Contrasting effects of Miocene and Anthropocene levels of atmospheric CO2 on silicon accumulation in a model grass

Grasses are hyper-accumulators of silicon (Si) which they acquire from the
soil and deposit in tissues to resist environmental stresses. Moreover,
given the high metabolic costs of herbivore defensive chemicals and
structural constituents (e.g. cellulose), grasses may substitute Si for
these components when carbon (C) is limited. Indeed, high Si uptake
grasses evolved in the Miocene when atmospheric CO2 concentration was much
lower than present levels. It is; however, unknown how pre-industrial CO2
concentrations affect Si accumulation in grasses. Using Brachypodium
distachyon, we hydroponically manipulated Si-supply (0.0, 0.5, 1, 1.5, 2
mM) and grew plants under Miocene (200 ppm) and Anthropocene levels of CO2
comprising ambient (410 ppm) and elevated (640 ppm) CO2 concentrations. We
showed that regardless of Si-treatments, the Miocene CO2 levels increased
foliar Si concentrations by 47% and 56% relative to plants grown under
ambient and elevated CO2, respectively. This is due to higher accumulation
overall, but also the reallocation of Si from the roots into the shoots.
Our results suggest that grasses may accumulate high Si concentrations in
foliage when carbon is less available (i.e. pre-industrial CO2 levels) but
this is likely to decline under future climate change scenarios,
potentially leaving grasses more susceptible to environmental stresses