Comparision Of Responses Of Winter Wheat(Triticum Aestivum L.) And Rape(Brassica Napus) To Arsenic Stress Pollution

Arsenic is a toxic element widely encountered in the environment and in organisms. Arsenic can enter terrestrial and aquatic environments through both natural formation and anthropogenic activities.In some areas of Hubei,Shanxi,Yunnan,and Hunan provinces of China,soil arsenic concentrations is much higher compared with other provinces because of coal fuels and metal smelters.In China both winter wheat and rape live through the winter and they are the main crops in these areas.High arsenic concentrations in soil or water may lead to elevated concentrations of arsenic in the grains of these two crops or in straw and give high risk to human's health.Based on the reviews of soil arsenic physical and chemical activities and arsenic bioavailability to plants,solution culture and soil culture experiments were conducted to investigate the different physiological responses between winter wheat and rape which are planted in arsenic polluted soil.The main results were as following:1.The effects of arsenic on the growth and yields of winter wheat and rape were investigated under arsenic stress.The results indicated that the growth of wheat shoots and roots stopped when solution arsenic was more than 30 mg/L but rape did not. Comparing with the control,the shoot and root biomass of wheat decreased by 66.7% and 78.3%,while those of rape decreased by 47.5%and 27.8%,respectively..It implied that rape has higher tolerance than wheat under arsenic stress.In soil culture experiments,wheat yields were elevated at low rates of arsenic addition(＜60 mg/kg) but reduced at high rates of As concentrations(80-100 mg/kg);the growth of rape hadn't showed significant responses to As addition.Phosphorus concentrations in wheat at jointing and ear sprouting stages increased with increasing soil As concentrations,and these increases were assumed to contribute a lot to enhanced growth of wheat at low As treatments.Arsenic did not significantly affect phosphorus concentrations in rape either.The highest arsenic concentrations in wheat shoot and rape leaf was 8.31 and 3.63 mg/kg,respectively.Arsenic concentrations in wheat and rape grains did not exceed the maximum permissible limit for food stuffs of 1.0mg/kg.When soil arsenic concentration is 0-60 mg/kg,both wheat and rape can be grown satisfactorily without adverse effects;when soil arsenic concentration is 80-100 mg/kg,rape is more suitable to be planted than wheat.2.The effects of arsenic on photosynthesis of wheat and rape under arsenic stress were investigated.The results indicated that arsenic significantly increased the Chlorophyll(Chla,Chlb and Catotenoids) contents in winter wheat and rape's leaves with solution arsenic increasing.However,Arsenic significantly decreased the photosynthetic rate(Pn),stomatal conductance(Cs),intercellular CO₂ concentration(Ci) and transpiration rate(Tr) of wheat under hydroponic conditions.Under arsenic stress, the stomatal limitation(Ls) of wheat leaf increased and Ci decreased.It suggested that the probable main factors to Pn decline were stomatal limitation.However,the Pn,Gs,Ci and Tr of rape leaves increased comparing with the control under hydroponic conditions. Because of the increasing chlorophyll and high tolerance to arsenic,rape's photosynthesis increased and enhanced the growth of rape biomass.Soil culture experiments indicated that wheat's Pn,Gs,Ci and Tr were affected by arsenic easily. They increased when soil arsenic concentration increased from 20 to 60 mg/kg but decreased under high soil arsenic treatments at different growth stages.But the photosynthesis of rape were not significantly affected by soil arsenic.It probably indicated that rape had high tolerance than wheat under arsenic stress.3.The effects of arsenic on the nutrients uptake of wheat and rape were investigated. The results indicated that the most of the arsenic accumulated in the roots of wheat and rape,and the shoot arsenic concentrations were much lower than those in roots.Arsenic increased Ca and Mg concentrations but decreased K and P concentrations in wheat shoots.The concentrations of Ca,Mg,K,P in wheat roots decreased with solution arsenic increasing.However,the nutrient elements of rape shoots were not significantly affected by arsenic.Under arsenic stress,rape has higher ability to transport nutrient elements from roots to leaves than wheat,especially for P and Fe transportation.Maybe this was one of the reasons that rape has higher tolerance to arsenic than wheat.4.The effects of phosphorus on the growth and yields of wheat and rape under arsenic stress were investigated.The toxicity of high soil arsenic concentrations to rape was more serious than wheat when soil P concentration was much lower.However,rape had high tolerance to arsenic than wheat and the growth and yield were not significantly affected by arsenic when 0.2 g/kg and 0.4 g/kg P fertilizers were added to this arsenic polluted soil.But the effects of P fertilizer on the toxicity of arsenic to wheat were not significant.Both arsenic concentrations in wheat and rape grains did not exceed the maximum permissible limit for food stuffs of 1.0mg/kg.When soil arsenic concentrations is very high(200mg/kg),P fertilizer could decrease the toxicity of arsenic to wheat and rape.Rape is more suitable to be planted in arsenic polluted areas compared with wheat.5.Concentrations and speciation of arsenic in wheat and rape tissues were investigated.The results indicated that wheat tissues could accumulate more arsenic than rape tissues could.Most arsenic accumulated in wheat and rape roots.Arsenic in roots and shoots of both two crops occurred as As³⁺ and As⁵⁺ species and As³⁺ was the main species.As solution culture As increased,As³⁺ concentration significantly increased but As⁵⁺ did not.6.The responses of non-enzymatic antioxidant and enzymatic antioxidant systems of wheat and rape tissues were investigated.The results indicated that both wheat and rape had high tolerance to arsenic.For rape,the contents of non-enzymatic antioxidants (glutathione-GSH,total acid-soluble thiol-SH,and phytochelatins-PCs) followed similar trends as root As concentrations,which is increasing with solution arsenic concentrations. The activities of enzymatic antioxidants(superoxide dismatase-SOD,catalase-CAT, peroxidase-POD) in rape increases with As concentrations up to 4 mg/L and then decreased.The results indicate that rape has a strong tolerance to arsenic.Enzymatic antioxidants are more important at low As exposure,whereas the non-enzymatic antioxidants are more critical at high As exposure.For wheat,the function of enzymatic antioxidant system's activities decreased and were not main system to decrease arsenic toxicity to this plant.But non-enzymatic antioxidants played an important role to decreased arsenic toxicity to wheat.