The following investigation revealed that when the gametophyte of extant hornworts is exposed to external CO2 concentrations of either 5%, ambient or 0.00175%, (for 14 days), the CCM showed no effect in its operation, unlike algal CCM’s where 5% CO2 after only 2-8 days switches the CCM off (REF). This indicates that a hornwort tissue response, to differing external CO2 concentrations, has not occurred, (maybe more time is needed in these CO2 environments). However, in hornworts grown at 300C for 14 days (compared to those at grown at lower temperatures) there was a sig. diff. in the operation of the CCM, appearing to increase the capacity of dissolved inorganic carbon (DIC) uptake and lower the CO2 compensation point and, lower K 0.5 CO2, (with hydrated thallus). Therefore an advantage of a CCM in a land plant such as the Anthocerotae would be to reduce the oxygenase reaction in the enzyme Rubisco by elevating the CO2 Concentration around its active site, reducing competition from oxygen.
The unique chloroplast architecture of the more “advanced” Anthocerotae possess thylakoids that cross the pyrenoid, termed channel thylakoids, Burr (1970), which have been speculated (through inference from work with algae), by Makay & Gibbs (1991), to be dominant in PSI, not PSII (water splitting side of the light reaction, releasing oxygen), therefore reducing further oxygenation events, possibly making up for a Rubisco with low specificity.
Furthermore, high light intensity (compared to low light intensity grown plants), can have a sig. effect on the capacity of the hornwort CCM; with a large dissolved inorganic carbon (DIC) uptake and low CO2 compensation point and low K 0.5 CO2. An active CCM will effectively reduce the light-utilization efficiency of photosynthesis, therefore increased CCM activity will optimise the supply of CO2 to Rubisco helping it deal with high PAR. The hornwort appeared to be able to photosynthesise at Photon Flux Densities (PFD) above 1500umols m2. s2, (in those grown at 120umol m2. s2 for 14 days); However a constituently expressed CCM at high PAR kept NPQ low, causing PSII damage, maybe acting as an electron sink to power the DIC pump, and/or as a proton sink in the thylakiod lumen, suplying protons to the reaction; (H+ )+ HCO3- CA CO2 + H2O, so lowering protonation of the xanthophylls reducing NPQ. However, at an extremely low light treatment, Pmax was greater than that of high light treated plants (lower Pmax could have been caused by chlorophyll loss), indicting growth at low PFD is preferred in the hornwort.
However light adaptability is possible with use of the CCM’s plasticity as a ''light use efficiency reducer'' a high PAR and an ''oxygenase reaction reducer'' at high temperature. Early land plants in the Silurian atmosphere, 450MYA, may well have made use of the already existing pyrenoid CCM (as possessed by the ancestral Coleochales), not because external CO2 was in short supply (Silurian aerial environment; [CO2] 5400 - 7000ppm) (REF), but as a way to deal with high PAR by increasing efficiency for CO2 at the cost of a reduced light use efficiency, the CCM acting as - a ''light use efficiency reducer'' - and in high temperatures the CCM up regulates, also increasing efficiency for CO2 acting as - a ''oxygenase reaction reducer''.
The hornwort CCM can be likened to a primitive stomata, regulating CO2 uptake, in response to light, temperature and thallus water content variations. The eventual loss of the pyrenoid CCM, and the move towards a more advanced morphology as seen in the C3 liverworts, meant a change in shape of the chloroplast from band shaped to discoid (REF). The discoid shape would allow more light adaptability; an example of this is the unusual light dependant changes in the chloroplast morphology in species of hornworts without pyrenoids (Burr 1968). Brown a lemon (19--) suggests the evolution of the multiplastic cell (away from the uniplastic cell found in Phaeoceros) may have meant the end of pyrenoid containing plastids. In the mutiplastidic cell the division of a number of plastids would have been more difficult to co-ordinate. An even distribution of Rubisco in the stroma would not require a pyrenoid division to insure that Rubisco is present in every chloroplast. With Rubisco no longer being pumped CO2 around the active site, the early C3 liverwort Rubisco kinetics would have improved efficiency, coupled with morphological specialisation to aid CO2 influx, and pores to reduce water loss, thus making the early C3 liverworts less dependant on water to aid CO2 in flux.
To retain a pyrenoid CCM on land meant adaptability to the Silurian atmosphere; higher temperatures and stronger light conditions (REF) than experienced in the water column. To retain a pyrenoid CCM on land today means hornworts with CCM’s have the plasticity to adapt to high and low temperatures and very low light, to higher light environments, ranging from the Australian and India rain forests, to, ditches in Scotland and Canada. However, full hydration of the thallus is essential for efficient CCM activity, so a damp to wet environment is the habitat where they are found. If environmental conditions are no longer favourable, hornwort tuber formation can occur and the thallus dies (REF); the plant to be resurrected when environmental conditions are suitable. This is another useful adaptation to cope with the high PAR and drying summer conditions experienced by Phaeoceros growing in the Mediterranean area.