Copper-deficiency-induced phytosiderophore release in the calcicole grass Hordelymus europaeus

Phytosiderophore (PS) release occurs under both iron and zinc deficiencies in representative Poaceae and has been speculated to be a general adaptive response to enhance the acquisition of micronutrient metals. We tested this hypothesis within an on-going study of the role of micronutrient metal nutrition for patterns of spontaneous vegetation in relation to soil pH and carbonate content. Hordelymus europaeus (L.) Hart, a negative grass species commonly found on soils rich in CaCO3 in Western Central Europe, was subjected to deficiencies of Fe, Zn, Mn and Cu using chelator-buffered nutrient solutions. PS release rates were determined at 3-5 d intervals during onset and development of deficiency symptoms. Plant dry matter yields and nutrient concentrations, measured at three time points were used to construct growth curves for calculation of PS release per unit root mass. In comparison with trace metal-sufficient control plants, dry matter production was markedly reduced in the Fe, Zn, Mn and Cu-deficiency treatments, with final relative yields of 6, 11, 15 and 31%, respectively. The phytosiderophore produced under Fe- and Cu-deficiency treatments was identified, using HPLC, as desoxymugineic acid. The highest rate of PS release (18 mu mol g(-1) root d. wt in 2 h) was measured in the Fe-deficiency treatment, and there was substantial release in the Cu-deficiency treatment (725 mu mol g(-1) root d. wt in 2 h). No PS release above control levels (2.6 mu mol g(-1) root d. wt in 2 h) was observed in the Zn- or Mn-deficiency treatments (1.5 and 2 6 mu mol g(-1) root d. wt in 2 h, respectively). It remains to be clarified whether PS release in response to Cu deficiency is a primary reaction to the deficiency, or is caused by a chain of events similar to that observed in Zn-deficient wheat, which involves internal Fe deficiency. Our results suggest that PS release in the native plant species H. europaeus is a specific response to Fe and Cu deficiency and is not significantly induced in response to deficiencies of Zn and Mn. Induction of the PS mechanism in different plant species might be more diverse than previously thought.