ZINC-EFFICIENT WILD GRASSES ENHANCE RELEASE OF PHYTOSIDEROPHORES UNDER ZINC-DEFICIENCY

The effect of the zinc (Zn) nutritional status on the rate of phytosid erophore release was studied in three wild grass species (Hordeum muri num, Agropyron orientale, and Secale cereale) grown in nutrient soluti on under controlled environmental conditions. These wild grasses are h ighly ''Zn-efficient'' and grow well on severely Zn-deficient calcareo us soils in Turkey (DTPA-extractable Zn was 0.12 mg/kg soil and CaCO3 was 37%). In all wild grasses studied, Zn deficiency reduced shoot gro wth but had no effect on root growth. Low amounts of phytosiderophores were released from roots of all wild grasses adequately supplied with Zn. In plants grown without Zn, release of phytosiderophores progress ively increased with the onset of visual Zn deficiency symptoms, such as inhibition of shoot elongation and appearance of chlorotic and necr otic patches on leaves. Compared to Zn-sufficient plants, phytosiderop hore release increased 18-20-fold in deficient plants. HPLC analysis o f root exudates showed that the dominating phytosiderophore in Zn-defi cient Agropyron and Hordeum was 3-epi-hydro-xymugineic acid (epi-HMA) and was 3-hydroxy-mugineic acid (HMA) in Secale. Besides HMA, epi-HMA and mugineic acid (MA) were also detected in exudates of Zn-deficient Secale. The results indicate the importance of phytosiderophores in ad aptation of wild grasses to Zn-deficient calcareous soils. Phytosidero phores might enhance mobilization of Zn from sparingly soluble Zn pool s and from adsorption sites, both in the rhizosphere and within the pl ants.