Mechanism of the inhibition of phloem loading by sodium sulfite: Effect of the pollutant on respiration, photosynthesis and energy charge in the leaf tissues

Abstract

As in other materials, sodium sulfite (Na2SO3) inhibited both respiration and photosynthesis in leaf tissues of broad‐bean L. cv. Aguadulce). Under our experimental conditions, photosynthesis was more sensitive (significant inhibition at 10 μM) to the pollutant than respiration (significant inhibition only for concentrations higher than 0.1 mM). Sulfite concentrations higher than 0.1 mM also caused the energy charge of leaf tissues to decline sharply. These results suggest that the long, term depolarization of the transmembrane potential difference noticed for concentrations of pollutant higher than 0.1 mM (Maurousset and Bonnemain, Physiol. Plant. 80: 233–237,1990) was mainly due to an indirect inhibition of the plasma membrane H+‐ATPase activity following the decrease of the available level of ATP.

Source-to-sink transport of sugar and regulation by environmental factors

“…In bean leaves (Minchin and Gould, 1986) and castor bean cotyledons (Lorenc-Plucinska and Ziegler, 1987), photo-assimilate translocation is also affected due to inhibited phloem loading, independently of reduced photosynthesis. In broad bean, H + /ATPase and SUTs have been identified as possible targets, both in leaf discs (Maurousset et al, 1992b) and plasma membrane vesicles (Maurousset et al, 1992a). More recently, an anatomical study showed that SO 2 led to a decreased number of phloem cells, but it was difficult to discriminate between general reduction of cambial activity and disturbances in the division of phloem mother cells (Kurczyńska, 1997).…”