Effects of the Phosphoenolpyruvate Carboxylase Inhibitor 3,3-Dichloro-2-(Dihydroxyphosphinoylmethyl)propenoate on Photosynthesis: C(4) Selectivity and Studies on C(4) Photosynthesis.
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 HCO(3) (-). When fed to leaf sections via the transpiration stream, DCDP at 1 millimolar strongly inhibited photosynthesis in C(4) species (79-98% inhibition for a range of seven C(4) species), but only moderately in C(3) species (12-46% for four C(3) species), suggesting different mechanisms of inhibition for each photosynthetic type. The response of P. miliaceum (C(4)) net photosynthesis to intercellular pCO(2) showed that carboxylation efficiency, as well as the CO(2) saturated rate, are lowered in the presence of DCDP and supported the view that carboxylation efficiency in C(4) species is directly related to PEP carboxylase activity. A fivefold increase in intercellular pCO(2) 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 CO(2) by the bundle sheath cells makes any significant contribution to photosynthetic CO(2) assimilation in C(4) species. The results support the concept that C(4)-selective herbicides may be developed based on inhibitors of C(4) pathway reactions.
Jenkins
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Commonwealth Scientific and Industrial Research Organization, Division of Plant Industry, GPO Box 1600, Canberra City, A.C.T. 2601, Australia.