Refixation of respiratory CO2 in leaves

Sammanfattning: Photosynthesis is a vital process for trees, one in which CO2 plays a major role. At the same time, the tree’s own metabolism produces CO2. For this PhD thesis, I investigated the fate of respiratory CO2 – is it all released back into the atmosphere or is it re-used in photosynthesis? The new LeafWeb-model uses gas exchange measurements of leaves to estimate the percentage of respiratory CO2 that is being refixed in photosynthesis (Pr). This thesis examined the effectson Pr of physiological, anatomical, and morphological traits; those of light availability and temperature; and Pr variations between species, functional groups or biomes. In addition, an experimental study explored the effect of drought on carbon allocation, which implied potential advantages of high Pr.I found that Pr is increased with high mesophyll resistance to CO2 diffusion (rm), high maximum carboxylation rate of Rubisco (Vcmax), and low stomatal conductance to CO2 diffusion (gs). This suggests that these physiological states slow CO2 diffusion out of the leaf (high rm and low gs) while increasing the draw on CO2 at the photosynthetic sites (high Vcmax). Thus, all three increase refixation probability. High leaf mass per area (LMA) and thick cell walls, traits known to correlate positively with rm, also increased refixation. The fact that both morphological and anatomical traits that are known to correlate with high rm also correlated with high Pr in my findings supports the assumption of the model regarding the relationship between Pr and rm. Furthermore, Pr increasing with rm is likely the reason Pr was highest in evergreen needle species and in the boreal biomes where this trait is prevalent. Species with high Pr might be less dependent on uptake of atmospheric CO2 and can close more of their stomata to conserve water. Models calculating terrestrial CO2-uptake should therefore consider including Pr, and assume that plants with high rm and high Vcmax refixate most of their (photo)respiratory-derived CO2.The thesis also includes a study that found that drought during early development of P.mariana shoots affected carbon partitioning and shoot morphology. The shoots allocated carbon away from structural components andtowards respiratory or osmoregulation processes. This might result in mature shoots with lower rm, which could reduce their Pr permanently. High Pr couldbe an advantage in conditions where stomata are closed; efficiently reusing(photo)respiratory CO2 during winter or mild drought could make it possible to maintain some photosynthesis even with very low gs. If this is true, new shoots with less efficient refixation might make the tree as a whole less resilient during future droughts. A possible implication is that while the higher Pr of boreal biomes and evergreen conifers may make them better able to tolerate future dry-periods, such periods may weaken this effect if they happen during shoot development.