Abstract Detail

Bryological and Lichenological Section/ABLS

Hanson, David T. [2], Andrews, T. John [1], Badger, Murray R. [1].

Evidence for a pyrenoid-based CO₂-concentrating mechanism in hornworts using membrane-inlet mass spectrometry.

Hornworts (Anthocerotophyta) are the o­nly group of land plants with pyrenoid-containing chloroplasts. CO₂ exchange and ∆¹³C values of plant tissue have previously demonstrated the presence of a CO₂ concentrating mechanism (CCM) in pyrenoid-containing species of the genus Anthoceros. However, the need for large amounts of tissue and difficulties in culturing hornworts has hampered more detailed studies. We have circumvented these problems by developing a membrane-inlet mass spectrometric system that can use less than 2 cm² of tissue per assay, making it ideal for bryophytes and other plant tissues with low photosynthetic rates o­n a leaf area basis. In addition, our combined fluorometer/mass spectrometer based technique can simultaneously measure net CO₂ exchange, ¹⁸O₂ uptake, ¹⁶O₂ evolution, chlorophyll fluorescence and P₇₀₀ absorbance. We have used this technique to compare hornwort CCM function in pyrenoid-containing (Phaeoceros Prosk. and Notothylas Sull.) and pyrenoid-lacking (Megaceros Campbell) hornworts, with photosynthetic function of the liverwort Marchantia polymorpha L. that has standard C₃ photosynthesis and a thalloid growth form similar to hornworts. We found that Notothylas has more CCM activity than Phaeoceros and that Megaceros has the least CCM activity of any hornwort. Notothylas and Phaeoceros had compensation points from 11-13 ppm CO₂, lower K_0.5 CO₂ than Marchantia, negligible photorespiration, and they accumulate a pool of dissolved inorganic carbon (DIC) between 19-108 nmol mg^-1 chlorophyll. Megaceros had an intermediate compensation point of 31 ppm CO₂ (compared to 64 ppm CO₂ in Marchantia), a lower K_0.5 CO₂ than Marchantia, and some photorespiration, but no DIC pool. We also determined the k_cat of Rubisco for all four species (Marchantia 2.6 s^-1, Megaceros 3.3 s^-1, Phaeoceros 4.2 s^-1, Notothylas 4.3 s^-1) and found that Rubisco content was 3% of soluble protein for pyrenoid-containing species, 4% for Megaceros and 8% for Marchantia.

1 - Australian National University, Research School of Biological Sciences, P.O. Box 475, Canberra, Australian Capital Territory, 2601, Australia
2 - University of New Mexico, Department of Biology, 167 Castetter Hall, Albuquerque, New Mexico, 87131-0001, USA

Keywords:
photosynthesis
hornworts
pyrenoid
Rubisco
CO₂-concentrating mechanism.

Presentation Type: Poster
Session: 32-6
Location: Special Event Center (Cliff Lodge)
Date: Tuesday, August 3rd, 2004
Time: 12:30 PM
Abstract ID:539