Fast Cytoplasmic pH Regulation in Acid-Stressed Leaves : ENVIRONMENTAL AND STRESS RESPONSES : MEMBRANES AND BIOENERGETICS

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Induction of photosynthesis in leaves was prolonged, and steady state photosynthesis was inhibited by very high CO_2 concentrations which cause cytoplasmic acidification. Prolonged exposure to high CO_2 relieved initially observed inhibition of photosynthesis at least partially. The sensitivity of carbon assimilation to high CO_2 was different in different plant species. Acidification by CO_2 (or subsequent alkalization) was detected by measuring rapid CO_2-release from the tissue and by monitoring fluorescence of pH-indicating dyes which had been fed to the leaves through the petiole. The results indicate that two different mechanisms operate in leaves to achieve and maintain pH homeostasis. Rapid and efficient pH-adjustment is provided by proton/cation exchange across the tonoplast. Slower and less efficient regulation occurs by formation or consumption of base. In the presence of high CO_2 concentrations, protons are pumped from the cytosol into already acidic vacuoles. In turn, vacuolar cations replace exported protons in the cytosol permitting bicarbonate accumulation and increasing the pH of the acidified cytosol. Similarly effective and fast proton/cation exchange relieves acid-stress in the chloroplast stroma and permits photosynthesis to proceed with high quantum efficiency or high light-saturated rates in the presence of CO_2 concentrations which would, in the absence of fast cytoplasmic pH regulation, inhibit photosynthesis. By inference, proton/cation exchange must also occur across the mitochondria boundary. After cytoplasmic pH adjustment in the presence of high CO_2, removal of CO_2 results in transient cytoplasmic alkalization and, subsequently, in the return of cytoplasmic pH values to levels observed prior to acid-stress. In addition to fast pH regulation by rapid proton/cation exchange across biomembranes, slow base production (e.g. NH_3-formation) also con-tributes to relieving acid stress. Base produced in the presence of high CO_2 is rapidly consumed after removal of CO_2-Implications of the findings in regard to forest damage by potentially acidic air pollutants such as SO_2 are briefly discussed.
日本植物生理学会の論文

Fast Cytoplasmic pH Regulation in Acid-Stressed Leaves : ENVIRONMENTAL AND STRESS RESPONSES : MEMBRANES AND BIOENERGETICS

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