Abstract
Plants rely on spectral cues present in their surroundings, generated by the constantly changing light environment, to guide their growth and reproduction. Photoreceptors mediate the capture of information by plants from the light environment over a wide range of wavelengths, but despite extensive evidence that plants respond to various light cues, only fragmentary data have been published showing patterns of diurnal, seasonal and geographical variation in the spectral composition of daylight. To illustrate patterns in spectral photon ratios, we measured time series of irradiance spectra at two distinct geographical and climatological locations, Helsinki, Finland and Gual Pahari, India. We investigated the drivers behind variation of the spectral photon ratios measured at these two locations, based on the analysis of over 400 000 recorded spectra. Differences in spectral irradiance were explained by different atmospheric factors identified through multiple regression model analysis and comparison to spectral irradiance at ground level simulated with a radiative transfer model. Local seasonal and diurnal changes in spectral photon ratios were related to solar elevation angle, atmospheric water-vapour content and total ozone column thickness and deviated from their long-term averages to an extent likely to affect plant photobiology. We suggest that future studies should investigate possible effects of varying photon ratios on terrestrial plants. Solar elevation angle especially affects the patterns of B:G and B:R ratios. Water vapour has a large effect on the R:FR photon ratio and modelled climate scenarios predict that increasing global temperatures will result in increased atmospheric water vapour. The development of proxy models, utilising available data from weather and climate models, for relevant photon ratios as a function of solar elevation angle and atmospheric factors would facilitate the interpretation of results from past, present and future field studies of plants and vegetation.
Highlights
Differences in spectral irradiance were explained by different atmospheric factors identified through multiple regression model analysis and comparison to spectral irradiance at ground level simulated with a radiative transfer model
Responses of plants to the red (R), far-red (FR), blue (B), green (G) and ultraviolet (UV) components of solar and artificial radiation have been studied for several decades, leading to the identification of more than a dozen plant photoreceptors grouped into five families: phytochromes (PHYs), cryptochromes (CRYs), phototropins (PHOTs), zeitlupe proteins (ZEITs), and UV RESISTANCE LOCUS 8 (UVR8)
To explore whether the non-ideal cosine response would affect the accuracy of our results, we evaluated its potential effect on spectral photon ratios at small solar elevation angles (-5° to 20°; SI Fig. 1)
Summary
A recent study has shown that photoreceptor interactions may depend on the time of the day at which different light cues are sensed and on the duration of exposure to these cues (Sellaro et al, 2018) This builds on earlier research showing that plant growth and development is greatly affected by both changing plant spacing at different times of the day in sunlight (Casal et al, 1990) and by using brief end-of-day illumination treatments to alter the received spectrum immediately prior to the day-to-night transition (Aphalo et al, 1991; Chia and Kubota, 2010; Yang et al, 2012). Interpretation of plants’ photomorphogenic responses in sunlight from an ecological perspective requires knowledge of how spectral irradiance at ground level varies through the day and through seasons at different latitudes and under different atmospheric conditions
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