Effects of CO₂ Concentration in the Air, and Shading, on the Utilization of NO₃ and NH₄ by Vegetable Crops

Abstract

Two experiments were carried out to investigate the effects of CO₂ concentration in the air and light intensity on the utilization of NO₃ and NH₄ by vegetable crops.
In the first experiment, lettuce, tomato, and kidney bean were grown in a nutrient solution composed of various NO₃/NH₄ mixtures under several concentrations (360-1800ppm) of CO₂ in the air. The growth of each crop was enhanced by CO₂ enrichment and was better when plants were provided with N as a combination of NO₃ and NH₄ than with NO₃ alone. Especially in tomato, growth enhancement by CO₂ enrichment was greater with NH₄ than with NO₃.
In the second experiment, cucumber, tomato, and spinach were grown in a nutrient solution containing N in various combinations of NO₃ and NH₄ under two levels of light intensity (unshaded, 40% shaded) and air CO₂ concentration (360, 1300ppm); lower growth was obtained under the shaded condition. The growth of each crop was markedly increased by CO₂ enrichment under unshaded higher light intensity. However, under the shaded condition, the effect of CO₂ enrichment on spinach growth was small. When plants were grown with NO₃ alone, increasing N concentration in the solution (2-12 me/l) produced little increase in dry weight at 360ppm CO₂ irrespective of light conditions, but produced marked increase at 1300 ppm CO₂. Greater growth increment owing to CO₂ enrichment was produced with NH₄ than with NO₃. Furthermore, growth reduction due to shading was smaller with NH₄ than with NO₃.
The results may suggest the possibility that NH₄ is a more effective N source than NO₃ under higher air CO₂ concentration or lower light intensity. Moreover, N concentration in the solution needs to be kept relatively high to produce the yield increase by CO₂ enrichment.