Effect Of Low Phosphorus On Light Energy Utilization Of Sweet Sorghum Under Salt Stress

Salt stress directly inhibit plant growth and development and arrests the process of photosynthesis.Phosphorus is one of the most abundant nutrients after nitrogen,and low soil phosphorus levels can seriously affect the yield of sweet sorghum.The study was undertaken to investigate the growth response of sweet sorghum to four different phosphorus concentrations of1 mM,0.5 mM,0.1 mM,and 0.01 mM under with and without 100 mM NaCl.Our study also aims to underpin the effects of salt stress and low phosphorus on light and to investigate the effects of salt stress and phosphorus deficiency on light energy utilization and carbon assimilation,changes in PSⅡ reaction center activity,removal of excess light energy,prompting the initiation of a series of photo-protective mechanisms to protect it from damage,the main findings are as follows.1.Effect of different phosphorus concentration treatments on physiological and biochemical parameters of sweet sorghum under salt stressUnder the same salt stress（100mM）,the growth conditions differed significantly with decreasing phosphorus concentrations.The height,biomass（dry and fresh weight）,leaf phosphorus content,and chlorophyll content of sweet sorghum all decreased to different degrees,and the above indicators were lower in salt-stressed sweet sorghum than in the control.The net assimilating rate Pn,stomatal conductance Gs,transpiration rate Tr and other photosynthetic indexes decreased significantly and the values under salt stress were generally lower than the control.Chlorophyll fluorescence parameters such as actual photochemical efficiencyφPSⅡ,maximum photochemical efficiency Fv/Fm,and photochemical burst q P also decreased with decreasing in phosphorus levels but the reduction was more significant in salt treated plants as compared to the control.Under low phosphorus stress number and length of root hairs increased slightly,thus the whole root system became shallow.This might be one of the adaptive strategies of sweet sorghum to low phosphorus.MDA content,SOD enzyme activity,peroxidase,and catalase content increased obviously with decreasing phosphorus concentration,and the values of salt stress treatment were higher than the control.While ATP content decreased with decreasing phosphorus content,and the decreasing trend was significant under salt stress.The content of soluble sugars and proline tended to increase and then decrease with the decrease of phosphorus concentration,reaching the maximum at 0.1 mM phosphorus concentration.Acid phosphatase content showed an increasing trend after low phosphorus treatment,and all controls were higher than the salt treatment.2.Effect of different phosphorus concentration treatments on light response curves and CO₂response curves of sweet sorghum under salt stressThe experimental results showed that the initial slopeαof the light response curve of sweet sorghum without salt stress increased as compared to the 100 mM NaCl treatment,and compared to the 1 mM and 0.5 mM low phosphorus treatments,the 0.1 mM and 0.01 mM treatments showed a significant decrease and the sudden decline in quantum yield,and the maximum photochemical efficiency and light saturation point of sweet sorghum decreased with the decrease in phosphorus concentration.The lowest values were found under salt stress and low phosphorus treatment（0.01mM P）,with smaller decreases in the 1 mM and 0.5 mM phosphorus treatments.Under low phosphorus stress,the light compensation point of sweet sorghum increased and the dark respiration rate declined.The photosynthetic rate of leaves reduced after low phosphorus treatment and the initial slope of the CO₂ response curve decreased for 100 mM salt-stressed sweet sorghum compared to the control.Compared to the 1 mM and 0.5 mM phosphorus treatments,the 0.1 mM and 0.01 mM low phosphorus treatments had lower slope values and reduced carboxylation efficiency,and the decreases were greater under low phosphorus treatments（0.1 mM and 0.01mM）than the 1 mM and 0.5 mM phosphorus treatments.Low phosphorus treatment also increased the CO₂ compensation point and its ability to utilize low CO₂ concentrations was diminished.3.Effect of different phosphorus concentration treatments on the photochemical properties of PSⅡ in sweet sorghum under salt stressAs compared to the control,the maximum photochemical efficiency PHI（Po）of sweet sorghum under 100 mM NaCl treatment decreased at all four phosphorus levels,except at 1 mM phosphorus concentration where the difference was larger,and the decrease was smaller for the other concentration treatments.The trend of the quantum yield PHI（Eo）and the ratio of absorbed photons in the reaction center for heat dissipation PHI（Po）were similar,both decreasing with phosphorus levels,and the magnitude of the decrease was more significant with increasing phosphorus deficiency levels,with the smallest PHI（Eo）for the 0.01 mM phosphorus treatment.As the phosphorus level decreased,the value of the heat dissipation energy ratio PHI（Do）increased,and the increase was more moderate in salt treated group as compared to the control.The relative variable fluorescence intensity Vj at point J,the excitation energy d VG/dto（MG）between reaction centers,the fluorescence intensity Wk at point K showed similar trend and the values gradually increased significantly with the decrease in phosphorus concentration.ETo/CSm,DIo/CSm,and DIo/CSm is a more accurate representation of PHI（Eo）,PHI（Do）,and the change trend was similar.With the decrease of phosphorus concentration,the values of the number of reaction centers per unit area RC/CSm,the value of PQ pool on the receptor side Sm,the driving force based on absorbed light energy D.F,and the performance index based on absorbed light energy PI_ABSdecreased quit clearly under control condition while the trend was relatively stable in salt stressed plants.