Effects of low and high levels of magnesium on the response of sunflower plants grown with ammonium and nitrate

The effect of the nitrogen source (ammonium and nitrate) and its interactio n with magnesium on various physiological processes was studied in sunflowe r plants (Helianthus annuus L.). Plants were grown in hydroponic culture wi th nitrate (5 mM) or ammonium (5 mM) and four concentrations of magnesium ( 0.1, 0.8, 5 and 10 mM). After 2 weeks, growth, gas exchange and fluorescenc e parameters, soluble carbohydrates, free amino acids, soluble protein and mineral elements were determined. Ammonium nutrition resulted in a reductio n of dry matter accumulation, as well as in a decrease in the CO2 assimilat ion. Moreover, ammonium-fed plants showed a greater content of free amino a cids, soluble protein, Rubisco and anions, and a lower cation content, most ly Mg2+. The presence of high levels of Mg2+ in the nutrient solution conta ining NH4+ resulted in a stimulation of growth and CO2 assimilation to the levels observed in nitrate-fed plants. The lower photosynthetic rate of amm onium-fed plants grown with low level of magnesium does not seem to be due to a lower photosynthetic pigment content, or a deficiency in Photosystem I I activity, or to lower Rubisco content. Hence, Rubisco activity or other e nzymes involved in CO2 fixation could have been affected in ammonium-fed pl ants.