Stem Xylem as a Possible Pathway for Mineral Retranslocation From Senescing Leaves to the Ear in Wheat

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Abstract
Retranslocation of mineral nutrients in wheat (Triticum aestivum L. cv. Kolibri) during the development phase from 19-22 days post-anthesis to maturity was investigated in isolated culms (with three leaves attached). From the amounts present in leaves and stem at the time when the culms were detached, 75% nitrogen, 86% phosphorus, 22% potassium, and 37% magnesium were retranslocated from vegetative plant parts to the ear. No calcium was retranslocated from the leaves, but up to 27% calcium in the stem was relocated. Steam girdling of the stem between the flag leaf and the ear did not change the amounts of nitrogen, phosphorus and magnesium retranslocated and transported to the ear but reduced calcium movement slightly. Potassium transport to the ear was, however, almost completely inhibited by the phloem discontinuity induced by steam girdling. The results for nitrogen and potassium were confirmed in short-term experiments (24-48 h) using 15N and 86Rb. In a comparison between girdled culms, it was found that removal or shading of the ear decreased nitrogen retranslocation from the leaves and delayed the time course of leaf senescence. The results show that a continuous connection by sieve elements between leaves as source and grains as sink in wheat is not a prerequisite for retranslocation of nitrogen, phosphorus or magnesium.