Collaborative Research: Recalibrating CO2 and water diffusion through leaves to improve models of photosynthetic responses to the environment

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Plant growth and crop productivity initially depend on carbon dioxide capture by photosynthesis. Rates of photosynthesis are measured directly to understand how efficient different plants are when grown across a range of environmental conditions. These data are then used in models that relate the rate of photosynthesis to the underlying leaf biochemistry in order to predict how plants will perform in different environments. The models require knowledge of the concentration of carbon dioxide inside the leaf, but that value is calculated from measurements of carbon dioxide and water vapor outside of the leaf. Recent work by the principle investigators has shown that the current methods for calculating carbon dioxide inside the leaf (in use for over fifty years) are often generating incorrect values. It is also necessary to understand resistances to carbon dioxide movement into leaf cells and the work of the principle investigators also shows current approaches may have major errors in the estimates of these resistances. This proposal will use new methods to re-calibrate the calculation of carbon dioxide levels inside leaves and the resistance to carbon dioxide movement into leaf cells. This will then improve understanding of photosynthesis in changing environments and assist in development of more productive crops, including crops that will use water and nutrients more efficiently.The 'Farquhar' model of photosynthesis was developed over 35 years ago and is the accepted standard for interpreting measurements of photosynthesis. Many of the variables in the model can be assigned from basic enzyme kinetics, but the model requires measurements of maximum rates of photosynthetic electron transport, carbon assimilation, resistance to CO2 diffusion into and through the leaf, and the rate of respiration during the day. Determination of each of these values requires accurate measurement of small fluxes of water or CO2 in order to avoid some potentially large errors in calculation of critical variables, namely the CO2 partial pressure inside the sub-stomatal cavities of leaves (ci) and the CO2 partial pressure in the chloroplast stroma (cc). Assumptions underpinning foundational methods for determining ci and cc are not holding under some conditions, but the extent of the deviations is not known. The major objectives of this proposal are to: 1) determine the relative contribution of leaf morphology (e.g. cuticle composition and epidermal stiffness) and turgor pressure to errors in ci and cc measurement during heat and drought stress, 2) develop or improve methods for correctly determining ci and cc that can be used broadly in both field and laboratory based measurements of photosynthesis.