiation course and the fluctuations of endogenous phytohormones of flower buds in apple. Nanjing Agricultural University, China
Cao, S.Y., Zhang, J.C. & Wei, L.H. 2000 Studies on the changes of endogenous hormones in the differentiation period of flower bud in apple trees J. Fruit Sci. 17 244 248
Cao, S.Y., Zhang, Q.M., Zhang, J.C. & Jiang, A.H. 2003 Effects of uniconazol spray on flower bud differentiation and endogenous hormone level in terminal bud of apple trees J. Plant Physiol. Mol. Biol. 29 375 379
Feng, F. & Yang, J.S. 2011 Relationship between floral bud differentiation and endogenous hormones in autumn-cutting Chrysanthemum morifolium ‘Jinba’ Sci. Agr. Sin. 44 552 561
Foster, T., Johnston, R. & Seleznyova, A. 2003 A morphological and quantitative characterization of early floral development in apple (Malus x domestica Borkh.) Ann. Bot. 92 199 206
Hanke, M.V., Flachowsky, H., Peil, A. & Hättasch, C. 2007 No flower no fruit-genetic potentials to trigger flowering in fruit trees Genes Genomes Genomics. 1 1 20
Kondo, S., Hayata, Y. & Inoue, K. 1999 Relationship between indole-3-acetic acid and flowering in two apple cultivars, Fuji and Ohrin J. Jpn. Soc. Hort. Sci 68 563 565
Li, Y.M., Zhang, D., Xing, L.B., Zhang, S.W., Zhao, C.P. & Han, M.Y. 2016 Effect of exogenous 6-benzylaminopurine (6-BA) on branch type, floral induction and initiation, and related gene expression in ‘Fuji’ apple (Malus domestica Borkh.) Plant Growth Regul. 79 65 70
Li, Y.M., Zhang, D., Zhang, L.Z., Zuo, X.Y., Fan, S., Zhang, X., Abdullah, S. & Han, M.Y. 2017 Identification and expression analysis of cytokinin response-regulator genes during floral induction in apple (Malus domestica Borkh) Plant Growth Regul. 83 455 464
Luo, Z.R., Xu, M.X. & Sun, Y.W. 1987 Studies on the changes of cytokinins during the flower-bud differentiation in ‘Starking’ apple trees J. Fruit Sci. 14 17 22
Qin, X.J., Huang, X.Y., Jiang, S.Y., Li, H., Dai, J., Wei, R.C. & Li, F. 2010 Changes in endogenous hormones during floral bud differentiation of Siraitia grosvenorii (Swingle) C. Jeffrey Plant Physiol. Commun. 46 939 942
Sanyal, D. & Bangerth, F. 1998 Stress induced ethylene evolution and its possible relationship to auxin-transport, cytokinin levels, and flower bud induction in shoots of apple seedlings and bearing apple trees Plant Growth Regul. 24 127 134
Su, H., Xu, K. & Liu, W. 2007 Changes of endogenous hormones during the process of flower bud differentiation of welsh onion Acta Hort. Sin. 34 671 676
Wang, L. 2010 Effects of branch bending on endogenous hormones, carbon and nitrogen nutrition for Red Fuji/Malus hupehensis. Shandong Agricultural University, China
Wang, S., Zhang, R.L., Hao, X. & Bao, H.P. 2013 Changes of endogenous hormone content in leaves of Tillandsia bergeri during the flower bud differentiation period Chin. Agr. Sci. Bull. 29 142 146
Weiler, E.W., Jourdan, P.S. & Conrad, W. 1981 Levels of indole-3-acetic acid in intact and decapitated coleoptiles as determined by a specific and highly sensitive solid-phase enzyme immunoassay Planta 153 561 571
Wu, M., Zhang, W.H., Wang, R., Dong, Y., Mao, Z.Q. & Shen, X. 2013 Changes of endogenous hormones of precocious crabapple during development Acta Hort. Sin. 40 10 20
Xing, L.B., Zhang, D., Li, Y.M., Zhao, C.P., Zhang, S.W., Shen, Y.W., An, N. & Han, M.Y. 2014 Genome-wide identification of vegetative phase transition-associated microRNAs and target predictions using degradome sequencing in Malus hupehensis BMC Genomics. 15 1125
Xing, L.B. 2016 Analysis of gene expression pattern of flower bud induction and molecular mechanism of promoting flowering withshoot bending in ‘Nagafu No.2’ apple variety. Northwest A&F University, Shaanxi, China
Yang, J.C., Zhang, J.H., Wang, Z.Q., Zhu, Q.S. & Wei, W. 2001 Hormonal changes in the grains of rice subjected to water stress during grain filling Plant Physiol 127 315 323
Yang, S., Hao, G.W., Zhang, X.W., Bai, M.D., Li, K., Shi, M.J., Cheng, P.H., Guo, H.P. & Li, L.L. 2015 Effects of endogenous hormone, carbon and nitrogen nutrition on development of wizened bud in ‘Yulu Xiangli’ pear Acta Hort. Sin. 42 1057 1065
Yang, Y. 2010 Effects of branch bending angle on carbon, nitrogen and endogenous hormones contents in buds and leaves of apple trees, Northwest A&F University, China
Zeng, H., Du, L.Q., Zou, M.H., Lu, C.Z., Luo, L.F. & Zhang, H.Z. 2008 Changes of endogenous hormones in macadamia during flower bud differentiation J. Anhui Agr. Sci 36 14949 14953
Zhang, S.L., Ruan, Y.L., Zhu, K.M. & Wu, G.L. 1990 Changes of endogenous zeatin and gibberellic acid in Citrus satsuma during the period of flower bud formation Acta Hort. Sin. 17 270 274
Zhang, S.W., Zhang, D., Fan, S., Du, L.S., Shen, Y.W., Xing, L.B., Li, Y.M., Ma, J.J. & Han, M.Y. 2016 Effect of exogenous GA3 and its inhibitor paclobutrazol on floral formation, endogenous hormones, and flowering-associated genes in ‘Fuji’ apple (Malus domestica Borkh.) Plant Physiol. Biochem 107 178 186
Zhou, X.M., Ma, H.P., Wang, F.Z., Wang, C.M., Wu, Z.X. & Cui, C. 1988 Variation of gibberellins, cytokinins and abscisic acid in vegetative and flower buds in various periods of apple tree Sci. Agr. Sin. 21 41 45
Zhu, Z.J., Jiang, C.Y., Shi, Y.H., Chen, W.Q., Chen, N.L., Zhao, M.J. & Wu, W.J. 2015 Variations of endogenous hormones in lateral buds of olive trees (Olea europaea) during floral induction and flower-bud differentiation Sci. Silvae Sin. 51 32 39