Effects of light quality and lighting duration on the quality of passion fruit in winter

【Objective】Current research regarding the effects of light quality and photoperiod on passion fruit primarily focuses on the vegetative growth stage, while studies addressing fruit quality management during the winter season remain relatively scarce. In winter greenhouse cultivation, balancing lighting costs with quality enhancement remains a key challenge in production practices. While lighting duration requires consideration of energy consumption costs, relying solely on one type of light quality fails to meet the multifaceted requirements for fruit component accumulation. By analyzing the impact of the light environment on fruit morphology, internal quality, and sugar- acid composition, this study aims to identify optimal supplementary lighting parameters, thereby providing a theoretical basis and technical reference for achieving high-quality, efficient, and cost-effective passion fruit production during the winter season.【Methods】Using Qinmi No.9 passion fruit as test material, six LED lighting treatments (R∶B=79∶21 (T1), R∶G∶B=70∶20∶10 (T2), R∶B=60∶40 (T3), R∶G∶B=50∶20∶30 (T4), R∶ B=41∶59 (T5), R∶B=20∶80 (T6)) and three night lighting durations (4 h, 6 h, 8 h) were established. The control group (CK) received no supplemental lighting. This study compared the effects of different lighting conditions and duration combinations on fruit morphology, quality, and sugar-acid composition of passion fruits.【Results】Lighting supplementation significantly improved the visual appeal of passion fruits. Under T2 (R∶G∶B = 70∶20∶10) conditions, single fruit harvested after 6-hour and 8-hour night lighting showed 18.2% and 19.8% higher weight (81.52 g vs 82.18 g) compared to CK (65.85 g), with edible rates rising to 50.2%-a 1.7 percentage point increase over CK. The fruit shape index remained stable at the range of 1.14-1.16, with simultaneous increases in both horizontal and vertical diameters. The fruit skin was thickened by 0.7-0.9 mm, exhibiting plump oil glands and uniform golden-yellow coloration, while the commercial grade rate increased by over 15%. All lighting treatments showed significantly higher single fruit weight, diameter measurements, and skin thickness than CK (P＜0.05), though no significant difference was observed between 6-hour and 8-hour treatments, indicating 6-hour exposure had reached the‘light saturation point’for morphological development. Total soluble solids (TSS) and total anthocyanins (TA) jointly determined flavor profiles. 8- hour lighting of T2 treatment achieved 15.8 Brix TSS, a 33% increase from CK's 11.9 Brix; the 6-hour treatment under identical light conditions also reached 15.2 Brix with a 28% increase, though no significant difference was observed. Under T4 (R∶G∶B = 50∶20∶30) lighting for 6 hours, TA dropped to 1.05%, down 30.5% from CK's 1.51%; while T2's 6-hour TA stood at 1.08%, showing a 28.5% decrease. The red, green, and blue light quality (T2, T4) enhanced total solids (TSS) while suppressing total anthocyanin (TA) synthesis. In contrast, high blue light (T5, T6) resulted in less than 15% TSS increase and under 20% TA reduction, showing limited flavor improvement. A sugar-to-acid ratio ≥15 is the threshold for commercial acceptance of passion fruit fresh-fruit varieties. T2 supplementation at 6 hours showed a sugar-to-acid ratio of 14.9, rising to 15.6 after 8 hours; T4 supplementation at 6 hours reached 15.3, both significantly higher than control (CK) at 9.8. T2 maintained the most stable performance with a coefficient of variation (CV%) of only 8.3% across 4, 6, and 8-hour durations. T4 peaked at 6 hours but plateaued with extended supplementation, confirming 6 hours as the“cost- effective window”for flavor regulation. HPLC analysis revealed a sweetness triangle composed of sucrose (35%), fructose (32%), and glucose (33%). At 8-hour T2 supplementation, sucrose, glucose, and fructose levels were 41.84 mg·g-1 , 31.38 mg·g-1 , and 38.79 mg · g- 1 respectively, representing increases of 102.81%, 47.95%, and 103.41% compared to CK. Under identical 6- hour light conditions, trisaccharide content reached 34.22, 28.45, and 27.16 mg · g- 1 , with 66% , 34% , and 42% increases respectively, showing no significant difference from the 8- hour treatment. While high red light (T1) exhibited higher sucrose levels, its fructose and glucose concentrations were significantly lower than those in T2, resulting in insufficient sweetness layers. The blue light treatment (T6) showed all trisaccharides at low concentrations with a bland taste profile. Citric acid constituted 65% of total acids, malic acid 25%, and vitamin C contributed the remaining 10%. After 8 hours of T4 supplementation, citric acid, malic acid, and vitamin C levels reached 1.67, 1.19, and 0.53 mg·g-1 respectively, representing increases of 38%, 25%, and 76.7% compared to CK. The vitamin C content in T2 after 6 hours was 0.49 mg · g- 1 , showing a 63% increase but no significant difference from the 8- hour group. Correlation analysis revealed highly significant positive correlations between vitamin C and fruit weight (r=0.82**), girth (r=0.75*), and length (r=0.71*), indicating synchronized accumulation of antioxidant compounds during fruit enlargement. No significant differences were observed in chlorophyll a/b and carotenoid levels across treatments (P＞0.05), suggesting that nighttime LED supplementation primarily influenced intrinsic quality rather than skin pigmentation. The fruits maintained typical golden coloration, meeting market standards for“Golden Passion Fruit”color specifications. 【Conclusion】Light treatment in winter can improve the fruit quality of passion fruit. It is recommended to utilize a light spectrum comprising 70% red light, 20% green light, and 10% blue light and light supplementation for 6 hours. While keeping energy costs under control, this practice can significantly improve both the morphological characteristics and flavor quality of the fruit.