Phloem translocation

The correlation between nicotine accumulation and its defensive role in M sylvestris has been convincingly demonstrated. Increased alkaloid production may also be demonstrated by true herbivory. Tobacco plants subjected to leaf damage showed a fourfold increase in the alkaloid content of their undamaged leaves. This resulted from increased alkaloid synthesis and, as a result, a tenfold increase in alkaloids in the xylem. Experimental evidence has indicated that alkaloid induction maybe triggered by a phloem-translocated signal (Hartmann, 1991 and references therein). 

Ouimette, D.G. and Coffey, M.D. (1990) Symplastic entry and phloem translocation of phosphonate. Pestic Biochem Physiol., 38, 18-25. 

Fromm and Bauer [36] found that action potentials in maize sieve tubes change phloem translocation. Using macro- and microautoradiography in mature leaves of maize, Fromm and Bauer [36] studied the inhibition of phloem translocation caused by electric and cold shock. They stimulated the leaf tip with ice water and found that the velocity of signal transmission was 3-5cm/s. Upon stimulation, the microelectrode recorded a basipetally propagating action potential with a depolarizing amplitude of 80 mV in the sieve tubes (Fig. 2). During electrical stimulation, the action potential was measured in the sieve-tube system with a speed of 5cm/s. 

BromUow, R. H., Chamberlain, K. and Evans, A. (1990). Physico-chemical aspects of phloem translocation of herbicides. WeedScience, 38,305-14. 

Figure 7. The time course of percent incorporation into ethanol insolubles of phloem translocated sucrose (o) and I -deoxy-l -fluorosucrose ( ) in soybean first trifoliolate leaves of various ages (%FLL is the percent of final leaf length of a comparable leaf at maturity). Radiolabled sucrose and/or 1 -deoxy-1 -fluorosucrose were supplied as described in Figure 3 and importing leaves were either removed from sampling at time points or parts of the leaf were removed at time points and extracted as described in the text. (Reproduced with permission from ref. 16. Copyright 1987 The American Society of Plant Physiologists.)...

A particular type of tissue is the vascular one. It contains xylem and phloem, which form bundles. Xylem is present in elongated cells with perforated end walls that no longer contain viable protoplasm. Xylem, in other words, forms open dead vessels that provide a way of transportation for minerals and water from roots to other parts of a plant. Phloem is present in elongated viable cells that have sieve end plates. Phloem translocates a solution of sugars, amino acids, and other nutritious substances. 

Gunning BBS, Pate JS, Briarty LG (1968) Specialized transfer cells in minor veins of leaves and their possible significance in phloem translocation. J Cell Biol 37 147-171... 

Canny MJ (1973) Phloem translocation. Univ Press, Cambridge... 

We determined the phloem systemicity of RH7988 and 15 commercial products using a procedure similar to that of Look et aL (9). Test solutions (in 1 1 acetone-methanol, v/v) were were applied onto cotyledon leaves of broccoli seedlings with a micropipet (10 pi/ leaf). The treated cotyledon leaves were detached from the plants 3 days after the treatment and the plant terminals artificially infested with green peach aphid. The percent aphid mortality was determined 2 days after infestation. Results of the test (Table VII) showed RH7988 to have outstanding phloem translocation. None of the commercial products showed efficacy at our use rate. 

For a full and detailed description of the vascular systems of plants, the reader is directed to a standard text such as Esau. The movement of solutes within plants takes place largely by two pathways. One route is via the extraprotoplasmic continuum (apoplast) of the plant and includes transport over short distances through the intercellular spaces and over long distances in the xylem vessels. Such transport is normally in an upward direction (acropetal). The second route is via the cytoplasmic continuum (symplast) of the plant and includes short-distance cell-to-cell transport through plasmodesmata and long-distance transport in the phloem sieve cells. Phloem translocation takes place in both upward and downward (basipetal) directions to the sites of new growth. 

Some groups of herbicides depend upon phloem translocation to reach their sites of action. For example, the phenoxyalkanoic acid herbicides, which act on cell growth and development, and glyphosate, which inhibits the biosynthesis of aromatic amino acids, are translocated from leaves to all sites of active growth within the plant. Similarly, the sulfonylureas and imidazolinones, which inhibit acetolactate synthase, are, when applied postemergence, translocated in the phloem to all parts of the plant. 

The use of different experimental systems and the lack of adequate detail in many articles concerning translocation make comparisons difficult a more consistent approach would be very helpful. While such consistency is less easy to achieve in the case of phloem translocation, results from studies on uptake and translocation following application to roots via nutrient solution can readily be presented in a standard way (see Sections 9.S.2.2 and 9.3.2.3). 

Bentazon, mefluidide, and perfluidone are of intermediate lipophilicity with pKa = 3.5, 4.6, and 2.5, respectively. Mefluidide is phloem translocated, but the contact herbicide bentazon is not moved symplastically perfluidone moves in the xylem following uptake by roots, but experiments testing its symplastic transport have not been reported. 

Fig. 2. Presumed pathways of incorporation of ammonia into uieides and amino compounds commonly generated during nitrogen metabolism of plant tissues. The scheme assumes that sucrose of phloem translocate provides the major source of carbon. Note carboxylation reactions involving additional inputs of carbon.

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