CHLOROPHYLLIAN PHOTOSYNTHESIS. — The remarkable process of foliar nutrition in plants has remained clear in its fundamental features ever since the classical experiments of De Saussure (1740–99) made it possible to establish that, in the presence of sunlight, the green plant emits a volume of oxygen equal to that of the carbon dioxide absorbed, so that the three fundamental conditions under which the phenomenon occurs could be identified: a) the presence of carbon dioxide in the environment; b) the presence of sunlight; c) the presence of green pigment.
Thus the process of foliar nutrition came to be well defined and limited to carbon nutrition, proceeding under the influence of light and in the presence of chlorophyll; hence it was called the process of c. p., or chlorophyllian photosynthesis.
The passage from inorganic carbon to organic carbon, the organization of carbon effected by c. p., was from that time onward regarded as a reduction process, in which completely oxidized and therefore non-combustible substances (carbon dioxide and water) are transformed into substances with a lower degree of oxidation and therefore combustible substances. It is consequently an endoenergetic process, that is, one which takes place with the absorption of energy—specifically, luminous energy (photons) supplied by the sun—which the plant absorbs and stores as chemical energy through the construction of organic matter. It is in this respect that plants may be defined as accumulators of solar energy.
The fruitful experimental investigations that have followed one another in large numbers since the dawn of the nineteenth century have shed light on the importance of the individual factors regulating the photosynthetic process: light (the intensity and quality of the radiation), the reacting raw materials (carbon dioxide and water), temperature, chlorophyll content, the content of mineral elements (macro- and microelements), and finally the other so-called “internal” factors, connected with the enzymatic and protoplasmic activity of the cell (in vitro, outside the chloroplasts, chlorophyll loses its characteristic action).
The numerous hypotheses concerning the first organic product originating from the photochemical reduction of carbon dioxide have, on the other hand, proved less satisfactory. Once the hypothesis put forward approximately a century ago—that starch is the first product of carbon assimilation—had been rejected, the hypotheses of Baeyer and of Willstaetter and Stoll, who regard formic aldehyde as the first reduction product, and the hypothesis of Maquenne, who excludes formic aldehyde and instead maintains that the first product of c. p. is directly a monosaccharide, that is, a simple sugar, remained accredited.
Recently in California, numerous researchers—Ruben, Randall, Kamen, and Hyde—administered to green algae of the genus Chlorella ordinary carbon dioxide and water containing the isotope O¹⁸ in place of oxygen O¹⁹. They demonstrated that the oxygen released during the photosynthetic process derives entirely from the water and not from the carbon dioxide. In the photosynthetic process, therefore, water undergoes dehydrogenation, while carbon dioxide, or one of its derivatives, represents the corresponding hydrogen acceptor.
Also in California, Calvin and his colleagues subsequently pursued these investigations in greater depth, working with the isotope C¹⁴. They reached the conclusion that the appearance of glucose as the first stable product of chlorophyllian synthesis is preceded by the formation of organic acids, which are progressively hydrogenated through photochemical action and also by virtue of reactions that proceed without the need for light.
According to these most recent investigations, c. p. may be regarded as the reverse process of respiration, not only in the energetic sense, but above all in the biochemical and enzymatic sense (Pratolongo).
With regard to the solar energy utilized by plants in c. p., according to recent calculations by Putter, who took into account the losses due to respiration, it varies within the low limits of 2–4%. Thus, from utilization values, per cent of solar energy, of 2.1 in beet, one passes to 2.6 in barley, 3.0 in potatoes, 3.2 in oats and wheat, and 3.7 in cabbage.
The quantities of carbon fixed annually by plants through c. p. are enormous. Today the vegetation present on the continents is attributed an annual assimilatory activity equal to 1.0–10¹⁰ tons of organic carbon, corresponding on average to 13 quintals of carbon per year and per hectare over the entire emerged surface. Chlorophyllian synthesis taking place in the oceans likewise exceeds that on land; it has been estimated at 15.1–10¹⁰ tons annually, corresponding on average to 37.5 quintals of carbon fixed per year and per hectare of marine surface (Riley, 1941).