Chloroplast movements

The rate of light collection by chloroplasts depends not only on their pigment complement but also on their position and orientation within the cell. In some aquatic plants this can vary with light intensity. Amongst the higher plants1415 the general pattern of behaviour is that in low-intensity light the chloroplasts move to a position in the cell such that light absorption is maximized - they spread themselves out adjoining, and parallel to, those cell walls that face the incident light. In high-intensity light (saturating for photosynthesis), the chloroplasts move to a position such that light absorption is minimized, perhaps to reduce photoinhibition: they move away from that cell wall that is directly exposed to the light and align themselves adjoining, and parallel to, those cell walls that are most shaded, typically the side walls so that not only are they edge-on rather than face-on to the light, but also some are shaded by others. In the case of the aquatic angiosperm Potamogeton crispus, growing in a Scottish loch, leaves collected from 0.25 m depth had their chloroplasts around the side walls of the cells, whereas leaves from 2.5 m depth had their chloroplasts distributed over the surface facing the light.1273 An alternative way of reducing light collection in high-intensity light is for the chloroplasts to assemble in a cluster around the nucleus: this has been observed in leaves of the seagrass Halophila stipulacea growing at 0.5 m depth in the Mediterranean.323 Sharon and Beer (2008) compared H. stipulacea plants grown under high light (450 mmol photons m~2s_1 at midday) or low light (150 mmol photons m~2s_1 at midday). At 06:00 h, before sunrise, the chloroplasts were evenly dispersed within the cytoplasm of the leaf cell. In the high-light plants, migration of the chloroplasts started at sunrise and the beginning of clumping was observed at 08:00 h. Full clumping had taken place by midday and continued throughout the afternoon. In the late afternoon the chloroplasts began to disperse through the cytoplasm, and full dispersal was achieved a few hours after sunset. In the low-light plants there was no clumping of chloroplasts during the day. In the high-light plants, chloroplast clumping was accompanied by a decrease in leaf absorptance from 0.56 in the early morning to 0.34 at midday, without any accompanying change in chlorophyll content. In plants of H. stipulacea in the Gulf of Aqaba (Red Sea), Schwarz and Hellblom (2002) found that chloroplast clumping in plants at 7 m depth reduced leaf absorptance to 55%, compared to 85% in plants at 30 m.

The action spectrum for chloroplast movement in higher plants has a peak in the blue region at about 450 nm: the photoreceptor may be a flavin or a carotenoid.544 The mechanism may be related to that of cytoplasmic streaming: the chloroplasts appear to move together with the cytoplasm rather than through it.

Nultsch and Pfau (1979) studied chloroplast movements in a large number of littoral and sublittoral marine macroalgae. In most of the brown algal species, the chloroplasts moved to the cell walls facing the light at low irradiance and to the side walls parallel to the light direction at high irradiance (Fig. 12.19): the change from one position to the other took 1 to 2 h. No clear-cut light-induced movements of the chloroplasts were observed in the green or red algal species studied. In the siphonac-eous green alga Caulerpa racemosa, which grows in tropical shallow water reef areas, Horstmann (1983) observed that in bright sunlight the chloro-plasts are retracted from the fronds into the stolon.

Fig. 12.19 Light-induced movement of the chloroplasts in the brown alga Laminaria saccharina (by permission from Nultsch and Pfau (1979), Marine Biology, 51, 77-82). Arrangement of the chloroplasts in (a) low-intensity (1000 lux) and (b) high-intensity (10 000 lux) light. Magnification ~4000 x.

In the filamentous green algae Mougeotia and Mesotaenium, which have a single, flat centrally located rectangular chloroplast in each cell, the characteristic movement of the chloroplast in response to a change in the light regime is to turn on its longitudinal axis, rather than to move around the wall. It turns face-on to moderate light, and edge-on to intense light. Haupt (1973) and coworkers have shown that the photoreceptor controlling this movement is phytochrome. In another filamentous, coenocytic alga, Vaucheria sessilis (Xanthophyceae), if the filament is illuminated at one point with low-intensity blue light, the chloroplasts and other organelles that are normally carried along by the streaming cytoplasm are caused to aggregate in the illuminated part of the cell.125 The photosyn-thetic implications of this are not yet clear.

In the case of floating, planktonic algae, oriented at random with respect to the light, movement alone to one part of the cell or another is not likely - if the cell contains only one or a few chloroplasts - to make much difference to the rate of light collection. In some cases the chloro-plast can reduce its absorption cross-section in bright light by shrinking: this has been observed in a dinoflagellate1329 and in diatoms.537,686 In the marine centric diatom Lauderia borealis, which has about 50 chloroplasts per cell, Kiefer (1973) observed that in the first 2min of exposure to intense light (244 Wm2) the chloroplasts contracted in size (Fig. 12.20a). During the following 30 to 60min the chloroplasts, which in low light were distributed evenly around the periphery, moved to the valvar ends of the cell forming two aggregates of equal size (Fig. 12.20b). The changes in size and position of the chloroplasts were accompanied by a decrease of about 40% in the absorbance of the suspension at 440 nm (probably an underestimate since scattering always contributes some spurious absorbance): thus by a combination of shrinkage and aggregation this diatom is able to substantially reduce its rate of energy collection in bright light.

Stephens (1995) investigated light-induced chloroplast migration, and the associated spectral changes, in the dinoflagellate, Pyrocystis lunula, a species abundant in the tropical and subtropical areas of the ocean, which has large crescent-shaped cells. The cells were grown on a 12 h light: 12 h dark cycle. During the dark cycle the chloroplasts were located among cytoplasmic strands in the two distal portions of the cell (the 'horns' of the crescent), and were absent from the granular central area of the cell. When cells were removed from the dark and placed in the light beam of the microscope, the chloroplasts began to move and after 2 to 3 min were aggregating in the central area of the cell. The absorption spectrum of the

Fig. 12.20 Chloroplast shrinkage and aggregation induced by high light intensity in the marine diatom Lauderia borealis (by permission, from Kiefer (1973), Marine Biology, 23, 39-46). In each case the series of micrographs shows increasing light-induced change from left to right. (a) Contraction of chloroplasts. (b) Movement of chloroplasts to valvar ends of the cell, followed by aggregation.

Fig. 12.20 Chloroplast shrinkage and aggregation induced by high light intensity in the marine diatom Lauderia borealis (by permission, from Kiefer (1973), Marine Biology, 23, 39-46). In each case the series of micrographs shows increasing light-induced change from left to right. (a) Contraction of chloroplasts. (b) Movement of chloroplasts to valvar ends of the cell, followed by aggregation.

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Fig. 12.21 Diurnal rhythm in the photosynthetic capacity (Pm) of phyto-plankton in the St Lawrence estuary, Canada, over a seven-day period (by permission, from Demers and Legendre (1981), Marine Biology, 64, 243-50).

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Fig. 12.21 Diurnal rhythm in the photosynthetic capacity (Pm) of phyto-plankton in the St Lawrence estuary, Canada, over a seven-day period (by permission, from Demers and Legendre (1981), Marine Biology, 64, 243-50).

aggregated chloroplasts in the centre of the cell showed a more marked package effect, i.e. flattening of the peaks, than chloroplasts in the distal regions of the cell. In this case also, chloroplast movement has the effect of reducing the rate of energy collection in bright light.

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