H2O2 accumulation in sunflower leaves as a consequence of iron deprivation

The present work aimed at evaluating in sunflower leaves: the relationship between Fe-deficiency and the possible accumulation of H2O2; the activity of some extracellular enzymatic and non-enzymatic systems involved in H2O2 production and scavenging. Iron-deficient leaves exhibited a significant H2O2 accumulation, monitored at extracellular level by electron microscopy analysis. Such an increase in H2O2 level could derive from the significant decrease in extracellular ascorbate peroxidase (APX) activity, accompanied by a lower APX protein content detected by immuno-electrophoresis. Also, extracellular ascorbic acid content was significantly affected by Fe-deficiency, implying a reduced capacity for H2O2 scavenging. The contribute of peroxidases (PODs) involved in lignification process in keeping H2O2 levels under the toxic threshold was dependent on their subcellular location: the activity of covalently-bound (CB) isoforms being increased while that of ionically-bound (CB) ones decreased. NAD(P)H oxidation activity, known to produce H2O2, was found to decrease in apoplastic fluid and IB fraction and to slightly increase in the CB fraction. In conclusion, H2O2 accumulation observed in Fe-deficient sunflower leaves seems to be mostly due to a reduced scavenging capacity of ASA, APX, and PODs, although the possibility of increased H2O2 production cannot be excluded.