Abscisic acid metabolism during development and maturation of rambutan fruit

Summary Endogenous abscisic acid (ABA), its 2-trans isomer (trans-ABA), phaseic acid (PA) and dihydrophaseic acid (DPA) were quantified in the skin, pulp and seed of the late-harvest cultivar Rongrien and the early-harvest cultivar Seechompoo to clarify the pathway of ABA metabolism in rambutan fruit (Nephelium lappaceum L.) and the relationship between ABA and fruit maturation. In the skin of ‘Rongrien’, ABA concentration reached a maximum 70.d after full bloom (DAFB) before maturation and then decreased torwards harvest. Trans-ABA and PA concentrations also followed a similar pattern to ABA. DPA increased from 84 DAFB and peaked at 98 DAFB. In the skin, although DPA increased after the decrease of ABA and PA, there was a long time lag, and the amount of increase was slight compared with the decrease of ABA. These results demonstrate that PA and DPA may not be the major metabolic pathway of ABA in rambutan skin. ABA metabolism in the seed of both cultivars followed a similar pattern. ABA and PA in the seed of ‘Rongrien' and ‘Seechompoo’ decreased from 56 DAFB and 42 DAFB towards harvest, respectively. Meanwhile DPA increased from 56 to 70 DAFB in ‘Rongrien’ and 42 to 56 DAFB in ‘Seechompoo’, and then decreased. The amount of ABA decrease coincided with that of DPA increase. These results suggest that in the seed PA may transform directly in DPA, and DPA may be the predominant metabolite of ABA. The ABA concentrations in the pulp from 84 to 105 DAFB (harvest) in ‘Rongrien’ and those from 70 to 91 DAFB (harvest) in ‘Seechompoo’ were higher than those in the skin respectively. The changes of ABA, trans-ABA, PA and DPA in the pulp were different for each cultivar. Although ABA, trans-ABA, PA and DPA in the pulp of ‘Rongrien’ showed almost no change between 84 and 105 DAFB, those of ‘Seechompoo’ decreased from 84 DAFB towards 91 DAFB. The difference of ABA metabolites in pulp of each cultivar, the relationships between lower ABA concentrations in maturing seed and dormancy, and between higher ABA concentrations in the pulp and chilling resistance are discussed.