Photosynthetic carbon fixation and changes in the levels of carbohydrate under the influence of diuron or atrazine in some crop and weed species^*

The effect of diuron or atrazine on the rate of photosynthetic carbon fixation and carbohydrate metabolism of two crop (Pisum sativum and Pennisetum typhoides) and weed species (Amarmthus viridis and Cyperus rotundus) was studied. The results indicated an initial inhibition of the rate of photosynthetic carbon fixation in all the plant species. The resistant plants were able to exhibit a recovery of the net photosynthetic rate subsequently which the susceptible plants failed to recover. The inhibition of ¹⁴CO₂ fixation was correlated with decreased carbohydrate levels. The resistant plants possessed enhanced levels of starch. The data suggest that the possible accumulation of starch in the guard cell chloroplasts leading to the stomatal closure and thereby develop resistance and that changes in starch content might be partly responsible for herbicide selectivity.