Introduction
Plants are sessile organisms, and many plants use physical or chemical
responses to defend against environmental stressors. For example, the
phenolic compound p -coumaric acid (p CA) is implicated as a
chemical defence mechanism with respect to exposure to solar radiation,
especially the UV-B wavelengths (280-315 nm). This compound is
stimulated through the phenylpropanoid pathway (PPP), and has been shown
to be an effective absorber of UV-B radiation (Blokker et al., 2006). It
is also an important building block for a number of essential plant
compounds, including sporopollenin, the major biomolecule that
constitutes the exine of pollen grains (Wehling et al., 1989, Blokker et
al., 2006, Rozema et al., 2001a, Jungfermann et al., 1997). Indeed,p CA has been found in higher abundances in the pollen-based
sporopollenin of plants exposed to higher levels of UV-B (Rozema et al.,
2001b, Blokker et al., 2005). Since sporopollenin is also highly
resistant to degradation under anoxic conditions, recent studies have
proposed using the pollen chemical record preserved in sediments as a
proxy for UV-B (Rozema et al., 2001b, Rozema et al., 2002, Fraser et
al., 2011, Mazza et al., 2000, Willis et al., 2011) or more generally
for total solar irradiance (Jardine et al., 2016) on centennial
timescales or longer. The development of such a tool would have major
implications for long-term ecological research, providing an independent
of solar radiation in the past, with a broad range of palaeoecological
and palaeoclimatological applications (Magri, 2011).
To enable a robust application of the chemical signatures of plants to
reconstruct past changes in solar radiation, an understanding of the
drivers of variability of these compounds within pollen grains is
required (Seddon et al., 2019). However, whilst a number of studies have
demonstrated positive relationships between exposure to solar radiation
and p CA along latitudinal and elevation gradients (Willis et al.,
2011, Jardine et al., 2016, Watson et al., 2007, Lomax et al., 2012),
the timing of the response of these compounds to UV-B radiation remains
poorly understood. For example, a number of studies have assumed thatp CA is a direct indicator of solar radiation during the growing
season (Rozema et al., 2001a, Rozema et al., 2001b, Lomax et al., 2008),
but this assertion has yet to be tested under field conditions. In fact,
in some species, such as Pinus sylvestris , tapetal cells produce
the membrane containing sporopollenin (including p CA) less than
11 days prior to pollen shed (Rowley et al., 2000) so significantly
shorter-term plastic responses than those currently proposed are
possible. Furthermore, the extent to which the p CA production is
driven by conditions experienced locally on the plant (i.e. if
variability is observed between branches or individual flowers shaded in
different ways, or if the signal represents a fully-integrated
whole-tree tree response) remains unknown. This has implications for how
the proxy is understood from the point of view from the palaeorecord
since it determines whether a signal derived p CA represents a
short-term or growing-season response, as well as whether within-sample
variability should be interpreted as reflecting the stand-, tree-, or
within-canopy patterns. Therefore, determining the timing and extent of
this plastic response remains an essential question in understanding how
pollen responds to changes in solar radiation.
A second aspect that remains poorly understood is the variation in both
the p CA content and in the response to environmental variation
between individuals and species. For example, the widespread abundance
of Pinus pollen grains found in sediments, combined with the fact
that Pinus is a light-demanding species, means that Pinushas become a focus for research into reconstruction of solar irradiance
using p CA. However, the pollen members of the European flora ofPinus spp. remain difficult to separate in palaeoecological
reconstructions using traditional light microscopic methods, and if
different species exhibit varying responses under equivalent radiation,
then this could have implications for both dose-response relationship ofp CA and reconstructions from pollen sampled from a sediment core.
An obvious source of species-level variation in p CA may be pollen
size. European pines vary considerably in pollen size (Beug, 1961), and
it is very likely that species may contain differences in totalp CA simply due to differences in their surface-area. In
palaeoecological reconstructions, such variation may be accounted for
simply by measuring and taking these size differences into account.
Species-level variation in phenolic acid content due to inherited
genetic differences in the ability to produce these acids, or in the
response to variation in radiation, would be more challenging to account
for using palaeoecological reconstructions. To date, the extent to which
species-level variation can potentially affect the dose-response
relationship between p CA and solar radiation remains poorly
understood.
Here, we present the results of two studies which aimed to investigate
the effect of short-term variations in ambient solar radiation on the
abundance of p CA in Pinus pollen grains. Using a
resampling of the same individuals over two consecutive years with
natural variation in ambient solar radiation, our first objective is to
investigate whether the abundance of p CA in pollen grains varies
in response to natural variability in radiation, and if the level or
response varies between Pinus species. Using shading cloths on 10
individuals of Pinus sylvestris , our second objective was
to investigate whether the abundance of p CA in pollen grains
varies in response to artificial reductions of ambient solar radiation
at the level of individual male cones.
Material and Methods