Effect Of Seeding Time On Accumulation And Distribution Of Heavy Metals In Rice

Cadmium,arsenic,lead is a toxic heavy metal element that is not essential for animals and plants.It easily migrates through the soil plant system and enters into the food chain,posing a serious threat to human health.For example,long-term consumption of cadmium exceeding the standard can cause connective tissue damage,reproductive system dysfunction,kidney damage and even cancer.Rice,as the main food crop in China.The farmland has been polluted seriously in recent years and the Cd?As?Pb strong enrichment of rice has increased by itself.Therefore,how to reduce the cadmium content in rice has become a major problem that needs to be solved urgently.A large number of studies have found that the cadmium content in late rice is higher than that in early rice,and the cadmium content in middle rice is the lowest,the reason and mechanism are still unclear.The field experiment of different sowing dates was carried out in Liuyang Comprehensive Teaching Experimental Base of Hunan Agricultural University in 2018,finding out the difference of Cd?As?Pb uptake and accumulation in rice in different seasons,understanding the effect of accumulated temperature on the absorption and accumulation of heavy metal Cd in rice and exploring the differences in physiological indexes during different sowing times.In the field experiment,two seedling-type early rice varieties ZhuLiangYou 819 and LuLiangYou 996 were used as experimental materials,and ten sowing dates were set up,including S1(March 23),S2(April 7),S3(April 22),S4(May 7),S5(May 22),S6(June 6),S7(June 21),S8(July 6),S9(July 21)and S10(August 5th).It was systematically studied that the growth,yield,physiological indexes and heavy metal absorption and accumulation trends of rice under different sowing dates.The correlation analysis between accumulated temperature and Cd content or accumulation of heavy metals was carried out,and the relationship between accumulated temperature during the key growth period and the Cd content or accumulation of various organs of the plant was discussed.The study provides technical theoretical support for large-scale pollution of heavy metal pollution.The main results are as follows:1.Effects of different sowing dates on growth period,yield and yield of riceThe effects of different sowing dates on the growth period,yield and yield components of the two rice varieties were roughly the same.The growth period of both varieties decreased first and then increased with the delay of sowing date.The highest yields of ZhuLiangYou 819 and LuLiangYou 996 were at S8,8112.08kg/hm~2and 8299.28kg/hm~2,respectively,which were mainly due to the highest seed setting rate and the higher effective panicles number.2.Effects of different sowing dates on dry matter accumulation of riceThe effects of different sowing dates on the total dry matter quantity of the two rice varieties at the tillering and jointing stages were basically the same.At tillering stage,V1 was the highest at S9(367.08g/m~2),V2 was at S7(392.25g/m~2),and at jointing stage,V1 and V2 were the highest at S7,499.63g/m2 and 541.41g/m2,respectively.At heading stage,the dry matter content of roots,stems and leaves was the highest at S9.At maturity stage,V1 had the highest dry matter content at S10,V2 had the highest dry matter of roots content at S9,and had the highest dry matter of stems and leaves at S7.3.Effects of different sowing dates on root activity and photosynthetic characteristics of riceThe root activity of S6-S8 at booting and full heading stages of two rice varieties showed the same change trend at different sowing dates,which decreased from S6 to S7 and then rose to S8.The transpiration rate and photosynthetic rate decreased with the delay of sowing date at jointing stage,heading stage and filling stage,while they were S7 > S6 > S9 at maturity stage.4.Effects of different sowing dates on the absorption and accumulation of heavy metals in riceDifferent sowing dates had different effects on Cd,As and Pb contents in rice plants,especially on the absorption and accumulation of Cd.At seedling stage,the Cd content of each organ of the two varieties showed the same rule under different treatments,and the Cd content of roots,stems and leaves of V1 was the highest at S8,51.31 mg/kg and 5.55 mg/kg,respectively.And V2 was the highest at S6,17.96 mg/kg and 1.47 mg/kg,respectively.At tillering stage,Cd content in organs of the two cultivars fluctuated alternately under different treatments.The content of Cd in roots,stems and leaves of V1 was the highest at S8,which was 4.29 mg/kg and 0.76 mg/kg,respectively.The content of Cd in roots,stems and leaves of V2 was the highest at S7,which was 2.97 mg/kg and 0.48 mg/kg,respectively.At jointing stage,the change of Cd content in different organs was the same as that at tillering stage.Cd content in roots,stems and leaves of V1 was the highest at S7,which was 4.83 mg/kg and 0.81 mg/kg,respectively.Cd content in roots of V2 was the highest at S4,4.47 mg/kg,and that in stems and leaves was the highest at S3,which was 1.67 mg/kg.The Cd content of organs at heading stage and maturity stage showed the same trend of decreasing first and then rising under different treatments,and the lowest was at S6-S7,the highest was at S10,and the decreasing range was lower than that of increasing range.The contents of As and Pb in different organs of rice at different sowing dates showed significant differences only at seedling stage,but not at other periods.5.Effects of accumulated temperature on the absorption and accumulation of heavy metal Cd in riceUnder the various sowing periods treatments,the changes in the accumulated temperature during the transplanting-tillering,heading-mature,and whole growth periods were first increased and then decreased,and the increasing range was lower than that of the declining.There was a positive correlation between Cd accumulation in each organ of two cultivars at tillering stage and accumulated temperature from transplanting stage to tillering stage.Take V2 for example,the correlation coefficient between root system,stem and leaf and accumulated temperature was 0.0004 and 0.0005,respectively,and the coefficient of determination(R~2)was 0.6461 and 0.7973.The cumulative temperature at heading-maturing stage and whole growth stage was negatively correlated with Cd content in all organs at maturity stage,and the coefficient of determination was higher and the degree of correlation was higher,which was higher than that at transplanting-tillering stage and the cumulative temperature at tillering stage.In conclusion,the suitable sowing date has a positive and significant effect on rice growth and development,high yield and reduction of heavy metal absorption and accumulation.