In this paper, the allelopathic effect of extracts of three citrus peels (Citrus sinensis, C. unshiu and C. grandis) on Microcystis aeruginosa PCC7806 was studied. Then, three experiments were performed. Firstly, one was the comparison of inhibitory effect between five fractions of extracts of three citrus peels. Secondly, the cytotoxicity test of five fractions of navel orange extract was taken. Finally, it was checked that the effect of extract of navel orange on the photosynthetic system of M. aeruginosa.
It was found that methanol extracts of three citrus peels could inhibit the growth of M. aeruginosa. The order of inhibitory activity was as followed: C. unshiu> C. grandis > C. sinensi. The activity increased with the concentration of all three extracts, but it could promote the growth of the algae at the concentration of 1.66 g/L in navel orange group. The inhibitory activity dropped down by the prolonging cultured time, which led the algal growth to recover. Therefore, it was suggested that allelochemicals of the citrus peels were degradable.
The anti-algal test of five fractions of extracts of citrus peels showed the comparison result of the inhibitory effect between five fractions as followed: in the navel orange group, petroleum ether and chloroform fraction > aqueous fraction > acetic ether and 1-butanol fraction; in the mandarin group, petroleum ether and chloroform fraction > 1-butanol fraction > aqueous and acetic ether fraction; in the grapefruit group, petroleum ether fraction > chloroform and acetic ether fraction > 1-butanol fraction > aqueous fraction. All fractions of three extracts, excluding both 1-butanol fractions in navel orange and grapefruit groups, showed low-dose stimulation and high-dose inhibition phenomenon. Consequently, it meant that allelochemicals in fractions showed hormesis to M. aeruginosa.
Five fractions of navel orange peel extract showed different cytotoxicity activity to fish blastulae embryonic cells. The order of activity was as followed: petroleum ether and chloroform fraction > acetic ether and 1-butanol fraction > aqueous fraction.
The extract of navel orange peel could destruct the photosynthetic system of M. aeruginosa. It appeared the decrease of chlorophyll a content, downturn of oxygen evolution rate and inhibition of activity of PSII reaction center. The inhibitory activity increased by increasing dose and prolonging cultured time. In contrast, there was no difference of the respiratory rate between treatment groups. After analysis of chlorophyll fluorescence parameters, it revealed allelochemicals of extract could block electron flow from QA to QB at PSII acceptor side, which led to the excess of electrons in PSII reaction center. Finally, it resulted in destroying the algal photosynthetic system.