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.