Which herbicide active ingredient inhibits photosynthesis?

The active ingredient that inhibits photosynthesis is Diquat. This herbicide functions by disrupting the photosynthetic processes in plants, leading to their death. Diquat acts quickly by generating reactive oxygen species that damage the chloroplasts in plant cells, effectively stopping their ability to photosynthesize and produce energy.

In contrast, Glyphosate works by inhibiting a specific enzyme pathway (the shikimic acid pathway) that is not present in animals, allowing it to selectively target and kill broadleaf and grassy weeds. Nicotine is a natural alkaloid that acts as a neurotoxin to insects but does not inhibit photosynthesis in plants. Lastly, 2,4-D is a synthetic auxin that mimics plant growth hormones, leading to uncontrolled growth and eventual death, rather than directly inhibiting the photosynthetic process. Thus, Diquat is specifically effective in targeting the photosynthetic pathways in plants, making it the appropriate answer for this question.