Effects of the Air Pollutant SO₂ on Leaves

Abstract

After S02 has entered leaves of spinach (Spinacia oleracea) through open stomata and been hydrated in the aqueous phase of cell walls, the sulfite formed can be oxidized to sulfate by an apoplastic peroxidase that is normally involved in phenol oxidation. The oxidation of sulfite is competitive with the oxidation of phenolics. During sulfite oxidation, the peroxidase is inhibited. In the absence of ascorbate, which is a normal constituent of the aqueous phase of the apoplast, peroxidative sulfite oxidation facilitates fast additional sulfite oxidation by a radical chain reaction. By scavenging radicals, ascorbate inhibits chain initiation and sulfite oxidation. Even after exposure of leaves to high concentrations of SO2, which inhibited photosynthesis, the redox state of ascorbate remained almost unaltered in the apoplastic space of the leaves. It is concluded that the oxidative detoxification of SO2 in the apoplast outside the cells is slow. Its rate depends on the rate of apoplastic hydrogen peroxide generation and on the steady-state apoplastic concentrations of phenolics and sulfite. The affinity of the peroxidase for phenolics is higher than that for sulfite.

Ascorbate is the natural substrate for plant peroxidases

“…The water-soluble apoplastic guaiacol peroxidases will catalyse the HzO2-dependent oxidation of phenolic compounds such as coniferyl alcohol. This oxidation is prevented and even reversed in the presence of ascorbic acid [25]. In this situation, hydrogen peroxide is reduced with the phenolic compound acting as the primary reducing agent and ascorbate acting as the secondary and ultimate reductant.…”

“…In the presence of 3,4-dihydroxyphenolic compounds, that contain a caffeic acid moiety, HRP will use ascorbic acid as a reductant (Table 1). This may be related to the ability of ascorbic acid to reduce o-quinone derivatives [30][31]. Hence, in stress situations peroxidases may contribute to H202 destruction in cellular compartments with neutral pH values.…”

Concerning a Dual Function of Coupled Cyclic Electron Transport in Leaves

“…Yet, because NADP drains electrons from the cyclic pathway, coupled cyclic electron transport is unlikely to occur when the chloroplast NADP system is largely oxidized. In isolated intact spinach chloroplasts, about 20% of extractable NADP is reduced even in the dark (22). On illumination, if CO2 is available, NADP reduction is increased only briefly.…”

“…After the induction phase, NADP is scarcely more reduced in the light than it was in the dark. In the absence of CO2, on the other hand, considerable reduction persists in the light (22). Such reduction, if it also occurs in leaves, would permit electrons to enter the cyclic pathway because Fd/NADP reductase is inhibited allosterically by NADPH, permitting electrons to accumulate at the level of Fd (23).…”

Changes in Dehydrodiferulic Acids and Peroxidase Activity against Ferulic Acid Associated with Cell Walls during Growth of Pinus pinaster Hypocotyl

“…Peroxidase activity was measured using ferulic acid as substrate (Takahama et al, 1992). The oxidation of ferulic acid was measured spectrophotometrically following the absorbance decrease at 310 nm (extinction coefficient = 11.3 miW1 cm-') in a reaction mixture (total volume 1 mL) containing 40 ~L M ferulic acid, 90 mM sodium-phosphate buffer, pH 4.0 to 5.5, 10 pg of protein extract, and 0.5 mM hydrogen peroxide.…”