Shoot Cadmium Absorption And Cd Transport In Wheat Plant

Cadmium（Cd）is a toxic heavy metal.Studies have shown that more than 50%of Cd in contaminated soil comes from atmospheric deposition.Compared with soil,particulate matters have greater Cd concentration,and the smaller the size of particulate matter,the higher the Cd concentration and bioavailability.The Cd in atmospheric deposition may be absorbed by the shoots and transported in plants.Wheat is a major food crop prone to cadmium accumulation in edible parts.Cadmium concentration in wheat grains in Cd contaminated areas can reach more than 3 times of the permissible value of the National Food Safety Standard.However,wheat shoot Cd absorption,the subsequent transport of the shoot-absorbed Cd and the effect of Cd on the absorption and transport of trace elements in wheat are not well understood yet.In this work,cadmium was applied to wheat shoot in pot and hydroponics experiments to investigate the absorption of Cd by wheat shoot,the transfer of Cd from wheat leaves to roots and grains,and to explore the effects of Cd absorption by leaves on the absorption of trace elements in wheat roots and accumulation of trace elements in grains.The main results are as follows:a)In the pot experiment,Cd solution was sprayed on wheat leaves from heading stage to filling stage,while wheat heads were covered with ziplocked bags to prevent the influence of head absorption on grain Cd.The results showed that foliar Cd application had no effect on grain yield and Cd concentration（p>0.05）,but the Cd concentration in wheat leaves was significantly increased by 1.06-2.77 times.Cadmium absorbed by wheat leaves was mainly stored in leaves and leaf sheathes,only 3.34-8.56%of leaf absorbed Cd was transferred to internode 1-2 and accumulated in the upper part of internode 1.Foliar Cd application affected the distribution and accumulation of metals in wheat grains and other tissues,and inhibited the Ni accumulation in roots.b)Pot experiments were conducted to explore the effects of different shoot tissues on Cd accumulation and heavy metal transport in grains by spraying 20 and 60 mg L^-1 Cd solution on wheat leaves（the wheat heads were covered with ziplocked bags）and the whole wheat shoots during heading to filling stage.The results showed that Cd application significantly reduced wheat grain yield and increased the Cd concentration in wheat grain.At the same Cd concentration,the grain Cd concentration of the wheat shoot Cd application was 69.3-157%greater than that of leaf spray only.The transfer factors（TF）of glume to grain were 3.53×10^-2 and 7.94×10^-2,and those of leaf to grain were 9.73×10^-4 and 4.66×10^-3.Shoot Cd application reduced Cu concentration in grain by reducing Cu transport from rachis to grain（p<0.05）.The wheat shoot Cd application reduced the accumulation of Cu,Zn and Mn in grains by 19.1-47.4%,13.5-40.4%and 12.5-37.5%,respectively.This indicated that Cd absorbed by wheat heads played a much more important role in grain Cd accumulation than wheat leaf absorption.c)In a hydroponic experiment,the leaves of 4-week-old wheat were immersed in Cd solution for 24 h,and then cultured in pollutant-free nutrient solution for 2 weeks.Samples were collected on the 1st,3rd,7th and 14th days after the treatment,respectively.The results showed that Cd absorbed by wheat leaves could be transferred to roots within a day,and more Cd was absorbed in wheat leaves with a cut.Compared with the results of leaves without cut,the Cd concentration of cut leaves was 23.0%higher（p<0.05）.The foliar Cd application increased the Cu absorption in wheat roots in the early stage,but inhibited the Cu absorption in the late stage.At all sampling times,Cd application inhibited Zn uptake in wheat roots,but promoted Mn uptake.The leaves Cd application increased the TF_leaves/root of Cu and Zn,but inhibited the TF_leaves/root of Mn.The results of this study show that,i)wheat leaves could absorb and store a large amount of Cd,but the Cd absorbed by leaves had little effect on the Cd accumulation of grain.The Ni absorption at root was inhibited by the wheat shoot Cd application;ii)The absorption of Cd by glume played an important role of Cd accumulation in wheat grains,and the shoot Cd application inhibited the accumulation of Cu,Zn and Mn in wheat grains;and iii)The Cd absorbed by wheat leaves can be transferred to the root and affect Cu,Zn and Mn concentrations in plants by increasing or decreasing root uptake and root-stem transport.