Mechanisms Of Melatonin Regulating Water Balance In Maize Seedlings Under Drought And Salt Stress

In agricultural production,environmental stress seriously affects crop growth and causes yield reduction.Therefore,how to improve the ability of crops to resist stress is critical to agricultural production.Melatonin（MEL）is a natural pleiotropic small molecule compound that increases the resistance of plants to a variety of biotic and abiotic stresses.Studies have found that MEL can maintain high relative water content by increasing osmotic adjustment ability when subjected to osmotic stress,but the effect of MEL on overall water regulation of plants under environmental stress and its mechanism is still less.This study will explore the regulation mechanism of MEL to improve the ability of maize seedlings resist drought and salt stress from the perspective of overall water regulation.The main results are as follows:（1）Mechanism of MEL regulating water balance under drought stress in maize seedlingsMEL can alleviate water shortage in plants induced by drought or osmotic stress.However,the effects and mechanisms of MEL on the detailed process of water uptake and transport in plants under water stress remain unknown.In this experiment,maize seedlings were pretreated with 1μM MEL for 24 h,and then subjected to 10%PEG simulated drought stress treatment.After 3 h of PEG stress,the photosynthesis parameters,leaf water characteristics,transpiration rate,whole water conductance,root water conductance,aquaporin gene（Zm PIPs）expression and root H₂O₂ content were determined.The results showed that drought stress significantly reduced photosynthetic parameters,leaf water status and transpiration rate of maize seedlings,while MEL treatment significantly reduced the inhibition of photosynthesis and water status by drought stress.Exogenous MEL increased the whole water conductance(K_plant)and root water conductance（Lp_r）of maize seedlings under drought stress,indicating that MEL relieves the drought induced water deficit by promoting root water absorption.Hg Cl₂（aquaporin inhibitor）treatment inhibit the transpiration rate in MEL-treated plants greater than those of MEL-untreated;after recovery by DTT（anti-inhibitor）,the transpiration rate in MEL-treated plants increased much higher than those of untreated plants.Moreover,under water deficiency,the transcription level of aquaporin genes was significantly up-regulated by MEL application,and the H₂O₂ was less accumulated in MEL-treated root.The results indicate that exogenous MEL promotes aquaporin activity,which contribute to the maintaining of Lp_r and K_plant under sudden water deficiency stress.The increased water uptake and transport lead to improved water status and thus increased maize seedling tolerance to PEG-induced sudden water deficiency in MEL-treated plants.（2）Mechanism of MEL regulating water balance under salt stress in maize seedlingsIt is widely reported that MEL can improve plant salt tolerance by improving antioxidant capacity and maintaining ion balance,but the effect of MEL on plant water status under salt stress and its regulation mechanism are still unclear.In this experiment,maize seedlings were pretreated with 1μM MEL for 24 h under hydroponic conditions,and then subjected to salt stress treatment with 125 m M Na Cl.The effects of MEL on the growth,photosynthetic parameters,leaf water status,root cell osmotic potential and xylem osmotic potential of maize seedlings after 3 h and 7 d of salt stress were analyzed.Results showed that salt stress for 7d significantly inhibited the growth of maize seedlings and caused severe senescence of old leaves,but MEL treatment significantly alleviated this phenomenon.Salt stress of 3 h and 7 d significantly reduced the photosynthetic parameters and water status of maize seedlings,however,1μM MEL treatment maintained the photosynthetic and water status of relatively high hydroponic culture by increasing stomatal conductance and leaf osmotic potential.Exogenous MEL increased the whole water conductance(K_plant)of maize seedlings after 3 h and 7 d of salt stress,indicating that MEL relieves salt stress by regulating water uptake and transport.After 3 h of salt stress,the increase in water absorption and transport of MEL-treated plants resulted in significantly higher xylem osmotic potential and Na⁺concentration than MEL-untreated seedlings.After 7 days of salt stress,the xylem osmotic potential and Na⁺concentration in MEL-treated seedlings were significantly lower than those of MEL-untreated seedlings.Moreover,exogenous MEL significantly increased the osmotic gradient between soil and root xylem.These results indicate that MEL can improve the water state of plants by regulating stomatal conductance and increasing transpiration in the early stage of salt stress（3h）.At the later stage of salt stress（7 d）,MEL can promote water absorption and transportation by regulating the osmotic gradient between soil and xylem.In summary,under drought stress,MEL improves water status and photosynthesis by increasing the activity of aquaporins,maintaining high root water conductance and whole water conductance.Under salt stress,MEL improves the transpiration rate and the osmotic gradient between soil and xylem by regulating stomatal conductance and xylem osmotic potential,thereby maintaining a higher whole water conductance(K_plant).This study provides a theoretical basis for the ability of MEL to improve crop stress resistance.