Effects Of Drought And Different Rewatering Time On Growth And Characteristics Of PS? Of Mulberry Seedlings

As the policy of "Mulberry should be planting from southeast region to northwest" have been implemented,the industry of mulberry are moving from the wet area to arid regions.In previous researches,plant injury and adaptation to drought have been paid more attention,but less attention to repair and compensation ability.For the reason,this paper was conducted to treatments with different soil moisture and different stages drought during mulberry seedling stage and grand stage.Planting mulberry?Morus alba?have both windbreak and sand-fixation ecological value,meanwhile,which have a variety of economic benefits to increase farmers' income.Taking the mulberry?Morus alba L.?variety "Qiuyu" as the experimental material,which was popular cultivars in Heilongjiang province,the compensation effects of mulberry under soil water drought with different levels and terms was carried using pot experiments.Mulberry compensation growth characteristics,photochemical reaction and carbon assimilation characteristics were investigated by using infrared CO₂ analysis and chlorophyll fluorescence technique,and changes of protective enzyme and osmoregulation substance.The results were as following:Mulberry growth appeared to compensation effects for rewatering after drought.Compensatory ability depended on drought level and days.During drought,mulberry height and leaf area reduced,and reduced in long-term drought more than in short-term drought of mulberry.After rewatering,short-term drought exceeded CK and produced super compensation significantly,there was no significant difference between CK and short-term drought after 14 days' rewatering,and produced super compensation after 21 days' rewatering;The same circumstances occurred in middle-term drought,but with no super compensation after 21 days' rewatering.Long-term drought only produced part compensation,and compensation effect was more obvious in short-term and middle-term drought.Especially the severe drought treatment in long-term drought,mulberry height and leaf area were never exceeded CK.SOD,POD and ASA,soluble protein?SP?,soluble sugar?SS?and proline?PRO?content increased during shortterm drought,which was useful to eliminate active oxygen and protect photosynthetic institutions.SOD,POD,ASA,soluble protein and soluble sugar content decreased slightly under severe drought treatment,cell osmotic regulation capacity reduced,which indicated that ROS damaged PS?response center.When the drought removed,each index were recovered to the normal level,and with no significant difference.Under severe drought stress,the water content of mulberry leaf?LWC?was significantly lower than CK,and leaf had already been wilting,curly and drooping,leaf base angle had been increased.Meanwhile,net photosynthetic rate?P_n?,stomatal conductance?G_s?and transpiration rate?T_r?were nearly to zero,and the value of stomatal limitation?L_s?and intercellular CO₂ concentration?C_i?were not significant difference with CK,which indicated that the photosynthetic capacity decline might be caused by factors of stomatal and non-stomatal limiting together.Compared with CK,the excess light energy?1-q_P?/NPQ had been increased in mulberry seeding leaves after 21 days' drought,the same trends had happened among mulberry leaf could consume excess light energy by increasing ratio of dependent thylakoid membrane on both sides of the proton gradient and the proportion of xanthophyll cycle(?_NPQ),the quantum yield of fluorescence and heat dissipation(?_f,D),which indicated that the excitation energy in mulberry seeding leaves were reduced mainly depend upon increasing ratio of xanthophyll cycle,the quantum yield of fluorescence and heat dissipation.The original fluorescence?F_o?had been increased,maximum fluorescence?F_m?,electron transport rate?ETR?and the proportion energy of absorbed the light in photochemical reaction yield(?_PS?)had been decreased,however,the amount of heat dissipation quantum yield(?_NF)in de-activation PS ? reaction center had been increased,which indicated that photosynthetic agencies accumulated too much excitation energy,PS?response center lost the ability to separate charge,the degradation of D1 protein and PS?response center was partly inactivation,which indicated that long-term drought made PS?response center injured.After rewatering for 5 h,the main fluorescence parameters value of mulberry leaf PS?was nearly to CK,the difference was not significant between rewatering and CK.After rewatering for 6 h,the main photosynthetic gas exchange parameters in rewatering treatment had no significant difference with CK.When the drought removed,ETR increased,which indicated that drought stress induced photosynthetic reaction center vitality,light energy from photosynthetic pigments was used more for the formation of NADPH and ATP and production of photosynthesis was enhanced.Therefore,rewatering made mulberry produced compensation after rewatering.Using pot test with control irrigating with installment and taking the chlorophyll fluorescence kinetics,the effects of drought stress on PS? functions of mulberry leaves were investigated.The results showed that maximum photochemical efficiency of PS??F_v/F_m?and photosynthetic performance index(PI_ABS)based on light absorption of mulberry seeding leaves were lower than CK,which indicated that PS? activity of leaf could be inhibited.The value of V_J and V_I was increased,which showed that QA to QB electronic transmission ability of PS?electronic receptor side were relatively decreased in standardization of OJIP curve.Which indicated that photoinhibition of PS?could be serious,because of PQ electronic library and electron-accepting ability of QB could be decrease and increased QA-accumulation.In receptor side,the electronic energy from QA to QB?S_m?and energy for non-photochemical quenching??D_o?had been increased,which led to the decrease of energy??E_o?for electronic transfer after QA-;the value of VK was increased,which showed that activity of oxygen complexes?OEC?was restrained in the donor side;the value of VL increase value showed that the stability of the PS?institutions had decreased,the main factors of PS?reaction center partly inactivation were the limitation of the electron transfer process.Electron transport in the receptor side and OEC activity in donor side of mulberry leaf PS? had been restrained under drought stress,which resulted in photosynthetic capacity decline;Drought stress effectively increased the dissociation degree of chloroplast thylakoid,decreased its adhesion of PS?protein complex to maintain photochemical activity of PS?.After rewatering,institutional stability of PS? had been enhanced,the light energy used for the radiant heat dissipation ratio had decreased,the number of electron transfer energy had increased from QA to the QB in receptor,meanwhile,the activity of oxygen complexes had also restored.And then,the values of photosynthetic gas exchange parameters and leaf shape parameters had been nearly to CK.Our results indicated that mulberry have a perfect mechanism for adapting to drought stress and repairing after injury,which could help mulberry surviving in the arid regions.