Possible mechanisms for diurnal variation in photosynthetic
capacity
We were able to rule out that repeated clamping could cause the
substantial diurnal declines in photosynthetic capacity in the three
species we tested (Fig. 6). Regarding biological mechanisms, the decline
in apparent Vcmax could be driven by declines in
Rubisco activation (Pérez et al. , 2005), diurnal changes in
mesophyll conductance (Bickford et al ., 2010), diurnal changes inKleaf (Brodribb & Holbrook, 2004), or possibly
from the movement of chloroplasts (e.g. Tholen et al ., 2008).
Rubisco activase is regulated by light through ATP:ADP (Streusand &
Portis, 1987) and redox potential (Zhang et al ., 2002), and is
responsible for removing the inhibitor 2-carboxyarabinitol 1-phosphate
(CA1P) from Rubisco (Robinson & Portis, 1988); therefore diurnal
changes in irradiance may be regulating apparentVcmax through changes in Rubisco activase
activity. CA1P could be further implicated in diurnal regulation of
apparent Vcmax , as it is exhibits diurnal changes
associated with changes in in vitro Rubisco activity in several
species – though not all species have CA1P (Kobza & Seemann, 1989).
Mesophyll conductance ultimately determines the supply of
CO2 from the intercellular airspace to the chloroplast,
and diurnal changes in mesophyll conductance may cause changes in the
slope of the A-Ci response, leading to a false
interpretation that there are diurnal changes in apparentVcmax . Mesophyll conductance is tightly linked toKleaf (leaf hydraulic conductance), which
exhibits diurnal dynamics (Brodribb & Holbrook, 2004) which could then
mediate diurnal changes in apparent Vcmax via
mesophyll conductance (Flexas et al ., 2013). If we assume that
the diurnal patterns observed in Kleaf are
general, then declines in Kleaf would reduce
gm (Flexas et al ., 2013), reducing apparentVcmax and Jmax . However,
these effects appear nonlinear, and we would have expected nonlinear
diurnal dynamics, which we only observed in P. deltoides .
Meanwhile, chloroplast movements (Tholen et al ., 2008) could also
mediate changes in mesophyll conductance. We note that applying the
model of Farquhar et al . (1980) in full, accounting for both
mesophyll conductance and Rubisco activation state, could allow forVcmax to remain constant while apparentVcmax could vary diurnally. Whether such
mechanisms explain diurnal dynamics in apparentVcmax remains to be seen.