FAST CYTOPLASMIC PH REGULATION IN ACID-STRESSED LEAVES

Induction of photosynthesis in leaves was prolonged, and steady state photosynthesis was inhibited by very high CO2 concentrations which cau se cytoplasmic acidification. Prolonged exposure to high CO2 relieved initially observed inhibition of photosynthesis at least partially. Th e sensitivity of carbon assimilation to high CO2 was different in diff erent plant species. Acidification by CO2 (or subsequent alkalization) was detected by measuring rapid CO2-release from the tissue and by mo nitoring fluorescence of pH-indicating dyes which had been fed to the leaves through the petiole. The results indicate that two different me chanisms operate in leaves to achieve and maintain pH homeostasis. Rap id and efficient pH-adjustment is provided by proton/cation exchange a cross the tonoplast. Slower and less efficient regulation occurs by fo rmation or consumption of base. In the presence of high CO2 concentrat ions, protons are pumped from the cytosol into already acidic vacuoles . In turn, vacuolar cations replace exported protons in the cytosol pe rmitting bicarbonate accumulation and increasing the pH of the acidifi ed cytosol. Similarly effective and fast proton/cation exchange reliev es acid-stress in the chloroplast stroma and permits photosynthesis to proceed with high quantum efficiency or high light-saturated rates in the presence of CO2 concentrations which would, in the absence of fas t cytoplasmic pH regulation, inhibit photosynthesis. By inference, pro ton/cation exchange must also occur across the mitochondrial boundary. After cytoplasmic pH adjustment in the presence of high CO2, removal of CO2 results in transient cytoplasmic alkalization and, subsequently , in the return of cytoplasmic pH values to levels observed prior to a cid-stress. In addition to fast pH regulation by rapid proton/cation e xchange across biomembranes, slow base production (e.g. NH3-formation) also contributes to relieving acid stress. Base produced in the prese nce of high CO2 is rapidly consumed after removal of CO2. Implications of the findings in regard to forest damage by potentially acidic air pollutants such as SO2 are briefly discussed.