Photosynthesis sensitivity to NH4--N change with nitrogen fertilizer type

Abstract

N-fertilization type affected differently tomato growth. In the field experiment, hydroponic cultures were conducted using NO3-N (5 mmol); mixture of KNO3-N (3 mmol) and (NH4)2SO4-N (2 mmol); NH+-N (5 mmol) or urea (5 mmol) as nitrogen source. Compared to nitrate, ammonium and urea had negative effects on morphology and dry matter production. Effects of the different nitrogen forms were investigated by measuring several photosynthesis parameters and chl a fluorescence. Two different significant types of reaction were found. When nitrogen was added as ammonium or urea, dry weight, chlorophyll tenor, transpiration rate, stomatal conductance and photosynthetic activity were inhibited. Supply of ammonium or urea, reduced the ratio (Fv/Fm), photochemical quenching and enhanced the non photochemical quenching. These data suggest that the adverse decrease in tomato growth under ammonium or urea supply may be related principally to inhibition of net photosynthesis activity. The high non photochemical quenching shown in tomato fed with ammonium or urea indicated that PS II was the inhibitory site of NH4--N which was directly uptaken by roots, or librated via urea hydrolysis cycle.