Fróðskaparrit - 01.01.1999, Page 167

V ATNPLANTUR í FØROYSKUM VØTNUM VIÐ BLEYTUM VATNI
TAKA LÆTTLIGA ÓLÍVRUNNIÐ KOLEVNIUPP
171
C02 concentration in the interstitial water,
the dependence on diffusive transport sets a
limit on the maximum leaf-length that can
be obtained by the isoetid as diffusion is
slow over longer distances even in air. Con-
sequently, the isoetids often dominate the
flora of oligotrophic, soft-water lakes, but
cannot compete with the taller elodeid veg-
etation in more alkaline systems.
Use of the more readily accessible at-
mospheric C02 is a characteristic of am-
phibious plants that have the ability to grow
submerged as well as emergent. The ex-
ploitation of atmospheric C02 requires that
the plants develop aerial leaves, either
floating on or raised above the water sur-
face. The aerial leaves have functional
stomata and resemble leaves of terrestrial
plants by being thicker and having a higher
photosynthetic capacity and C02 affinity
than their submerged counterparts when
measured in air (Sculthorpe, 1967; Sand-
Jensen and Frost-Christensen, 1999).
Biochemical and Physiological Adaptations
A number of aquatic macrophyte species
have C02 uptake characteristics that devi-
ate from those expected for plants relying
on diffusive entry of C02 with subsequent
fixation by the C-3 carbon fixation path-
way. In particular, the rate of photorespira-
tion and the C02 compensation point are
lower than predicted. This indicates that
these plants possess a carbon acquisition or
concentrating system that can overcome
the limitation imposed by CO0 diffusion
and C-3 photosynthesis. Among these sys-
tems, bicarbonate use is the most wide-
spread (Madsen and Sand-Jensen, 1991),
whereas C-4 photosynthesis and Crassu-
lacean Acid Metabolism (CAM) is em-
ployed by only a few species.
In addition to C02, inorganic carbon in
water exists as bicarbonate (HC03 ) and
carbonate (C032 ) and the three carbon
species form a buffering system, where the
relative distribution among species is deter-
mined by pH and, to a lesser extent, by tem-
perature and salinity (Stumm and Morgan,
1996). With increasing pH, the equilibrium
shifts from C02 to bicarbonate and carbon-
ate with pKa values of 6.38 and 10.38
(20°C) (Prins and Elzenga, 1989). Thus, in
waters with pH above 6.4, the concentra-
tion of HC03‘ is higher than the C02 con-
centration. In alkaline waters, plant species
with the ability to use bicarbonate, there-
fore, have an advantage compared to plants
that use C02 only, an advantage that is
greater at higher alkalinity and pH. Based
on characteristic electro-chemical poten-
tials of plant cells, it can be calculated that
passive entry of HC03' only takes place if
the extemal HC03' concentrations is 102
—104 times higher than intemal concentra-
tions, an observation that has led to the con-
clusion that HC03" uptake is active (Raven,
1970).
Among freshwater species possessing
the ability to use bicarbonate, the affinity
for HC03' in general is much lower than the
affinity for C02, when expressed as the
concentration needed to half-saturate pho-
tosynthesis and as the initial slope of the
dose-response curve (Madsen and Sand-
Jensen, 1991; Madsen, 1993; Maberly and
Madsen, 1998). The lower affinity for bi-
carbonate compared to C02 potentially re-