Text and pics by Linda-Liisa Veromann-Jürgenson
I am overjoyed that I have my first article for my PhD accepted! It is now published in Journal of Experimental Botany.
As everyone in our group knows, mesophyll conductance is a key player in net assimilation rates in plants, but it’s not too well studied. Recent research, several from this department, has highlighted the importance of mesophyll structure in determining its conductance. Although several papers have been published on the mesophyll conductance of angiosperms and its underlying ultrastructural characteristics, only two studies had been conducted on gymnosperms that consider gas-exchange as well as mesophyll anatomy. We studied evolutionarily old plants including three divisions of gymnosperms, a whisk fern and a clubmoss to understand the role of mesophyll conductance on net assimilation rate as well as the impact of mesophyll anatomy on gas-exchange with an evolutionary perspective. Additionally, we examined the leaf economics spectrum (LES) correlations in these species as gymnosperms have been found to differ significantly from angiosperms in the LES parameters. We got several important results. Firstly, mesophyll conductance is a very important limiter of photosynthesis in gymnosperms and it depends on mesophyll anatomy. Secondly, the cell wall thickness as well as chloroplast area exposed to intercellular airspaces influence mesophyll conductance significantly and in opposite directions. Furthermore, chloroplast shape and size can play an important role in limiting CO2 diffusion in the liquid phase. Thirdly, although anatomy plays a key role, high leaf mass per area did not relate low net assimilation or high cell wall thickness. Interestingly, we measured extremely thick cell walls in several species, which may indicate a preservation of ancient traits through time due to some evolutionary constraints.
Citation: Veromann-Jürgenson, L. L., Tosens, T., Laanisto, L., & Niinemets, Ü. (2017) Extremely thick cell walls and low mesophyll conductance: welcome to the world of ancient living!. Journal of Experimental Botany, DOI: https://doi.org/10.1093/jxb/erx045 (link to full text)
Mesophyll conductance is thought to be an important photosynthetic limitation in gymnosperms, but they currently constitute the most understudied plant group in regard to the extent to which photosynthesis and intrinsic water use efficiency are limited by mesophyll conductance. A comprehensive analysis of leaf gas exchange, photosynthetic limitations, mesophyll conductance (calculated by three methods previously used for across-species comparisons), and the underlying ultra-anatomical, morphological and chemical traits in 11 gymnosperm species varying in evolutionary history was performed to gain insight into the evolution of structural and physiological controls on photosynthesis at the lower return end of the leaf economics spectrum. Two primitive herbaceous species were included in order to provide greater evolutionary context. Low mesophyll conductance was the main limiting factor of photosynthesis in the majority of species. The strongest sources of limitation were extremely thick mesophyll cell walls, high chloroplast thickness and variation in chloroplast shape and size, and the low exposed surface area of chloroplasts per unit leaf area. In gymnosperms, the negative relationship between net assimilation per mass and leaf mass per area reflected an increased mesophyll cell wall thickness, whereas the easy-to-measure integrative trait of leaf mass per area failed to predict the underlying ultrastructural traits limiting mesophyll conductance.