What effect do post-emergent contact herbicides have on plants?

Post-emergent contact herbicides are designed to target and damage the tissues of the plants they come into contact with. These herbicides work by being absorbed through the leaves and stems, leading to cell destruction and eventual plant death. The key point is that they cause injury to any part of the plant that they touch, making them effective against unwanted vegetation. This direct contact action differentiates them from systemic herbicides, which are absorbed and transported throughout the plant to affect all parts.

The other choices do not accurately reflect the nature of post-emergent contact herbicides. For instance, stating that they only affect newly sprouted leaves overlooks their capability to damage all exposed plant tissues regardless of age. Indicating that they prevent root development misrepresents their mechanism of action, as the injury occurs through direct contact, not through prevention of root growth. Lastly, asserting that they enhance plant growth would be incorrect, as their primary function is to damage or destroy unwanted plants. Therefore, the primary effect of these herbicides is indeed to cause injury to any plant part they touch.