Effects of Cr6+ On Chlorophyll and Photosynthesis of Cyperus Alternifolius and Coix Aquatica Roxb Grown in Constructed Wetlands.

Constructed wetlands are common and well received for treatment of municipal sewage in rural area. Hexavalent chromium (Cr⁶⁺) has been increasingly reported in rural domestic sewage. The effects of Cr⁶⁺on physiological characteristics of plants grown in constructed wetland are not fully understood. In this study, different concentrations of Cr⁶⁺(0, 10, 20 and 40 mg L^-1) were spiked to domestic wastewater and the effects of Cr⁶⁺ on chlorophyll fluorescence and photosynthesis two plants (Cyperus alternifolius and Coix aquatica Roxb) were evaluated with greenhouse experiments.  The contents of leaf chlorophyll (CHL) and soluble protein (SP) increased under Cr⁶⁺ stress, but the promoting effect on Coix aquatica Roxb was more significant than Cyperus alternifolius. With increasing Cr⁶⁺ concentration, net photosynthetic rate (Pn ), transpiration rate (Tr ), stomatal conductance (Gs ), maximal photochemical efficiency of PSII (Fv/Fm ), maximal fluorescence (Fm ) and Photochemical quenching coefficient (qP ) in Cyperus alternifolius leaves tended to increase at the vigorous tillering stage; but they increased initially and then decreased at the vigorous flowering stage. Intercellular CO₂ concentration (Ci ), non-photochemical quenching coefficient (qN ) and initial fluorescence (Fo ) in Cyperus alternifolius leaves decreased at the vigorous tillering stage, but at the vigorous flowering stage, Ci and Fo were significantly higher under 40 mg L^-1 Cr⁶⁺ treatment as compared to the control, whereas qN was significantly lower under 20 or 40 mg L^-1Cr⁶⁺ treatments. Pn, Tr and Gs in Coix aquatica Roxb leaves increased under the treatment of 10 or 20 mg L^-1Cr⁶⁺; but Ci was significantly lower under Cr⁶⁺ stress at the vigorous tillering stage. Compared with the control, Fv/Fm, Fm and qP in Coix aquatica Roxb leaves were significantly higher at the vigorous tillering stage, but significantly lower at the vigorous flowering stage under Cr⁶⁺ stress. Fo and qN were significantly lower at the vigorous tillering stage under Cr⁶⁺ stress, but they were significantly higher under 20 mg L^-1 Cr⁶⁺ treatment than the control at the vigorous flowering stage. In general, the photosynthesis of the two plants was promoted at the vigorous tillering stage, but inhibited at the vigorous flowering stage under Cr⁶⁺ stress.