Physiological significance of faster CO2 response curves with the LI-COR LI-6800 photosynthesis system

Background/Question/Methods

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 (C_i). 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 C_i. Here, we will compare results for typical CO₂ response curve between the LI-6400XT and LI-6800 photosynthesis systems and discuss implications on measurements.

Results/Conclusions

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 C_i values. Thus, response curves collected over similar ranges of atmospheric CO₂ concentrations (Ca) resulted in a larger range of C_i 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.