Abstract
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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