Thermo-radiative conditions and nitrogen availability are to be considered important factors in controlling plant feeding and hence growth. In this research their effects were studied on tomato and lettuce in the greenhouse. Changes in dry matter weight, total nitrogen uptake and nitrate nitrogen content were recorded at two different growth stages in relation to: temperature (cold and heated greenhouse with set point 13 degrees C) and light (100% and 60% radiation) conditions, and nitrogen content in the nutrient solution (3, 8, 13, 18 meq.l(-1) of NO3-). Reduced light level negatively affected dry matter production and proportionally the amount of absorbed nitrogen in both species. Lower temperatures level negatively modified these parameters only in tomato plants, in this species the increase in nitrogen content, due to a higher availability of the element, reduced the negative effect of lower temperature on dry matter production. Nitrate nitrogen to total nitrogen increased with an increase of the element in the nutrient solution, this increase was in lettuce similar in all conditions; the trend was not so regular in tomato. In conclusion, an appreciable interaction between thermo-radiative conditions and nitrogen availability, was observed only in tomato plants with reference to temperature.
