AOBPreview originally published online on July 25, 2009
Annals of Botany 2009 104(5):853-861; doi:10.1093/aob/mcp177
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Mesophyll distribution of ‘antioxidant’ flavonoid glycosides in Ligustrum vulgare leaves under contrasting sunlight irradiance
1 Istituto di Fisica Applicata ‘Carrara’, IFAC, Consiglio Nazionale delle Ricerche, Via Madonna del Piano 10, I-50019, Sesto Fiorentino, Firenze, Italy
2 Dipartimento di Ortoflorofrutticoltura, Università di Firenze, Viale delle Idee 30, I-50019, Sesto Fiorentino, Firenze, Italy
3 Istituto per la Valorizzazione del Legno e delle Specie Arboree, IVALSA, Consiglio Nazionale delle Ricerche, Via Madonna del Piano 10, I-50019, Sesto Fiorentino, Firenze, Italy
* For correspondence. E-mail m.tattini{at}ivalsa.cnr.it
Received: 30 March 2009 Returned for revision: 15 May 2009 Accepted: 4 June 2009 Published electronically: 25 July 2009
Background and Aims: Flavonoids have the potential to serve as antioxidants in addition to their function of UV screening in photoprotective mechanisms. However, flavonoids have long been reported to accumulate mostly in epidermal cells and surface organs in response to high sunlight. Therefore, how leaf flavonoids actually carry out their antioxidant functions is still a matter of debate. Here, the distribution of flavonoids with effective antioxidant properties, i.e. the orthodihydroxy B-ring-substituted quercetin and luteolin glycosides, was investigated in the mesophyll of Ligustrum vulgare leaves acclimated to contrasting sunlight irradiance.
Methods: In the first experiment, plants were grown at 20 % (shade) or 100% (sun) natural sunlight. Plants were exposed to 100 % sunlight irradiance in the presence or absence of UV wavelengths, in a second experiment. Fluorescence microspectroscopy and multispectral fluorescence microimaging were used in both cross sections and intact leaf pieces to visualize orthodihydroxy B-ring-substituted flavonoids at inter- and intracellular levels. Identification and quantification of individual hydroxycinnamates and flavonoid glycosides were performed via HPLC-DAD.
Key Results: Quercetin and luteolin derivatives accumulated to a great extent in both the epidermal and mesophyll cells in response to high sunlight. Tissue fluorescence signatures and leaf flavonoid concentrations were strongly related. Monohydroxyflavone glycosides, namely luteolin 4'-O-glucoside and two apigenin 7-O-glycosides were unresponsive to changes in sunlight irradiance. Quercetin and luteolin derivatives accumulated in the vacuoles of mesophyll cells in leaves growing under 100 % natural sunlight in the absence of UV wavelengths.
Conclusions: The above findings lead to the hypothesis that flavonoids play a key role in countering light-induced oxidative stress, and not only in avoiding the penetration of short solar wavelengths in the leaf.
Key words: Confocal laser scanning microscopy (CLSM), flavonoid glycosides, fluorescence microimaging, fluorescence microspectroscopy, hydroxycinnamates, intra-cellular flavonoid localization, Ligustrum vulgare, photoprotection, UV stress
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