Changes in electron carriers and carboxylase

In higher plant species that are able to adapt to low light conditions, the increase in pigment during shade adaptation is not accompanied by an increase in the content of the photosynthetic electron transfer components - the cytochromes, ferredoxin and plastoquinone. The levels of these per unit mass may remain about the same or may decrease some-what.128,1467 The level per unit mass of leaf is markedly lowered in shade-adapted plants.895 These changes are of adaptive value since at low light intensities the plant will not be able to carry out high rates of electron transfer and carboxylation anyway and so can achieve biosyn-thetic economies by refraining from increasing, or preferably decreasing, its content of electron transfer components and carboxylase at the same time as it increases its pigment content and photosynthetic unit number. Thus, in shade-adapted higher plants there is an increase in the ratio of pigment assemblies to the pools of electron carriers, and an even bigger increase in the ratio of pigment assemblies to carboxylase.

The same sort of changes have been shown to accompany shade adaptation in some unicellular algae. In the chlorophyte Scenedesmus obliquus, while the chlorophyll content per g fresh weight increased by 64% (in the low-light-grown, relative to the high-light-grown cells), the cytochrome f content decreased by 33%, and the carboxylase activity also decreased.400,1203 In shade-adapting Chlamydomonas reinhardtii, while the cellular chlorophyll content doubled, the cytochrome f level remained about the same.979 In another chlorophyte, Tetraedron minimum, reduction of growth irradiance from 500 to 50 mmol photons m~2s_1 led to a five-fold increase in cellular chlorophyll and photosynthetic unit number, but the amount of Rubisco per cell remained the same.398 In partial contrast to some of the above observations, in the marine chlorophyte Dunaliella tertiolecta, the cellular cytochrome f concentration increased several-fold in parallel with chlorophyll and photosynthetic unit number during shade adaptation, but in this case also Rubisco content per cell remained about the same.1322 In the marine diatom Phaeodactylum tricornutum, grown in continuous culture, while the chlorophyll and carotenoid content rose progressively by about 100% as light intensity was lowered from 12 to 0.5 klux, the Rubisco activity per cell fell to less than 25% of the value present in high-light-grown cells.82 Thus, in algae the preferred strategy for coping with low light levels appears to be an increase in the number or size (or both) of the pigment assemblies (photo-synthetic units) with no increase, or an actual decrease, in the synthesis of carboxylase.

Lin and Carpenter (1997) used epifluorescence microscopy of immuno-fluorescent-stained cells to study the distribution of Rubisco within the chloroplast of the chlorophyte Dunaliella tertiolecta, as a function of light intensity during growth. Over a seven-fold range of intensity the fraction of the cell population that displayed distinct Rubisco staining in the pyrenoid was positively correlated with irradiance. Lin and Carpenter propose that D. tertiolecta possesses an adaptive mechanism that brings about the redistribution of Rubisco between the pyrenoid - the probable site of Rubisco activation and CO2 fixation - and the stroma (which putatively acts as a reservoir of the enzyme in its inactive form) in response to change in light intensity.

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