Physiological And Metabolic Regulation Mechanisms Of Wood Formation Underlying The Different Phosphorus Levels In Populus Species

Phosphorus(P)is a basic nutrient element that limits the production of plant biomass and plays a central role in plant metabolism.Populus L.is one of the main tree species in plantations in northern China,which is often planted in P-deficient soils,thus greatly limiting its economic and ecological value.At present,the regulation of nitrogen,potassium,and calcium on wood development has been deeply studied,however,the research on the physiological regulation mechanisms of wood formation underlying the different P have not been reported.Therefore,in this study,the annual cuttings of Populus alba × Populus glandulosa were used as experimental materials for pot experiment.The P level was adjusted to 1 m M(adequate P treatment),4 m M(excess P treatment)and 0.1 m M(limiting P treatment).After 7 weeks,the plants were harvested.Only the part of the plant which grew after the P treatment beginning were used for further experiments.The main results are as follows:(1)Compared to the control P treatment,P-deficient inhibited net photosynthetic rate,and the high growth and radial growth of poplar seedlings were weakened,which resulting in reduced leaf and wood biomass and increased root biomass.Compared to the control P treatment,the net photosynthetic rate of poplar seedlings increased under excess P treatment,as well as the poplar high growth and biomass of each tissues was increased.(2)PEPC and MDH activities were higher in P-deficient poplar seedlings than control P treatment,and the ATP content of P-deficient poplar were less than that control P treatment;Excess phosphorus treatment inhibited the activity of MDH,and the ATP content of each tissue was increased.(3)P-deficient plant had less cellulose compared to the control P treatment;Excess P treatment resulted in wider vessel elements as well as shorter fibre length,but had no effect on cell wall polysaccharide.(4)We profiled polar metabolites from wood of poplar using liquid chromatography-mass spectrometry(GC-MS).The results showed that P-deficient plants accumulated di-and trisaccharides and oligosaccharide(sucrose,raffinose,rhamnose,cellopentaose,rutinose),but reduced the levels of phosphorylated intermediates(glucose-6-P,fructose-6-P and glucose-1-P),small P-containing metabolites(Adenosine monophosphate(AMP),phosphocholine glycerol and 4-Phospho-D-erythronate),UDP-Glc and various gibberellins.P-excess plant increased the levels of small P-containing metabolites(AMP,phosphocholine glycerol and 4-Phospho-D-erythronate)and UDP-Glc.In addition,the results revealed that both P-deficient and P-excess plants modified the shikimic acid pathway and ammonium metabolism,which increase the levels of secondary metabolite(quercetin,flavonols,salicylic acid,anthocyanidin and isoflavone)and decrease the concentrations of aspartic acid,L-glutamic acid and L-arginine.(5)Based on these data,P-deficient promotes the production of plant secondary metabolites and inhibites the primary metabolism,which decrease the growth rates and productivity of the poplar.P-excess promotes both the primary and the secondary metabolism of poplars,and increase the moisture supply to above-ground organs by expanding the diameters of the vessel element,which increased poplar growth and biomass accumulation.Because of the plant secondary metabolism were both increased under P treatment conditions,we speculate that there is a relatively stress effect under P-deficient and P-excess conditions.Therefore,moderate P application is conducive to wood formation and excessive P application may inhibit the growth of poplar to certain extent.