Variations in Leaf Intercellular Air Spaces

During the course of evolution, the invasion of lands by  plants exposed plant tissues to air, 
which dramatically lowered the resistance for CO₂ diffusion to chloroplasts by ~10,000-fold. The evolutionary development of the leaf intercellular airspace was a key innovation that allowed land plants to profit from the lowered resistance to CO₂ diffusion afforded by the terrestrial environment. Terrestrial inhabitation, however, also exposed leaves to the risk of desiccation. Plants presumably navigated this trade-off by developing a complex spatial cellular 
arrangement in order to produce a more or less tortuous 3D intercellular airspace network that allowed for the rapid 
delivery of CO₂ to the chloroplasts, while maintaining a humid environment inside the leaf 
to limit evaporative water loss. 
Leaves utilizing Crassulacean acid metabolism (CAM) exhibit a major contrast in 
their intercellular airspace properties to those engaging the C₃ metabolic pathway for photosynthesis CAM plants frequently occupy the lower end of the intercellular airspace volume spectrum. In 
comparison to C₃ leaves, CAM leaves tend to have larger cells that are more densely packed, which typically results in lower porosity. The tendency of CAM plants toward a higher cell volume fraction, enables greater nighttime storage of CO₂ as malic acid in vacuoles, which is then decarboxylated and released as CO₂ when stomata are closed during the light 
period. C₃ plants, on the other hand, tend toward 
smaller cell sizes, which presumably facilitates greater mesophyll surface exposure to the intercellular airspaces, allowing more efficient carbon fixation by Rubisco. In this issue, Earles, Théroux-Rancourt et al. (10.1104/pp.18.00550) take a closer look at the 3-dimensional (3D) intercellular airspace traits that potentially influence intercellular airspace conductance in 19 bromeliads with CAM 
or C₃ photosynthetic pathways. They have re-evaluated the standard equation for intercellular airspace conductivity by incorporating tortuosity, lateral path lengthening, and intercellular airspace connectivity. They report the occurrence of substantial variation in porosity, tortuosity, lateral path lengthening, and intercellular airspace connectivity across all bromeliad leaves. Their data show that intercellular airspace traits that better 
capture the 3D complexity of leaves strongly influence the conductivity of the intercellular airspace and that the impact of the intercellular airspace on mesophyll conductance should be carefully considered with respect to leaf 
anatomy.