Dynamic Profiling of Fruit Quality Attributes During Development Reveals the Early-Ripening Advantage in ‘Longhuihong’ Navel Orange, a Bud Mutant of ‘Newhall’

Citrus bud mutants provide valuable genetic resources for breeding early-ripening cultivars with improved fruit quality. However, the physiological mechanisms underlying early ripening traits remain poorly understood. To elucidate the physiological basis for the early-ripening phenotype of the bud mutant ‘Longhuihong’ navel orange, fruit development was systematically monitored from 60 to 240 days after full bloom over two consecutive growing seasons, with the maternal cultivar ‘Newhall’ serving as a control. The results demonstrate that the precocity of ‘Longhuihong’ arises from the coordinated optimization of multiple fruit quality traits in this cultivar. The mutant exhibited enhanced fruit growth potential, with an average increase of 12.07–15.92% in single fruit weight. Peel coloration was significantly accelerated, as reflected by the faster coloring rate. Internal quality development followed a distinct pattern, characterized by high sugar accumulation, rapid acid degradation, and elevated vitamin C content. Notably, citric acid metabolism in ‘Longhuihong’ displayed a unique biphasic profile: substantial accumulation in the early stage, followed by rapid degradation in the later stage, which advanced the peak of the TSS/TA ratio by approximately 15 days. Principal component analysis further confirmed that the early ripening trait represents a systemic and integrated advancement in fruit size, sugar–acid balance, and peel pigmentation. Collectively, these findings provide a comprehensive understanding of the physiological mechanisms underlying precocity in ‘Longhuihong’ and offer key indices for breeding high-quality, early-ripening citrus cultivars.