We have a colourful leaf this week. Capsicums grow green on the vine but in many varieties, other colours are revealed as their chlorophyll disappears and their chloroplasts morph into pigment-containing chromoplasts. The colours are due to phytochemicals such as β-carotene (βC, orange), lutein (yellow), zeaxanthin (Zx,orange), violoxanthin (Vx, orange) capsorubin (red), to name just a few. All these carotenoids are synthesized in plants from a long 40-carbon isoprenoid chain called phytoene, with cyclic groups at both ends that are metabolically altered to give the different colours. The process typically ends with the familiar red colour that gives red peppers their name. Peppers of other colours have been bred by selecting varieties that stop the metabolism at different stages.
Long before fruits even form, carotenoids serve to protect the photosynthetic apparatus. βCis a protective component of photochemical reaction centres, while Vx⇄Zx interconversion (the xanthophyll cycle) serves a vital protective function in the chlorophyll-rich “light-harvesting” complexes that collect muchof the energy for photosynthesis. When the xanthophyll in the complexes is Vx, the light energy is directed to photosynthesis. When the light is too strong for photosynthesis to keep up, Vx converts to Zx and light energy is safelydissipated to prevent damage.
In fruits (not just capsicum), the colours attract grazers that spread the seeds. Dietary carotenoids are even the source of “retinoid” the photoreceptor of the visual system. But they are also important, protective antioxidants. Carotenoid-based supplements are thus proposed as therapy for cancers and inflammatory disease and for protecting the skin against UV damage.
To summarise, plantcarotenoids are good for the plant, good for our health, and provide some of the best and safest natural colours for use in food and cosmetics.
SOURCES:
Mohd Hassan, Norazian et al. “Carotenoids of Capsicum Fruits: Pigment Profile and Health-Promoting Functional Attributes.” Antioxidants(Basel, Switzerland) vol. 8,10 469. 9 Oct. 2019, http://doi:10.3390/antiox8100469
Latowski, Dariusz et al. “Xanthophyll cycle--a mechanism protecting plants against oxidative stress. ”Redox report : communications in free radical researchvol.16,2 (2011): 78-90. http://doi:10.1179/174329211X13020951739938
