Influence of N form on growth and photosynthesis of Phaseolus vulgaris L. plants

Many plant species are characterized by pronounced sensitivity to sole ammonium supply and exhibit growth depression and particularly reduced leaf growth rates. Stress symptoms under sole ammonium supply may be related to perturbation of photosynthetic processes, e.g., low rates of net CO2 assimilation, low quantum yield, reduced stomatal conductance, and carboxylation capacity. The results of three experiments with French bean plants supplied with an N concentration of 5 mM illustrate significantly lower dry mass and specific leaf area, reduced leaf expansion, and higher chlorophyll and N content of ammonium-compared to nitrate-supplied plants. Light-saturated rates of CO2 assimilation (A(max)) per unit leaf area were higher under ammonium compared to nitrate supply while no significant effects of N form on quantum yield and A(max) per unit leaf weight and chlorophyll were found. Maximal carboxylation (V-cmax) and electron-transport (J(max)) rates were significantly higher under ammonium supply only in one of three experiments. Vcmax was linearly related to total leaf N, the slope of the regression was similar with both N forms, the x-axis intercept was significantly higher for ammonium-compared to nitrate-supplied plants. The ratio V-cmax : Jmax was not affected by N form. It is concluded that ammonium supply had no negative effects on the operation of photosynthetic protein-enzyme complexes.