Xylem transport

Xylem transport has been reported for tropane alkaloids and nicotine, which are synthesized in roots but accumulate in aerial parts. 

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). 

The photoreduction of xylem-transported ferric carboxylates, like citrate, is thought to be an important driving force in the reduction of iron in shoots thereafter, the distribution of iron to the leaves is probably mediated again by the NA-iron complex (Fig. 8.9). 

Table 29.2.3.6 shows the physicochemical properties of thiamethoxam (13). Its properties favor rapid and efficient uptake in plants and xylem transport [37, 53]. Through this systemic activity all plant parts situated acropetally from the application site can be protected. 

Kreuzwieser, J., Graus, M., Wisthaler, A., Hanse, A., Rennenberg,H.,Schnitzler, J.P. (2002) Xylem-transported glucose as an additional carbon source for leaf isoprene formation in Quercus robur. The New Phytologist, 156, 171-178. 

Bradford KJ, Yang SF (1980) Xylem transport of 1-aminocyclopropane-l-carboxylic acid, an ethylene precursor, in waterlogged tomato plants. Plant Physiol 65 322-326... 

It is clear that we must look around for another mechanism of xylem transport. Root pressure can only be a reinforcing measure, particularly in the spring, when very heavy demands are made on the xylem before the leaves have burst forth and the transpiration surface of the leaves is not yet present. This gives us our cue transport in the xylem occurs, in the main, as a result of suction from above rather than pressure from below. The precondition for this suction is transpiration. 

Once the chemical passes through the root membrane, and depending on its properties, it can be transported to other parts of the plant by way of the flow of sap in the xylem and phloem channels. Xylem channels conduct the unidirectional flow of water and nutrients from roots to the photosynthetic sections of the plant while phloem is the bidirectional flow that distributes sugars and other photosynthetic products throughout the plant. However, within the xylem, lateral movement to adjacent cells occurs and may provide a pathway for contaminants to partition into the phloem (Hendrix, 1995 Marschner, 1995). Xylem transport rates are on the order of 10 cm/min while phloem transport rates are much slower, 1 cm/min (Lang, 1990). Xylem transport rates are directly related to transpiration rates while the rate of phloem transport is governed by differences in solute concentrations between sites of solute synthesis (source) and consumption (sink) (Marschner, 1995). 

An enhanced uptake of metals into the root symplasm was found in T. caerulescens compared to the related non-accumulator, T. arvense [68,69], and a reduced sequestration into the root vacuoles was associated with the higher root to shoot translocation efficiency of T. caerulescens [49,69,70]. Xylem loading and xylem transport are key steps in Cd hyperaccumulation, as it will be discussed in detail in the section about transport proteins below, and as it was commented by White et al. already 10 years ago [71]. Also in non-hyperaccumulators, the degree of Cd accumulation in above-ground tissues mainly depends on xylem transport [72]. 

---