Once inside the cytoplasm, the esters are hydrolyzed to form the ionized acid which is trapped, accumulates and then translocates with sugars. Buildup of herbicide within the sieve cell is the critical factor involving herbicide movement with sugars through the symplast. Injury progresses from new to older growth. Apoplastically translocated herbicides move with water and nutrients moving upward through the xylem into older leaves and concentrate in the leaf tips and leaf margins. Thus net movement is upward only.
Injury progresses from older to newer growth.
Herbicides that do not translocate are called contact herbicides and fail to move in the plant because of rapid cell membrane destruction. Injury is localized with each spray droplet. Thus, top kill with a contact herbicide requires good coverage of the foliage. The way in which a herbicide kills a plant is called its mode of action. I. Modes of action of foliar applied, translocated herbicides causing initial symptoms on new growth.