Photosynthetic consequences of lightshade adaptation

The physiological consequences of these biochemical changes are manifested as changes in the dependence of photosynthetic performance on light intensity. If photosynthetic rate per unit chlorophyll is measured as a function of irradiance then it is in some cases found that rates exhibited by low- and high-light-adapted cells or tissues are much the same at low irradiance, but the low-light-adapted plants level off and reach light saturation first. It is generally the case that high-light-adapted cells require a higher light intensity to reach saturation and achieve a higher light-saturated photosynthetic rate, as is shown for S. obliquus in

Chlorophyceae Diagram

Fig. 12.6 Photosynthetic characteristics (P versus Ed) of cells of Scenedesmus obliquus (Chlorophyceae) grown under high (o) or low (•) light intensity. The specific photosynthetic rate is expressed either per mg chlorophyll

(-) or per unit cellular biomass (packed cell volume, ). The cells were grown in continuous culture under high (28 W m~2) or low (5 W m~2) irradiance, and contained 7.8 and 12.8 mg chlorophyll ml-1 packed cell volume, respectively. Plotted from data of Senger and Fleischhacker (1978).

Fig. 12.6 Photosynthetic characteristics (P versus Ed) of cells of Scenedesmus obliquus (Chlorophyceae) grown under high (o) or low (•) light intensity. The specific photosynthetic rate is expressed either per mg chlorophyll

(-) or per unit cellular biomass (packed cell volume, ). The cells were grown in continuous culture under high (28 W m~2) or low (5 W m~2) irradiance, and contained 7.8 and 12.8 mg chlorophyll ml-1 packed cell volume, respectively. Plotted from data of Senger and Fleischhacker (1978).

Fig. 12.6: the Ek values were about 40 and 110 Wm~2 for the low- and high-light-adapted cells, respectively, in this green alga. Comparable increases in Ec during high-light adaptation have been observed in a wide range of algal types.

In the case of S. obliquus, the high-light-adapted cells also achieve a higher light-saturated photosynthetic rate per unit of cellular biomass (packed cell volume), presumably reflecting the higher cellular content of electron transfer components and carboxylase. This is not observed in all species. In some, such as the dinoflagellate Glenodinium,1080 and the chlorophyte, Chlamydomonas reinhardtii,979 the high-light-adapted forms achieve a higher light-saturated rate per mg chlorophyll but only about the same light-saturated rate per unit cellular biomass as the low-light-adapted forms. It seems likely that in such species the carboxylase content does not change during light adaptation.

We have noted that both high- and low-light-adapted forms of S. obliquus and certain other algae have about the same photosynthetic rate per mg chlorophyll at low light intensities: the P versus Ed curve has about the same slope for both types of cells in the light-limited region. This is because at such intensities, electron transfer and carboxylation capacity are present in excess. Photosynthetic rate is determined entirely by the rate of photon capture, and so is determined by the amount of pigment present. Strictly speaking, it is the rate of photosynthesis per unit absorptance rather than per unit chlorophyll that we would expect to be the same for both types of cell in low light, and the ratio of absorptance to chlorophyll can change. The slope, a, of the P versus Ed curve in the light-limited region is equal to df*fm where af* is the specific absorption coefficient (per mg chlorophyll) for PAR, and fm is the maximum quantum yield for the algal cells (§10.2). We do not expect fm to change during shade adaptation; however, we saw earlier (§9.5) that af* is a function of the intracellular pigment concentration as well as of cell size and shape. The more concentrated the pigments within the cell, the less efficiently they collect light, and so the lower the value of af*. Thus the increase in pigment content that takes place during shade adaptation is likely (assuming no change in the ratio of pigments present) to lead to some reduction in af* and therefore in a: moderate decreases (7-34%) in a have in fact been observed in certain diatom species during shade adaptation.1049 In an alga in which there was a substantial increase in the ratio of some other light-harvesting pigment to chlorophyll during shade adaptation, and therefore a diminution in the proportionate contribution of chlorophyll to light absorption, we might expect the shade-adapted cells to show a higher rate of photosynthesis per unit chlorophyll at low irradiance than cells grown at high light intensity. Such an effect is in fact found in, for example, those cyanobacterial and red algal species that increase their biliprotein/chlorophyll ratio during shade adaptation.408,409,1107,675,799

When shade-adapted cells achieve about the same photosynthetic rate per unit pigment at low irradiance as high-light-grown cells, but contain more pigment per unit cellular biomass, they consequently have a higher photosynthetic rate per unit biomass than the high-light-grown cells under these conditions. This can be seen for Scenedesmus obliquus in Fig. 12.6. The difference becomes more pronounced in accordance with the extent to which pigment content increases and is particularly marked, for example, in the case of Chlorella vulgaris.1291 It is at these low light intensities, when the supply of excitation energy is the limiting factor, that the higher concentrations of light-harvesting pigment within the low-light-grown cells gives them the advantage over the high-light-grown cells.

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