Endogenous Free Polyamines of Mangos in Relation to Development and Ripening

Changes in endogenous free polyamines (putrescine, spermidine, spermine) were monitored from fruit set (fruit diameter 4.6 ± 0.5 mm, wt 0.09 ± 0.05 g) until 1 week before the expected harvest time in 'Kensington Prideʼ and 'Glenʼ to examine their role during mango (Mangifera indica L.) fruit development. Polyamines (PAs) in the pericarp tissues (exocarp and mesocarp) were estimated throughout the fruit development period, while estimations from growing ovules were started from 41 days after fruit set (DAFS). During fruit ripening, ethylene production and endogenous free PAs in skin and pulp of 'Kensington Prideʼ mango were also monitored. PA contents of pericarp declined between fruit set and maturity from 788 to 101 nmol·g-1 fresh weight (FW) in 'Kensington Prideʼ and from 736.6 to 89.6 nmol·g-1 FW in 'Glenʼ during fruit development. Spermidine (SPD) and spermine (SPM) were higher than putrescine (PUT) during the initial phase of fruit growth. The highest levels of free PAs, especially SPD and SPM, at the initial stages of fruit growth suggest a potential role during the cell division phase and not in subsequent fruit development. Ovule seems to be a rich source of PAs as evident from 2.3- and 2.7-fold higher total PAs than pericarp tissues in 'Kensington Prideʼ and 'Glenʼ, respectively. During fruit ripening of 'Kensington Prideʼ, total PAs increased in skin and pulp tissues along with the climacteric rise of ethylene, and reached maximum levels (skin 796, pulp 314 nmol·g-1 FW) on day 4 of ripening. Skin exhibited 55.8% higher mean free PAs than the pulp. PUT dominated both in skin and pulp tissues. The simultaneous increase of ethylene and free PAs during fruit ripening suggests that their biosynthesis may not be competitive, and free PAs may have evolved as a response to increased biosynthesis of ethylene. Mango fruit growth and development have been associated with changes in endogenous plant growth substances (Ram, 1992). Endogenous levels of auxins, gibberellins, cytokinins, abscissic acid (ABA) and ethylene have been determined during mango fruit growth and development (Murti and Upreti, 1995; Ram, 1992). However, biosynthesis and distribution of PAs in relation to mango fruit growth and development is yet unknown. PAs are implicated in fruit growth and development because of their ubiquitous pres- ence in all cells (Smith, 1985). Putrescine (PUT), spermidine (SPD), and spermine (SPM), are the most commonly found polyamines (PAs) in plants. Changes in PAs during fruit development and ripening imply a signifi cance in such processes, however, the precise role of PAs is not clear (Shiozaki et al., 2000). Exogenous application of PAs have been reported to increase the endogenous levels of PAs and fruit size (Biasi et al., 1991), indicating that the lower levels of these com- pounds could be growth limiting. Higher endogenous PAs level is also correlated with tolerance against a number of biotic and abiotic stresses (Bouchereau et al., 1999). Polyamines are synthesized from ornithine, arginine, and S-adenosyl methionine (SAM) and three of the enzymes involved are amino acid decarboxylases (Malmberg et al., 1998). PAs share a common precursor with ethylene (SAM), but show opposite effects in relation to the senescence processes