| The quality of rice seed is an important factor to determine the yield of rice.It is directly related to the safety of grain production.Obtaining the seeds of high vigor is the prerequisite to judge the optimal harvest time,which is very important to improve seed quality.The seed vigor was influenced by seed maturity,and was formed during seed development.Duringseed storage,seed aging is common situation.As too long time storage and other reasons will lead to decline of seed vigor,seed priming can be used as a technology to restore its vitgor partially.In present experiment,two hybrid rice(Oryza sativa L.)varieties,Zhuliangyou 06(ZLY06,indica)and Chunyou 84(CY84,japonica),were used to carry out studies on the optimal harvest time of hybrid rice seeds,the effects of physiological and biochemical indexes of hybrid rice seeds on seed vigor and the mechanism of reactive oxygen species priming to improve natural and artificial aged rice seeds.The main findings are as follows:The seed germination,seedling quality,electrical conductivity(EC),seed size,glume chlorophyll content,fresh and dry weight and moisture content of different maturity seeds were studied,and the optimal harvest time was determined.The results of germination test in 2014 and 2015 showed that vigor related parameters of ZLY06 seeds reached a high level from 20 to 45 days after pollination(DAP),the seedling quality was better,while those of CY84 maintained the higher levels from 25 to 45 DAP.For ZLY06,EC decreased dramatically and then changed a little from 20 to 45 DAP;while for CY84,EC reached the lower value and kept slightly decreasing from 25 to 55 DAP,which was consistent with the results of seed germination test.With the seed maturation of ZLY06 and CY84,a rapid increase in seed size were observed at the initial filling periods,especially in seed thickness.The glume chlorophyll content declined rapidly and kept very low and stable level after 35 or 40 DAP.Moisture content presented a sharp decline at the early stage after pollination,while seed fresh and dry weight increased sharply and seed fresh weight decreased in the late matu.re period slightly.In addition,Seed moisture content had relatively low value,and seed dry and fresh weight reached the higher levels in ZLY06 and CY84 at 20 and 25 DAP,respectively.The results showed that seeds could be harvested as early as 20 DAP for ZLY06,25 DAP for CY84,respectively,that is,in the waxy ripening stage suitable for harvest.The changes of the contents soluble sugar,soluble starch,soluble protein and hormone,amylase activity,gene expression,antioxidant enzymes activities and their relationship with seed vigor were studied in different maturity seeds.Starch content exhibited an increasing trend during seed maturation;while the contents of soluble sugar,soluble protein,gibberellins(GAs)and abscisic acid(ABA),amylase activity decreased at early stages of seed development and then remained unchanged.Superoxide dismutase(SOD)activity in the two hybrid rice seeds rose sharply,and then declined slightly.While catalase(CAT),peroxidase(POD)and ascorbate peroxidase(APX)activities had the similar downward trends.Moreover,there were significant correlations between glume chlorophyll content,dry weight,soluble sugar,soluble protein,GA3,ABA,APX and seed vigor,respectively,in different maturity.Sucrose synthase-related genes both SUS2 and SUS4 expressed abundantly at the early stage after pollination,and gradually decreased.The ADPase related genes AGP-S1,AGP-S2and AGP-L2 had different expressions in two hybrid rice varieties.Amy4A decreased at the early stages of seed maturation and then increased.The effects of reactive oxygen species(ROS)solution priming on natural and artificial aged rice seed vigor were studied,and the best priming solution and its concentration were selected.The optimal concentration of the priming solution was different for different varieties or the same variety with different aging treatments.0.2%hydrogen peroxide(H2O2),0.2%Fenton and 0.4-?M methyl viologen(MV)were better for the two aging treatments of ZLY06 seeds.For CY84,natural aged seeds were better treated with 0.8%H2O2,0.4%Fenton and 0.2 M MV,and 0.2%H2O2,0.4%Fenton and 0.4?M MV were better for artificial aged seeds.Moreover,Fenton priming had better effect than the other two treatments,and germination energy(GE),germination index(GI)and vigor index(VI)of two rice varieties aged seeds were significantly improved.On the one hand,Fenton priming triggered the promotion of ?-amylase activity,enhanced GA3 and H2O2 content of aged seeds,reduced ABA content and ABA/GA3 ratio and promoted seed germination.At the molecular level,Fenton priming could significantly increase the expression of genes related to GAs synthesis and ABA catabolism in natural and artificial aged seeds and inhibit the expression of GAs catabolism-related genes and ABA synthesis-related genes in aged seeds,so that it alleviated inhibition of aging on seed germination and accelerated seed germination.On the other hand,Fenton priming increased the antioxidant enzymes activities,decreased H2O2 content of aged seedlings,increased chlorophyll content and promoted seedling growth.In addition,this study showed that Fenton priming had a similar promotion effect on natural and artificial aged seeds.The effects of natural aging and artificial aging on hybrid rice seed vigor were studied.The germination percentage(GP),GE,GI and VI of CY84 hybrid rice seeds were significantly higher than those of ZLY06 seeds after 12 months of natural aging.And after 6 days artificial,there was a similar germination result that CY84 seeds were significantly better than ZLY06.After imbibition for 24h and 48h in two varieties,both artificial and natural aging significantly reduced GA3 content,?-amylase activity and H2O2 content in two hybrid rice seeds during germination,increased ABA content,ABA/GA3 ratio and inhibited seed germination.The results showed that the artificial aging seeds could better match the natural aging seed in germination results.However,the effects of the two aging treatments on the enzymes activities in seedlings were different.?-amylase activity of natural aging rice seeds was significantly lower than that of artificial aged seeds.Compared with seeds without aging,the natural aging significantly reduced SOD activity,while the artificial aging had no significant difference.And SOD and POD activities after natural aging were significantly lower than those of artificial aging.At the molecular level,artificial aging suppressed the genes expression related to GAs synthesis and ABA catabolism,or promoted the genes expression related to GAs catabolism.However,after natural aging,the expression of genes related ABA synthesis mainly increased to inhibit seed germination.This showed that there were some essential differences between natural and artificial aging,which still need further studies. |
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