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Metabolism and Enzymology

Effects of the Phosphoenolpyruvate Carboxylase Inhibitor 3,3-Dichloro-2-(Dihydroxyphosphinoylmethyl)propenoate on Photosynthesis

C₄ Selectivity and Studies on C₄ Photosynthesis

Commonwealth Scientific and Industrial Research Organization, Division of Plant Industry, GPO Box 1600, Canberra City, A.C.T. 2601, Australia

The effect of 3,3-dichloro-2-(dihydroxyphosphinoylmethyl)-propenoate (DCDP), an analog of phosphoenolpyruvate (PEP), on PEP carboxylase activity in crude leaf extracts and on photosynthesis of excised leaves was examined. DCDP is an effective inhibitor of PEP carboxylase from Zea mays or Panicum miliaceum; 50% inhibition was obtained at 70 or 350 micromolar, respectively, in the presence of 1 millimolar PEP and 1 millimolar HCO₃^–. When fed to leaf sections via the transpiration stream, DCDP at 1 millimolar strongly inhibited photosynthesis in C₄ species (79-98% inhibition for a range of seven C₄ species), but only moderately in C₃ species (12-46% for four C₃ species), suggesting different mechanisms of inhibition for each photosynthetic type. The response of P. miliaceum (C₄) net photosynthesis to intercellular pCO₂ showed that carboxylation efficiency, as well as the CO₂ saturated rate, are lowered in the presence of DCDP and supported the view that carboxylation efficiency in C₄ species is directly related to PEP carboxylase activity. A fivefold increase in intercellular pCO₂ over that occurring in P. miliaceum under normal photosynthesis conditions only increased net photosynthesis rate in the presence of 1 millimolar DCDP from zero to about 5% of the maximal uninhibited rate. Therefore, it seems unlikely that direct fixation of atmospheric CO₂ by the bundle sheath cells makes any significant contribution to photosynthetic CO₂ assimilation in C₄ species. The results support the concept that C₄-selective herbicides may be developed based on inhibitors of C₄ pathway reactions.

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