The Cold Resistance Mechanism Of Dimocarpus Longan Lour Seedlings Inoculated With Arbuscular Mycorrhizal Fungi

Arbuscular Mycorrhiza fungi?AMF?can form mutualistics symbiosis with most plants,which improve the ability of absorbing water and ion from soil,promote their vegetative growth,increase photosynthesis,osmotic balance and peroxidase activity,and finally improve stress resistance of plants.Longan?Dimocarpus longan Lour?is a species with high economic value because of its fruits and medicines.Longan trees grow vigorously and root deep.Their crown keep evergreen all the year round.It is not only the best trees for landscape greening and beautiful countryside construction,but also is the important candidate trees for soil and water conservation.However,longan prefers to warm climate and has poor cold resistance.Chongqing is the northernmost longan planting area in China.Here the extreme low temperatures often occur in winter due to the increase of latitude in production areas.So low temperature stress is a bottleneck problem restricting the longan industrial development in Chongqing.Owing to AMF plays an important role in promote plant growth and improve cold resistance,two main longan varieties?Shuguan and Youtanben?seedlings planted in Chongqing were inoculated with AMF?Glomus intraradices?.The relationship between AMF inoculation and cold tolerance of longan seedlings was explored in order to provide theoretical basis for the application of AMF to longan production practice and cultivation of cold tolerant longan seedlings,and promote the vigorous development of longan industry in Chongqing.Experiments were designed as goals above:mature,healthy and fresh longan fruit samples were provided by the Yongchuan district Duojing-station in Chongqing.The seeds were taken out for surface disinfection in the laboratory and soaked in sterile water about four to seven days.After the seed coat cracks,the seeds were fostered in the sterile germination system of a culture dish laied with several layers of wet filter paper at 25? for three or four days.The germinated seeds were planted in a nutrient pot loading with two-thirds sterilization medium.Four agents per pot.One hundred pots were planted for Shuguan and Youtanben,respectively.Then move them in culture room for one month at 25? under 7000 lux.Selected the same size of longan seedlings with five-leaf-age and nine gram inoculation?about forty AMF active spores,some mycelia and mycorrhizal segments?were added to each seedling rhizosphere.The controls were treated with nine gram sterilized inoculation.Then they was incubated at 25-28? for12 h/12 h?day/night?under 7000 lux,an appropriate irrigation of 25% improved sterilized Hoagland nutrient solution every fifteen days.The growth indices of longan were determined by random sampling at 60,90,120,150 and 180 days after inoculation to explore the effect of AM fungi on longan growth.Subsequently,eighty pots of 180-day-old longan mycorrhizal and sterile seedlings?forty pots of Shuguan and Youtanben individulally?were selected to carry out low temperature stress experiments at 4? in artificial climate incubators with light intensity of 7000 lux,photoperiod of 12 h/12 h?day/night?and relative humidity of 65%.The controls at 25?.The physiological indexes related with resistance were determined after 0,2,4 and 6 days.The experimental results are as follows:1.Promoting effect of AMF inoculation on potted longan seedlingsAfter inoculation with AMF,the growth indexes?plant height,ground diameter,leaflets number,leaves number and leaf area?,root growth index?root length,ratio of middle roots to coarse roots?of the mycorrhizal longan seedlings and their biomass?fresh weight and dry weight?were significantly higher than those of uninoculated seedlings.These results indicated that inoculation of AMF would have a significant promoting influence on the growth of longan seedlings such as branches leaves and root.2.Effects of inoculation with AMF on the biological characteristics of longan roots under low temperature stressThe mycorrhizal infection rate,mycorrhizal dependence,root activity,root-shoot ratio and specific root length of longan seedlings under low temperature stress were lower than those under normal temperature treated.The results showed that low temperature stress inhibited the growth of longan root system.Nevertheless,the mycorrhizal dependence,root activity,root-shoot ratio and specific root length of longan mycorrhizal seedlings were obviously better than those of uninoculated seedlings,whether under normal temperature treated or low temperature stress.This suggested that AMF could improve the cold resistance of longan by increasing its specific root length,root-shoot ratio and root activity under low temperature stress,in order to alleviate the adverse effects of low temperature stress on longan.3.Effects of inoculation with AMF on ion absorption of longan seedlings under low temperature stressUnder low temperature conditions,with the extension of stress days,the contents of macronutrients N,P,K,Ca,Mg and trace elements Fe,Mn,Zn and Cu in the stems,leaves and roots of the two longan seedlings were all lower than those under normal temperature treated.It was concluded that low temperature stress inhibited the absorption of different mineral elements by longan roots,stems and leaves.But the contents of mineral elements in different parts of longan mycorrhizal seedlings were evidently greater than those of uninoculated seedlings,no matter at room temperature or at low temperature stress.These manifested that AMF could resist the cold damage caused by low temperature by promoting the absorption of various mineral elements in different parts of longan,further improving the cold resistance of longan.4.Physiological effects of AMF inoculation on longan seedlings under low temperature stressUnder low temperature stress conditions,the relative water content,chlorophyll a,chlorophyll b,chlorophyll a+b,net photosynthetic rate,intercellular CO₂ concentration,soluble sugar,soluble protein,SOD,POD and CAT of longan seedlings were lower than those of normal temperature treated.Yet the leaf water saturation deficit,MDA content and relative conductivity were higher than those of normal temperature treated.More importantly,regardless of low temperature or room temperature treated,the relative water content,chlorophyll a,chlorophyll b,chlorophyll a+b,net photosynthetic rate,soluble sugar,soluble protein,SOD,POD and CAT of leaves of longan mycorrhizal seedlings were superior to those of uninoculated seedlings.While,intercellular CO₂ concentration,water saturation deficit,MDA content and relative conductivity were far less than those of uninoculated seedlings.It seemed that inoculation of AMF significantly promoted the growth of longan seedlings below low temperature stress.Improveing the water absorption of longan leaves,enhancing water retention capacity,increasing relative water content?RWC?,chlorophyll content?SPAD?,net photosynthetic rate?Pn?,osmotic regulators and activities of SOD,POD and CAT enzymes,the natural water saturation deficit?WSD?,intercellular carbon dioxide concentration?Ci?,MDA content and relative conductivity of leaves were reduced.Effective mitigation of the damage caused by adversity,specifically enhancing the cold tolerance of longan.5.Comparison of cold tolerance between Shuguan and YoutanbenComprehensive comparing experimental results,it was obvious that both longan variety seedlings could grow at the condition of 4?.But the cold resistance of Shuguan was superior to that of Youtanben.High cold-tolerant varieties possessed more sensitive and efficient scavenging ability of reactive oxygen species under low temperature stress,which could effectively resist low temperature stress.Inoculation of AMF could effectively alleviate the damage caused by low temperature by promoting growth,regulating the growth correlation between roots and branches and leaves,regulating ion absorption balance and physiological metabolism mechanism.Thereby improving the cold resistance of the longan.AMF increased root absorptive surface area and absorptive capacity through a large number of mycelia,and promoted longan to absorb more mineral nutrients?N,P,K,Ca,Mg,Fe,Mn,Cu,Zn?from soil.Which could further promote root growth,increase specific root length,root-shoot ratio and root activity of longan seedlings,improve water absorption and water retention of longan leaves,increase relative water content?RWC?,chlorophyll content?SPAD?,net photosynthetic rate?Pn?,osmotic regulators and activities of SOD,POD and CAT.The content of natural water saturation deficit?WSD?,intercellular carbon dioxide concentration?Ci?and MDA in leaves regulated down.More precisely,the damage caused by low temperature stress was effectively reduced and the cold resistance of longan seedlings was improved.Both the Shuguan and the Youtanben had a strong cold resistance,but the Shuguan's cold resistance was stronger than Youtanben's.