Abstract
The relationship between the apoplastic pH in young sunflower roots ( Helianthus annuus L.) and the plasmalemma ferric chelate reductase (FC‐R; EC 1.16.1.7) activity in roots was investigated. The hypothesis was tested that a high apoplastic pH depresses FC‐R activity, thereby restricting the uptake of Fe 2+ into the cytosol. Until recently, little has been known about this relationship, because pH and redox reaction measurements are difficult to perform within the confines of the root apoplast. We recorded the apoplastic pH by means of the fluorescence ratio in conjunction with video microscopy by covalently tagging fluorescein boronic acid to OH groups of the root cell wall. Fe III reduction was measured using a similar approach by tagging ferrozine diboronic acid with OH groups of the cell wall. Ferrozine forms an Fe 2+ complex, thus indicating the reduction of ferric iron. In roots bathing in buffered outer solutions of different pH, a high pH sensitivity of apoplastic Fe III reduction was found, with the highest ferric iron reduction rates at an apoplastic pH of 4.9; above an apoplastic pH of 5.3, no reduction was observed. Nitrate in the bathing solution increased the apoplastic pH and hence depressed the Fe III reduction; ammonium had the reverse effect. Nitrate together with HCO 3 – , a combination which is typical of calcareous soils, had the strongest depressing effect. From the results, it can be concluded that the main reason for the frequently occurring iron deficiency chlorosis of plants grown on calcareous soils is the inhibition of Fe III reduction in the apoplast, and hence Fe 2+ uptake into the cytosol.
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Claims 7
Claim · model-written summary
Apoplastic ferric reduction was most rapid at pH 4.9.
- Taxon
- sunflower
Helianthus annuust0 - Outcome
- ferric iron reduction ratese0
- Direction
- higheste0
- Value
- 4.9e0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“a high pH sensitivity of apoplastic Fe III reduction was found, with the highest ferric iron reduction rates at an apoplastic pH of 4.9”
t0“sunflower”
Claim · model-written summary
No apoplastic ferric reduction was observed above pH 5.3.
- Taxon
- sunflowerHelianthus annuust0
- Outcome
- reductione0
- Direction
- no reductione0
- Value
- 5.3e0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“above an apoplastic pH of 5.3, no reduction was observed”
t0“sunflower”
Claim · model-written summary
Nitrate increased apoplastic pH.
- Taxon
- sunflowerHelianthus annuust0
- Treatment
- Nitratee0
- Outcome
- apoplastic pHe0
- Direction
- increasede0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Nitrate in the bathing solution increased the apoplastic pH and hence depressed the Fe III reduction”
t0“sunflower”
Claim · model-written summary
Nitrate depressed apoplastic ferric reduction.
- Taxon
- sunflowerHelianthus annuust0
- Treatment
- Nitratee0
- Outcome
- Fe III reductione0
- Direction
- depressede0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Nitrate in the bathing solution increased the apoplastic pH and hence depressed the Fe III reduction”
t0“sunflower”
Claim · model-written summary
Ammonium had the reverse effect on apoplastic pH and ferric reduction relative to nitrate.
- Taxon
- sunflowerHelianthus annuust0
- Treatment
- ammoniume0
- Outcome
- reverse effecte0
Not reportedComparatorDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“ammonium had the reverse effect”
t0“sunflower”
Claim · model-written summary
Nitrate with bicarbonate had the strongest depressing effect.
- Taxon
- sunflowerHelianthus annuust0
- Treatment
- Nitrate together with HCO 3 –e0
- Outcome
- depressing effecte0
- Direction
- strongest depressinge0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Nitrate together with HCO 3 – , a combination which is typical of calcareous soils, had the strongest depressing effect.”
t0“sunflower”
Claim · model-written summary
The authors attributed iron deficiency chlorosis on calcareous soils to inhibition of apoplastic ferric reduction and ferrous uptake.
- Taxon
- sunflowerHelianthus annuust0
- Outcome
- iron deficiency chlorosise0
- Qualifier
- inhibition of Fe III reduction in the apoplaste0
- Setting
- calcareous soilse0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“the main reason for the frequently occurring iron deficiency chlorosis of plants grown on calcareous soils is the inhibition of Fe III reduction in the apoplast, and hence Fe 2+ uptake into the cytosol.”
t0“sunflower”
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