Transient Changes in Cellular Gas Exchange and the Problem of Maximum Efficiency of Photosynthesis

Abstract

PLANT PHYSIOLOGY P32 labeled compounds of the xylem exudate. Only radioactive inorganic sulfate was present on chromatograms of the exudate collected over 24 hours. The S35 inorganic sulfate in the exudate fluid was isolated chromatographically as easily as the phosphate, but no detailed study of time versus amount of S35 in the exudlate has been run. SUMMARYThe xylem exudate of several species of plants contained three P32 labeled compounds-inorganic phosphate, and two unknown phosphorus containing compounds. As much as 20 % of the total radioactive phosphorus has been observed in the unknown compounds from the exudate on decapitated tomato stems. Sulfur was present only as inorganic sulfate.The two unknown phosphorus-containing compounds are normal constituents of the xylem sap, but they have not been detected in the exudate from squash peduncle. The unknown present in largest amounts in the exudate from decapitated plants was also found in barley guttate. The two unknowns are normal constituents of the xylem exudate rather than wound compounds on the cut surface of the stem or leaf base or products formed by radiation injury from the p32.The percentages of p32 in the unknowns reached a maximum value about 4 hours after the addition of P32 to the nutrient solution bathing the roots. This time lag was independent of when the plant was decapitated to collect exudate. It is interpreted as indicative of the rate of incorporation of phosphate into these compounds.Both the seeds of seedlings and the roots of older plants contributed to the formation of the two unknown phosphorus-containing substances. Removal of the seed from young seedlings severely reduced the amount of these compounds present. The amount of the unknowns in the xylem exudate decreased rapidly with age of the barley seedling, but remained present in considerable quantity in older plants. LITERATURE (6,7,31,34) has published a series of papers in which he reports efficiencies of 70 % and upwards to 100 %. We refer

Quantum efficiency of photosynthetic energy conversion.

“…One concept, espoused by Warburg et al (3), was that photosynthetic quantum conversion has an efficiency of about 90%-i.e., that energy equivalent to that of 3 einsteins of red quanta (42 kcal each) is sufficient to liberate 1 mol of 02 (corresponding to 1/6 mol of glucose, for which AGO' = 686/6 = 114 kcal). In contrast, Emerson (4) and his followers (5) concluded that photosynthetic efficiency was much lower, of the order of 8 to 12 quanta per 02, a range that is widely accepted today even though values less than 8 have, at times, been obtained by investigators (6,7) who did not share Warburg's conclusions.…”

“…Using the total number of quanta absorbed by both cyclic and noncyclic photophosphorylation, we calculated a requirement of about 6 quanta per NADPH for chloroplasts from young plants and about 8 quanta for chloroplasts from old plants (Table 3).…”

Tansley Review No. 36 Excited leaves

“…That much is not in doubt but the world of photosynthesis agonized for years over the question of how many photochemical events combine to bring about tbe fixation of one molecule of CO^, tbe liberation of one molecule of oxygen. The German Nobel Laureate, Otto Warburg (Warburg & Negelein, 1923;W'arburg & Burk, 1950;Warburg, Geleick & Briese, 1952;Warburg, 1958;W'arburg, Krippahl & Lehman, 1969), said 4 (or even less), bis erstwhile American student, Robert Emerson, said at least 8, possibly 12 (Emerson & Green, 1938;Emerson & Lewis, 1939, 1941, 1943Emerson & Chalmers, 1955;Emerson, 1958;Kok, 1960;Radmer & Kok, 1977). The debate was passionate, even bitter.…”

The Minimum Photon Requirement for Photosynthesis

“…Short-term manometric techniques used by Warburg were severely criticized by Emerson and co-workers because of transients in COg and O^ exchange associated with light-dark transitions (the so-called bursts). On the basis of these artifacts, as elaborated by Emerson (Emerson & Chalmers, 1955;Emerson, 1958), much of the other experimental work where this was thought to be a problem has been discredited. The two vessel manometric technique used by Warburg in other studies (Warburg & Bruk, 1950) was not prone to these errors, nor were gas exchange transients a problem in those experiments conducted over long time intervals.…”

“…Both investigations show widely diverging values of the measured photon requirement which varies in conjunction with changes in the gas exchange quotient (COg flux divided by the Og flux). In order to avoid confusing experimental observations, which may have been made under suboptimal conditions, with the true minimum photon requirement, we will call the measured values apparent photon requirements (^co o) ^^ distinguish them from the true minimum value (^cog ^^ Og)-This formality is necessary because much of the controversy over the correct value of the minimum photon requirement has been exacerbated by questions regarding methodological problems and the interpretation of results (Nishimura, Whittingham & Emerson, 1951;Kok, 1952Kok, , 1960Emerson & Chalmers, 1955) and because the reactant (i.e. COg) and product (i.e.…”