In xylem sap

In ordinary plants the evidence is for the involvement of anionic complexes of Cu and Ni for uptake and transport. This is generally taken as resulting from citrate rather than amino add complexation [as neutral bis(amino acidato)metal(II) complexes]. However, this conclusion ignores the presence of aspartic add, which is often found in exudates in relatively large amounts and would form bis-anionic spedes. A metal binding role for aspartic acid in xylem sap was proposed in some earlier work.37... 

Most of the sulfate obtained through the root system is transported in the xylem (Pate, 1965 Tolbert and Wiebe, 1955) to leaf tissue where it is reduced and assimilated into cysteine in light-dependent reactions (Schmidt and Trebst, 1969 Schwenn and Trebst, 1976 Trebst and Schmidt, 1969). Some sulfate reduction occurs in nonphotosynthetic tissue (Ellis, 1%3 Ferrari and Renosto, 1972) but this is of minor importance. In the field pea for example Pate (1965) found that only 0.2-0.4% of the S label of plants supplied with p S]sulfate via their root systems was present in xylem sap as organic sulfur. 

As noted above, the explant system readily serves to analyze the effects of the CK and mineral supply from the roots on leaf senescence. The mineral mixture used approximates the elemental levels in xylem sap during early pod development [3, 24]. Minerals enhance the ability of CK to delay leaf yellowing, especially at lower CK concentrations (Table 1) [11]. Thus, the xylem sap mineral constituents need to be included along with any CKs that are to be tested for activity in the explants. 

Table 2. Changes in the levels of cytokinin bases, ribosides, ribonucleotides and O-glucosides in xylem sap collected from soybean rootstocks taken at various stages during pod development...

Carr DJ, Burrows WJ (1966) Evidence for the presence in xylem sap of substances with kinetin-like activity. Life Sci 5 2061-2077 Carr DJ, Reid DM (1968) The physiological significance of the synthesis of hormones in roots and of their export to the shoot system. In Wightman F, Setterfield G (eds) Biochemistry and physiology of plant growth substances. Runge, Ottawa, pp 1169-1185... 

Davison RM (1965) Some properties of a plant growth inhibitor present in xylem sap of woody plants. Austr J Biol Sci 18 475-486 Davison RM, Young H (1973) Abscisic acid content of xylem sap. Planta 109 95-98 Davison RM, Young H (1974) Seasonal changes in the level of abscisic acid in xylem sap of peach. Plant Sci Lett 2 79-82... 

Greene DW (1975) Cytokinin activity in xylem sap and extracts of MM 106 apple rootstocks. Hort Science 10 73-74... 

Johnsson A (1965) Photoinduced lateral potentials in Zea mays. Physiol Plant 18 574-576 Johnsson A (1967) Relationships between photo-induced and gravity-induced electrical potentials in Zea mays. Physiol. Plant 20 562-579 Jones OP (1973) Effect of cytokinins in xylem sap from apple trees on apple shoot growth. J Hortic Sci 48 181-188... 

As the concentration of compound in xylem sap is not readily measured, it is usually estimated from the amount of compound moved to the shoot for a measured volume of water transpired a correction needs to be applied if there is appreciable metabolism of the compound in the shoots over the period of the test. 

Irtelli, B., Peprucci, W.A., Navari-lzzo, F., 2009. Nicotianamine and histidine/proline are, respectively, the most important copper chelators in xylem sap of Brassica carinata under conditions of copper deficiency and excess. J. Exp. Bot. 60, 269—277. 

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