CHLOROPHYLLIAN PHOTOSYNTHESIS. - The suggestive process of foliar nutrition in plants has been clarified in its fundamental outlines ever since the classic experiments of De Saussure (1740-99) made it possible to establish that, in the presence of sunlight, the green plant releases 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 fixed: a) presence of carbon dioxide in the environment; b) presence of sunlight; c) presence of green pigment.
Accordingly, the process of foliar nutrition came to be well defined and limited to carbonic nutrition, occurring under the influence of light and in the presence of chlorophyll: for which reason it received the name of the process of chlorophyllian photosynthesis.
The passage from inorganic carbon to organic carbon, the organization of carbon brought about by chlorophyllian photosynthesis, has ever since been regarded as a process of reduction, in which from completely oxidized substances, and therefore non-combustible ones (carbon dioxide and water), one passes to substances with a lower degree of oxidation and therefore combustible, a process thus of an endoenergetic character, that is, one that takes place with the absorption of energy, and precisely of luminous energy (photons) supplied by the sun, which the plant absorbs and stores as chemical energy through the construction of organic matter. And it is by virtue of this aspect that plants can be defined as accumulators of solar energy.
The fruitful experimental researches, which followed one another in abundance from the dawn of the nineteenth century, have shed light on the importance of the individual factors that regulate the photosynthetic process, namely light (intensity and quality of the radiations), the reacting raw materials (carbon dioxide and water), temperature, chlorophyll content, content of mineral elements (macro and micro) and finally the other so-called "internal" factors, tied to the enzymatic and protoplasmic activity of the cell (in vitro, outside the chloroplasts, chlorophyll loses its characteristic action).
Far less conclusive, on the other hand, are the numerous hypotheses concerning the first organic product that originates from the photochemical reduction of carbon dioxide. Once the hypothesis put forward about a century ago, that starch is the first product of carbon assimilation, was discarded, the hypotheses of Baeyer and of Willstaetter and Stoll remained credited, which regard formaldehyde as the first product of reduction, and the hypothesis of Maquenne, which excludes formaldehyde and admits instead that the first product of chlorophyllian photosynthesis is directly a monosaccharide, that is, a simple sugar.
Recently in California numerous researchers, Ruben, Randall, Kamen, and Hyde, by administering to green algae of the genus Chlorella ordinary carbon dioxide and water containing the isotope O¹⁸, in place of the oxygen O¹⁹, revealed that the oxygen released during the photosynthetic process comes entirely from the water and not from the carbon dioxide. Water therefore in the photosynthetic process undergoes a dehydrogenation and carbon dioxide or a derivative thereof represents the corresponding hydrogen acceptor.
Again in California, Calvin and colleagues subsequently deepened the research by working with the isotope C¹⁴ and reached the conclusion that the appearance of glucose as the first stable product of chlorophyllian synthesis is preceded by the formation of organic acids that are progressively hydrogenated, through photochemical action and also by virtue of reactions that proceed without the need for light.
According to these very recent researches, chlorophyllian photosynthesis can 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 chlorophyllian photosynthesis, according to recent calculations by Putter, who took into account the losses due to respiration, it turns out to vary within the low limits of 2-4%. From values indeed of utilization, per hundred of solar energy, of 2.1 in the beet, one passes to 2.6 in barley, to 3.0 in potatoes, to 3.2 in oats and in wheat, to 3.7 in cabbage.
The quantities of carbon that are fixed each year by plants through the action of chlorophyllian photosynthesis are enormous. Today an annual assimilatory activity equal to 1.0-10¹⁰ tons of organic carbon is attributed to the vegetation present on the mainland, which corresponds on average to 13 quintals of carbon per year and per hectare over the entire emerged surface. The chlorophyllian synthesis that takes place in the oceans moreover exceeds that on land; it has been estimated at 15.1-10¹⁰ tons per year, equal on average to 37.5 quintals of carbon fixed per year and per hectare of sea surface (Riley, 1941).
Bibliography
Luigi Marimpieri