NADP –ME is the key enzyme for decarboxylation reactions in C4 CO2 concentration pathways. So, Amaranthus viridis has been evaluated with regards to photosynthetic NADP-malic enzyme for its response under light and darkness. Illumination (1000–1200 µEm-2s-1) for 40 minutes under 2 mM bicarbonate (HCO3-) sensitivity increased activity by 1.97 & 3.77-fold over darkness under 4.0 mM and 0.01 mM malate respectively. Limiting (0.01 mM) and saturated (4.0 mM) malate concentration had significant changes in enzyme activities. The different kinetic parameters indicated had the feedback inhibition under illumination. The activity with the inducer (citrate and succinate) and inhibitor (pyruvate and oxalate) was significant with substrate concentrations. Dithiol had reduced the activity by inhibition of the diminishing effect of light activation treatment. Therefore, NADP-ME is stringently regulated by redox changes with illumination as a key factor. Moreover, the pattern of polymorphic gene expression may be supportive in molecular modulation under light/darkness. This study may support the role of NADP-ME as a biomarker for C4 weed species under oxidative stress through light/darkness.