Humic Substances Delay Aging of the Photosynthetic Apparatus of Chara hispida

In freshwaters, dissolved humic substances (HSs) distinguish apparently HS-avoiding Charophytes from apparently HS-tolerant ones, but the underlying mechanisms so far remain obscure. In this contribution, we tested direct and indirect effects of HSs on Chara hispida (L.) Hartm. Using Rhodamine B, we showed that C. hispida is able to adsorb or even uptake and, subsequently, desorb and depurate organic compounds in the molecular mass range of the applied fulvic acids. To classify direct and indirect HS-mediated effects due to reduced light quantities, or to effects more strongly elicited by red relative to blue light, plants were exposed to HSs directly as well as through a neutral foil, or shaded by means of an external HS-containing reservoir (low-light variant). We showed that the apparently HS-tolerant C. hispida exhibited reduced lipid peroxidation and non photochemical quenching of chlorphyll fluorescence when exposed to HSs. Plants directly exposed to HSs were significantly different from control as well as to foil-shaded plants in terms of chl a+b, VAZ/chl, and β-Car/chl; yet, in low-light plants these variables did not differ from control and HS-exposed plants, suggesting that the shift in favor of red lights in the low-light variant led to a reduction in its cells' internal antioxidant content. However, the Fv/Fm ratio in HS-exposed plants decreased more slowly than in all other exposure variants, indicating that the photosynthetic apparatus aged more slowly, by a mechanism yet to be discovered. Our study indicates that both direct and indirect effects contribute to the HS tolerance of C. hispida.