Morphogenetic adaptation to depth

We have noted earlier that the adaptation of angiosperms to increased shade includes a morphogenetic response: the leaves increase in area while simultaneously becoming thinner. Spence, Campbell and Chrystal (1973) studied five submerged species of Potamogeton in several Scottish lochs. They found that specific leaf area (SLA, cm2mg-1 leaf dry mass) increased linearly with water depth. The rate of increase with depth appeared to be typical of the loch (and therefore, presumably, of the water type) rather than of the species, the more rapid rates of increase being observed in the more highly coloured and therefore more attenuating waters. For example, the specific leaf area of P. perfoliatus increased from about 0.6 to 1.3 cm2mg-1 from near the surface to 4m depth in the brown water of L. Uanagan, but increased only from about 0.7 to 1.1 cm2 mg-1 from the surface down to 6 m depth in the comparatively colourless waters of L. Croispol. It seemed that a given specific leaf area could be achieved in light fields of markedly different spectral quality, and it was thought likely that the total irradiance (400-750 nm) was of more importance in determining the SLA than any aspect of the spectral quality such as the blue/red or red/far-red ratios. Lipkin (1979) found that in communities of the seagrass Halophila stipulacea growing in the northern Red Sea, leaf area increased about 2.5-fold with depth from the surface down to 30 m (Fig. 12.14). Although this was at first considered to be an ontogenetic adaptation controlled by light intensity, culture experiments indicated that the differences were genetically controlled and Lipkin concluded that true ecotypes of this seagrass were present in the same locality within short distances, i.e. adaptation was apparently phylogenetic rather than ontogenetic.

The siphonous green alga Halimeda tuna growing in the Adriatic Sea shows depth-dependent changes in thallus morphology. Mariani Colombo et al. (1976) observed an increase in the total surface number

Fig. 12.14 Change in length and area of leaves of the seagrass Halophila stipulacea (Sinai, northern Red Sea) with depth (by permission, from Lipkin (1979), Aquatic Botany, 7, 119-28). ITZ = intertidal zone.

Fig. 12.14 Change in length and area of leaves of the seagrass Halophila stipulacea (Sinai, northern Red Sea) with depth (by permission, from Lipkin (1979), Aquatic Botany, 7, 119-28). ITZ = intertidal zone.

of branches and number of segments with increasing depth over the range of 7 to 16 m. The overall effect is to increase the photosynthetic surface exposed to light.

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