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Abstract
Plants commonly respond to UV radiation through the accumulation of flavonoids and related phenolic compounds which potentially ameliorate UV-damage to crucial internal structures. However, the seasonal dynamics of leaf flavonoids corresponding to epidermal UV absorbance is highly variable in nature, and it remains uncertain how environmental factors combine to govern flavonoid accumulation and degradation. We studied leaf UV-A absorbance of species composing the understorey plant community throughout two growing seasons under five adjacent tree canopies in southern Finland. We compared the relationship between leaf flavonol index (Iflav—repeatedly measured with an optical leaf clip Dualex) and measured spectral irradiance, understorey and canopy phenology, air temperature and snowpack variables, whole leaf flavonoid extracts, and leaf age. Strong seasonal patterns and stand-related differences were apparent in Iflav of both understorey plant communities and individual species, including divergent trends in Iflav during spring and autumn. Comparing the heterogeneity of the understorey light environment and its spectral composition in looking for potential drivers of seasonal changes in Iflav, we found that unweighted UV-A irradiance, or the effective UV dose calculated according to the biological spectral weighting function (BSWF) for plant growth (PG action spectrum), in understorey shade had a strong relationship with Iflav. Furthermore, understorey species seemed to adjust Iflav to low background diffuse irradiance rather than infrequent high direct-beam irradiance in sunflecks during summer, since leaves produced during or after canopy closure had low Iflav. In conclusion, we found the level of epidermal flavonoids in forest understorey species to be plastic, adjusting to climatic conditions, and differing according to species' leaf retention strategy and new leaf production, all of which contribute to the seasonal trends in leaf flavonoids found within forest stands.
Highlights
Spectral irradiance changes in forest understoreys by season with solar elevation angle and with canopy leaf-out, as incident solar radiation is selectively absorbed in leaves (Grace, 1983; Endler, 1993)
Seasonal Trends in Iflav From Plants in Forest Stands Across Consecutive Years The seasonal trend in Iflav with day of the year (DOY) was similar between consecutive years, Iflav was slightly higher in the spring of 2015 than 2016
Stand-related trends in Iflav were related to spectral irradiance in deciduous stands, to effective UV dose calculated according to biological spectral weighting function (BSWF) for plant growth (PG) and unweighted ultraviolet-A radiation (UV-A), measured in understorey shade
Summary
Spectral irradiance changes in forest understoreys by season with solar elevation angle and with canopy leaf-out, as incident solar radiation is selectively absorbed in leaves (Grace, 1983; Endler, 1993). There is ample evidence that plants can perceive and respond to changes in the spectral composition of received irradiance in forest understoreys (Smith, 1982; Valladares, 2003). Most research has focussed on the response of understorey plants to photosynthetically active radiation (PAR, 400–700 nm), while ultraviolet radiation (UV-B 280–315 nm, and UV-A 315–400 nm) in forests has received less attention (Grant, 1997; Flint and Caldwell, 1998; Grant et al, 2005). Kolb et al, 2001; Hectors et al, 2007; Brelsford et al, 2019) These realistic UV radiation conditions often act as eustress, stimulating responses and possible cross-tolerance through mechanisms that are not yet well elucidated (Hideg et al, 2013)
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