Effects Of Trace Element Spraying After Artificial Pollination On Seed Vigor Of Hybrid Rice(Oryza Sativa L.)

Rice production in China is in the major transition period.Modern agricultural technologies based on high-vigor seeds are constantly emerging such as machine-transplanted seeding or seedling.However,hybrid rice still has low seed vigor such as low field seedling rate,poor seedling quality,and inconsistent growth rate,which is difficult to meet the urgent demand for high-quality seeds of modern rice production technology,and has become a‘Stuck neck’problem in the development of seed industry.Field experiments were conducted with different hybrid rice cultivars including‘Zheyou 12’（‘Zhe 04A’בZhehui 414’）,‘Zheyou 18’（‘Zhe 04A’בZhehui 818’）,‘Chunyou 927’（‘Chunjiang 16A’בC84’）and‘Chunyou 927’（‘Chunjiang 16A’בC927’）.Different concentrations of Zn SO₄,Fe SO₄,and Mn SO₄solutions were sprayed after artificial pollination to explore the effect of trace element spraying on seed vigor,and reveal the underlying regulatory mechanism from physiological and molecular levels,meanwhile,to screen out the suitable spraying concentrations of trace elements.This will provide the theoretical basis and technical support for the high vigor seed production of hybrid rice in China.The main results are listed as follows:1.Spraying suitable concentrations of trace elements after artificial pollination significantly promoted seed vigor of four hybrid rice cultivars.The results of field experiments conducted in 2019and 2020 showed that the germination rate,germination potential,germination index,vigor index of hybrid rice seeds after Zn SO₄,Fe SO₄,and Mn SO₄spraying were increased by 0.6-14.0%,2.0-37.7%,0.6-37.7%,0.2-36.6%,and the optimal concentration was 0.5,4,1.25 g/L,respectively.When artificial accelerated aging test was applied,the germination rate and germination potential of‘Chunyou 927’and‘Zheyou 18’with 0.5 g/L Zn SO₄spraying were significantly increased by 3.2%and 5.1%on average.The germination rate and vigor index of both cultivars with 4 g/L Fe SO₄spraying were markedly improved by 6.0%and 24.3%on average.The germination index and vigor index of both cultivars with 1.25 g/L Mn SO₄spraying were obviously promoted by 7.5%and 12.2%on average.In addition,‘Chunyou 927’and‘Zheyou 18’sprayed with the best concentration of trace elements at 14 d after artificial assisted pollination had better effect than at 7 d and there was no significant effect between combination treatment and single treatment in 2021.2.Spraying trace elements with an appropriate concentration at 14 days after artificial pollination significantly increased the activities of antioxidant enzymes and decreased the content of MDA in both‘Chunyou 927’and‘Zheyou 18’.With the increase of spraying concentration of Zn SO₄,Fe SO₄,Mn SO₄,the activities of CAT,SOD,POD of seeds germinated for 3 days increased firstly and then decreased in both cultivars.Spraying trace elements promoted the activities of CAT,POD,SOD,and reduced the MDA content.The optimum concentration was basically consistent with the phenotypic results.The SOD,POD activities of both cultivars with 0.5 g/L Zn SO₄spraying were significantly increased by 17.2%,24.8%,and the MDA content was decreased by 41.3%on average.The activity of CAT of both cultivars with 4 g/L Fe SO₄spraying was significantly improved by 162.7%,and the MDA content was reduced by 35.7%on average.The CAT activity after 1.25 g/L Mn SO₄spraying was significantly promoted by 57.2%,and the MDA content was decreased by 18.2%on average.3.Spraying trace elements with an appropriate concentration at 14 d after artificial pollination promoted the expression of antioxidant enzyme genes in both‘Chunyou 927’and‘Zheyou 18’.The Real-Time PCR results showed that spraying appropriate concentrations of trace elements significantly increased the expression of Os APX2 and Os CSD3 genes.When the spraying concentration of Zn SO₄was 0.5 g/L,the expression of Os APX2 and Os CSD3 was increased by 42.6%and 237.8%on average,respectively.When the spraying concentration of Fe SO₄was 4 g/L,the expression of Os APX2 and Os CSD3 was increased by 23.2%and 182.6%on average,respectively.When the spraying concentration of Mn SO₄was 1.25 g/L,the expression of Os APX2 and Os CSD3was increased by 68.4%and 262.2%on average,respectively.Furthermore,the expression level of Os APX2 in’Chunyou 927’and’Zheyou 18’seeds germinated for 5 d was generally higher than that of seeds germinated for 3 d and 4 d,and the expression level of Os CSD3 in seeds germinated for 4 d and5 d was in general higher than that of seeds germinated for 3 d.In conclusion,spraying appropriate concentrations of Zn SO₄,Fe SO₄,and Mn SO₄at 14 d after artificial pollination increased the expression of Os APX2 and Os CSD3 genes,and promoted activities of CAT,SOD,POD and reduce the MDA content of seeds,which improved seed antioxidant capacity and seed vigor.In the production of high vigor seeds of hybrid rice,it is suggested that the optimal spraying concentrations of Zn SO₄,Fe SO_4,Mn SO₄is 0.5,4,1.25 g/L,respectively.