Effects Of Different Sowing Dateson Heavy Metal Absorption And Accumulation In Rice

The quality and safety of rice production in China—the largest rice producer and consumer in the world—directly affect people’s daily life and health at home and abroad.In contrast to other crops,rice is more susceptible to Cadmium(Cd)pollution.Under the influence of environmental pollution,the heavy metal polluted cultivated land in China is nearly 2×10~7 hm~2(about 1/5 of the total cultivated land area),in which Cd pollution is the most severe.Through the accumulation of biological migration,the increasing Cd content in irrigation water and soil leads to increasing Cd content and heavy metal over-specification in rice.For the health of human beings,how to reduce Cd content in rice is a major problem to be solved urgently.Previous studies showed that the content of Cd in early season rice was lower than that in late season rice,but the reason and mechanism were not clear.In order to study the distinctions of Cd contents of rice in different seasons and the roles of the temperatures in heavy metal absorption of rice,the field experiments,were conducted at Liuyang comprehensive teaching experiment base of Hunan Agricultural University,with two indica rice varieties LuLiangYou996(LLY996)and ZhuLiangYou819(ZLY819)as test materials,on 10 sowing dates(from April 2nd to August 7th),tring to systematically analyze the features of growth,yields and absorption and accumulation of heavy metals in rice on different sowing dates,and provide technical reference for large-area metal pollution control.The main results were as follows:1.The effects of different sowing dates on yields and yield components of rice.The effects of different sowing dates upon yields and yield components of two rice varieties were inconsistent.The highest yield of LLY996 was in S3(7744.92kg/hm~2),because of the peak grain per panicle.The highest yield of ZLY819 was in S7(8234.41kg/hm~2),related to the higher productive panicle number.2.The effects of different sowing dates on the accumulation of dry matter in rice.The effects of sowing dates on the total dry matter quality of two rice varieties at heading stage were basically consistent.The highest values were all in S7,1293.85g/m~2 and959.03g/m~2 respectively.The effects of different sowing dates on the total dry matter qualities of two rice varieties at maturity stage were different.The highest value of total dry weight of LLY 996 was in S3(1614.15g/m~2),while that of ZLY 819 in S1(1555.65g/m~2).3.The effects of different sowing dates on heavy metal uptake and accumulation in rice.The sowing dates exert more influence on the Cd content than the other three(Pb,Cr,As)heavy metals.At heading and mature stage,the Cd content of organs first decreased from S1 to S6,(with S6 as the lowest),and then increased rapidly.At the first seeding stage of LLY 996,Cd content in root,stem,leaf and panicle(19.26mg/kg,4.81 mg/kg,0.90 mg/kg,1.01 mg/kg,0.91 mg/kg respectively),first reduced,in which the lowest value appeared in S6,(5.05 mg/kg,1.07 mg/kg,0.45 mg/kg,0.17mg/kg,0.20mg/kg respectively),and then it showed a rapid rising trend from S6 to S10,up to the peak value in S10,(46.63 mg/kg,9.60mg/kg,1.70 mg/kg,1.63 mg/kg,and 3.42 mg/kg).In the process,the change speed from S6to S10 was faster than that from S1 to S6.And the Cd contents in plant body were higher in the latter three stages.But the effects of sowing dates on Pb,Cr and As contents were not obvious.4.The effects of accumulated temperatures on the absorption and accumulation of heavy metals in rice.From S1 to S10,the accumulated temperatures surged first and dropped later in the whole growth stage,before and after heading,with larger range in increase than in decrease.According to the correlation between the accumulated temperatures and the contents of four heavy metals,Cd,Pb,Cr and As,the accumulated temperatures of the two varieties in the whole growth stage were negatively correlated with Cd content in mature period.The correlation coefficient between the Cd contents in the root,stem,leave,filled grain and unfilled grain of LLY996 and the accumulated temperatures were-0.0045、-0.0243、-0.0025、-0.0035、-0.0077,and the determinant coefficient(R~2)were 0.8594、0.8086、0.9106、0.8406、0.9044 respectively.The accumulated temperatures after heading stage were negatively correlated with Cd contents in the mature period.And the correlated degree was higher.The correlation before heading was the same as after heading.The accumulated temperatures were negatively correlated with Pb content in all periods,and were positive correlated with Cr and As content.Generally speaking,the differences of sowing dates show the distinctions mainly in temperature,sunshine and water.But in this study,there was no correlation between sunshine and rainfall and heavy metal content in rice plants,while the accumulated temperature significantly affected the uptake and accumulation of heavy metals in rice,especially Cd.It was found that the accumulated temperature had a negative correlation with Cd content in the rice plant,which may be one of the reasons that Cd in late season rice was more than in early season and middle season rice,with Cd in middle season as the lest.The difference of Cd content in rice plants was very significant on different sowing dates.So in production,the sowing time of middle season rice and one season late rice can be properly adjusted to reduce the accumulation of Cd in rice plants.For double cropping rice,agronomic and technical measures can be used to change the micro ecological environment of rice,thus affecting the absorption and accumulation of heavy metals by rice.