Photosynthetic CO2Fixation in the Second Leaf of Wheat Seedlings Grown at Different Conditions of Nitrogen Nutrition

Abstract

The rates of photosynthetic СО2 assimilation were determined in fully expanded second leaves of 21-day-old wheat (Triticum aestivum L.) seedlings grown on media supplied with nitrate or ammonia and on a nitrogen-free medium (NO3 –- or NH4 +-treatments and N-deficit treatment, respectively). The maximal quantum efficiency of photosynthesis was independent on conditions of nitrogen nutrition. When leaves were exposed to 0.03% СО2 and high-intensity light, the lowest photosynthetic rate was noted for N-deficit treatment and the highest rate was characteristic of NH4 + treatment. The elevation of the СО2 concentration in the gas phase to 0.1% stimulated photosynthesis at high-intensity light in all treatments. The rate of СО2 uptake by the leaf of N-deficient seedlings increased with СО2 concentration to a larger extent than in other treatments and approached the СО2 uptake rate characteristic of the NO3 – treatment. In plants grown on a nitrogen-free medium, the leaf accumulated lesser amounts of reduced nitrogen and higher amounts of starch, but the content of chloroplast protein corresponded to that of NO3 – treatment. In the leaf of NH4 +-treated seedlings, the rate of СО2 assimilation was higher than in the leaf of NO3 – treated plants, regardless of the composition of the gas mixture. The ammonium-type nutrition, as compared to the nitrate-type nutrition, elevated the amount of reduced nitrogen in the leaf and promoted accumulation of chlorophyll and protein, the chloroplast protein in particular.