Evaluation of the photoprotective role of quercetin to selected light-adapted and shade-tolerant plant species

Plants respond to different light intensities depending on their genetic make-up, mutation and other environmental conditions. Mikania micrantha, Clidemia hirta, and Tetracera sarmentosa were selected so as to analyse how varying light intensity affects some gas exchange characteristics, pigment and production of total flavonoid content. Besides, the study was aimed at evaluating the photoprotective role of quercetin to the selected plants, in addition to correlating how the production of flavonoids affects the plants photosynthesis. The photosynthetic rates of the selected plants were determined using LI-6400. The chlorophyll, carotenoid, anthocyanin, flavonoid, antioxidant enzymes, malondialdehyde, soluble sugar and soluble protein contents were quantified using spectroscopic techniques. Quercetin was quantified using high-performance liquid chromatography (HPLC). Sun-exposed plants were having the maximum photosynthesis and quercetin content compared with semi-shaded or fully shaded plants. The highest quercetin content was recorded for sun-exposed C. hirta (0.950 ± 0.023 μg/ml) while the lowest was recorded for shaded T. sarmentosa (0.13 ± 0.007 μg/ml). The highest oxidative stress was recorded for sun-exposed T. sarmentosa (6.19 ± 019 μg/ml) which was also having the lowest quantum efficiency of photosystem II (0.509 ± 0.003). Superoxide dismutase activity was lowest under sun-exposed C. hirta (1.86 ± 0.06 U/mg protein), while peroxidase and catalase were lowest under Sun-exposed T. sarmentosa (59.59 ± 2.67 and 3.75 ± 0.17) U/mg protein respectively. The result obtained makes it possible to accept the generated hypothesis of the study because the quercetin content was higher when the antioxidant enzymes of the plants were low. This leads to a conclusion that increase in the production of secondary metabolites at high light intensity is not due to high CO2 assimilation rate, but rather due to the production of photoprotective metabolites to conquer the light stress