H₂O₂ Accumulation in Sunflower Leaves as a Consequence of Iron Deprivation

Abstract

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 (IB) 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.

Keywords:

• Ascorbic acid
• Iron deprivation
• Helianthus annus L.
• Hydrogen peroxide
• NAD(P)H oxidation
• Peroxidase

Acknowledgments

We gratefully acknowledge Dr. Kubo for the gift of APX antibody. This research was supported by a grant from MURST (National Project) Rome, Italy.