Xylem systemicity

Picoxystrobin, first announced in 2000 [45], is an outcome of ICI s optimization work on enol ethers. It combines two physicochemical properties, namely a relatively low octanol-water partition coefficient and an relatively high vapor pressure, which give it both systemic xylem mobility and episystemic distribution properties. Regarding episystemicity, it closes a gap in the biokinetic and biological properties of azoxystrobin. 

Some roots, however, display considerable ability to acclimate to subopti-mal temperatures. For example, in rye or barley acclimated by previous exposure to 8°C, net transport of K, Ca-, and H PO into the xylem sap of detached roots was enhanced by factors of 2 to 3 as compared to controls maintained at 20°C (193,194), with a threefold increase in flux of water to the xylem. These changes almost completely compensated for the smaller root systems that devel-... 

D. T. Clarkson. The influence of temperature on exudation of xylem sap from detached root systems of rye (Secaie cereale) and barley (Hordeum vidgare), Planta 752 297 (1976). 

D. T. Clark.son. M. G. T. Shone, and A. V. Wood, The effect of pre-treatment temperature on the exudation of xylem sap by detached barley root systems, Planta I213 (1974). 

Two complex tissues, the xylem and phloem, provide the conducting network or "circulatory system" of plants. In the xylem or woody tissue, most of the cells are dead and the thick-walled tubes (tracheids) serve to transport water and dissolved minerals from the roots to the stems and leaves. The phloem cells provide the principal means of downward conduction of foods from the leaves. Phloem cells are joined end to end by sieve plates, so-called because they are perforated by numerous minute pores through which cytoplasm of adjoining sieve cells appears to be connected by strands 5-9 pm in diameter.154 Mature sieve cells have no nuclei, but each sieve cell is paired with a nucleated "companion" cell. 

Plants possess a kind of circulatory system by which fluids are transported from the roots upward in the xylem and downward from the leaves through the phloem. Many compounds are carried between cells in this manner, while others are transported across cell membranes and against concentration gradients by active transport. A number of compounds that move between cells in either of these two manners have been classified as hormones.366-369 The major plant... 

Ros-Barcelo, A., Pomar, F., Lopez-Serrano, M., Martinez, P., Pedreno, M. A., 2002, Developmental regulation of the H2C>2-producing system and of a basic peroxidase isoenzyme in the Zinnia elegans lignifying xylem, Plant Physiol. Biochem. 40 325-332. 

Microautoradiographs of the harley leaf immediately after spraying showed the radioactive substance on the epidermis (Figure 6). However, a sequential time series of microautoradiographs of cryostat leaf sections (8 ym) shows that triadimefon and/or its metabolites are rapidly taken up (Figure 7). Subsequent transport takes place almost exclusively with the transpiration flow in the xylem. This transport system is then rapidly abandoned again so the active substance and/or metabolites move out of the transportation stream and are found in all leaf cells (29). 

These examples clearly show that it is not always desirable to develop fully systemic plant protection compounds, i.e. substances which migrate equally well both in the xylem and in the phloem. Transport in the xylem is often sufficient to effectively protect the plant. Due to the physiology of the plant, the storage organs which... 

Evidence was obtained recently that pesticide vapors may enter the air by still another mechanism, involving plant circulation and water loss (57). Rice plants were found to efficiently transport root-zone applied systemic carbamate insecticides via xylem flow to the leaves, eventually to the leaf surface by the processes of guttation and/or stomatal transpiration, and finally to the air by surface volatilization. Results from a model chamber showed that 4.2, 5.8, and 5.7% of the residues of carbaryl, carbofuran, and aldicarb, respectively, present in rice plants after root soaking vaporized within 10 days after treatment. The major process was evaporation of surface residues deposited by guttation fluid. 

Selective systemic herbicide, absorbed by roots and leaves with rapid translocation in xylem and phloem to the meristematic tissues... 

Systemic herbicide absorbed by roots and foliage with translocation in xylem and phloem and accumulation in meristetic regions... 

Systemic foliar fungicide with translocation acropetally in the xylem... 

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