A Simple Theory Regarding Ambimobility of Xenobiotics with Special Reference to the Nematicide, Oxamyl

Abstract

A theory is presented to explain the phloem mobility of certain systemic xenobiotics that are not weak acids. It is shown that there is a theoretically optimum permeability that permits optimum circulation through the symplasm and apoplast (including the phloem and xylem) of Solanum tuberosum plants. The optimum permeability is large enough to permit substantial passive permeation into sieve cells in the source leaf and yet is small enough to permit phloem ransport with some retention. The optimum permeability is a function of the velocity of sap flow in sieve tubes, the radius of the sieve tube, the over-all length of the plant, and the length of the carbohydrate and xenobiotic sources. It is argued that the nematicide, oxamyL is near the optimum permeability under some experimental conditions. It is shown that depending on the strength of the carbohydrate sink in roots or growth points and depending on the permeability of the xenobiotic, there can be passive accumulation of xenobiotics in the sieve tubes in the carbohydrate sink regions.The patterns of translocation which various xenobiotic substances display after entering a plant have been described for many chemicals, especially herbicides (1). Two distinctive patterns of transport were discerned, namely the apoplastic pattern and the symplasmic pattern. Movement in the apoplast occurs mainly via the xylem and the cell walls of the parenchyma cells, while movement in the symplasm occurs via the phloem and the interconnected protoplasts of the parenchyma cells. Chemicals moving by both routes are called ambimobile (5). It has been assumed that the potential for a chemical to penetrate the plasmalemma of the cells is correlated with the over-all pattern of translocation it achieves in the plant as a whole. It was originally thought that chemicals displaying an apoplastic transport pattern were unable to penetrate the plasmalemma of plant cells and thus were excluded from the plant symplasm but recent investigations have shown that several chemicals termed apoplastic do penetrate plant cell membranes (4, 7-10, 13). Peterson and Edgington (7) have called such chemicals pseudoapoplastic. They suggested that the permeability of the plasmalemma to the pseudoapoplastic chemical is so great that it cannot be retained in the symplasm for long

Study of Root Uptake and Xylem Translocation of Cinmethylin and Related Compounds in Detopped Soybean Roots Using a Pressure Chamber Technique

“…Both factors would make the partitioning of these compounds into xylem sap less than optimal. It is known that the phloem delivery of a xenobiotic can be strongly influenced by many plant parameters (14,19). The differences in plant species, plant size, and experimental conditions between this study and the one by Briggs et al (3) may account for the small difference in the observed optimal log Kow values for TSCF.…”

“…It should be pointed out that the root pathway to xylem for the foliarly applied ambimobile xenobiotics (11,19) differs considerably from that of a soil applied xenobiotic. Following a foliar application, the phloem-mobile xenobiotic gets unloaded (symplastically, apoplastically, or both; [9]) from phloem in, or near, the stele ofthe root.…”

Short‐ and Long‐distance Transport of Boric Acid in Plants

“…at least an order of magnitude less than the concentration in the phloem (0-5 to 1 mM). Further analysis can be made using the model of Tyree, Peterson and Edgington (1979) for the distribution within plants of 'plant protection' compounds of varying permeability values. Their analysis showed that continued cycling around aim tall plant of a foliarsupplied compound with a phloem transport velocity of 0-3 m h~^ was maximal w^hen 9-2 244 J-P was 3 X 10~* cm s~^.…”

Plasmodesmata: Symplastic Transport of Herbicides Within the Plant

“…Translocation through phloem is fundamental in the distribution of either natural or synthetic chemical compounds from mature leaves to growth regions in roots and stem (Vidal, 2002). There are mathematical models that allow efficient calculation of translocation rates of xenobiotics via phloem, according to membrane permeability, size of the phloem loading area, and other parameters (Tyree et al, 1979). After the herbicidal molecule is translocated via phloem and entered into an adjacent cell, neighboring cells also need to be achieved to allow for proper action of the herbicide.…”