Abstract: Solution culture was conducted to study the relationship between decrease in tougor pressure in top leaf and ammonium nitrogen on inhibition of sunflower growth. The results showed that compared with nitrate supply, ammonium nutrition led to the significant decrease in biomass, leaf area of top expanded leaves and water content of shoot and top expanded leaves. No difference in osmotic potential between nitrate and ammonium treatments was found, while the water potential and tougor pressure in top expanded leaves supplied with nitrate were obviously higher than those of ammonium treatment. In addition, the K+,Ca2+,NO3-- concentration in top expanded leaves fed with ammonium was lower than that of nitrate treatment. In conclusion, the decrease in tougor pressure in top expanded leaves was one of the main mechanisms leading to biomass decline of sunflower plants under the conditions of this experiment.