Effect Of Exogenous Mannitol On Physiological Characteristics Of Quinoa(chenopodium Quinoa Willd.)under Low-temperature Stress

Global climate change has led to frequent extreme temperatures,and low temperature stress is one of the main environmental factors affecting plant distribution,growth,and development.Quinoa（Chenopodium quinoa Willd.）is a resilient crop that is rich in nutrients and has promising applications.Mannitol is an important osmoregulatory substance and free radical scavenger,and it has been shown that spraying mannitol can improve plant stress resistance,but few studies have shown that exogenous mannitol has a mitigating effect on quinoa under low temperature stress.Therefore,this paper elucidates the effects of mannitol on the physiological mechanisms of quinoa by analyzing the effects of low temperature stress and exogenous mannitol pretreatment on photosynthesis,antioxidant capacity,and protective substance content of quinoa.In this study,quinoa was used as the experimental material.The experiment was divided into two groups.One group was treated with low-temperature alone,which lasted12h,24h,and 48h in 8℃and 4℃climate chamber,respectively.The other group was treated with exogenous mannitol at the same condition 5 g·L^-1mannitol was sprayed on quinoa leaf surface before low-temperature stress.The same low-temperature treatment was carried out after natural drying for 12h.All treatments were recovered at room temperature for 24h after the end.The main findings of the study are as follows:1.Studies on the growth status of quinoa under low temperature showed that the leaves of quinoa in the low temperature treatment group alone showed wilting,wrinkling and plant collapse.The seedlings were always upright or semi-erect in the exogenously applied mannitol treatment group.Low-temperature had negatively effect on the growth status of quinoa,and exogenous mannitol alleviated this effect.2.Studies on photosynthesis of quinoa at low temperature showed that SPAD,q P,ETR,AQY,PC_max,LSP,and R_pshowed a decreasing trend,while NPQ and LCP showed an increasing trend under low temperature conditions.The results indicated that low-temperature stress led to the blockading of photosynthesis,and reduction of chlorophyll synthesis,the destruction of PSII reaction center,the reduction of the original light energy utilization efficiency,electron transfer rate,and light reaction assimilation power,resulting inphotoinhibition.The conversion of excess light energy to heat may be a self-protective measure.Moreover,the CE,PL_maxand CSP of quinoa showed a decreasing trend,while CCP and R_dshowed an increasing trend under low-temperature conditions.This indicated that low-temperature stress reduced CO₂utilization,assimilation rate and organic matter synthesis,which inevitably affected the process of carbon assimilation and organic matter accumulation.The low-temperature stress severely inhibited the light reaction and carbon assimilation process,which led to the decrease of gas exchange parameters（P_N,E,g _S）and WUE and increase of Ci in quinoa.This indicated that the decrease of net photosynthetic rate was mainly caused by non-stomatal factors.The percentage changes of SPAD,P_N,E,WUE,and Ci of the low-temperature treatment with exogenous mannitol were better than those of the low-temperature treatment alone.The percentage changes of NPQ,q P,ETR,CSP,and R_pthe low-temperature treatment with exogenous mannitol were better than those of the low-temperature treatment alone.This indicated that exogenous mannitol pretreatment improved the ability of quinoa to resist dehydration and maintain chlorophyll content.The photosynthetic machinery was less damaged,which alleviated the decrease in photosynthetic rate caused by low-temperature stress.This also indicated that exogenous mannitol enhanced the ability of quinoa to utilize high CO₂concentration,played a positive role in protecting photosystem stability,enabled the PSII reaction center to maintain a high.It also showed that exogenous mannitol enhanced the ability of quinoa to utilize high CO₂concentrations and played a positive role in protecting photosystem stability,enabling the PSII reaction center to maintain high activity and stay open,and regulating PSII to dissipate excess light energy to reduce photoinhibition.3.Studies on the antioxidant enzyme activities of quinoa showed that the antioxidant enzyme activities（SOD,POD,CAT）and MDA contents of quinoa were consistently higher than the control during the low temperature treatment at 8°C and 4°C.The activities of SOD,POD,and CAT of quinoa increased by 2.37%（p>0.05）,46.27%,86.99%,and 43.62%（p<0.05）,respectively,at 4°C treatment for 48h compared to the control.The percentage increase in CAT activity,SOD and POD of quinoa in the exogenous mannitol pretreatment group was higher than that of the low temperature treatment group alone;at 48 h of treatment at 4°C,SOD,POD,and CAT of quinoa increased by 4.16%,97.37%,and 98.88%,respectively（p<0.05）.This indicates that exogenous mannitol treatment increased the antioxidant enzyme activity of quinoa,alleviated the membrane lipid peroxidation caused by low temperature,and improved the tolerance of quinoa to low temperature.4.Studies on protective substances of quinoa showed that the proline,soluble sugar and sucrose contents of quinoa were significantly higher than the control at low temperatures of 8°C and 4°C.Proline,soluble sugar and sucrose contents of quinoa increased by 143.21%,238.49%（p<0.05）and 8.79%（p>0.05）,respectively,when treated at low temperature of 4°C for 48h compared to the control.The percentage increase of soluble sugars and sucrose in quinoa in the exogenous mannitol pretreatment group was higher than that in the low temperature treatment group alone,indicating that exogenous mannitol treatment improved the osmoregulatory capacity and lowered the freezing point of quinoa,which in turn improved the tolerance of quinoa to low temperature.In conclusion,exogenous mannitol had a modulatory effect on quinoa under low temperature stress and improved the cold tolerance of quinoa.Under low temperature stress,exogenous mannitol pretreatment slowed down the decrease of chlorophyll content,gas exchange parameters and fluorescence parameters of quinoa caused by low temperature stress,and also increased the antioxidant enzyme activity and protective substance content of quinoa.Therefore,exogenous mannitol pretreatment could improve photosynthetic rate,antioxidant activity and maintain osmoregulatory balance in quinoa under low-temperature stress.This provides some reference for subsequent in-depth studies on cold resistance in quinoa,and hopefully provides a theoretical basis for the wide application of mannitol in different plants.