Effects of different nitrogen forms on photosynthetic irate and the chlorophyll fluorescence induction kinetics of flue-cured tobacco

Net photosynthetic rate (P-N) of tobacco plants grown with NH4-N as the only N source was the lowest all the times, while P-N grown only with NO3-N was the greatest until 22(nd) day, and PN grown with both NO3-N and NH4-N (I : 1) was the greatest. Maximal photochemical efficiency of photosystem 2 (PS2), F-v/F-m, and actual quantum yield of PS2 under actinic irradiation (Phi(PS2)) in plants grown with only NH4-N were greatest at early stage and then decreased and were smaller than those of other treatments. Photochemical quenching coefficient (q(p)) and non-photochemical quenching coefficient (q(NP)) in the NH4-N plants were the greatest at all times. Hence excessive NH4-N can decrease not only photochemical efficiency but also the efficiency of utilization of photon energy absorbed by pigments for photosynthesis. Therefore, excessive NH4-N is a hindrance to photosynthesis of flue-cured tobacco. On the other hand, tobacco cultured with an appropriate mixture of NO3-N with NH4-N can sufficiently utilize photon energy and increase the efficiency of energy transformation.