Antagonism or synergism? Combined effects of enhanced UV-B radiation and acid rain on photosynthesis in seedlings of two C4 plants

Abstract

Enhanced ultraviolet-B (UV-B) radiation and acid precipitation (AP), abiotic stress factors that may have a strong impact on plants because of their detrimental effects on photosynthetic components. Therefore, understanding the mechanisms of UV-B radiation and acid rain on plant photosynthesis is a crucial question in plant ecology. However, research regarding their combined effects on photosynthesis of C4 plants is still scarce. Here we monitored the response of net photosynthetic rate, chlorophyll content and enzyme activities of NADP-ME C4 photosynthetic pathway monocotyledon plant waxy corn (Zea mays L. certain Kulesh), and NAD-ME C4 photosynthetic pathway dicotyledon plant edible amaranth (Amaranthus mangostanus L.) to enhanced UV-B and acid rain and their interaction. We grew them in the green house under 3 levels of UV-B radiation (0, 2.88, 5.76 kJ·m−2·d−1 for Z.mays and 0, 2.88, 4.32 kJ·m−2·d−1 for A.mangostanus) and 3 levels of acid rain (pH 6.5, pH 4.5, pH 3.5) in a full three-way randomized block design. The key results were as follows: Enhanced UV-B radiation and acid rain and their interaction all decreased the net photosynthesis (Pn), PEPCase and RuBPCase activity of both species, compared with the control A0B0. UV-B decreased Chla, Chlb and Car in Amaranth seedlings, however, increased them in Maize seedlings, compared with those of the control. As a whole, acid rain had negative effects on chlorophyll content, however, positive effects on Car in two plants seedlings. Both UV-B radiation and (or) acid rain did not affect Chla/Chlb and Car/Chl. In conclusion, the negative effect of UV-B radiation was greater than that of acid rain. Whereas, more synergism was observed in monocotyledonous C4 plants, Maize seedlings, and more antagonism was observed in dicotyledonous C4 plants, Amaranth seedlings, which could explain why the Maize was affected less by the combined UV-B irradiation and acid rain.