The effect of acid rain on ultrastructure and functional parameters of photosynthetic apparatus in pea leaves

Abstract

The ultrastructure and functional parameters of the photosynthetic apparatus in leaves of 14-day-old pea seedlings were studied in conditions of laboratory simulated acid rain (SAR). Pea seedlings were sprayed with an aqueous solution containing NaNO₃ (0.2 mM) and Na₂SO₄ (0.2 mM) (pH 5.6, a control variant), or with the same solution, which was acidified to pH 2.5 (acid variant). Functional characteristics were determined by chlorophyll fluorescence analysis. There was reduction in the efficiency of the photosynthetic electron transport by 25% accompanied by an increase in the quantum yield of thermal dissipation of excess light quanta by 85% without significant change in maximum quantum yield of PSII photochemistry (F_v/F_m). Ultrastructural changes in chloroplasts were revealed by transmission electron microscopy (TEM) 2 days after the SAR treatment of pea leaves. In this case, changes in the structure of the grana and heterogeneity of the thylakoids packing in the granum, namely, an increase in thylakoid intraspace widths and thickness of granal thylakoids compared to the control, were found. It was shown also that carbonic anhydrase activity was significantly inhibited in chloroplast preparations isolated from SAR-treated pea leaves. We hypothesize possible involvement of chloroplast carbonic anhydrase in thylakoid granal structure maintenance. The structural disturbances and the inhibition of photochemical activity of chloroplasts are possible consequences of the carbonic anhydrase inactivation by SAR treatment leading to violation of HCO₃⁻–CO₂ equilibrium. The data obtained suggest that acid rains negatively affect the photosynthetic apparatus by disrupting the membrane system of the chloroplast.