DIURNAL-VARIATIONS IN RELEASE OF PHYTOSIDEROPHORES AND IN CONCENTRATIONS OF PHYTOSIDEROPHORES AND NICOTIANAMINE IN ROOTS AND SHOOTS OF BARLEY

The effects of the iron (Fe) nutritional status and time of the day on the release of phytosiderophores (PS) and on concentrations of PS and their precursor nicotianamine (NA) in roots and shoots were studied o ver a 24-hour cycle for barley (Hordeum vulgare L. cv. Europa). Phytos iderophores were released in a diurnal rhythm with maximum rates 5 to 6 hours after onset of the light. Compared with Fe-sufficient plants, PS release rates were up to 250 times higher in Fe-deficient plants. T he dominant PS in root exudates and root and shoot extracts was epi-hy droxymugineic acid (HMA). In root extracts of Fe-deficient plants, con comitantly to release of epi-HMA, concentrations of epi-HMA decreased until the end of the highest release rate. Thereafter, epi-HMA concent rations in root extracts increased with time and were as high at the e nd of the light period as at the onset. In root extracts of Fe-suffici ent plants, tpi-HMA concentrations were up to 10 times lower and diurn al variations in epi-HMA concentrations were not observed. Nicotianami ne concentrations were 1.5 times higher in root extracts of Fe-deficie nt plants than of Fe-sufficient plants and were independent of time of the day. In root extracts of Fe-deficient plants NA concentrations we re up to 62 times lower than epi-HMA concentrations. In shoot extracts epi-HMA and NA concentrations were similar in Fe-sufficient and Fe-de ficient plants and no consistent diurnal variations were observed. App arently, depending on Fe nutritional status, PS are synthesized in roo ts at constant rates over the whole light period. No marked accumulati on of NA or other precursors of epi-HMA such as deoxymugineic acid (DM A) or mugineic acid (MA) occurs under Fe deficiency.