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An Ascorbate-induced Absorbance Change in Chloroplasts from Violaxanthin De-epoxidation
H Y Yamamoto1, L Kamite, Y Y Wang
1Department of Food Science and Technology, University of Hawaii, Honolulu, Hawaii 96822.
Plant Physiology
|February 1, 1972
Summary
A new light-dependent absorbance change in chloroplasts, linked to violaxanthin de-epoxidation, acts as a probe for chloroplast acidification and photosystem 1 activity. This process, influenced by pH and ascorbate, reveals insights into plant photosynthesis.
Area of Science:
- Plant Physiology
- Photosynthesis Research
- Chloroplast Biochemistry
Background:
- Carotenoids play crucial roles in photosynthesis, including light harvesting and photoprotection.
- Violaxanthin de-epoxidation is a key process in the xanthophyll cycle, regulating light energy dissipation.
- Chloroplast acidification is a known phenomenon linked to light-dependent reactions.
Purpose of the Study:
- To describe a novel ascorbate-induced absorbance change in chloroplasts.
- To investigate the relationship between this absorbance change, carotenoid shifts, and chloroplast acidification.
- To identify the underlying mechanisms and conditions affecting violaxanthin de-epoxidation.
Main Methods:
- Spectrophotometric analysis of chloroplasts treated with ascorbate.
- pH-dependent measurements of light-induced absorbance changes.
- Inhibition studies using nigericin, KCl, and photosystem inhibitors.
- Comparison of difference spectra with carotenoid solvent spectra.
Main Results:
- A distinct absorbance change peaking at 505 nm was observed, characteristic of a carotenoid shift.
- The change was light-dependent at pH 7 but not pH 5, and linked to violaxanthin de-epoxidation.
- Chloroplast acidification, driven by photosystem 1, mediated the 505 nm change.
- Ascorbate's role varied with pH, influencing reducing potential and electron transport.
Conclusions:
- The 505 nm absorbance change serves as an endogenous probe for chloroplast acidification.
- Violaxanthin de-epoxidation is confirmed to require acidic conditions within chloroplasts.
- The findings elucidate the link between photosystem 1 activity, proton transport, and carotenoid dynamics.
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