Minisymposium 28: Photosynthesis II

Abs # 41005: Assessing carbon concentrating mechanism function with a tunable diode laser absorption spectrometer: real-time analyses of ¹³CO₂ and C¹⁸O¹⁶O discrimination by hornworts

Presenter:	Hanson, David T, Contact Author
Authors	Hanson, David T (A)   McDowell, Nate G (B)
Affiliations:	(A): University of New Mexico (B): Los Alamos National Laboratory

Hornworts are early divergent land plants closely related to mosses and liverworts. The photosynthetic apparatus of hornworts is unique among plants due to the presence of a pyrenoid (an electron dense body in each chloroplast). This structure is an aggregation of all the Rubisco (ribulose-1, 5-bisphosphate carboxylase/oxygenase) in the cell, plus a few other enzymes, and is often traversed by thylakoid membranes. Despite their scarcity among plants, pyrenoids are common in algae and are believed to be an integral part of a carbon concentrating mechanism (CCM), though many mechanistic details are unclear. The presence of a CCM correlates with less negative δ¹³C tissue values than are found for organisms that only use the C₃ photosynthetic pathway. Carbon isotope composition in the air surrounding photosynthetic tissues is also affected by the presence of a CCM, though these analyses have previously required painstaking collection of gas samples for subsequent processing with an isotope-ratio mass spectrometer generating single data points a few times per hour at best. In this study, we used a new method for real-time analysis of carbon isotope composition in conjunction with a Li-Cor LI-6400 open gas exchange system. We acquired real-time isotopic measurements using a tunable diode laser absorption spectrometer (TDLAS) from Campbell Scientific that is capable of measuring absolute concentrations of ¹²CO₂ and ¹³CO₂ at a frequency of 10Hz. This method can achieve a resolution around 0.1 per mil every 10-60s through averaging of the 10Hz data. Our TDL is also capable of simultaneously measuring the oxygen isotopic composition of CO₂ with a resolution around 0.5 per mil. With this system we examined the regulation of the hornwort pyrenoid-based CCM by growing hornworts at 1% and 100 ppm CO₂ before measuring their gas exchange characteristics. We report the first real-time leaf-level measurements of carbon and oxygen isotope ratios of photosynthetic and respiratory fluxes in pyrenoid containing and pyrenoid lacking species and compare these values to those of C₃ and C₄ crops using both isotopomers. When we incorporated our data into existing models, we found that the pyrenoid based CCM was leakier than C₄ based CCMs and that differences in pyrenoid structure influence leakiness.