All the adaptive changes we have considered so far are in the biochemical composition and consequent functioning of the photosynthetic apparatus. There are also other forms of ontogenetic adaptation, particularly in multicellular species, which can enable plants to cope with variation in the intensity of the light field. In higher plants, both aquatic and terrestrial, adapting to shade, at the same time as chlorophyll content (% of dry mass) increases, the leaves increase in area so as to intercept more light and also become thinner. Because of the increase in specific leaf area (area per unit dry mass of leaf), the chlorophyll content per unit area increases less than content per unit mass during shade adaptation, or may remain the same, or may decrease. Chlorophyll content (per g fresh weight) has been shown to increase not only with shade, but also with growth temperature, in certain submersed freshwater angiosperm species.71
Spence and Chrystal (1970) studied light-intensity adaptation in certain species of the freshwater angiosperm genus Potamogeton from Scottish lochs. In P. polygonifolius, a shallow-water species (occurring from the waters edge to 0.6 m depth), shade plants (grown at 6% of full sunlight) had a specific leaf area three times that of plants grown in full sunlight: the shaded leaves were 0.04mm and the sunlit leaves 0.12mm thick. The chlorophyll content in the shaded leaves was about one third higher than that in the sunlit leaves on a dry mass basis. Because of the great increase in area, however, the shade leaves contained only about half as much chlorophyll per unit area as the sunlit leaves. The dark-respiration rate per unit area was 27% lower in the shaded leaves - a greater reduction might have been expected given that leaf thickness was reduced by two thirds. Presumably as a consequence of the diminution in respiration rate, the light compensation point in the shaded leaves was lowered by about the same amount.
Potamogeton obtusifolius, a deeper water species (depth range 0.5-3.0 m) appeared to lack the ability to change its specific leaf area in accordance with light intensity, there being little difference in this respect between plants grown at 100% and 6% of full sunlight. There was also little change in the chlorophyll content per unit mass or leaf area. However, the specific leaf area of P. obtusifolius (^2cm2mg_1) is already greater than that of P. polygonifolius (sunlit leaves, 0.48 cm2mg-1, shaded leaves, 1.43 cm2 mg-1), and so we may suppose that P. obtusifolius has already taken this step during evolution, as part of its phylogenetic adaptation to the deeper water, and it may be that further increases in specific leaf area would not be possible. The respiration rate of P. obtusifolius per unit area of leaf was, even in the sunlit leaves, as low as one third, and the light compensation point about half that of the shaded leaves of P. polygoni-folius, further evidence for the superior phylogenetic adaptation of P. obtusifolius to low light. This species still retained some capacity for ontogenetic adaptation, however, since its shaded leaves had a very much lower respiration rate than its sunlit leaves, so much so that its light compensation point was reduced by ^90%.
The submersed macrophyte Potamogeton perfoliatus, a species inhabiting turbid, brackish tidal waters in Chesapeake Bay, responds to a diminution in ambient light by increasing its chlorophyll concentration. Goldsborough and Kemp (1988) found that in plants transferred to a light intensity 11% of ambient, Chl a cm~2 leaf area increased by 20% in 3 days and 50% after 17 days; on returning the plants to normal light intensities, chlorophyll content reverted to normal in about 3 days. The increased pigment content led to a marked increase in the photosyn-thetic rate per g dry weight of shoot at low light intensity, with a consequent reduction in the light compensation point: Pmax in saturating light did not change during the shade treatment. Shading was accompanied by very marked stem elongation, as well as an increase in specific leaf area.
While P. perfoliatus seems to have the capacity to adapt to a reduction in available light, many submersed macrophytes cannot cope so well. Seagrasses, for example, which typically occur in clear, relatively colourless waters, are particularly sensitive to increased turbidity in the water, resulting from human activity such as dredging or effluent disposal.894,988
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