THE CARBON DIOXIDE FACTOR 1901 



value to be dependent on light intensity — despite the remark by Heath (1951) that simi- 

 lar limiting pressures had been found at 25 klux (Heath), 1 1 klux (C.abrielsen), and 2.8- 

 9.3 klux (Heath and Milthorpi'). 



A related question is that of the possibility of stopping photosynthesis 

 altogether by rapid removal of respiratory carbon dioxide. This problem 

 is of importance for the interpretation of the quantum yield measurements, 

 since a positive answer (c/. chapter 29, p. 901) would mean that respiration 

 intermediates are not drawn into photosynthesis, as substitute oxidants, 

 if carbon dioxide (or the CO2 acceptor complex) is available. The relevant 

 experiments are therefore discussed in section 4 below. 



(d) Carbon Dioxide Supply through Roots 



Reference to this phenomenon was made in Vol. II Part 1 (p. 910). It was since 

 studied quantitatively by Kursanov, Kuzin and Mamul (1951) and Kursanov, Krju- 

 kova and Vartapetjan (1952). Phaseolus plants, with roots immersed in C(14)-tagged 

 carbonate solution or placed in a gas chamber containing CO2, were found to take up C* 

 and convey it up the stalk into the leaves. With roots immersed in water, the rate of 

 C* supply through the roots was several times higher than that of the transpirational 

 flow of water; in contrast to the latter, it occurred only in light. If the (green) stalk was 

 illuminated, most of the C* was intercepted and assimilated there, before reaching the 

 leaves. The absolute rate of CO2 uptake through the root, observed in 9-15 day old 

 plants, from an atmosphere containing 0.8-1% C*02 (a concentration common in podzol 

 soils), corresponded to 3-5 mg. CO2 per 100 cm.'' leaf surface— about one fourth of the 

 ([uantity these leaves can utilize if supplied with carbon dioxide from the air. 



These measurements indicate the possibility of a substantial contribution of root- 

 absorbed carlion dioxide (or bicarbonate?) to the photosynthesis of leaves, and espe- 

 cially of green stalks. This may be the explanation of the occurrence of chlorophyll in 

 stalks, whose shape is unsuitable for the uptake of carbon dioxide from the air. 



Kursanov (1954) summarized the studies of his group on the C*02 up- 

 take by roots and its transportation and utilization in the plant. Chroma- 

 tographic analysis indicated the conversion of the CO2, absorbed into roots, 

 to oxalacetic acid, by combination with pyruvic acid, and reduction of the 

 latter to malic acid, which appeared as the first stable tagged compound; 

 later, C(14), taken in through the roots, could be found also in citric and 

 ketoglutaric acid. In the absence of enough light, a part of these acids, 

 conveyed to the leaves, was decarboxylated there, liberating tagged carbon 

 dioxide into the atmosphere. 



(e) Isotopic Discrimination 



The observations concerning the discrimination, in photosynthesis, 

 between the carbon isotopes 12, 13 and 14 will be described in section 3 of 

 this chapter {cf. p. 1927). 



