Transport of Rb and Sr to the ear in mature, excised shoots of wheat: Effects of temperature and stem length on Rb removal from the xylem

Abstract

Wheat (Triticum aestivum L. cv. ‘Arina’) shoots grown in the field were excised post-anthesis and incubated in the laboratory for 72 h standing in 2 mM RbCl+2 mM SrCl2. Strontium is a phloemimmobile, xylem-mobile element and indicates the distribution of the xylem sap in the plant. Rubidium is easily transported in the phloem and behaves similarly to the highly mobile K as far as the redistribution within the plant is concerned, although Rb cannot substitute physiologically or biochemically for K. The Sr contents in the ear were hardly affected by stem length or by steam-girdling (phloem-interruption). Rubidium on the other hand accumulated in the stem. A peduncle length of 5 cm was sufficient to decrease the Rb concentration in the xylem by more than 50% at 25°C. Only a minor quantity of Rb reached the ear after passing through 20 cm of stem without nodes and this transport was prevented by steam-girdling. A remarkable flux of Rb into the ear was observed in shoots with a vascular connection between the flag leaf lamina and the ear. Our results suggest that Sr was transported with the transpiration stream, while Rb was rapidly eliminated from the xylem and reached the ear via the phloem. The temperature optimum for the removal of Rb from the xylem was around 35°C. The nodes may further contribute, but are not prerequisites for this redistribution. The observed transfer processes could allow a solute specific transport via the xylem and phloem of maturing cereals and may be an important factor influencing the nutrient economy in the field.