Generala Rodina, Orel, 302019 Russia, e-mail borpli@rambler.ru Study was carried out on the equipment of Сenter for collective equipment use, Institute of Biology, the Karelian Scientific Center of Russian Academy of Sciences Received January 11, 2012 Abstract 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 °С and 10 °С (for A. hypochondriacus and A. cruenthus, respectively). <...> 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 СО2/(g⋅h); Е as illumination, W/m2; Т as air temperature, °С; a0-a5 as the coefficients calculated basing on the experimental data. <...> 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. <...> Sultan variety was more light- and heat-loving, while Krepysh variety demonstrated more coldresistance. <...> Plant hardening increased their tolerance to low temperature, decreased significantly the peak of net photosynthesis and led to a narrowing the range of light intensity and temperature, necessary to achieve the optimal net photosynthetic parameter. <...> Keywords: Amaranthus L., preplanned multifactor experiment, net <...>