Research articleNumerical research on the effects the skyglow could have in phytochromes and RQE photoreceptors of plants
Graphical abstract
Introduction
Skyglow includes an interaction of artificial light from cities and constituents of the atmosphere, such as air molecules and aerosol dispersed basically at troposphere level. Skyglow is well known to be a central environmental issue in the new global economy and a source of different impacts to biodiversity. Skyglow relies on inaccurately situated or designed artificial night lighting systems that degrade the nocturnal environment. It relates with climate change because fossil fuels are the primary sources of energy inputs to street lighting, thus all the light radiated to the upper hemisphere is a completely wasteful use of energy. In addition, a considerable reduction of luminance contrast between the affected sky and stellar bodies’ luminance is due to skyglow. Among such issues artificial light at night disturbs biological processes of most photosensitive living organisms.
Contemporary developments in Solid State Lighting (SSL) have directed to an increased interest in using light emitting diodes (LED) having an affordable price and allowing important energy savings (Humphreys, 2008, Pimputkar et al., 2009, Haitz and Tsao, 2011). As documented by several authors, these technologies often stand for a mismanaged source of artificial radiation that could cause an alteration of nightlife environment (Longcore and Rich, 2004, Navara and Nelson, 2007, Falchi et al., 2011, Gaston et al., 2013, Pawson and Bader, 2014).
The effects of skyglow on biodiversity are day by day more known and the studies more frequent from ecological and biological perspectives. Nevertheless, those studies primarily consider the effects of artificial light on insects and vertebrates (Moore et al., 2001, Anisimov, 2003; Vera et al., 2010, Fox, 2012, Cho et al., 2015, Solano Lamphar and Kocifaj, 2015, Solano Lamphar and Kocifaj, 2013), including ecosystem services (Lyytimäki, 2013).
Animals are not the only organisms influenced by the light of the night sky (Neil and Wu, 2006, Solano Lamphar, 2010), thus a valuable research has arisen around the skyglow effects on plants. Plant photoreceptors play various roles in leaf and stem growth, timing of flowering, fruit production, and other life developments (Briggs, 2013). These organisms generate their own food over the process of photosynthesis. They capture radiant energy from different light sources through chlorophyll and convert the carbon dioxide and water into sugars which are used as energy font (Krause and Weis, 1991). Such mechanism is crucial in the plant development (including aquatic ecosystems) (Kirk, 1994). For instance, a popular laboratory plant the mustard Arabidopsis thaliana has nine photoreceptors that interact with visible radiation. This is why the skyglow should not be ignored e.g. in analysis of how different levels of light can affect the endogenous clock of plants (Fankhauser and Staiger, 2002).
Photoperiodism is an evolving response of plants to the light-dark cycles, yet another essential process to their development (Smith, 1982). Three types of photoperiods control the flowering reaction of many plants. In this sense, short day plants flower in response to long periods of night darkness, meanwhile long day plants flower in response to short periods. Moreover, neutral day plants flower without considering the length of the night, but they flower earlier and abundantly under long daylight regimes. Roman et al. (2000) have reported that artificial light can be an important effect to the impairment of both photosynthetic efficiency and photoperiod. Unfortunately, there are only a few studies that have investigated the consequences that low illumination levels can cause in plants (Whitman et al., 1998).
According to Bennie et al., “understanding the impacts of artificial night-time light on wild plants and natural vegetation requires linking the knowledge gained from over a century of experimental research on the impacts of light on plants in the laboratory and glasshouse with knowledge of the intensity, spatial distribution, spectral composition and timing of light in the night-time environment” (Bennie et al., 2016). Therefore, the modeling of the brightness of the night sky, in conjunction with theoretical knowledge, is crucial in the characterization of nocturnal radiation. Hence, it is suitable to determine how the skyglow affects the photoreceptors of plants through innovative simulations.
Plants have an enormous importance in the life of our planet. Such significance relies on the trophic chain: animal herbivores eat plants to sustain themselves and are eaten by carnivores. In this process, plants obtain energy that rises along the food chain. The electromagnetic radiation that plants receive from sunlight reaching the earth and penetrating the atmosphere is the key ingredient in the photosynthesis process, which is fundamental for plants to get energy.
However, the response of plants to day and night light are quite different. Unnatural night lighting can affect different activities that can only happen during nightlife settings. Light and darkness have an essential influence on the vital functions of plants periods of the day throughout the year. Different reactions occur on plants due to illumination. Those reactions are expressed in the processes of growth and development, which are commonly called photoperiodism. In this paper, we model skyglow taking into account different atmospheric conditions and artificial light emissions that can disturb the natural processes of plants during the night. Specifically, we simulate the downward radiative fluxes as perceived by plants near the city of Bradford for which the experimental data on light pollution exist (Somers-Yeates et al., 2016). This work is intended to model plant responses to the artificial light produced by a lighting system in the region. As indicated earlier in this document, there is a plenty of works studying the negative impacts that light pollution could have on animals. However, the way in which the plants react when exposed to the light at night is largely unknown. In spite of a number of indirect indicia, no serious quantification of the phenomena exists. To the best of our knowledge, no controlled experiments have been performed, and the theoretical development in the field is in its beginnings. This pioneering paper addresses a fundamental phenomenon in light interaction with plant sensors and it is the first time the effects are quantified for phytochromes and RQE photoreceptors of plants. Such numerical experiment must be performed prior to field experiments that are expensive and time-consuming. The results achieved here could be useful for Botanist, Plant Molecular Biologists but also illumination engineers who deal with artificial lights. The modeling performed in this study can be made in any geographical area and for various sources of artificial light.
Section snippets
Methods
Modeling the diffuse light field in an inhomogeneous nocturnal atmosphere is a non-trivial task that is difficult to solve theoretically and numerically due to the complexity of the radiative transfer methods. On the one hand, the atmosphere is a random disperse medium with highly heterogeneous physical and optical properties. On the other hand, ground-based light sources have dissimilar distributions making the problem more challenging. The sky brightness depends on many different factors, in
Results and discussions
The product of the night sky radiance and plant spectral response functions is determined numerically in order to simulate the effects that light sources would have on 2 plants processes (photoperiodism and photosynthesis). Interrelating the results from this work one can get a better scenario about the quantity of artificial light the plants can perceive in night. Our aim is produce results that can appear useful to researchers who carry out studies related to changes in plant phenology due to
Conclusions
This paper specifically contributes to the field by interrelating the spectral features of artificial light sources with plant sensor's sensitivities, while showing that skyglow need to be analyzed quantitatively (with focus on the amount of ground-reaching light) and qualitatively (with a special attention to the spectral characteristics of diffuse night light). This analysis is essential for better understanding of all living organisms' behavior because they usually differ markedly in visual
Acknowledgments
This work was supported by the Slovak Research and Development Agency under contract No: APVV-14-0017. Computational work was supported by the Slovak National Grant Agency VEGA (grant No. 2/0016/16). H.A.S.L. received support from the National Council of Science and Technology of Mexico under the project Cátedras CONACYT # 2723.
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