Herbicides with these groups include sethoxydim, bentazon, and sulfonylureas. They pass through the plasma membrane at special specialized protein sites, called carriers. These herbicides accumulate in the sieve cells and move source to sink with sugars. 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.