Two experiments were carried out to investigate the effects of CO
2 concentration in the air and light intensity on the utilization of NO
3 and NH
4 by vegetable crops.
In the first experiment, lettuce, tomato, and kidney bean were grown in a nutrient solution composed of various NO
3/NH
4 mixtures under several concentrations (360-1800ppm) of CO
2 in the air. The growth of each crop was enhanced by CO
2 enrichment and was better when plants were provided with N as a combination of NO
3 and NH
4 than with NO
3 alone. Especially in tomato, growth enhancement by CO
2 enrichment was greater with NH
4 than with NO
3.
In the second experiment, cucumber, tomato, and spinach were grown in a nutrient solution containing N in various combinations of NO
3 and NH
4 under two levels of light intensity (unshaded, 40% shaded) and air CO
2 concentration (360, 1300ppm); lower growth was obtained under the shaded condition. The growth of each crop was markedly increased by CO
2 enrichment under unshaded higher light intensity. However, under the shaded condition, the effect of CO
2 enrichment on spinach growth was small. When plants were grown with NO
3 alone, increasing N concentration in the solution (2-12 me/
l) produced little increase in dry weight at 360ppm CO
2 irrespective of light conditions, but produced marked increase at 1300 ppm CO
2. Greater growth increment owing to CO
2 enrichment was produced with NH
4 than with NO
3. Furthermore, growth reduction due to shading was smaller with NH
4 than with NO
3.
The results may suggest the possibility that NH
4 is a more effective N source than NO
3 under higher air CO
2 concentration or lower light intensity. Moreover, N concentration in the solution needs to be kept relatively high to produce the yield increase by CO
2 enrichment.
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