Abstract
It is intended to elucidate the interaction between photosynthesis and nitrogen metabolism in C_3 and C_4 plants. In the present study, the effects of nitrogen nutrition on photosynthetic rates (P_0), RuBP- and PEP-carboxylase activities and photorespiration rates were examined. Using newly expanded leaves of C_3 and C_4 plants grown at graded nitrogen level or at certain nitrogen level with ammonium or nitrate in culture solutions, their rates of apparent photosynthesis (P_0), nitrogen contents, RuBP- and PEP-carboxylase activities, CO_2 compensation points and the rates of photorespiration and dark respiration were measured. The results obtained were as follows: 1. Total nitrogen contents were highly correlated to P_0 of forage leaves. Both P_0 and Po per unit nitrogen increment were significantly higher in C_4 plants than in C_3 plants. 2. Reasons why P_0 of C_4 plants was enhanced more than that of C_3 plants with an increase of nitrogen content in leaves were (a) C_4 plants had higher proportion of soluble protein fraction to the total nitrogen compounds, (b) Activities of enzymes related to C_4 cycle remarkably increased with an increase of nitrogen content in leaves. 3. No significant differences were observed in P_0 of C_3 and C_4 plants between forms of nitrogen source. In C_3 plants, the rate of photorespiration, as determined by the rate of CO_2 released in CO_2 free air, of plants supplied with nitrate was approximately 30 percent lower than that of those supplied with ammonium. However, the rate of true photosynthesis was also lower in plants supplied with nitrate than with ammonium.
