Effects of Blue or Red LED Light Irradiation on Postharvest Degreening of Citrus Fruit

Objective: Postharvest degreening treatments with ethylene are commonly used to improve citrus fruit peel color, especially early in the season when declining field temperatures have not yet induced sufficient natural color development. Nevertheless, peel color of degreened fruit is lighter than that of fruit with on-tree maturation. To improve peel color of degreened citrus fruit, the present study evaluated the effects of combining ethylene exposure with red or blue light-emitting diodes (LEDs) on color changes and internal qualities of citrus fruit. Methods: In 2015, ‘Fallglo' tangerines were harvested on 12 October, navel oranges on 12 October, and grapefruit on 5 November and 3 December, all from groves near Fort Pierce, Florida. Fruits were immersed into 1000 mg L⁻¹ ethephon solution for 1 min, dried in air and then put in a growth chamber. The fruit were continuously treated at 29◦C for 10 h under 600 µmol m⁻² s⁻¹ red LED lights (630 nm), 400 µmol m⁻² s⁻¹ blue LED lights (465 nm), or in the dark (control). Another control consisted of fruit stored in dark at 20◦C. After the 10-hr treatments, the fruit were stored under ambient room conditions for subsequent evaluations. Results: The 29^oC blue and red LED light treatments significantly accelerate changes in hue (H◦), Chroma, lightness (L*), a*, b*, and a*/b*, resulting in better fruit flavedo color development during storage compared to the 20^oC control. When compared with the 29^oC control and red LED light treatments, the use of blue LED light significantly decreased peel H◦ and increased L*, a*, b*, a*/b* and Chroma. Harvests of navel orange and grapefruit in November responded greater to the blue LED light than Fallglo and Grapefruit harvested in December. The light treatments had no effect on stem end rot caused by Lasiodiplodia theobromae or on internal quality (soluble solids, titratable acid, juice content, firmness, and weight loss). Conclusion: The results showed that blue or red LED light treatments improve color development of degreening citrus fruit, with blue LED light showing the most promise.