Effect of Different Nitrogen Levels on the Photosynthesis and Growth of Sweet Sorghum Seedlings under Salt Stress

Abstract

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 M0) all decreased with nitrogen level. The results indicated that the optimum nitrogen level for sweet sorghum is about 5 mmol L-1.