Ethoxyzolamide repression of the low photorespiration state in two submersed angiosperms

Net photosynthesis in the submersed angiosperms Myriophyllum spicatum L. and Hydrilla verticillata (L.f.) Royal was inhibited by 21% O₂, but the degree of inhibition was greater for plants in the high than in the low photorespiratory state. Increasing the CO₂ concentration from 50 through 2,500 l l^-1 decreased the O₂ inhibition of the high-photorespiration plants in a competitive manner, but it had no effect on the O₂ inhibition of plants in the low photorespiratory state. Carbonic-anhydrase activity increased by almost threefold with the induction of the low photorespiratory state. Ethoxyzolamide, an inhibitor of carbonic anhydrase, reduced the net photosynthesis of low-photorespiration Myriophyllum and Hydrilla plants by 40%, but their dark respiration was unaffected. This ethoxyzolamide inhibition of net photosynthesis exhibited a competitive response to CO₂ concentration, resulting in a decrease in the apparent affinity of photosynthesis for CO₂. The net photosynthesis of plants in the high photorespiratory state was inhibited only slightly by ethoxyzolamide, and this inhibition was independent of the CO₂ level. Ethoxyzolamide treatment caused an increase in the O₂ inhibition of net photosynthesis of plants in the low photorespiratory state. Ethoxyzolamide increased the low CO₂ compensation points of low-photorespiration Myriophyllum and Hydrilla, but the values for the high-photorespiration plants were unchanged. In comparison, the CO₂ compensation points of the terrestrial plants Sorghum bicolor (C₄), Moricandia arvensis (C₃-C₄ intermediate) and Nicotiana tabacum (C₃) were unaltered by ethoxyzolamide treatment. These data indicate that the low photorespiratory state in Myriophyllum and Hydrilla is repressed by ethoxyzolamide treatment, thus implicating carbonic anhydrase as a component of the photorespiration-reducing mechanism in these plants. The competitive interaction of CO₂ with ethoxyzolamide provides evidence that the low photorespiratory state in submersed angiosperms is the result of some type or types of CO₂ concentrating mechanism. In Myriophyllum it may be via bicarbonate utilization, but in Hydrilla it probably takes the form of an inducible C₄-type system.Abbreviations PEP phosphoenolpyruvate - RuBP ribulose bisphosphate