Abstract
Boreal forests undergo a strong seasonal photosynthetic cycle; however, the underlying processes remain incompletely characterized. Here, we present a novel analysis of the seasonal diffusional and biochemical limits to photosynthesis (Anet ) relative to temperature and light limitations in high-latitude mature Pinus sylvestris, including a high-resolution analysis of the seasonality of mesophyll conductance (gm ) and its effect on the estimation of carboxylation capacity ( ). We used a custom-built gas-exchange system coupled to a carbon isotope analyser to obtain continuous measurements for the estimation of the relevant shoot gas-exchange parameters and quantified the biochemical and diffusional controls alongside the environmental controls over Anet . The seasonality of Anet was strongly dependent on and the diffusional limitations. Stomatal limitation was low in spring and autumn but increased to 31% in June. By contrast, mesophyll limitation was nearly constant (19%). We found that limited Anet in the spring, whereas daily temperatures and the gradual reduction of light availability limited Anet in the autumn, despite relatively high . We describe for the first time the role of mesophyll conductance in connection with seasonal trends in net photosynthesis of P. sylvestris, revealing a strong coordination between gm and Anet , but not between gm and stomatal conductance.
Highlights
Terrestrial biosphere models (TBMs) typically use the Farquhar, von Caemmerer, and Berry (FvCB) model (Farquhar et al, 1980) to predict photosynthetic carbon (C) assimilation by C3 plants, including responses to rising temperatures and atmospheric CO2
We considered Anet as too low when it was below 1 μmol CO2 m−2 s−1 and the CO2 concentration difference as too low when it fell below 9 μmol mol−1
Using high-resolution continuous measurements of gas exchange and C isotope discrimination, we present for the first time the seasonality of mesophyll conductance, and VCmax estimates corrected for mesophyll conductance, together with photosynthesis and stomatal conductance in mature P. sylvestris under natural conditions
Summary
Terrestrial biosphere models (TBMs) typically use the Farquhar, von Caemmerer, and Berry (FvCB) model (Farquhar et al, 1980) to predict photosynthetic carbon (C) assimilation by C3 plants, including responses to rising temperatures and atmospheric CO2. 40 yr old, the FvCB model has been eveloped and improved over time. It defined C assimilation rate as determined by the more limiting of two biochemical processes: carboxylation capacity of Rubisco (VCmax) and the capacity of ribulose 1,5bisphosphate (RuBP) regeneration by electron transport. A third biochemical process, the capacity for triose phosphate utilization (Sharkey, 1985), was identified and incorporated into the model later. TBMs commonly represent photosynthetic capacity in terms of VC . TBMs commonly represent photosynthetic capacity in terms of VC . max
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