What is the mode of action of contact herbicides?

The mode of action of contact herbicides primarily involves causing rapid damage to the cell membranes of plant tissues. When contact herbicides are applied, they penetrate the plant cells and can lead to the disruption of cellular integrity, which results in physiological stress and ultimately the death of the plant. This direct action on the plant tissues means that contact herbicides are effective on the foliage and other green parts of the plant but do not affect the root system directly.

The other options describe mechanisms that are characteristic of different classes of herbicides. For example, disrupting the root system is more aligned with systemic herbicides that are absorbed and translocated throughout the plant. Promoting new growth is not associated with herbicide action, as these products are meant to inhibit or kill unwanted plants. Inhibiting photosynthesis pertains to another type of herbicide, typically systemic ones that impact the plant's ability to produce energy effectively. Thus, the action of contact herbicides is specifically characterized by their immediate effect on cell membranes, leading to plant death.