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ENDOGENOUS INORGANIC CARBON SOURCES IN PLANT HOTOSYNTHESIS
Author(s) -
RAVEN J. A.
Publication year - 1972
Publication title -
new phytologist
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.742
H-Index - 244
eISSN - 1469-8137
pISSN - 0028-646X
DOI - 10.1111/j.1469-8137.1972.tb01978.x
Subject(s) - photorespiration , algae , decarboxylation , endogeny , metabolism , carbon dioxide , botany , substrate (aquarium) , carbon fibers , biology , oxygen , chemistry , photosynthesis , environmental chemistry , biochemistry , ecology , materials science , organic chemistry , composite number , composite material , catalysis
S ummary The quantity of CO 2 arising from photorespiration in angiosperms and in algae has been estimated. Much of such CO 2 arises from the metabolism of glycolate so that estimates are based on the rate of formation of glycolate and the rate at which it is metabolized (Table 1). The total CO 2 production in leaves or algae in the light can then be taken as equal to the photorespiratory CO 2 plus the CO 2 produced by‘dark’ respiratory pathways. Methods of estimating CO 2 release from illuminated green tissues are considered and the rate of decarboxylation deduced from biochemical evidence of substrate turnover is compared with the observed rate of CO 2 release; the difference between these values is reassimilation (Table 1). In angiosperms with the PGA pathway, in air, reassimilation is never complete and ranges from o to 0.6 of the endogenous CO 2 production. This implies that the resistance to refixation is as great or greater than the resistance to loss of endogenous CO 2 to the medium and is in accord with the relative values of these resistances based on other evidence. In law oxygen environment less glycolate is produced and hence less photorespiratory CO 2 ; the resistance to refixation is smaller and so fractional reassimilation in increased. Unicellular algae have the PGA pathway and some appear to have more efficient reassimilation than the corresponding angiosperms. The special problems of metabolism in four carbon acid plants are discussed. Such plants show essentially complete reassimilation since they do not lose CO 2 when illuminated. In their leaves the resistance to escape of endogenously produced CO 2 must be very much greater than the resistance to refixation. The low resistance to refixation is probably related to high levels of CO 2 in chloroplasts which are generated by the‘CO 2 ‐pump’ rather than to any peculiarities of carboxydismutase. The importance of reassimilation is discussed in relation to the influence of CO 2 on various aspects of metabolism, to isotopic discrimination in CO 2 fixation and to the mechanism of the use of bicarbonate in photosynthesis.