Effect of different potassium levels on the growth and photosynthesis of sweet sorghum seedlings under salt stress

In order to investigate the effect of potassium on the growth of sweet sorghum [Sorghum bicolor (L.) Moench] seedlings under salt stress and the optimum potassium level under salinity. Seedlings of sweet sorghum were supplied with 0.1 mmol L-1, 3 mmol L-1, 6 mmol L-1 and 9 mmol L-1 potassium fertilizers under 0% or 0.6% NaCl conditions. Then the growth parameters of the seedlings including fresh and dry weight, photosynthetic gas exchange, leaf chlorophyll a fluorescence as well as the content of MDA and relative electric conductance were determined. Results showed that plant growth and photosynthesis decreased under salt stress. Furthermore, with the increase of potassium level, plant fresh and dry weight, net Photosynthetic rate (Pn), Transpiration rate (Tr), stomatal conductance (Gs), the maximum photochemical efficiency of PSII (PHIPo), quantum yield under light (phi PSII), 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 PS II (Sm) all increased and reached the peak at about 6-9 mmol L-1 potassium level. On the other hand, the close extent of PS II Reaction center (Vj) and the content of MDA as well as relative electric conductance all decreased. It is found that potassium at 6-9 mmol L-1 was beneficial to the growth of sweet sorghum, under both control and saline conditions. Though sweet sorghum grew much better under 9 mmol L-1 potassium level under salinity, it is not significant compared with 6 mmol L-1 potassium level.