Effect of Different Nitrogen Levels on the Photosynthesis and Growth of Sweet Sorghum Seedlings under salt stress

Seedlings of sweet sorghum were supplied with 0.1 mmol L-1, 2 mmol L-1, 5 mmol L-1 and 10 mmol L-1 nitrogen fertilizers under 0% or 0.6% NaCl conditions. Then the growth parameters of the seedlings including photosynthetic gas exchange and leaf chlorophyll a fluorescence were determined. Results showed that the seedlings grew worse under 0.6% NaCl. Furthermore, with the increase of nitrogen level, the net photosynthetic rate (Pn), transpiration rate (Tr), the maximum photochemical efficiency of PSII (PHI(Po)), the driving force on light absorption basis (D.F), the active reaction centers per cross section (RS/CSm), trapped energy per cross section (TRo/CSm) and PQ size of photosystem (PS) II (Sm) all increased and reached the peak at 5 mmol L-1) nitrogen level. On the other hand, heat dispassion per cross section (DIo/CSm) and the close extent of PS II reaction center (Vj and M-0) all decreased with nitrogen level. The results indicated that the optimum nitrogen level for sweet sorghum is about 5 mmol L-1