It will be very interesting to see how light will influence the rate of photosynthesis in plants and what will happen if they do not get the required light in order to produce starch .
Two groups of carotenoid pigments can compete for light with photopigments (chlorophylls, peridinin, etc.) - Carotenes and Xanthophylls (Latin for '.)
Beta-cryptoxanthin is a xanthophyll carotenoid that is also provitamin A. It can be a source of vitamin A, but it produces half as much as beta-carotene. Premkumar listed papaya, mango and oranges as good sources of it. Beta-cryptoxanthin is typically found in yellow foods, such as corn and bell peppers, and is present in yellow-colored dairy products, such as egg yolks and butter.
Algae also use chlorophyll, but various other pigments are present as phycocyanin, carotenes, and xanthophylls in green algae, phycoerythrin in red algae (rhodophytes) and fucoxanthin in brown algae and diatoms resulting in a wide variety of colors.
Carotenoid: Any of about 600 naturally occurring pigments in plants. Carotenoids can be divided into two sub-categories: Carotenes and Xanthophylls.
The Junior-PAM fluorometer exploits the relationships between competing processes for light energy. The energy can be used in photosynthesis, or dissipated as heat in a process called Non-Photochemical Quenching (NPQ) or Other Pathways (collectively called NO). Figure 34 details these relationships. In all cases, Photochemistry (called Yield of Photosystem II, or Y (II)), plus Y (NPQ) - where excessive energy is shunted away from the photosynthetic apparatus by the Xanthophyll Cycle) - and Y (NO), where energy is diverted in ways other than NPQ = 1. In short,
Figure 28. These xanthophylls (diadinoxanthin and diatoxanthin) are important photosynthesis regulators in zooxanthellae. They also absorb mostly violet and blue light. After Jeffrey et al., 1997.
There is another xanthophyll reportedly found in stony corals and Tridacnid clams - dinoxanthin. Jeffries et al. (1997) state it is a minor pigment (relative to chlorophylls and other photopigments.) There is some debate about early reports of dinoxanthin in corals - some feel advances in analytical procedures would find that dinoxanthin is actually another xanthophyll, perhaps diadinoxanthin or diatoxanthin. . In any case, the pigment in Caribbean stony corals is about 7% of total photopigment content. See Figure 27. See Figure 29 for absorbance data for dinoxanthin.
Xanthophylls (oxygenated carotenoids) are also found in zooxanthellae. Two xanthophylls (diadinoxanthin and diatoxanthin) play an important role in protecting symbiotic algae and coral hosts from excessive light energy. When light energy is sufficient enough to effect pH changes within the photosynthetic apparatus of zooxanthellae, diadinoxanthin is converted to diatoxanthin. This conversion shunts light energy away from photosynthesis. In darkness, the process reverses, and diatoxanthin becomes diadinoxanthin. Note that these xanthophylls both absorb some violet but most strongly blue wavelengths at ~450 - 490nm. See Figure 28.
Figure 27. Relative concentrations of pigments found in select Caribbean stony corals. The amount of carotenoids (xanthophylls and carotenes) is not insignificant (9%). After Gil-Turnes and Corredor (1981).
Light sources of different 'colors' produce different photosynthetic responses but these are not due to the quality of the light per se, but instead the absorption of violet and blue light by carotenoids (including -carotene,and xanthophylls diatoxanthin, diatoxanthin, and diadinoxanthin.) -carotene is known to transfer collected energy to chlorophyll and hence photosystem reaction centers, in an inefficient manner.
Figure 27 shows the relative concentration of -carotene in some Caribbean stony corals. Considering the date of this work (1981), it is possible that future improvements in laboratory techniques would identify the pigment dinoxanthin as other xanthophylls such as diadinoxanthin and diatoxanthin (see next section.)
Perhaps the real story here is the photo-protective responses (the xanthophyll cycle) of zooxanthellae to different light sources, especially red light. Why is there little, if any, protective cycling of xanthophylls when strong red light is used? Strong doses of red light can regulate zooxanthellae densities even to the point of bleaching (Kinzie et al., 1984). What are the long term effects of red light on captive corals? This deserves investigation, and will be the subject of a future report.