What makes a molecule a pre- or a post-herbicide - how valuable are physicochemical parameters for their design?

Pre-emergence herbicides are taken up by seeds before germination and by roots, hypocotyls, cotyledons, coleoptiles or leaves before emergence, whereas post-emergence herbicides are taken up primarily by foliage and stems. Most modern pre-emergence herbicides are lipophilic, but post-emergence herbicides may be lipophilic or hydrophilic. The metabolic conversion of herbicides to inactive or active metabolites after plant uptake is of major importance for some compound classes. Several herbicides are proherbicides as for example some acetyl-coenzyme A carboxylase (ACCase)-inhibitors. The physicochemical characteristics of proherbicides and herbicides are usually unrelated. A major role can be attributed to the site of action at a cellular level. A great number of herbicides such as photosystem II (PS II)-inhibitors, protoporphyrinogen oxidase (PPO)-inhibitors or carotenoid biosynthesis inhibitors require light for activity. Others, such as cellulose-biosynthesis and mitotic inhibitors seem to be primarily active in belowground organs. Several lipophilic barriers against the uptake of xenobiotics exist in aboveground and belowground plant parts. The relevance of these barriers needs, however, further clarification. Uptake and translocation models are valuable tools for the explanation of the potential movement of compounds. Many factors other than uptake and translocation have, however, to be considered for the design of herbicides. For post-emergence herbicides, ultraviolet (UV) light stability, stability in formulations, and mixability with other agrochemicals have to be kept in mind while, in addition to the aforementioned factors soil interaction plays a major role for pre-emergence herbicides. In our opinion, general physicochemical characteristics of pre- or post-emergence herbicides do, unfortunately not exist yet.