Studies On The Mechanism Of The Effects Of Low Phosphorus On Leaf Growth And Photosynthesis In Wheat

Phosphorus is one of the essential nutrient elements for plant growth and development.It is mainly present in phospholipids,nucleic acids,nuclear proteins,participates in many metabolic processes including photosynthesis.However,the diffusion rate of phosphorus in soil is very slow,and it is easy to combine with soil minerals,including iron,aluminum and calcium,to form insoluble chemicals.Therefore,a large proportion of phosphorus in soil cannot be absorbed by plants,resulting in phosphorus deficiency symptoms in plants.A large number of studies have shown that phosphorus deficiency can lead to significant decreases in leaf photosynthetic rate and leaf area.However,few studies have systematically analyzed the effects of low phosphorus stress on photosynthesis and leaf growth from the perspective of leaf anatomical structure.In addition,the thresholds of phosphorus concentration for photosynthesis and leaf growth were not clear.In present study,hydroponic experiments with four phosphorus concentrations(12.500 mg L^-1、2.500 mg L^-1、0.500 mg L^-1、0.125 mg L^-1)were conducted in two wheat varieties（SL26and JM22）,to study the mechanisms underlying of the effect of low phosphorus on wheat leaf growth and photosynthesis.The main results were as follows:（1）Phosphorus deficiency significantly reduced plant biomass in the two wheat varieties.There was no significant difference in plant biomass in SL26 or JM22 between the first two phosphorus concentrations(12.500 mg L^-1 and 2.500 mg L^-1).However,the plant biomass in the last two phosphorus concentrations(2.500 mg L^-1 and 0.125 mg L^-1)was significantly decreased in both genotypes.In comparison with the treatment of 12.500 mg L^-1,total dry weight,leaf dry weight,stem dry weight,root dry weight,leaf area per plant in the treatment of 0.125 mg L^-1 in SL26 were decreased by 90.2%,92.1%,92.6%,77.2%and92.8%,respectively;and,in JM22,they were decreased by 88.4%,92.3%,92.7%,62.0%and 93.8%,respectively.On the contrary,the root/shoot ratio increased by 2.2 times and4.2 times,respectively,in SL26 and JM22.（2）Phosphorus deficiency significantly reduced individual leaf area in the two wheat varieties.In comparison with the treatment of 12.500 mg L^-1,the individual leaf area in treatment of 0.125 mg L^-1 were decreased by 75.9%and 76.2%,respectively,in SL26 and JM22.By analyzing the leaf transverse section structures,phosphorus deficiency significantly reduced the mesophyll cell size in two varieties,but had no significant effect on the fraction of leaf mesophyll volume occupied by intercellular air space(f_ias).The effect of phosphorus deficiency on mesophyll cell number between major veins was genotypic dependent.Phosphorus deficiency had significantly decreased mesophyll cell number between major veins in SL26,while had significantly increased mesophyll cell number between major veins in JM22.Therefore,the smaller mesophyll cells were one of the reasons for the decrease in individual leaf area under low phosphorus conditions.（3）The thresholds of phosphorus concentrations were different for photosynthesis and leaf growth.With the decrease in leaf phosphorus content,the decrease in leaf area preceded the decrease in leaf photosynthetic rate in two varieties.The thresholds of phosphorus concentration for photosynthesis were 3.89 mg g^-1 and 4.03 mg g^-1,respectively,in SL26and JM22;and they were 5.87 mg g^-1 and 6.19 mg g^-1,respectively,for leaf growth.（4）In comparison with the treatment of 12.500 mg L^-1,the leaf photosynthetic rate,stomatal conductance and mesophyll conductance in treatment of 0.125 mg L^-1 were decreased by 50.9%,42.6%,and 41.1%,respectively,in SL26;while they were decreased by 27.0%,36.5%,and 47.1%,respectively,in JM22.The response of the maximum carboxylation capacity of Rubisco(V_cmax)and the maximum electron transport capacity(J_max)in two wheat varieties to phosphorus treatments were different.With the decrease in phosphorus concentration,the V_cmax and J_max decreased by 18.7%and 20.6%,respectively,in SL26,however,the V_cmax and J_max in JM22 decreased first and then increased.Under low phosphorus stress,the decrease in leaf photosynthetic rate in SL26 was related to the decrease in stomatal conductance,mesophyll conductance and biochemical capacities.However,the decrease in leaf photosynthetic rate in JM22 was mainly related to the declines in stomatal conductance and mesophyll conductance,but not related to the biochemical capacities.（5）Low phosphorus stress had no significant effect on stomatal size and density of two wheat varieties,but significantly decreased leaf hydraulic conductance of two wheat varieties.With the decrease in phosphorus concentration,the leaf hydraulic conductance decreased by 59.9%and 17.6%,respectively,in SL26 and JM22,and there was a significant positive correlation between leaf hydraulic conductance and stomatal conductance.These results showed that leaf water supply capacity was insufficient to maintain large stomatal apertures under low phosphorus conditions,which should be the reason for the decrease in stomatal conductance.（6）In present study,phosphorus deficiency significantly increased the surface area of chloroplast facing intercellular airspace per unit leaf area（S_c）in two wheat varieties.The effect of phosphorus treatments on cell wall thickness(T_cw)was different between two varieties.Phosphorus deficiency significantly increased cell wall thickness in JM22,but had no obvious effect on cell wall thickness in SL26.Therefore,the decrease in mesophyll conductance under phosphorus deficiency was not related to the S_c or the cell wall thickness.