The Function of Flavonoids in the Diurnal Rhythm under Rapidly Changing UV Conditions-A Model Study on Okra.

Susanne Neugart,Mark A Tobler,Paul W Barnes

doi:10.3390/plants10112268

Susanne Neugart, Mark A Tobler + Show 1 more

Open Access

https://doi.org/10.3390/plants10112268

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Abstract

Flavonoids are favored compounds in plant responses to UV exposure and act in UV absorption and antioxidant activity. Here, it was investigated, with okra as a model species, how fast plants can react to changing UV conditions and to what extent these reactions take place. Okra (Abelmoschus esculentus) plants were exposed to either full or nearly no UV radiation. The diurnal rhythm of the plants was driven by the UV radiation and showed up to a 50% increase of the flavonoid content (measured optically in the +UV plants). This was reflected only in the trends in UV-absorption and antioxidant activity of the extracts but not in the soluble flavonoid glycosides and hydroxycinnamic acid derivatives. In a second experiment, a transfer from a −UV to a +UV condition at 9:00 CDT showed the immediate start of the diurnal rhythm, while this did not occur if the transfer occurred later in the day; these plants only started a diurnal rhythm the following day. After an adaptation period of seven days, clear differences between the +UV and -UV plants could be found in all parameters, whereas plants transferred to the opposite UV condition settle between the +UV and -UV plants in all parameters. Broadly, it can be seen that the flavonoid contents and associated functions in the plant are subject to considerable changes within one day and within several days due to the UV conditions and that this can have a considerable impact on the quality of plant foods.

Highlights

UV radiation is an important regulator of adaptation processes in plants
Quercetin glycosides show increased antioxidant activity compared to kaempferol glycosides [4,5], but these compounds may not necessarily differ in UV-absorbance properties [4]
The present study aims to clarify whether rapid changes in flavonoid contents measured by optical methods are related to (1) soluble flavonoid glycosides and hydroxycinnamic acids, (2) UV-absorption, and (3) antioxidant activity

Summary

Introduction

UV radiation is an important regulator of adaptation processes in plants. Plants are often very well adapted to the UV conditions of their regions of origin [1]. Studies show that due to changes in cloud formation, higher UV radiation intensities are more likely to occur in mid-latitude regions in the coming years, accompanied by shading due to the formed clouds [2]. One of the best-known plant responses to increased UV radiation is the formation of flavonoids and other phenolic compounds [3]. Not all compounds are affected to the same degree by UV as they exhibit variations in specific characteristics, which are linked to functions such as UV-absorbance and antioxidant activity. Quercetin glycosides show increased antioxidant activity compared to kaempferol glycosides [4,5], but these compounds may not necessarily differ in UV-absorbance properties [4]. Glycosylation decreases the antioxidant activity of flavonoids [4,10]

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