The Effects Of Drought Stress On Seed Germination And Physiological And Biochemical Characteristics And Cuticular Wax In The Seedling Stage Of Brassica Napus L.

Rapeseed is one of the main oil crops in the world and is an important source of daily edible oil and protein.It can be used not only as feed,vegetable,green manure,ornamental and honey crops but also as food,biofuel,lubricant and surfactant,which has high economic value.However,with the limited rainfall and irrigation,the production and quality of rapeseed have declined.Studies have shown that plants under drought stress made adaptive adjustments on the morphological and physiological level,such as root/shoot ratio,plant height,osmosis regulator,antioxidant enzyme activity,etc.,to reduce water loss.Cuticular wax is a layer of hydrophobic organic compounds covering the surface of plants,mainly composed of fatty acids and their derivatives such as alkanes,alcohols,esters and so on.The increase of plant cuticular wax deposition has shown to be related to drought stress.However,different plant cultivars have different responses to drought stress.Therefore,understanding the resistance of different rapeseed cultivars to drought and revealing the physiochemical response mechanism are very important for the high and stable production of rapeseed.We carried out two experiments in this paper to explore the responsive mechanism of different rapeseed cultivars to drought stress.In the first experiment,we analyzed the response of the germination of 18 rapeseed cultivars planted in Chongqing to water stress(10% and 15% PEG),by measuring the germination parameters and main indexes related.Drought sensitivity and tolerance of rapeseed cultivars were classified according to these 12 indicators.And then,the drought coefficient(DC)and membership function value(MFVD)of drought resistance were calculated.Finally,cluster analysis was used to rank the drought resistance of different rapeseed cultivars.In the second experiment,9 rapeseed cultivars with different drought-tolerance were further evaluated.Plants were subjected to drought stresses at seedling stages to analyze the responses of leaf physical and chemical properties and cuticular wax to drought.Finally,we compared the differences of leaf physiochemical properties and cuticular wax among cultivars to explore the drought resistance mechanism.The main results were as follows:The results of the seed germination test showed that cultivars,polyethylene glycol(PEG)and their interactions significantly affected the germination index.Compared with the control(0%PEG),PEG stress significantly inhibited the germination rate of P010,P047,P160 and ZS11,however,10% PEG stress had no significant effect on the germination rate of P047,P160 and ZS11.PEG treatments significantly promoted the germination rate of P065.Overall,PEG stresses prolonged the germination time,and 15% PEG inhibited seed germination index more than 10%.Different from the results of germination rate,PEG stresses promoted the growth of root and inhibited the shoot growth of the seedling,which indicated that PEG-induced drought could promote the early development of root.However,PEG stress significantly reduced the seed vigor index except for P107.By calculating the membership function value(MFVD)and cluster analysis of seed germination index,18 rapeseed cultivars were divided into three groups,drought-tolerant cultivars(P010,P107,P019,P038,P194,P061),moderate drought-tolerant cultivars(P191,P065,P141,P177,P149,ZS11)and drought-sensitive cultivars(P024,P195,P171,P160,P153,P047).In the drought-stressed pot experiment,the physiological indexes of rapeseed leaves were significantly affected by drought,variety and their interactions.The content of MDA and proline in rapeseed leaves were significantly increased under drought stress,but there was no significant change in the MDA content of P107 and no significant change in the proline content of P019 and P065.The photosynthesis rate of rapeseed significantly reduced under drought stress,but there was no significant change in the drought-sensitive cultivars such as P024 and P047.Furthermore,leaf peroxidase activity increased,while catalase activity reduced under drought stress.APX and GR content were enhanced and significantly increased more in water stress more than the control.These results showed that rapeseed seedlings could alleviate the effects of drought stress by increasing osmosis regulation and antioxidant enzyme activity.However,overall,there was no obvious difference in drought stress responses at the seedling stage among different droughtresistant cultivars screened from the seed germination experiment.The results showed that the cuticular waxes of rapeseed leaf have mainly consisted of aldehydes,primary alcohols,alkanes,secondary alcohols,ketones,and small amounts of unknown ingredients.Among them,alkanes were the predominant components,with their relative abundance ranging from 47%-61%,followed by ketones(9-31%),secondary alcohols(2%-15%),primary alcohols,and aldehydes(1%-12%).Compared among cultivars,the relative abundance of alkanes of drought-resistant cultivars was more than60%,while the sensitive cultivars were below 58%.However,the total amount of wax in drought-resistant cultivars was lower than that of sensitive cultivars.After drought stress,the total amount of leaf wax increased,which indicated that rapeseed improved its resistance to drought by increasing wax deposition.The results also showed that the contents of total wax and wax components were positively or negatively correlated with the physicochemical index,indicating that the change of wax composition and contents were directly involved in the response of rapeseed to drought stress.