Seedling Growth Inhibitors
There are three major chemical classes of herbicides which are termed seedling growth inhibitors: the acetanilides (e.g. alachlor, dimethenamid and metolachlor), dinitroaniline herbicides (e.g. ethalfluralin, oryzalin, pendimethalin and trifluralin) and thiocarbamates (e.g. butylate, EPTC and triallate). Seedling growth inhibitors are soil applied herbicides that prevent seedling from developing normally. After application, these herbicides must be moved downward in soil by mechanical incorporation, rainfall or irrigation to the depth where weed seed germinates.
The dinitroaniline herbicides inhibit root growth by binding to the protein responsible for the production and function of the spindle apparatus. The spindle apparatus pulls replicated chromosomes apart during mitosis (cell division) ensuring that the new developing cells have their full compliment of DNA. Disruption of the spindle apparatus caused cell division/replication to stop. Symptoms on plants affected by dinitroaniline herbicides include roots that appear stunted, swollen or club shaped. Stems near the soil line may appear swollen. Reddening of foliage may be seen. Seedlings appear stunted and do not develop normally. The dinitroaniline herbicides are primarily thought of as grass herbicides but many small seeded dicots are controlled depending on the herbicide/dicot combination. Translocation of these herbicides within seedlings is very limited.
The acetanilides and the thiocarbames primarily inhibit shoot growth. Symptoms on plants affected by these herbicides include shoots of monocots that unfurl underground or fail to emerge. If monocot shoots emerge, they fail to unfurl and may have a "buggy whip" appearance. Affected dicot plants have leaves that are crinkled, rough in appearance and may be "heart" shaped. Roots can appear stunted. Affected crops are slow to emerge or emergence is not uniform. Like dintroanailine herbicides, these herbicides are more active on grasses than dicots.
Photographs by Dr. Michael Pfeiffer
Pendimethalin - Wheat
click for a larger image
Pendimethalin - Ryegrass
click for a larger image
Prodiamine - Ryegrass.
click for a larger image