Pre-harvest spray application of abscisic acid (S-ABA) regulates fruit colour development and quality in early maturing M7 Navel orange

Poor fruit colour development at harvest in early maturing M7 sweet orange cultivar causes economic losses to the growers. The responses of different concentrations (50, 100, 200, 300 or 500 mgL(-1)) of abscisic acid (S-ABA) and its biosynthesis inhibitor nordihydroguaiaretic acid (NDGA) (0.01, 0.02, 0.04 mM) at pre-harvest stage (6 or 3 weeks before anticipated harvest) on peel colour development particularly from yellow to deep orange and on the fruit quality of M7 were studied during 2015 and 2016 in Western Australia. S-ABA treatments during both years irrespective of the concentrations applied exhibited significantly lower hue angle (h) with enhanced citrus colour index (CCI) and higher levels of total carotenoids in the flavedo during 2015 and 2016. Spray application of S-ABA (300 and 500 mg L-1) resulted in higher level of total carotenoids (35.0 and 71.5 mg kg(-1)) in the flavedo during 2015 and 2016. A single spray application of S-ABA applied at 6 weeks before anticipated harvest (WBAH) showed higher mean CCI (10.1) and level of total carotenoids (37.6 mg kg(-1)) as compared to its single application at 3 WBAH and double spray at 6 WBAH followed by 3 WBAH in 2015. However, nordihydroguaiaretic acid (NDGA) restricted colour development indicated by higher le and reduced CCI with lower levels of total carotenoids in the flavedo during 2015. S-ABA treatments exhibited significantly reduced total organic acids in the juice, whilst total sugars were not affected by any of the treatments. S-ABA treatments (200 and 300 mg L-1) showed increased SSC/TA ratio (12.8%) as a result of reduction in total acidity (TA) (0.96%). In conclusion, pre-harvest spray application of S-ABA promoted fruit colour development from yellow to deep orange, indicated by reduced h* and increased CCI as well as the levels of total carotenoids in the flavedo of M7 Navel orange. Promotion of fruit colour development (yellow to deep orange) with the pre-harvest application of S-ABA and its down regulation with the application of ABA biosynthesis inhibitor (NDGA) suggested the involvement of S-ABA in flavedo colour development in M7 Navel orange fruit.