TARJETAS DE CONTROL DE ARMADURA: ENTRADAS Y SALIDAS

However, most of the work has centered around a consideration of the g

supply of exogenous carbon for photosynthesis. Raven (1970) and Wium-Andersen and Andersen (1972) have shown that the carbon dioxide concentration in the sediments in the habitats normally colonised

by isoetids is at least an order of magnitude greater than the water #

above. This is so in lochs with a low pH, which is the usual habitat of the isoetids (Spence, 1964). However, most species are capable of growing at higher pH (up to pH 9.0) and in lochs of high

alkalinity (Spence et al., 1979; Seddon, 1965) where such a condition may not apply. Roeloffs et al. (1984) also showed that at a very low pH, associated with the acidification of lakes, the supply of sediment

CO2 may be considerably decreased.

Luther (1983) noted that isoetids have a root:shoot ratio of at least 0.5. This was considered large for aquatic macrophytes. This is consistent with the studies of Steemann-Nielsen (1960), Wium-Andersen

(1971), S^hdergaard and Sand-Jensen (1979), Richardson et al. (1984)

and Keeley et al, (1984) who have shown that Lobelia dortmanna,

Littorella uni flora, Isoetes lacustris and Stylites andioola absorb

CO2 from the sediment via the root system for photosynthesis. Sand-

Jensen et al. (1982) also showed that these plants show a considerable

oxygen efflux from the root systems. In L, dortmanna almost 100% of

the gas exchange was found to occur via the root system. L, uniflora

and J, laoustris show some gas exchange between the leaves and the

lake water.

The isoetids have also been noted by their possession of

98

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deacidification and photôsynthetic fixation of the CO2. L, uniftora,

all submerged Isoetes species and S, andioola have been shown to

possess such diurnal fluctuations in acidity (Keeley, 1982; Keeley 4

and Morton, 198 2 and Keeley et al., 1984), D. dortmanna, however,

lacks such a feature (Boston and Adams, 1983; Richardson et al., 1984).

Keeley (1981a, b) considered that such dark CO2 fixation is a %

particular adaptive advantage to isoetids growing in lakes low in dissolved inorganic carbon, Boston and Adams (1985) have shown in .d

study of the relative contribution of daytime and nightime CO2

fixation in I, maorospora and L, uniflora across the growth seasons

that night fixation contributes approximately a half of the carbon fixed in these species,

A further adaptation of the isoetids to enhance carbon gain was shown by Sçzindergaard (1979) . He found that the large lacunae in the

leaves of L. dortmanna and L, uniflora are efficient at trapping COg

generated in the surrounding cells by photorespiration, instead of it being lost to the surrounding water.

The general morphology of the isoetids has been cited as being functionally extremely important in these physiological attributes. In particular the possession of large lacunae in the leaves and roots

and a thick cuticle in X, dortmanna and L, uniflora have been

suggested as being necessary for the diffusion of CO2 and O2 between

the leaves and the sediment (Sand-Jensen.et al,, 1982). A detailed examination has not, however, been undertaken.

The physiology of aquatic macrophytes has been shown to be 4

extremely important in determining zonation. In general these studies have concentrated on the effect of changing light regimes on photo­

synthesis. Spence, Campbell and Chrystal (1973) found that the

specific leaf area (SLA) of five Fotamogeton species increased

linearly with depth. The degree of increase in SLA depended on the 4

99

spectral character of the lochs co;<=>;=?@ cha;Aes i; SLA being

correlated with the attenuation in PAR rather than any effect of the R:FR,ratios. The degree to which any species could alter its SLA determined the zonation of that species in a loch. For instance,

P. poVygonifolius was least able to increase its SLA and was found to be the species distributed in the most shallow water. Spence and

a.

Chrystal (1970) also found that deep water Potamogeton exhibited

a lower dark respiration rate than shallow water leaves of the same species. This is an obvious adaptation to reduce the loss of fixed carbon in deep water where carbon-fixation in the light is reduced.

Sand-Jensen (1978) found that the SLA of I, lacustris (from 2 m depth)

was greater (0,195) than that of L, uni flora (0.240) (from 0.5 m

depth). Also that I, laoustris was able to photosynthesise at

slightly higher rates in low light and exhibited much lower leaf and root dark respiration rates. Sand-Jensen suggested that this

adaptation of P. laoustris enabled it to colonise to greater depths

in a lake that X. uniflora. It is unfortunate, however, that the two

species were not studied from the same depths where a direct comparison could be made.

Kirk (1983) also records further adaptations that aquatic plants may exhibit to allow for photosynthesis at depth. Some macrophytes

exhibit changes in tiieir photosynthetic pigments. Thus the total chlorophyll content may rise with depth, the a.:b ratio may decline. Other antennal pigments such as caroteneids, apart from chlorophyll b, may also increase in concentration. Kirk (1983) states that this feature varies amongst macrophytes, and that further studies are

necessary. S^hdergaard (unpublished data) found plants of L, uniflora

from 2.3 m depth had nearly twice the chlorophyll content of plants