Effect of zinc deficiency in wheat on the release of zinc and iron mobilizing root exudates

Abstract

The effect of Zn deficiency in wheat (Triticum aestivum L. cv. Ares) on the release of Zn mobilizing root exudates was studied in nutrient solution. Compared to Zn sufficient plants, Zn deficient plants had higher root and lower shoot dry weights. After visual Zn deficiency symptoms in leaves appeared (15–17 day old plants) there was a severalfold increase in the release of root exudates efficient at mobilizing Zn from either a selective cation exchanger (Zn‐chelite) or a calcareous soil. The release of these root exudates by Zn deficient plants followed a distinct diurnal rhythm with a maximum between 2 and 8 h after the onset of light. Re‐supply of Zn to deficient plants depressed the release of Zn mobilizing root exudates within 12 h to about 50%‐, and after 72 h to the level of the control plants (Zn sufficient plants). The root exudates of Zn deficient wheat plants were equally effective at mobilizing Fe from freshly precipitated Fe III hydroxide as Zn from Zn‐chelite. Furthermore, root exudates from Fe deficient wheat plants mobilized Zn from Zn‐chelite, as well as Fe from Fe III hydroxide. Purification of the root exudates and identification by HPLC indicated that under Zn as well as under Fe deficiency, wheat roots of the cv. Ares released the phytosiderophore 2′‐deoxymugineic acid. Additional experiments with barley (Hordeum vulgare L. cv. Europa) showed that in this species another phytosiderophore (epi‐3‐hydroxymugineic acid) was released under both Zn and Fe deficiencies. These results demonstrate that the enhanced release of phytosiderophores by roots of grasses is not a response mechanism specific for Fe deficiency, but also occurs under Zn deficiency. The ecological relevance of enhanced release of phytosiderophore also under Zn deficiency is discussed.