The submersed angiosperms Myriophyllum spicatum L. and Hydrilla verticillata (L.f.) Royal exhibited different photosynthetic pulse-chase labeling patterns. In Hydrilla , over 50% of the 14 C was initially in malate and aspartate, but the fate of the malate depended upon the photorespiratory state of the plant. In low photorespiration Hydrilla , malate label decreased rapidly during an unlabeled chase, whereas labeling of sucrose and starch increased. In contrast, for high photorespiration Hydrilla , malate labeling continued to increase during a 2-hour chase. Thus, malate formation occurs in both photorespiratory states, but reduced photorespiration results when this malate is utilized in the light. Unlike Hydrilla , in low photorespiration Myriophyllum , 14 C incorporation was via the Calvin cycle, and less than 10% was in C 4 acids. Ethoxyzolamide, a carbonic anhydrase inhibitor and a repressor of the low photorespiratory state, increased the label in glycolate, glycine, and serine of Myriophyllum . Isonicotinic acid hydrazide increased glycine labeling of low photorespiration Myriophyllum from 14 to 25%, and from 12 to 48% with high photorespiration plants. Similar trends were observed with Hydrilla . Increasing O 2 increased the per cent ( 14 C)glycine and the O 2 inhibition of photosynthesis in Myriophyllum . In low photorespiration Myriophyllum , glycine labeling and O 2 inhibition of photosynthesis were independent of the CO 2 level, but in high photorespiration plants the O 2 inhibition was competitively decreased by CO 2 . Thus, in low but not high photorespiration plants, glycine labeling and O 2 inhibition appeared to be uncoupled from the external (O 2 )/(CO 2 ) ratio. These data indicate that the low photorespiratory states of Hydrilla and Myriophyllum are mediated by different mechanisms, the former being C 4 -like, while the latter resembles that of low CO 2 -grown algae. Both may require carbonic anhydrase to enhance the use of inorganic carbon for reducing photorespiration.
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