The Influence Of Cadmium Stress On Rice Growth And Quality Formation As Affected By Different Nitrogen Forms

A pot experiment was conducted to study the influence of four N fertilizer forms(CO(NH₂)₂(Urea),Ca(NO₃)₂,(NH₄)₂SO₄ and organic N on cadmium stress onrice growth and quality formation under two Cd levels(0 and 100 mg Cd kg^-1 soil).The main results are as follows:The addition of Cd significantly reduced photosynthetic rate(Pn) of rice plants atboth heading and milking stages,and the reduction varied with N fertilizer forms,with NH₄⁺-N and urea-N treated plants having more reduction.There was nosignificant difference in chlorophyll content expressed by SPAD value betweenCd stress and the control at milking and heading stages.However,N form had adistinct effect on SPAD value,and the effect was also dependent on Cd level andgrowth stage.There was a significant difference in the size of flag and thesecond leaves among four N forms.Cd-stressed plants had significantly smallerflag leaf area than the control,irrespective of N forms,but for the second leaf,only the plants supplied with organic N showed the difference between two Cdlevels.There was a significant difference in plant height among four N forms,withNH₄⁺-and NO₃^--treated plants having the highest and lowest height,respectively.The addition of Cd also significantly reduced plant height.Urea-and NH_<sup>4+-Ntreated plants had significantly more panicles per plant than the other two Nform-treated ones.There were significant differences among N forms in N and Cd uptake of theplants subjected to Cd stress,with NH₄⁺-N treated plants having highest N andlowest Cd concentrations and NO₃^--treated plants having lowest N and highestCd uptake.The concentration of Cd in plant parts was in the ordershoot＞panicle＞leaf.However,the accumulation of N in leaf,stem and paniclewas dependent more on N form and to a less extent Cd contamination.Both Nform and Cd stress affected SOD activities at both stages.There was asignificant increase in leaf SOD activity and peroxidase(POD) activities among the three N fertilizers relative to control at both stages except with NO₃^--N treatedplants where there was decrease or no significant difference at heading andmilking stages respectively.It was shown that Cd induced the increasingoxidative damage and that the extent of damage caused depended on the N formapplied to plants.Cd addition significantly reduced grains per panicle and totalplant weight,and the reduction varied with N form,with organic N treatmentshowing more reduction.Results showed that both N form and Cd stress affectedyield.Urea-N and NH₄⁺-N treatments had significantly higher grain yields than theother two treatments under normal conditions.Cd addition caused a significantreduction of grain yield in comparison with the control in NH₄⁺-N and organic-Ntreatments but there was no significant difference for urea-N and NO₃^--Ntreatments between the two Cd levels.N form had an effect on Brown Rice Rate(BRR) only in stressed plants,withNH₄⁺-N treated plants having highest BRR value and urea-N having the lowestBRR value.Similarly,addition of Cd to the soil also only significantly reducedHead Rice Rate(HRR) of rice grains treated with urea-N form relative to thecontrol.Under Cd stressed condition,urea-N and NH₄⁺-N treated plants had asignificant reduction in Head Milled Rice Rate(HMRR).There was a significantdifference in gelatinization temperature(GT) when plants were subjected to Cdstress compared to the control plants.Meanwhile,significant difference was alsofound among the four N forms and the effect of N form on GT varied with Cdtreatment.Thus under no Cd stress condition,organic-N treated plants had thehighest GT,being significantly higher than NO₃^--N treated plants.Under Cdstress condition,urea-N treated plants had the highest GT,being significantlyhigher than NH₄⁺-treated plants.Amylose content(AC) was not significantlyaffected by Cd stress,but was dramatically affected by N forms,with NO₃^--Ntreated plants having the highest AC.A significant difference was found inProtein Content(PC) between Cd stressed and the control plants,with Cd-stressed plants having significantly lower PC than the control.There was asignificant difference in PC among the four N forms,with NH₄⁺-treated plantshaving the highest PC,followed by urea-N treated plants. Out of the seventeen amino acids examined,only five(aspartic acid,tyrosine,phenylalanine,lysine and arginine) were significantly affected by N forms only,with NH₄⁺-and urea-N treated plants having significantly higher values.