Polyamine and ethylene changes during floral initiation in response to paclobutrazol in mango (Mangifera indica L.)

Mango (Mangifera indica L.) is considered one of the important widely cultivated fruit crops of India in an estimated area of 2.54 million hectare with 18.08 million tonnes of fruit production. However, productivity (6.8 t ha-1 ) and market share of mango export in India is low due to the problems of alternate bearing, poor fruit set, early fruit drop, absence of efficient size controlling rootstock etc. Flowering is the key developmental event for crop yield and production. The intensity and timing of flowering show strong dependence on physiological status of growing buds, hormonal interactions, environmental factors and nutrient availability (Bernier and Perilleux 2005). In mango, the flowering is a complex process that involves differentiation of apical buds under the influence low temperature and/or attaining of certain degree of shoot maturity followed by bud burst and panicle emergence (Davenport 2007). Nunez-Elisea and Davenport (1995) reported that the temperature around 15-18 °C and 6-8 month old matured shoots exhibit strong behavior for floral growth initiation in mango. Ramirez and Davenport (2010) suggested involvement of leaf synthesized and phloem mobile florigenic promoter which moves to buds under the influence cold inductive conditions for exhibiting of floral growth in mango. Upreti et al. (2013) showed high accumulation of abscisic acid (ABA) and cytokinins and reduction in gibberellins in the growing buds linked to floral induction in mango. Similarly Upreti et al. (2014) reported high levels of sucrose and glucose contributed to the formation of generative buds in mango. However, flowering process in mango still remains unelucidated because of fragmentary information on various aspects of floral development including physiological, biochemical and molecular aspects. 

Use of growth retardants is an important horticultural practice for the management of reproductive growth and productivity enhancement in number of fruit crops including mango. Among the growth retardants, use of paclobutrazol [(2RS, 3RS)-1- (4-chlorophenyl)-4,4 dimethyl-2-(1,2,4-triazol)-1-yl)-pentane-3-ol] has been shown beneficial in restriction of vegetative growth and successful induction of floral growth in many mango cultivars (Yadav et al. 2005; Kishore et al. 2015). Evidences have shown that the paclobutrazol induced floral development is linked to suppression of gibberellins and increase in ABA and cytokinins besides increases in shoot C: N ratio and leaf water potential (Upreti et al. 2013). Several investigations have described polyamines, important polycationic growth regulatory molecules, as facilitators of reproductive development by sensitizing floral induction, floral differentiation, floral initiation and pollination in number of crops (Pritsa and Voyiatzis 2004; Kakkar and Sawhney 2002, Liu et al. 2006; Aloisi et al. 2016). Rey et al. (1994a) suggested the spermine accumulation as potential physiological marker for ascertaining timing of flower induction. In another study, Rey et al. (1994b) showed that high endogenous spermidine and spermine levels with low putrescine in buds and leaves are vital to flowering process in hazelnut trees. In strawberry, polyamines are reported to be involved in regulating floral initiation (Tarenghi and Martin-Tanguy 1995). Zhu et al. (1999) in apple stated active role for spermine in the modulation of floral bud growth activity. Similarly, Kushad and Orvos (1990) reported that the reproductive structures in citrus accumulated high polyamine levels. Wang et al. (1985) in the flower buds of cherry species (Prunus avium L. and P. serrulata L.) reported that the polyamines were actively present in all stages of bud development stages and their levels were low during dormancy, which increased rapidly upon the dormancy break and floral induction. Importance of polyamine involvement in flowering process has also been confirmed through their exogenous applications in varied crops. In apple trees, polyamines application through cut pedicels enhanced the number of flower buds (Rohozinski et al. 1986) and spraying polyamines favouring flower bud formation (Costa and Bagni 1983). Similarly, promotion of flowering by exogenous polyamines has been demonstrated in Spirodela punctata and morning glory (Liu et al. 2006). Tarenghi and Martin-Tanguy (1995) on the other hand by employing inhibitor of polyamine biosynthesis, α-difluoromethylornithine (DFMO) reported that the inhibition in flowering in strawberry was related polyamine decrease, which was restored by exogenous application of putrescine. Despite the importance of polyamines in floral development of different fruit crops, studies attributing polyamine involvement in floral induction of mango are lacking. Considering the fact that the biosynthesis of polyamines and ethylene are corregulated as a result of sharing of common precursor, s-adenosine methionine (SAM) (Yang 1987), and importance of ethylene in the promotion of flowering in many fruit crops including mango (Bukovac et al. 2006; Turnbull et al. 1999; Davenport and Nunez-Elisea 1990), we in the present investigation studied the effects of paclobutrazol on polyamine contents, ethylene production, ethylene precursor 1-aminocyclopropane carboxylic acid (ACC) content and activity of enzyme facilitating ACC to ethylene conversion- ACC oxidase at different stages of floral bud growth in the cv. Totapuri to delineate the role of polyamines in paclobutrazol induced flowering in mango.