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B422: Leaf mesophyll constraints on global water and carbon cycles (Lead Supervisor: Wanne Kromdijk, Plant Sciences)

Supervisors: Wanne Kromdijk (Plant Sciences) and Howard Griffiths (Plant Sciences

Importance of the area of research:

The expansion of the human population is having a severe impact on biogeochemical carbon and water cycles. To understand and interpret the magnitude of human impact on both cycles, it is imperative to have precise constraints on water and carbon fluxes through the biosphere. Plant leaves form the most important intersection of water and carbon cycles, with water and CO2 fluxes transiting the biosphere-atmosphere interface through a stomatal pore, representing 70% of terrestrial evapotranspiration and 40% of global net primary productivity. Curiously, the interaction between carbon and water fluxes within the mesophyll of plant leaves is very poorly understood, and the tortuous diffusion pathlength in air, and liquid phase limitations imposed by cell wall and leaf ultrastructure, have important implications for our understanding of atmospheric changes (Keeling et al. 2017) and the size of the terrestrial carbon sink (Sun et al. 2014).

Project summary:

This project aims to characterize the interaction between water and carbon fluxes in the mesophyll of plant leaves. The project will utilize previously generated tobacco lines with specific modifications affecting carbon and water transfer as model experimental material to provide basic mechanistic insights. The programme will then scale to natural vegetation and communities by comparing the interaction of water and carbon cycles between selected C3 and C4 species, where substantial variation is present due to internal leaf structure associated with the presence or absence of a carbon concentrating mechanism, and interplay between bundle-sheath and mesophyll cells.

What the student will do:

The student will design and execute experiments, emplopying a wide range of eco-physiological techniques to characterize the interplay between mesophyll diffusion resistance, water and CO2 exchange under a range of environmental conditions. The student will measure photosynthetic gas exchange to evaluate CO2 assimilation and transpiration. In parallel, chlorophyll fluorescence, discrimination against carbon-13 and oxygen-18 during photosynthetic CO2 assimilation and oxygen-18 in leaf and transpiration water will be used to assess mesophyll diffusion resistance. Measurements will also be made of leaf hydraulic conductance and partitioning between xylem and extra-xylary components. The student will use the experimental results to parameterize and refine existing leaf-level models for mesophyll limitation to leaf carbon exchange and stable isotope biomarkers (carbon-13, oxygen-18). The results will also be implemented in large-scale earth system models to evaluate the impact that contrasting biomes impose on global biogeochemical cycles of water and carbon.

Please contact the lead supervisor directly for further information relating to what the successful applicant will be expected to do, training to be provided, and any specific educational background requirements.


Keeling RF, Graven HD, Welp LR et al. 2017. Atmospheric evidence for a global secular increase in carbon isotopic discrimination of land photosynthesis. Proceedings of the National Academy of Sciences 114: 10361-66.

Sun Y, Gu L, Dickinson RE et al. 2014. Impact of mesophyll diffusion on estimated global land CO2 fertilization. Proceedings of the National Academy of Sciences 111: 15774-79.


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Other projects available from the Lead Supervisor can be viewed here.

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