Changes in polyamine levels in relationship to the double-sigmoidal growth curve of peaches

Polyamines and the activities of their biosynthetic enzymes were evaluated during peach (Prunus persica L. 'Biscoe') mesocarp (pulp) and seed growth starting at full bloom and until full fruit maturity at 14 weeks after full bloom (AFB). Mesocarp fresh mass exhibited a double-sigmoidal pattern characteristic of peaches. Seed fresh mass increased to a maximum of ≃1 g at 4 weeks AFB then remained unchanged during the remaining weeks of sampling. Free putrescine, spermidine, and spermine levels were significantly higher in the flower bud, declined in the mesocarp tissue during the first 2 weeks AFB, then exhibited another increase between 2 and 6 weeks AFB. In contrast, conjugated spermidine and spermine levels were low in flower buds, then increased to their maximum level at 6 weeks AFB, then declined at full fruit development. Ornithine decarboxylase (ODC, EC 4.1.1.17) activity was high in flower buds (89.3 nmol · h^-1 · mg^-1 protein) and in early stages of mesocarp development then declined to its lowest level (5.8 nmol · h^-1 · mg^-1 protein) at full-fruit development. Arginine decarboxylase (ADC, 4.1.1.19) activity did not change during the first 6 weeks of mesocarp growth but declined later, reaching its lowest (1.95 nmol · h^-1 · mg^-1 protein) at 14 weeks AFB. During the first 5 weeks AFB, ODC activity was 3.0- to 4.5-fold that of ADC activity; however, at full-fruit maturity (14 weeks AFB) the activities of both enzymes were similar. The slowdown in mesocarp growth during pit hardening between 6 and 9 weeks AFB did not change polyamines concentrations or their biosynthetic enzymes. Free spermidine and spermine levels declined during seed development; however, between 7 and 9 weeks AFB an increase in putrescine was observed. Similarly, conjugated putrescine increased substantially during seed growth reaching its highest level of 680 nmol · g^-1 fresh mass at week 8 then declined at the later weeks, while conjugated spermidine and spermine peaked at week 10 to 1,169 and 2,148 nmol · g^-1 fresh mass. ODC and ADC activities declined between 3 and 5 weeks AFB. However, a significant increase in ADC but not ODC activity in the seed tissue was observed during pit hardening between 6 and 10 weeks AFB. Based on the rapid increase in putrescine and ADC activity in the seed tissue, it appears that pit hardening may be a stress-related phenomenon. Data also suggest that polyamine levels in the mesocarp and seed tissue are independently regulated.