Influence of monuron on photosystem II and light-dependent 14CO2 fixation in isolated cells of C3 and C4 plants

Cells were isolated from the developing leaves of Ipomoea aquatica (water spinach), a C₃ plant, and three kinds of C₄ plants, namely, Digitaria sanguinalis (NADP⁺-specific malate dehydrogenase type), Panicum miliaceum (NAD⁺-specific malic enzyme type), and Panicum texanum (phosphoenopyruvate carboxy kinase type), to study the effect of monuron on light-dependent ¹⁴CO₂ fixation and oxygen evolution. Bundle sheath cells, except for those of D. sanguinalis, and mesophyll cells of all plants fixed approximately the same amount of ¹⁴CO₂. Monuron, at the range used (2 to 10 × 10^?7M), showed strong inhibition in the mesophyll cells of water spinach and in bundle sheath cells of P. miliaceum and P. texanum and moderate inhibition in the mesophyll cells of all C₄ plants. In the bundle sheath cells of D. sanguinalis the low rate of ¹⁴CO₂ fixation was stimulated to some extent by the addition of malate and ribose 5-phosphate. The I₅₀ value was 6 × 10^?7M for the sensitive cells. Monuron inhibited the oxygen evolution of all seven cell types and their I₅₀ values varied between 3 × 10^?7 to 6 × 10^?7M. The differential response of isolated plant cells from different species to light-dependent CO₂ fixation in the presence of monuron may also be involved in urea herbicide selectivity and undoubtedly is due to the differential photosynthetic pathways present nn them.