Quantitative traits associated with adaptation of three barley (Hordeum vulgare L) cultivars to suboptimal iron supply

Iron stress responses were examined for 3 barley cultivars (Hordeum vulgare L. cvs. Steptoe, Morex and Dicktoo) grown at 5 iron levels (pFe 16.5, 17, 17.5, 18, 19) in chelator-buffered hydroponic solutions. Among the 3 cultivars, Steptoe produced the highest biomass and maintained the highest leaf chlorophyll content at each incremental decrease in Fe supply, followed by Dicktoo and Morex. All 3 cultivars responded to Fe limitation by early release of phytosiderophore prior to onset of Fe-deficiency chlorosis sp., but differed in the length of time prior to maximum phytosiderophore release. With Steptoe, near maximum release [42 mumol g(-1) root (2 h)(-1)] occurred at day 6 for plants grown at pFe 18, and the plants maintained the same relative growth rate as Fe-sufficient plants (pFe 16.5) until day 15. In contrast, maximum rates of phytosiderophore release in Dicktoo and Morex peaked higher at 67 and 56 mumol g(-1) root (2 hr)(-1), but not until day 12 and day 18, respectively. By day 10, both Dicktoo and Morex were chlorotic and had a similar to50% decrease in relative growth rate. Under moderate Fe stress, all 3 cultivars decreased their shoot:root ratios by 25-30%, but differed with respect to specific root length and numbers of root tips per unit root length. The results suggest that differences in plant adaptation to Fe limitation cannot always be attributed to differences in quantities of phytosiderophore release, but also reflect several additive responses including timing of phytosiderophore release, root morphological adaptations and efficiency in Fe metabolism at low tissue Fe concentrations.