Effects Of Drought Stress On Germination And Growth Of Rapeseed And Mechanism Of Seed Priming To Improve Drought Resistance

The edible oil production in China is highly dependent on the successful production of rapeseed.However,the excessive occurrence of autumn drought caused obstacles to the germination and normal growth of rapeseed.Drought stress is the main reason for crop failure.Water stress encumbers the rapeseed growth and development primarily by oxidative alteration to biological membranes and interrupted tissue water status.Seedling tolerance to water stress is vital for better crop development through the entire period under drought stress condition.Priming seed is an approach through which seeds are exposed to numerous unfavorable situations to confer stress variation in seedlings germinating and growing under stressful conditions.However,its physiological mechanism research is not systematic.The present study aimed to elucidate how seed priming affected rapeseed germination,early seedling growth,seed yield and quality by conducting a series of incubator,pot and field experiments.The main findings are summarized below.(1)Drought stress inhibited germination and early seedling growthTo better predict about the adaption of rapeseed to drought stress,the current study explored the presumed roles of enzymatic,non-enzymatic antioxidants and osmolytes in improving rapeseed drought-tolerance.The comparative analysis in terms of germination and early seedling growth of six rapeseed cultivars was assessed under normal(0.00 MPa)and osmotic stresses(-0.1,-0.2,-0.3,-0.4,and-0.5 MPa simulated by polyethylene glycol 6000).Subsequently,based on the process of germination and early seedling growth,Shenguang 127(SG 127)and Zhongyou 36(ZY 36)were chosen as drought-tolerant and-sensitive rapeseed varieties.The results indicated that drought stress rigorously hindered germination and early seedling growth of rapeseed cultivars.On the other hand,SG 127 exhibited less reduction in seedling growth paralleled with ZY 36.SG 127 revealed lower levels of hydrogen peroxide(H2O2),lipids peroxidation(MDA),electrolyte leakage(EL %)and less reduction in chlorophyll(Chl)content than ZY 36.The drought-tolerance of SG 127 may be correlated with enhanced activities of enzymatic(superoxide peroxidase,catalase,and dismutase),non-enzymatic antioxidants(ascorbic acid,glutathione),more accumulation of proline;total soluble sugar and total soluble protein.Furthermore,our study highlighted the significance of enzymatic and non-enzymatic antioxidants and osmolytes in the establishment of rapeseed seedlings under drought stress.(2)Effective priming treatments promoted rapeseed drought-tolerance during germination and early seedling growthWe studied the influences of seed priming(induced by 6-BA,ABA,BRs,GA3,GB,IAA,Me Ja,MT,PA and SA)on subsequent seedling establishment and growth attributes of rapeseed seedlings under the drought stress induced by polyethylene glycol(PEG).Seed priming treatments including GA3,MT,GB,BRs,and IAA were effective at improving seedling establishment and early seedling growth of rapeseed under the PEG simulated drought stress.The effective seed priming treatment such as GA3 and MT were further elucidated from the perspectives of physiological and biochemical mechanisms involved in droughttolerance during early seedlings stage.GA3-and MT-priming tended to alleviate the oxidative damage and the amount of chlorophyll in rapeseed.Seed priming with gibberellic acid and melatonin strengthened the antioxidant defense system in rapeseed by regulating the activities of antioxidant enzymes(SOD,POD,and CAT)and levels of nonenzymatic antioxidants(GSH and As A).Nonetheless,gibberellic acid and melatonin priming also played an important role in osmotic adjustment under drought stress by increasing the accumulation of osmolytes including proline,soluble protein and soluble sugar contents.The results from scanning electron microscopy(SEM)revealed that stomatal traits such as number,stomatal length and width were significantly reduced under drought stress conditions in no-primed treatment.However,GA3-and MT-priming greatly improved stomatal number;stomatal length and width.Ultrastructure analysis showed that in no-primed treatment,chloroplast autopepsia(self-digestion)was observed.Under drought stress,the no-primed treatment also exhibited cell wall degradation.Nonetheless,GA3-and MT-priming preserved the chloroplast from chlorophagy and the chloroplast exhibited ellipse shape,well-arranged and clung to the cell wall.Similarly,under the drought stress conditions,GA3-and MT-priming protected cell wall from degradation and exhibited a typical structure with complete and clear edges;and was well-formed.(3)Gibberellic acid and melatonin priming significantly increased seed yield and improved seed quality under drought stress and normal growing conditionsPot and field experiments were conducted to investigate whether seed priming could improve rapeseed yield and quality under drought-stress and non-stress conditions.We primed the seeds of two rapeseed cultivars viz.,Shenguang 127(SG 127)and Huayouza 9(HZ 9)with gibberellic acid(GA3)and melatonin(MT).The potted plants were subjected to a severe drought stress of 35-40% at flowering stage in well-controlled glasshouse conditions;while in the field experiment,the plants were grown under the open field conditions.In the pot experiment,significant reduction in morphological traits,yield and yield components;and deterioration in seed qualitative attributes was recorded under the drought stress in both cultivars.However,the GA3 and MT-primed plants exhibited less reduction in morphological traits,yield and yield components;and deterioration in seed qualitative attributes under the drought stress compared to no-primed plants.The results of our study reveals that the better performance of primed-issued plants was associated with better scavenging of ROS through the activation of antioxidant enzymes and more accumulation of proline as compared to no-primed plants both under normal and drought stress conditions.Nonetheless,GA3 and MT-priming also improved the morphological traits,seed yield,and seed qualitative attributes under the open field conditions compared to no-primed treatment.Conclusively,effective priming treatments such as GA3 and MT promoted rapeseed drought-tolerance during germination and different growth stages.GA3 and MT-priming potentially regulated antioxidant defense system and osmotic adjustment which ultimately modulated stomatal morphology and chloroplast ultrastructure under drought stress.Meanwhile,GA3 and MT-priming significantly increased seed yield and improved seed quality.Taking together,the finding of our study may explore new avenues and advancing priming-induced drought tolerance helping to improve rapeseed productivity and quality under water scarcity and semi-arid cropping system.