Phloem transport

It has long been recognized that boron is required by higher plants [61, 62], and recent research indicates the involvement of boron in three main aspects of plant physiology cell wall structure, membrane function, and reproduction. In vascular plants, boron in solution moves in the transpiration stream from the roots and accumulates in the stems and leaves. Once in the leaves, the translocation of boron is limited and requires a phloem transport mechanism. The nature of this mechanism was only recently elucidated with the isolation of a number of borate polyol compounds from various plants [63-65]. These include sorbitol-borate ester complexes isolated from the floral nectar of peaches and mannitol-borate ester complexes from the phloem sap of celery. The implication is that the movement of boron in plants depends on borate-polyol ester formation with the particular sugar polyol compounds used as transport molecules in specific plants. 

While the evidence presented herein cumulatively supports the Involvement of the phloem In translocating PIIF out of wounded tomato plants, the velocity of transport appeared to be much slower than expected for phloem transport. When carefully measured from point to point In petiole tissue the velocity of assimilate out of leaves. In general. Is In the order of 1-5 cm/mln (7, . Our techniques do not allow direct measurements... 

Dreyer, D. L., Jones, . C. and Molyneux, R. L. (1985). Feeding deterrency of some pyrrolizidine, indolizidine, and quinolizidine alkaloids towards pea aphid (Acryrthosiphon pisum) and evidence of phloem transport of indolizidine alkaloid swainsonine. Journal of Chemical Ecology 11 1045-1051. 

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... 

It has been nearly a century and a half since Boussingault (1868) presented the hypothesis that the accumulation of assimilates in an illuminated leaf may be responsible for a reduction in the net photosynthetic rate of that leaf. According to the Munch hypothesis for phloem transport, the greater the sink strength, the greater the depression in solute concentration in the phloem at the sink. This increases the concentration differential between the source and sink, creating the hydrostatic pressure head that drives the system. 

Phloem transport is known for quinolizidine, pyrrolizidine alkaloids, and aconitine. 

As a rule of thumb, we can assume that all parts of an alkaloidal plant contain alkaloids, although the site of synthesis is often restricted to a particular organ, such as the roots or leaves. Translocation via the phloem, xylem, or apoplastically must have therefore occurred. Phloem transport has been demonstrated for quinolizidine, pyrrolizidine, and indolizidine alkaloids, and xylem transport for nicotine and tropane alkaloids 36,39,511). 

Table 1.3 summarizes the evidence for xylem and phloem transport of some SM. 

Table 1.3 Examples of xylem and phloem transport of secondary metabolites (SM)...

Chen, S., Retersen, B.L., Olsen, C.E., Schulz, A. and Halkier, B.A. (2001) Longdistance phloem transport of glucosinolates in Arabidopsis. Plant Physiol, 127, 194-201. 

Our results have led us to suggest a role for L-ascorbic acid in phloem transport (9) in which ascorbic acid is translocated from its biosynthetic site in the leaf to a catabolic site in the fruit cluster where C4-C5 cleavage of the carbon chain produces tartaric acid and a putative C2 precursor of carbohydrate biosynthesis. The sugar/organic acid balance that influences grape quality in winemaking may well be determined by the role of ascorbic acid as the precursor of tartaric acid. 

The capillary and porous system of the body exists in vascular tissue and intercellular spaces. Xylem forms an open conduit of relatively low hydraulic resistance that is filled with diluted mineral solution. Phloem exists in cells with a width ranging from 10 to 70 pm and a length from 100 to 500 pm in dicotyledons [4]. Their turgor is around 2 MPa (beetroot is 1.83 MPa) with a pressure gradient of 0.02-0.03 MPa/m [5,6]. As phloem transports substances of very different molecular weight, shape, charge, and surface activity along with water, it is presumed that the mechanism is an osmotically driven solution flow [6]. 

N.V. Parthasarathy, Sieve-element structure. In Encyclopedia of Plant Physiology. Vol. 2. Transport in Plants I. Phloem Transport (M.H. Zimmermann and J.A. MUbum, eds.). Springer Verlag, Berlin, Germany, 1975, p. 3. 

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