Tree Age-Dependent Modulation of Fruit Endogenous Hormones and Quality Attributes in ‘Kinnow’ Mandarin (Citrus nobilis × C. deliciosa)

The study was carried out to investigate the influence of tree age on endogenous hormones and fruit quality traits of ‘Kinnow’ Mandarin (Citrus nobilis × C. deliciosa). Fruits were harvested in January at commercial maturity stage (total soluble solids/titratable acidity [TSS/TA] ratio 12:1) from four tree age groups (6–10, 11–15, 16–20 and 21–25 years old). Results revealed that fruits harvested from 16–20-year-old trees exhibited the highest average fruit weight, fruit diameter, juice content, TSS, TSS/TA ratio, total phenolic content (TPC), and total antioxidant content, whereas the lowest peel thickness was observed in trees of other tree ages. Maximum fruit smoothness and juice pH, along with minimum TA and vitamin C, were recorded in 21–25-year-old trees. Regarding endogenous hormones, fruits harvested from 16–20-year-old trees had a higher level of indole-3-acetic acid (IAA), gibberellic acid (GA₃), and zeatin (Zt), and a minimum level of ABA as compared to other tree ages. A positive correlation was recorded between endogenous hormones IAA, GA₃, Zt, and fruit quality attributes, including average fruit weight, fruit diameter, juice %, TSS, TSS/TA ratio, vitamin C, TPC, and total antioxidant content. Abscisic acid (ABA) positively correlated with fruit smoothness, peel thickness, TA, and juice pH. Principal component analysis accounted for 55.3% of total variability and showed that the fruits of 16–20-year-old trees were strongly positively associated with IAA, GA_3, and Zt, and better fruit quality characteristics, whereas older tree age, 21–25 years old, were positively aligned with ABA and reduced quality indices. Overall, it is concluded that 16–20-year-old trees exhibited better endogenous plant hormone and fruit quality attributes of ‘Kinnow’ mandarin.