Influences Of Cadmium And Chromium And Lead Multiple Stresses On Wheat And Corn And A New Classifying Method Of Interactions Between Heavy Metals

The study was conducted about the effects on wheat and corn of cadmium (Cd) and chromium (Cr) and lead (Pb), which are main pollutants among toxic heavy metals to crop in sludge applied soils of loess-like drab soil type in Shanxi. With principles and procedures of pollution ecology and ecological toxicology, a series of experiments were carried out including soil column culture and solute culture and germination tests of wheat and corn. Effects of stresses of the three heavy metals were evaluated with the agronomic and physiological and healthy characteristics of wheat and corn in different growth and development stages. A new classifying method of interactions between heavy metals on crops was also proposed based on the results from this study.I. Study on influences of Cd and Cr and Pb multiple stresses to wheat and corn1.1 Toxicity order of Cd stress alone to germination is that root length(RL) > root fresh weight(RW) > bud fresh weight(BW) > root number(RN) > germination rate(G) for wheat, and that RW > RL > BW > BL > RN > G for corn. Corn was more sensitive than wheat. Toxicity order of Cr stress alone is that RL > BL > RW > BW > RN > G for wheat, and that RL > RW > BW > BL > RN > G for corn. Toxicity order of Pb stress alone is that RL > BL > BW > RW > G > RN for wheat, and that RL > RW > BW > BL > RN > G for corn. The order of toxicity to the same traits of wheat and corn is that Cr > Cd > Pb. Relations between the inhibition to crop traits and heavy metals concentrations were well fitted with math models of hyperbola equation Y = x/(a + x/100) or exponential curve Y = a[1-exp(-bx)]. EC50s were estimated from these models and used to order toxicity of the heavy metals. 1.2. Any double or triple stresses of Cd and Cr and Pb caused weaker joint inhibition actions on germination of wheat and corn than the sums of their independent actions. In most cases, there were interactions of type 2 among the heavy metals. In other words, there was antagonism among them.1.3. Toxicity order of the heavy metals stresses alone to wheat seedlings is that top weight (TW) = leaf area (LA) > root dry matter (RW) > plant height (PH) for Cd, and that TW > LA > RW > PH for Cr, and that LA > TW > RW > PH for Pb. The order of toxicity to TW and LA and PH is that Cr > Cd > Pb, but Cd > Cr > Pb to RW. Relations between inhibitions to wheat agronomic traits and the heavy metals concentrations were well fitted with linear equation, or hyperbola equation Y = x/(a + x/100) or exponential curve Y = a[1-exp(-bx)].1.4. Joint inhibition actions of heavy metals on leaf area and weights of top shoots and roots of wheat showed that joint actions of Cd and Cr combined stress were smaller than sums of their independent actions on day 5 post treatment, but larger on day 10, and that Cd and Pb joint actions were larger than their independent action sum on both days. Joint actions of Cr and Pb combined stress and of the triple stress of Cd and Cr and Pb were larger than sums of their independent actions on root weight of wheat, but smaller on leaf area and top shoot weight.1.5. Stresses of 0.3 mM Cd or 0.25 mM Cr or 0.8mM Pb resulted in that wheat chlorophyll content and soluble protein content went down, stomatal resistance up, leaf osmotic potential down, root and leaf solute leakage up, MDA content up, activities of SOD and CAT up. However, Cr or Pb caused root vigor to go down, and POD activity up, while Cd had the opposite effects to Cr and Pb.1.6. Single stresses of 0.3 mM Cd and 0.8mM Pb changed POD isoenzyme spectrum compared to the control, but single stress of 0.25 mM Cr did not. Their multiple stresses of two elements or three elements both increased and decreased the isoenzyme spectrum compared to the control or single stresses.1.7. Interactions among heavy metals brought out four patterns of joint actions of heavy metals multiple stresses (JA) compared to sums of their independent actions (SIA). (1) JAs were larger than SIAs, including joint inhibitions of Cd and Cr combined stress and of Cd and Pb combined stress to root vigor, and leaf osmotic potential decrease exerted by Cr and Pb combined stress and of Cd and Cr and Pb triple stress, and and chlorophyll content decrease caused by any double or triple stresses of Cd and Cr and Pb. (2) JAs were smaller than SIAs, including leaf stomatal resistance up and soluble protein and MDA contents down because of stresses by any double or triple combinations of Cd and Cr and Pb, decrease of root vigor and SOD activity stressed by Cr and Pb combination. (3) JAs varied with stress time from smaller than to larger than SIAs, including root and leaf solute leakage up stressed by any double or triple combinations of Cd and Cr and Pb. (4) JAs changed from larger than to smaller than SIAs, including leaf osmotic potential down stressed by Cd and Cr combination and of Cd and Pb combination, root vigor down exerted by the triple stress of Cd and Cr and Pb. (5)Uptake by wheat roots and sheaths and leaves of Non-essential elements of Cd and Cr and Pb each was higher under multiple stresses of these heavy metals than under stresses alone of them. Translation of the heavy metals from roots to top parts of wheat were higher under multiple stresses than single stresses.1.8. Toxicity order of soil Cd and Cr and Pb stresses alone to grain yield of wheat and corn is that Cd > Cr > Pb. Interactions between any double stresses of the heavy metals on grain yield ranked as Cd*Cr > Cd*Pb > Cr*Pb, which mainly were interactions of type 2, i.e. antagonism. Direct path coefficients of wheat grain yield to its components under multiple stresses of Cd and Cr and Pb were that 0.637 for ear number, 0.406 for ear grain number and 0.154 for grain weight, of which only 0.154 was not significant.1.9. Relations between Cd uptake in grains of wheat and corn and concentrations of Cd and Cr and Pb in soil was multiple linear. All regression coefficients were positive, of which the regression coefficient of Cd was much larger than others. Soil critical concentrations of Cd were 3.2mg/kg for wheat, and 3.7mg/kg for corn. These critical values would become smaller if Cr and/or Pb stresses exist in soil. The order of Cd uptake by different organs of wheat and corn ranked as roots > stalks > grains.1.10. Relations between Cr uptake in grains of wheat and corn and concentrations of Cd and Cr and Pb in soil was multiple linear. All regression coefficients were positive, of which the regression coefficient of Cr was much larger than others. Soil critical concentrations of Cr were 37.2mg/kg for wheat, and 61.2mg/kg for corn. These critical values would become smaller if Cd and/or Pb stresses exist in soil. The order of Cr uptake by different organs of wheat and corn ranked as roots > stalks > grains.1.11. Relations between Pb uptake in grains of wheat and corn and concentrations of Cd and Cr and Pb in soil was multiple linear. All regression coefficients were positive, of which the regression coefficient of Pb was much larger than others. Soil critical concentrations of Pb were 738mg/kg for wheat, and 2731mg/kg for corn. These critical values would become smaller if Cd and/or Cr stresses exist in soil. However, these critical concentrations should be validated because their values were beyond maximum concentration in the experiment. The order of Pb uptake by different organs of wheat and corn ranked as roots > stalks > grains.II. A new classifying method of interactions between heavy metals on crops2.1. The interaction between two heavy metals on crops is defined asinteraction = joint action– sum of independent actions where joint action equals to observation of two heavy metals combination minus observation of control, and independent action equals to observation of one heavy metal minus observation of control. Every action has its own direction and magnitude.2.2. Interactions have three action modes: (1) strengthening the effect of one factor; (2) weakening the effect of one factor; (3) inducing the effect of one factor. Based on theses modes, interactions between two heavy metals on crop are classified into seven types.2.3. Joint actions of two heavy metals are classified into four types: opposability, homogeneity, zero-non, and equivalence. Joint actions are further classified into thirteen subtypes based on associations with interactions.