Leaf anatomical properties in relation to differences in mesophyll conductance to CO₂ and photosynthesis in two related Mediterranean Abies species

Abstract

Abies alba and Abies pinsapo are closely related species with the same ribulose 1·5-bisphosphate carboxylase/ oxygenase (Rubisco) large subunit (rbcL) but contrasting hydraulic traits and mesophyll structure occurring in the Iberian Peninsula under contrasting conditions. As photosynthesis and hydraulic capacities often co-scale, we hypothesize that these species differ in mesophyll conductance to CO2 (gm). gm and key anatomical traits were measured in both species. Drought-adapted population of A. pinsapo has higher photosynthesis than the more mesic population of A. alba, in agreement with its higher hydraulic capacity. However, A. alba exhibits the largest stomatal conductance (gs), and so water use efficiency (WUE) is much higher in A. pinsapo. The differences in photosynthesis were explained by differences in gm, indicating a correlation between hydraulic capacity and gm. We report a case where gm is the main factor limiting photosynthesis in one species (A. alba) when compared with the other one (A. pinsapo). The results also highlight the discrepancy between gm estimates based on anatomical measurements and those based on gas exchange methods, probably due to the very large resistance exerted by cell walls and the stroma in both species. Thus, the cell wall and chloroplast properties in relation to CO2 diffusion constitute a nearfuture research priority.

Leaf anatomy and photosynthesis : unravelling the CO2 diffusion pathway in C3 leaves

“…The assumption that m does not change with i was confirmed in a recent study (Tazoe et al, 2009) in wheat, but several other recent studies report considerable changes in the estimate of m if the above methods to determine it are applied at different CO 2 levels (Flexas et al, 2007;Hassiotou et al, 2009;Vrabl et al, 2009;Bunce, 2010;Douthe et al, 2011;Tazoe et al, 2011) as reviewed by Flexas et al (2012). Importantly, this reported variability violates the definition of a physical conductance and implies that m is not a lumped conductance but instead an apparent variable that depends on i .…”

“…An alternative is to calculate m from leaf anatomical properties, curvature factors, and assumed diffusion coefficients and/or conductances. In these models, the physical definition of a conductance (equation (2.8)) is directly applied to quantify the conductance of some of the components in the liquid phase for CO 2 transport in the mesophyll (Evans et al, 1994;Niinemets and Reichstein, 2003;Peguero-Pina et al, 2012;Tosens et al, 2012a;Tosens et al, 2012b;Tomas et al, 2013) in order to calculate m . Once the conductance of each component is quantified, the liquid phase conductance, liq can be calculated as (Tosens et al, 2012b;Tomas et al, 2013):…”

“…Importantly, this reported variability violates the definition of a physical conductance and implies that m is not a lumped conductance but instead an apparent variable that depends on i . The mechanism of the dependence of m on i is largely unclear (Flexas et al, 2012), which makes it hard to mechanistically model it. One solution to this issue is to describe the variability of m by a phenomenological model instead (Yin et al, 2009):…”

“…This makes it impossible to assess to what extent each structure and biochemical process affects c and N , particulalry in response to environmental variables. This problem can be tackeled somewhat by the partitioning of mesophyll conductance in subconductances (Evans et al, 1994;Niinemets and Reichstein, 2003;Peguero-Pina et al, 2012;Tosens et al, 2012a;Tosens et al, 2012b;Tomas et al, 2013), but these models do not consider variability of m with i . Mesophyll conductances are useful to estimate parameters of the FvCB model.…”

“…Also, (2) they require leaf anatomical data to reconstruct the computational domain. The collection of these data is considerably more laborious than gas exchange measurements if it is done by TEM and/or light microscopy (Thain, 1983;Evans et al, 1994;Peguero-Pina et al, 2012;Tosens et al, 2012a;Tosens et al, 2012b;Tomas et al, 2013) or it requires access to advanced 3-D visualization technology like X-ray synchrotron microscopy (Verboven et al, 2008;Verboven et al, 2015). This latter problem may become obsolete over the years due to technological advancement of visualization technology.…”