Photosynthesis, Photorespiration and Productivity of Wheat and Soybean Genotypes

The results of the numerous measurements obtained during the last 40 years on gas exchange rate using an infrared gas analyzer URAS-2T (Germany), photosynthetic carbon metabolism by exposition in 14СО2 and activities of enzyme of primary carbon fixation, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPC/O), in various wheat and soybean genotypes grown over a wide area in the field and contrasting in photosynthetic traits and productivity are presented in this paper. It was established that high productive (7-9 t ha-1) wheat genotypes with the optimal architectonics possess higher rate of CO2 assimilation during leaf ontogenesis. Along with the high rate of photosynthesis, high values of photorespiration are characteristic for high productive genotypes. There is a parallel increase in the rates of true photosynthesis and photorespiration in ontogenesis. Genotypes with moderate (4-5 t ha-1) and low (3 t ha-1) grain yield are characterized by relatively low rates of both CO2 assimilation and photorespiration. The ratio of true photosynthesis to photorespiration in genotypes with different productivity is equal on average to 3:1. A value of photorespiration constitutes 28-35% of photosynthetic rate in contrasting wheat genotypes. The activities of RuBP carboxylase and RuBP oxygenase were changing in a similar way in the course of the flag leaf and ear elements development. RuBP oxygenase activity was higher in high productive wheat genotypes than in low productive ones. The rates of sucrose (the main transport metabolite in plants) biosynthesis and products of glycolate metabolism also correlate with the CO2 assimilation rate and the activity of RuBP oxygenase. High productive genotypes are also characterized by a higher rate of biosynthesis and total value of glycine-serine and a higher photosynthetic rate. Pattern of changes in biosynthesis rate and total value of glycine-serine as well as ratio of RuBP carboxylase to oxygenase activities and CO2 assimilation rate predisposes to parallel change in the rates of photosynthesis and photorespiration during leaf ontogenesis. High rates of photosynthesis and photorespiration in conjunction with favourable photosynthetic traits, an optimum leaf area index and the best architectonics define high productivity of wheat genotypes. Therefore, contrary to conception arisen during many years on wastefulness of photorespiration, taking into account the versatile investigations on different aspects of photorespiration it was proved that photorespiration is one of the evolutionary developed vital metabolic processes in plants and the attempts to reduce this process with the purpose of increasing the crop productivity are inconsistent. Phosphoglycolate phosphatase, a key enzyme of photorespiration was first homogeneously purified from eukaryotic green algae Chlamydomonas reinhardtii with subsequent determination of complete nucleotide and deduced amino acid sequences (NCBI Nucleotide 1:AB052169). Since metabolic processes of photorespiration in the leaf occur in the light simultaneously with photosynthesis, it is evident that released energy is used in certain reactions of photosynthesis.