Phloem mobility

Grayson, B.T., Kleiser, D.A. (1990) Phloem mobility of xenobiotics. IV. Modeling of pesticide movement in plants. Pestic. Sci. 30, 67-79. 

Jerusalem artichokes temporarily store assimilates in several locations within the plant that are in excess to the amount needed for structural and maintenance purposes. Most of these reserves are reallocated to the tubers during bulking. While a cross section of assimilates is found in these sites, carbohydrates predominate, of which inulin is the primary storage form. In addition to mono- and disaccharides and small amounts of starch, a number of nutrients are found, many of which are phloem mobile and reallocated to the tubers during the latter part of the growing season. 

In this review I address the phloem mobility of agrichemicals from the viewpoint of a phloem physiologist. First, I will present an overview of the physiological basis of translocation by using... 

Edgington and Peterson (4) have subdivided apoplastic xeno-biotics into two classes. Euapoplastic (only transported in the apoplast) and pseudoapoplastic (transport occurs mainly in the xylem but entry into the symplast occurs). Most traditional "apoplastic" chemicals are now known to really be pseudoapoplastic chemicals, e.g., atrazine, diuron, oxamyl, etc. The unresolved question is why don t these pseudoapoplastic chemicals which cross the cell membranes and enter the symplast remain in the symplasm of the phloem There have been numerous studies focusing on the molecular requirements for phloem mobility (1-5), In general, there is not a good correlation between phloem mobility and water solubility, metabolism of the xenobiotic, or the presence of various substitution groups in a molecule. 

There is qualified support for the weak-acid hypothesis, particularly for compounds such as 2,4-dichlorophenoxyacetic acid. Crisp and Look (.5) compared the phloem mobility of several synthetic 4-chlorophenoxy derivatives. The carboxyl derivative was loaded and transported in the phloem, whereas derivatives in which the COOH group was replaced by an ethyl ester, amide, ketone, alcohol, or amino group were not translocated. 

Although the weak acid hypothesis appears to explain the mobility of compounds such as chlorophenoxy derivatives, there are several exceptions to the weak-acid hypothesis (4, 1A> 15). For example, some xenobiotics are phloem mobile but are not weak acids and do not appear to be converted to a weak acid prior to transport (e.g., amitrole, oxamyl). Also, some xenobiotics (e.g., glyphosate) which have an ionizable COOH group are loaded into the phloem independently of apoplast pH. These should lose their mobility under pH conditions which ionize the chemical in the free space. Furthermore, accumulation of the weak acid glyphosate against a concentration gradient does not occur (14). 

The "intermediate diffusion" hypothesis proposes that the critical determinant of phloem mobility is the optimum permeability coefficient of a given molecule, P. P is calculated as ... 

In summary, the intent of this review was to examine the systemic transport of xenobiotics from the viewpoint of a phloem physiologist in order to highlight certain biochemical, physiological, and structural features of the translocation system that may be relevant to the future design of phloem mobile chemicals. I hope the review has taken a modest step in that direction. 

It is suggested that Mo in Vigna mungo is phloem-immobile at a low Mo supply, but is phloem-mobile in all plant parts (with the possible exception of stem segments) at an adequate Mo supply (Jongruaysup,... 

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]. 

Whereas 78 only translocated in the phloem of plants, compound 79 is xylem and phloem mobile. 

Table 2 summarizes the redistribution properties of strobilurins from the two references and our experimental HPLC-logPfi and gas chromatography vapor pressuref5 data for some of the referenced compounds. Literature values for the logP and vapor pressure were taken from the 13 Edition of the Pesticide Manual. The calculations of RCF and TSCF were calculated according to Equations 1 and 2. The phloem mobility was calculated for a IS cm potato plant, according to the equation developed by Daniel Kleier and co-workers. ("/7 ... 

All of the compounds in Table 2 are too lipophilic to be phloem mobile with the possible exception of metominostrobin. When the log of the phloem concentration factor is greater than -4, it is considered to be phloem mobile. We have often seen that when the HPLC-logP is less than about 2.5 that phloem mobility occurs. This is in agreement with Kleier s phloem mobility model when solved for the phloem mobility of a neutral compound, as seen in Figure 4. 

If the logP of metominostrobin is truly represented by a logP of 2.32, then it might be expected to exhibit limited phloem mobility. If the logP is better represented by a higher logP, then one is apt to see little phloem activity because the log of the concentration factor falls very quickly when the logP exceeds 2.5. 

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