Herbicides phloem mobility

More than 50% of the active ingredient that can penetrate into the leaf is taken up within the first 4—6 h after foUar apphcation and more than 90% is taken up within 24 h [37, 38]. Ullrich et al. [39] showed evidence for active uptake of the compound which is mediated by amino acid carriers. As already mentioned, air humidity modulates the uptake. Under conditions that favor rapid symptom development (high temperatures and high light intensity), the translocation of the compound is limited, whereas in plants kept in the dark after apphcation the active ingredient shows a considerable phloem mobility. Under field conditions glufosinate-ammonium is regarded as a contact herbicide with partial phloem mobility [40]. 

The physical properties of pyridine-containing acids make them particularly attractive starting points for combinatorial libraries for herbicide screens. Pyridines are relatively polar (log P = 0.60 for pyridine vs. 2.13 for benzene) (4) and the sinq>le unsubstituted acids are weakly acidic (nicotinic acid pKa 4.8). (5) Perhaps for these reasons, pyridine-containing acids have found considerable utility as herbicides. Representative examples of commercially successful, phloem mobile, pyridine acids (or dieir derivatives) include picloram, clopyralid, and triclopyr (synthetic auxins) imazethapyr and nicosulfuron (ALS inhibitors) clodinafop and haloxyfop (ACCase inhibitors). Each has a direct counterpart in the conesponding benzene series but, in these instances, the pyridine analog is invariably the superior herbicide. 

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