Mesophyll conductance and photosynthesis

Leaf uptake of CO2 (photosynthesis) is the basis for plant productivity. Guillaume Théroux-Rancourt (an Esau post doctoral fellow at UC Davis) and I are interested in understanding photosynthetic mechanisms that allow plants to have higher net CO2 uptake rates, while minimizing the consequences due to tradeoff’s. Thus, in order to optimize the photosynthetic rate of a crop, the mechanistic basis for photosynthetic response to CO2, O2, light, temperature, water, nitrogen and leaf structure must be understood.

Of course, processes at the level of a leaf are not necessarily indicative of those for a canopy. Thus a more holistic approach is needed where field, scale validations are done and physiological experiments are done in realistic environments.

Our current goals are:

  • To understand the basis for the diffusion of CO2 to Rubisco – one determinant of photosynthetic capacity,
  • Find field appropriate screening techniques for maximizing photosynthetic capacity, and
  • Testing whether photosynthetic capacity is independent of variation in other desirable traits.

Ultimately our goal is to provide crop breeders with information that may allow them to select crops for high photosynthetic capacity inorder to ameliorate the negative effects of a desirable trait reducing water use.

Imaging of the changes in photosystem II efficiency after a signal – the effect rapidly radiates out from the leaf vascular bundlesImaging of the changes in photosystem II efficiency after a signal – the effect rapidly radiates out from the leaf vascular bundles. Exciting results, but how do we probe the leaf to find out what is causing the change in PSII? Our lab uses a variety of innovative gas exchange, fluorescence and other methods to do this often without invasive sampling.

 

Mesophyll conductance to CO2 (gm)

The Guillaume  and I have long focused on how CO2 diffuses into the leaf.

Measurement issues

My initial efforts showed that the fluorescence-gas exchange method of measuring (gm) is very sensitive to poor calibration:

  • Gilbert, ME. Pou, A. Zwieniecki, M. Holbrook, NM. (2012). On the measurement of mesophyll conductance to carbon dioxide with the variable J method. Journal of Experimental Botany 63(1): 413-425. link

However, Guillaume and co-workers developed a way of making the fluorescence-gas exchange method less sensitive (although isotopic or curve-fitting methods should still always be used to confirm the results):

  • Théroux-Rancourt G, Éthier G, Pepin S. Threshold response of mesophyll CO2 conductance to leaf hydraulics in highly transpiring hybrid poplar clones exposed to soil drying. Journal of Experimental Botany. 2014;65(2):741-753. link

Does gm respond to light?

Yes! Sort of. We have recently resolved this important question:

  • Théroux-Rancourt, G. & Gilbert, ME. (2017). The light response of mesophyll conductance is controlled by structure across leaf profiles. Plant, Cell and Environment. link

gm responds strongly to light, but this is likely not a regulated process, but the consequence of different depths of light penetration into the leaf – a result of leaf structure.

Click on image to see full poster:

What determines CO2 diffusion through the leaf intercellular airspaces?

Guillaume and Mason Earles have developed a library of microCT (3D) images of diverse leaves. They are using these to investigate the nature of CO2 diffusion in the leaf:

Guillaume and Mason’s microCT images of leaves allow them to investigate CO2 diffusion through the intercellular airspaces