Supplemental blue light and vapour pressure deficit modulate antioxidant metabolism in spinach (Spinacia oleracea L.)

Spinach (Spinacia oleracea L.) is a leafy vegetable rich in antioxidants such as glutathione, vitamin C, vitamin B2 and carotenoids. These compounds are important for plant performance but also for human health upon consumption. In plants, antioxidants are produced to mitigate harmful metabolites generated in response to environmental stresses. Therefore, optimising antioxidant production may be achieved by modulating the environmental factors such as light spectrum and vapour pressure deficit (VPD). Supplemental blue light is known to influence carotenoid accumulation, while VPD affects various physiological processes. However, combined effects of these factors on antioxidant content have not been thoroughly investigated. In this study, we examined their interactive effects on antioxidant levels in spinach. In a phytotron, four spinach varieties (two green varieties, Green1 and Green2, and red-veined varieties, Red1 and Red2) were grown under standard light (ST) and supplemental blue (HB) light combined with three different VPD treatments. Reduced glutathione levels were significantly lower in all varieties under HB, whereas oxidised glutathione increased only in Green1 grown under HB. Elevated VPD was the main factor associated with higher vitamin B2 content, although HB was also contributing to a higher content. Vitamin C levels were also higher under HB. Lutein content was increased under high VPD. An interactive effect between variety and light spectrum on total carotenoid content was found, with contrasting responses in Green2 and Red2. Overall, this study highlights the novel finding that the interaction between genotype, light spectrum and VPD plays a key role in antioxidant content in spinach.