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Among the numerous taxonomic group of Amaranthus L., 12 species are cultivated as vegetable, cereal and fodder crops, medicinal and ornamental plants. A limitation for their introduction in Russia results particularly from lack of data on the physiological and ecological parameters of plant growth and development, which prevents the effective breeding investigations, especially with a view to create varieties for cultivation in northern regions, where the temperatures during spring vegetation are low. In controlled conditions of two factorial preplanned experiment we studied the effect of light intensity and temperature to net photosynthesis in intact Amaranthus cruenthus L. plants (Sultan variety) and A. hypochondriacus L. plants (Krepysh variety). At 6-7 leaf phase, the experimental plants were subjected to hardening for 3 days at 8 °N and 10 °N (for A. hypochondriacus and A. cruenthus, respectively). To evaluate the CO2 concentration, the infrared gas analyzer Infralot-IV (SAXON Junkalor GmbH, Germany) was used according to differential scheme. To describe the relationship between CO2 exchange and external factors, the nonlinear equations (a model) were obtained: NP = a0 + a1E + a2T + a3ET + a4E2 + a5T2, with NP as the intensity of observed photosynthesis, mg NI2/(g⋅h); A as illumination, W/m2; O as air temperature, °N; a0-a5 as the coefficients calculated basing on the experimental data. A reliability of the equations was verified by the multiple determination index (R2 = 85-94) and F-Fisher test (F = 4.1-5.6) at ð = 0.05. This model can be applied for estimating photosynthetic activity at definite vegetation phase for each cultivar under different conditions and predicting parameters necessary to achieve definite net photosynthesis even under limiting factors. Thus, using multiple regression analysis, the equations were obtained, which allow estimating favorable combinations of the light intensity and temperature for maximal and optimal net photosynthesis at natural CO2 concentration in the air. In both examined varieties, the net photosynthesis value of 37-38 ia NI2/(g⋅h) was registered as a potential maximum. Sultan variety was more light- and heat-loving, while Krepysh variety demonstrated more coldresistance. The photosynthetic optimum was provided within the limits of 26.7-47.0 °N and 335580 W/m2 for Sultan variety and at 23.5-39.6 °N and 284-501 W/m2 for Krepysh variety. Plant

LIGHT AND TEMPERATURE PARAMETERS TO OPTIMIZE PHOTOSYNTHESIS IN TWO Amaranthus L. SPECIES