Flux of SO₂ into Leaf Cells and Cellular Acidification by SO₂

Abstract

ABSTRACIA comparison of fluxes of SO2 from the atmosphere into leaves with fluxes across biomembranes revealed that, apart from the cuticle, the main barrier to S02 entry into leaves are the stomates. S02 fluxes into leaves can be calculated with an accuracy sufficient for many purposes on the assumption that the intracellular S02 concentration is zero. SO2 entering green leaf cells is trapped in the cytoplasm. In the light, the products formed in its reaction with water are processed particularly in the chloroplasts. Flux of SO2 to the acidic central vacuole of leaf cells is insignificant. Intracellular acidification of barley mesophyll protoplasts by S02 was measured by the uptake of '4C-labeled 5,5-dimethyl-oxazolidine-2,4-dione. The measured acidification was similar to the acidification calculated from known buffer capacities and the rate of SO2 influx when the H@/SO2 ratio was assumed to be 2. A comparison of photosynthesis inhibition by S02 with calculated acidification revealed different mechanisms of inhibition at low and at high concentrations of SO2. At very low concentrations, inhibition by S02 was even smaller than expected from calculated acidification. The data suggest that, ifacidification cannot be compensated by pH-stabilizing cellular mechanisms, it is a main factor of S02 toxicity at low SO2 levels. At high levels of SO2, anion toxicity and/or radical formation during oxidation of SO2 to sulfate may play a large role in inhibition.Diffusion of gases into a leaf is hindered by several barriers. Stomata can regulate entry of a gas into the interior of a leaf. Inside, aqueous phases of cellular systems are less important in restricting gas flux than lipid phases, if the solubility of a gas in an aqueous phase is high or if the gas can react with water in a reversible reaction to form ionic species which can contribute to and may actually dominate diffusional fluxes towards a sink. Both conditions are fulfilled by gases such as C02, which is the substrate of photosynthesis, and the air pollutants SO2 and NO2.When aqueous phases offer much less resistance to the diffusional flux of a gas or its reaction products than lipid phases, it may be sufficient for a flux analysis to know the resistance of biomembranes which have to be crossed by a gas on its way into a leaf cell or its organelles. In the previous publication (29) thereby releasing protons. Protons are trapped by alkali. As the pK, of sulfurous acid is about 1.8 and the pK2 about 7, the HS03-anion dominates below pH 7 and the S032-anion above pH 7. Thus, the products of the reaction of S02 with water will accumulate particularly in alkaline cell compartments. Release of protons in these compartments will decrease pH. The extent of the decrease is a function of the amount of SO2 trapped (i.e. of the flux of S02 into the leaf), the pH of the compartment and its buffering capacity. If the decrease in pH caused by influx of SO2 cannot be compensated by cellular mechanisms aimed at stabilizing intracellular pH, metabolism will be affected. For ...

Uptake and metabolism of sulphur dioxide by Arabidopsis thaliana

“…The sulphite/bisulphite formed when SOd issolves might ititerfere with several tnetabolic processes (Malhotra & Hocking, 1976). On the other hand, the sulphuric acid, which is formed upon oxidation of sulphite, might acidify the plant interior and disturb plant futictioning (Alscher et al 1987;Pfanz et al, 1987). The formation of active ox>'gen species and free radicals has also been proposed as a factor in the phytotoxicity of SO.^ (Alscher & Anithor, 1988), but their occurrence and significance at realistic SO., concentrations have been questiotied (De Kok & Stulen, 1993).…”

“…Platit shoots form a sink for atmospheric SO.^, which is predominantly taken up through the stomata. The liigh uptake rate of SO., is caused by its high solubility and rapid hydration iti the aqueous pbase in the plant (Pfanz et al, 1987;De Kok, 1990).…”

Kinetics Analysis of the Plasma Membrane Sucrose-H+ Symporter from Sugar Beet (Beta vulgaris L.) Leaves

“…Vd. 106, 1994 pH units or more have been observed following exposure to acidic gases such as SOz or NOz (Pfanz et al, 1987), although acidification of this magnitude is likely responsible for the tissue damage associated with exposure to low concentrations of these gases. Values for the apoplastic pH vary widely between 5.0 and 6.5 with extreme values of 4.0 and 7.0 having been reported (reviewed by Grignon and Sentenac, 1991).…”

Sulfite Reductase Protects Plants against Sulfite Toxicity

“…Being a strong nucleophile, sulfite/SO 2 can harm the plant tissue via sulfitolysis, which leads to chlorophyll destruction, suppression of photosynthesis, necrotic damage, and growth retardation (Pfanz et al, 1987;Nandi et al, 1990;Noji et al, 2001).…”