Glycolate, Glycine, Serine, and Glycerate Formation during Photosynthesis by Tobacco Leaves

Prying into the green black-box

Life-long efforts of the Tartu photosynthesis research group have been summarized. The measurements were facilitated by self-designed instruments, distinct in multifunctionality and fastresponse time. The black-box type kinetical analysis on intact leaves has revealed several physiologically significant features of leaf photosynthesis. Rubisco studies reflected competition for the active site between the substrates and products, linearizing in vivo kinetics compared with the low-K_m in vitro responses. Rubisco Activase usually activates only a small part of the Rubisco, making the rest of it a storage protein. Precisely quantifying absorbed photons and the responding transmittance changes, electron flow rates through cytochrome b₆f, plastocyanin and photosystem I were measured, revealing competition between the proton-uncoupled cyclic electron flow from PSI to Cyt b₆f to P700⁺ and the proton-coupled linear flow from PSII to Cyt b₆f to P700⁺. Analyzing responses of O₂ evolution and Chl fluorescence to ms-length light pulses we concluded that explanation of the sigmoidal fluorescence induction by excitonic connectivity between PSII units is a misconception. Each PSII processes excitation from its own antenna, but the sigmoidicity is caused by rise of the fluorescence yield of the Q_A-reduced PSII units after their Q_B site becomes occupied by reduced plastoquinone (or diuron). Unlike respiration, photosynthetic electrons must prepare their acceptor by coupled synthesis of 3ATP/4e^-. Feedback regulation of this ratio leads to oscillations under saturating light and CO₂, when the rate is P_i-limited. The slow oscillations (period 60s) indicate that the magnitudes of the deflections in the 3ATP/4e^- ratio, corrected by regulating cyclic and alternative electron flow (including the Mehler type O₂ reduction), are only a fraction of a per cent. The P_i limitation causes slip in the ATP synthase, slightly increasing the basic 12H⁺/3ATP requirement.

Enzymes of serine and glycine metabolism in leaves and non-photosynthetic tissues of Pisum sativum L

A comparative study is presented of the activities of enzymes of glycine and serine metabolism in leaves, germinated cotyledons and root apices of pea (Pisum sativum L.). Data are given for aminotransferase activities with glyoxylate, hydroxypyruvate and pyruvate, for enzymes associated with serine synthesis from 3-phosphoglycerate and for glycine decarboxylase and serine hydroxymethyltransferase. Aminotransferase activities differ between the tissues in that, firstly, appreciable transamination of serine, hydroxypyruvate and asparagine occurs only in leaf extracts and, secondly, glyoxylate is transaminated more actively than pyruvate in leaf extracts, whereas the converse is true of extracts of cotyledons and root apices. Alanine is the most active amino-group donor to both glyoxylate and hydroxypyruvate. 3-Phosphoglycerate dehydrogenase and glutamate: O-phosphohydroxypyruvate aminotransferase have comparable activities in all three tissues, except germinated cotyledons, in which the aminotransferase appears to be undetectable. Glycollate oxidase is virtually undetectable in the non-photosynthetic tissues and in these tissues the activity of glycerate dehydrogenase is much lower than that of 3-phosphoglycerate dehydrogenase. Glycine decarboxylase activity in leaves, measured in the presence of oxaloacetate, is equal to about 30-40% of the measured rate of CO2 fixation and is therefore adequate to account for the expected rate of photorespiration. The activity of glycine decarboxylase in the non-photosynthetic tissues is calculated to be about 2-5% of the activity in leaves and has the characteristics of a pyridoxal-and tetrahydrofolate-dependent mitochondrial reaction; it is stimulated by oxaloacetate, although not by ADP. In leaves, the measured activity of serine hydroxymethyltransferase is somewhat lower than that of glycine decarboxylase, whereas in root apices it is substantially higher. Differential centrifugation of extracts of root apices suggests that an appreciable proportion of serine hydroxymethyltransferase activity is associated with the plastids.

Photosynthesis and increased production of protein

Photosynthesis, the use of light energy in the conversion of CO2 and inorganic nutrients into plant material, is the ultimate source of the food protein necessary to man's existence. Given certain assumptions, the overall maximal theoretical photosynthetic efficiency of agricultural plants can be calculated. Actual measured maximal growth rates of plants are equivalent to efficiency levels well below that theoretical maximum. In air, C4 plants can some closer to the theoretical value than C3 plants, perhaps because C4 plants avoid the occurrence of measurable photorespiration and oxygen inhibition of photosynthesis. Alfalfa, a C3 legume, is an extremely productive protein source. Its protein yield per acre can surpass that of commonly grown C4 crops (corn, sorghum) and C3 seed crops (soybean, wheat, rice). Alfalfa leaf protein is of high nutritional quality and can apparently be used directly in the human diet, eliminating the protein loss involved in animal production. Plant protein productivity can be raised as part of an increase in overall crop yield. The growth of plants in atmospheres with elevated CO2 levels can result in increased yields. In C3 plants this is due, at least in part, to the suppression of photorespiration and oxygen inhibition of photosynthesis. We have investigated the effect of CO2 concentration on alfalfa photosynthetic metabolism. Our results support the contention that alfalfa productivity can be increased by an environment of elevated CO2. A second approach toward increased plant protein productivity is through regulation of carbon flow during photosynthesis so as to increase portein production relative to that of other plant constituents. In particular, we have investigated whether ammonia (the form in which plants first incorporate nitrogen) can act to regulate leaf carbon metabolism. Our results indicate that NH4+, in part through stimulation of pyruvate kinase, brings about increased production of amino acids at the expense of sucrose production in alfalfa. The effect may be of considerable importance in the regulation of green leaf protein synthesis.