Mechanisms Underlying The Effects Of Seed Priming On The Establishment Of Direct-Seeded Early Season Rice Under Chilling Stress

The poor crop establishment of direct-seeded early rice due to chilling stress is one of the main obstacle for the extension of double direct-seeded rice in central China.While pre-sowing seed treatment refers to the technology that treated the seeds physically or chemically prior to sowing to enhance the crop establishment under various abiotic stresses.However,the effects of pre-sowing seed treatments on the seed germination and seedling growth of direct-seeded early rice under chilling stress has never been evaluated before.Series of experiments were conducted to unravel the effects of two seed coating treatments,two seed priming treatments and 4 combinations of coating-priming treatments on emergence,seedling growth of direct-seeded early rice under chilling stress.And the results showed that seed priming was more useful in enhancing the establishment of direct-seeded early rice than seed coating and the combined treatments.Then,mechanisms associated with the seed priming treatments were explored by measuring alpha-amylase activity,total soluble sugar content,relative gene expression,GA₃ and ABA content,respiratory rate and ATP content.Lastly,two early rice varieties,EZ18?inbred?and LY287?hybrid?were used to evaluate effects of seed priming treatments on seedling establishment,yield and yield components of direct-seeded early rice.The main results are as follows:1.The daily mean temperature and the night mean temperature during 1 DAS to 10DAS were 18.8°C and 12.7°C,respectively.The minimum temperature even reached6.3°C.Suggesting that the direct-seeded early rice was exposed to chilling stress during the course of study.The chilling stress severely hampered the process of seedling establishment,which resulted in 17.7%reduction in seed germination percentage,and82.6%-92.2%reduction in seedling growth attributes.Se-priming and SA-priming significantly enhanced the seed germination and seedling growth of rice under chilling stress,which increased the germination percentage by 102.8%-103.8%,and increased the root length,shoot length,root fresh weight and shoot fresh weight by 15.2%-38.9%.Nevertheless,seed coating treatments did not promote the establishment of direct-seeded early rice under chilling stress,which might be attributed to the impermeable polymer materials of the coating agent that would have reduced the gas and water exchange from soil to the seeds.The combination of seed priming and coating treatments also increased the seed germination and seedling growth of direct-seeded early rice,but its effects showed no significant difference to that of seed priming.2.Seed priming enhanced the?-amylase activity and total soluble sugars of direct-seeded early rice under chilling stress.Besides.The relative expression of GA controlled amylase gene OsRamy1A and OsRamy3B in primed rice seeds was up-regulated by 6.7 and 7.1 folds,respectively.Further researches revealed that the expression of OsGA₃ox1 and OsGA₂₀ox1 was up-regulated by seed priming,which resulted in the increase of GA₃ content in rice seeds during germination.In contrary,the ABA content and the expression of ABA synthesis genes was decreased and down-regulated in primed rice seeds.Suggesting that the regulatory effect of seed priming on starch metabolism was associated with the increases of GA/ABA ratio in rice seeds during germination,which triggered the expression of specific alpha-amylase genes and ultimately promoted the starch metabolism and accelerate the germination of rice seeds under chilling stress.3.Seed priming significantly improved the respiration rate of rice seeds by76.5%-191.2%,and increased the ATP contents by 62.1%-23.0%.Further researches explored that the enhanced respiratory metabolism induced by seed priming was associated with the increased activity of PK,HK,PFK and MDH in EMP and TCA pathways.Moreover,seed priming initiates the efficient development of mitochondria,which may also accelerated the respiration of rice seeds and seedling under chilling stress.4.Under field condition,seed priming enhanced the seed germination and seedling growth of mega early rice varieties in central China.No significant difference was found between Se-priming and SA-priming on enhancing the establishment of early direct-seeded rice.However,the promoting effect of seed priming was more pronounced in hybrid early rice variety than in inbred one.At maturity stage,seed priming increased the grain yield of early direct-seeded rice,which was attributed to the increase in panicle numbers per unit area.In summary,seed priming was effective in accelerating seed germination,promoting seedling growth,and maintaining the grain yield of direct-seeded early rice under chilling stress.The beneficial effect of seed priming on seed germination was associated with the enhanced starch degradation and respiratory metabolism.Further studies should be focused on the optimization of seed priming agents and procedure,and the combination of seed priming treatments with other seed treatments.Besides,the variances among different type of varieties in response to seed priming should also be addressed in future studies.