A common measurement to understand biochemical controls of photosynthesis is the analysis of the response of carbon dioxide uptake (A) to inter-cellular CO2 concentrations (Ci). The CO2 response curve data are used to calculate important physiological parameters, commonly using the Farquhar von Caemmerer Berry model of C3 photosynthesis. Points on the response curve are placed into one of three regions based on the limiting factor of photosynthesis, either Rubisco, RuBP-regeneration or triose-phosphate (TPU) limitation. Response curves are often performed using portable gas exchange systems, where good measurement practice includes rapid alteration of the in-coming, or reference, CO2 concentration and little time at each concentration. Several impacts on data quality can occur if too much time is spent at CO2 concentrations far different from ambient concentrations. Long exposure to low CO2 concentrations can result in de-activation of Rubisco, which alters the maximum rate of carboxylation, a key parameter calculated by the model. Long exposure to high CO2 concentrations can cause reduced stomatal conductance, which also reduces Ci. Here, we will compare results for typical CO2 response curve between the LI-6400XT and LI-6800 photosynthesis systems and discuss implications on measurements.
Using similar measurement protocols, The LI-6800 completes each CO2 concentration 20 - 25% faster than the LI-6400XT.. The increased speed results from a number of instrument differences, including improvements to the CO2 control system, a different method of matching the sample and reference infrared gas analyzers, and more precise gas analyzers. There are several important benefits resulting from faster CO2 response curves. First, the reduced leaf exposure to low CO2 concentrations reduces the risk of de-activating Rubisco during the response curve. Second, reduced time at higher CO2 concentrations reduces impacts of stomatal closure, which prevents data points at high Ci values. Thus, response curves collected over similar ranges of atmospheric CO2 concentrations (Ca) resulted in a larger range of Ci in the LI-6800. This increased range may allow for better curve-fitting, particularly in the TPU-limited region of photosynthesis. Finally, the decreased time for each CO2 response curve allows for increased biological replicates over the same time period, increasing statistical power.