The effect of salicylic acid (SA, 100 μM) of various duration on some indexes of CO2-gas exchange, water regime, growth and cold resistance of wheat plants (Triticum aestivum L.) under optimal (22 °C) and low hardening (4 °C) temperatures were studied. For this, week-long seedlings throughout the whole experiment (7 days) were kept on a nutrient solution with added SA at 22 and 4 °C (prolonged SA action) or kept for 1 day at 22 °C on a nutrient solution with SA and then on a solution without SA (within 6 days) exposed to temperatures of 22 and 4 °C (pre-treatment of SA). It was shown that at a temperature of 22 °C, the prolonged action of SA led to a decrease in the rate of visible photosynthesis and the ratio of respiration in the process of CO2-gas exchange, did not affect water use efficiency (WUE), increased the biomass of shoot and root. On the contrary, diurnal exposure to SA had virtually no effect on the dynamics of CO2 assimilation, reduced the ratio of respiration in CO2-gas exchange and transpiration, but increased the WUE. At a temperature of 4 °C, the continuous action of the SA contributed to maintaining the rate of photosynthesis at a higher level than in the control, reduced the ratio of respiration in CO2-gas exchange, increased WUE, and also stabilized the accumulation of plant biomass. In contrast, the diurnal pretreatment of the SA increased the negative effect of low temperature on the assimilation of CO2, led to increase in expenses of respiration, did not affect the level of transpiration, caused a decrease in WUE. It is important, that the exogenous SA contributed to the growth of the cold tolerance of wheat in conditions of normal and low hardening temperatures. Thus, the response of wheat plants to the effect of SA depends on its duration, while the nature of changes in physiological processes differ under optimal and low temperatures.