Xylem exudate

A. Pich, G. Scholz, and U. W. Stephan, Iron-dependent changes of heavy metals, nicotianamine, and citrate in different plant organs and in the xylem exudate of two tomato genotypes. Nicotianamine as possible copper translocator. Plant Soil 765 189 (1994). 

Gartner S, Le Faucheur L, Roinel N, Paris-Pireyre N. Preliminary studies on the elemental composition of xylem exudate from two varieties of wheat by electron probe analysis. Scanning Electron Microsc 1984 IV 1739-1744. 

Distribution of compounds in barley and wheat tissues. Tissues of barley and wheat leaves were mechanically separated under the microscope. It was observed that in barley gramine was more concentrated in the epidermis than in the entire leaf (Table II). Hydroxamic acids in wheat were absent in epidermic tissues and were more concentrated in the vascular tissues than in the entire leaf. Neither compound was detected in xylem exudates nor in guttation drops. 

The individual reactions affected by iron stress can be considered as regulated biochemical pathways, although regulation by iron is not understood. The mechanism of iron absorption and transport involves the release of hydrogen ions by the root, which lowers the pH of the root zone. This favors Fe3+ solubility and reduction of Fe3 to Fe2+. Reductants are released by roots or accumulate in roots of plants that are under iron stress. These "reductants, along with Fe3+ reduction by the root, reduce Fe3+ to Fe2+, and Fe2+ can enter the root. Ferrous iron has been detected throughout the protoxylem of the young lateral roots. The Fe2+ is probably kept reduced by the reductant in the root, and it may or may not have entered the root by a carrier mechanism. The root-absorbed Fe2+ is believed to be oxidized to Fe3, chelated by citrate, and transported in the metaxylem to the tops of the plant for use. We assume Fe2+ is oxidized as it enters the metaxylem because there is no measureable Fe2+ there (13), and Fe3+ citrate is transported in the xylem exudate (30, 31,32). 

Much information concerning the assimilatory activity of roots comes from analyses of the sap collected from a bleeding root system or of the tracheal sap recovered by perfusion or vacuum extraction of shoot segments. The former system is utilized for herbaceous species, the latter for wood material. The fluid obtained is considered to be a xylem exudate although there is always a danger that the sample may be contaminated with phloem and the contents of living cells. 

The amount of N fertilizer applied to the roots of the plant also affects the amount and ratio of compounds transported increased amounts of NO3 applied to wheat increased the total amount of N transported in the xylem and also the proportion of NOj-N amino-N (Kirkman and Miflin, 1979). In maize xylem exudates (Ivanko and Ingversen, 1971) over 10-fold less N is transported in plants starved for 5 days. On the addition of NO3 to the plants the N content of the sap increased rapidly after 4 h. The addition of NHJ to the roots causes a rapid synthesis of the two amides glutamine and asparagine, presumably as a detoxification mechanism. 

Table 1. Cytokinin levels in the xylem exudate of pigeonpea plants inoculated with three different strains of Rhizobiunf...

The differing levels of cytokinins in tobacco leaves of varying age could be due to one or a combination of the following factors (1) differential translocation of xylem cytokinins (2) differential metabolism of xylem cytokinins (3) differential retention of xylem cytokinins and (4) differential biosynthesis of cytokinins in situ in the leaves. These four factors were assessed. By RIA, the principal cytokinins in tobacco xylem exudate were identified as Z, (diH)Z, [9R]Z and (diH)[9R]Z. The distribution and metabolism of the two ribosides in the tobacco shoot were determined after supply via the xylem. The major metabolites of [9R]Z in tobacco leaf laminae were adenine, adenosine and AMP, while the principal metabolite of (diH)[9R]Z was the 7-glucoside of (diH)Z. However, expanded pre-senescent and early senescent laminae did not differ in their cytokinin metabolism, while small expanding laminae showed a higher rate of metabolism. There was no differential distribution of the ribosides to these laminae, and no differential retention in leaves of differing maturity. 

Roots are generally considered as the major site of cytokinin biosynthesis in vegetative plants [23]. There is thus the possibility that the extra-cytokinins found in induced leaves were formed in roots and translocated in the xylem sap. Indeed, analyses showed that (a) the cytokinin levels extracted from root tissue decreased at 16 h (Lejeune, unpublished), and (b) the cytokinin activity (mostly at the Rt of ZR) was increased in the root (xylem) exudate at 9, 16 and 22 h, but not at 12 and 30 h (Fig. 2) [15]. 

Effects on Cytokinin, Gibberellin, and Auxin. Desiccation causes a reduction in the cytokinin activity of the root exudate of sunflower (Itai and Vaadia 1965) as well as the leaf tissue and xylem exudate of Nicotiana rustica (Itai and Vaadia 1971). In the experiments with Nicotiana there is evidence for a reversible chemical transformation of " C-kinetin as the means of altering cyto-... 

Atkin RK, Barton GE, Robinson DK (1973) Effect of root-growing temperature on substances in xylem exudate of Zea mays. J Exp Bot 24 475-487 Atsmon D, Lang A, Light EN (1968) Contents and recovery of gibberellins in monoecious and gynoecious cucumber plants. Plant Physiol 43 806-810 Backus GE, Schrank AR (1952) Electrical and curvature responses of the Avena coleoptile to unilateral illumination. Plant Physiol 27 251-262 Bandurski RS (1978) Chemistry and physiology of myo-inositol esters of indole-3-acetic acid. In Wells W, Eisenberg F (eds) Cyclitols and phosphoinositides. Academic Press, London New York, pp 35-54... 

Van Staden J, Dimalla GGA (1977) A comparison of the endogenous cytokinins in the roots and xylem exudate of nematode-resistent and susceptible tomato cultivars. JExp Bot 28 1351-1356... 

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