Research On The Dynamic Characteristics Of Physiology And Ecology Of Soybean (Glycine Max) During The Growth Period Under Cd Stress

A pot experiment and solution culture experiment were conducted to study the following:(ⅰ) dynamic characteristics of main growth and physiological-ecological indicators of soybean plants during the whole growth period under Cd stress; (ⅱ) dynamic characteristics of Cd uptake and distribution in soybean plants during the whole growth period under Cd stress; (ⅲ) the effects of Cd stress on physiological and ecological indicators and their differences in soybean plants at different growth stages; (ⅳ) the effects of Cd on growth of soybean seedlings and subcellular distribution and localization of Cd in soybean leaves; (ⅴ) cadmium accumulation in soybean seedling leaves and correlation between cadmium accumulation and physiological-biochemical indicators;(ⅵ) physiological and ecological responses of two soybean cultivars at flowering-poding stage under Cd stress. The main results were summarized as follows:1. The pot experiments with red soil showed that during the whole growth period of soybean plants, the influences of Cd stress on chlorophyll contents, SOD activities, POD activities, and MDA contents in the leaves were very significant (ρ<0.01). The growth of the plants was enhanced under low concentrations and short time of Cd stress, and restrained under high concentrations and long time of Cd stress. Cd concentration to restrain the plant heights was 1.00 mg·kg-1, which was far lower than that (2.50 mg·kg-1) to restrain the biomass of the plants. When Cd concentration reached a certain level, there was a very significant positive correlation between the restraining effects on biomass and height of soybean plants and Cd concentrations (ρ<0.01), which could be used to indicate soil Cd pollution, especially by using the correlation concerning the plant height. The sensitivity of chlorophyll content to Cd stress was higher at seeding stage than that at flowering-poding and mature stages. Increase of POD and SOD activities could reduce, to some extent, the injury effects of soybean plants due to membrane-lipid peroxidation caused by Cd stress. However, when Cd concentration reached 2.50 mg·kg-1, further increasing activities of plant protective enzyme system did not make up enough for the soybean plant injury caused by Cd stress. The POD activities of soybean at seedling stage and flowering-poding stage, or the activities of SOD at flowering-poding stage and mature stage, could indicate well Cd pollution level in soil. MDA contents in soybean plants increased under Cd stress, implying peroxidation of membrane reinforced.2. The pot experiments showed that under certain Cd concentration stress, the Cd contents in the plant roots at different stages were in the following sequence of flowering-poding stage< seeding stage< mature stage; the Cd contents in the shoots were in seeding stage< flowering-poding stage< mature stage. At seeding stage and flowering-poding stage, the Cd contents in the soybean leaves increased gradually with the time, but decreased at mature stage. During the whole growth period, when the time of Cd stress was certain, Cd contents in all the organs of soybean plants increased with the increasing of Cd concentrations added in soils, and showed power function correlation.3. The pot experiments indicated that the characteristics of physiological and ecological effects were different greatly during the growth of soybean plants under Cd stress. The patterns of MDA contents, biomasses, and heights of soybean plants showed almost the same trends, namely, stimulating effects at low Cd concentrations and inhibitory effects at high Cd concentrations. However, change trends of activities of POD and SOD in the protective enzyme systems and chlorophyll contents in soybean leaves were quite different. At the seedling stage, chlorophyll contents and SOD activities were inhibited obviously, POD activities were activated rapidly, and the mutual coordination of these processes relieved Cd toxicity to soybean plants. At the flowering-poding stage, the antioxidant defense system of soybean plants was excited effectively by Cd stress, resulting in activities of protective enzymes POD and SOD increased rapidly with chlorophyll contents increasing. At the mature stage, SOD and POD activities and chlorophyll contents of soybean plants decreased sharply due to a long-term toxicity of Cd, especially under Cd stress with high concentrations. It was obvious that the indicators of the soybean growth and the physiological and biochemical characteristics behaved a significant pattern of three stages under Cd stress.4. The solution culture experiments showed that the capacity of soybean seeding leaves for Cd accumulation was strong. Cd accumulation in the leaves increased greatly with increasing of Cd concentrations added to the culture solutions, and showed power function correlation (y=10.2x0.308, R2= 1.000, n=4). Most Cd associated with the cell walls and soluble fractions, and a minor part of Cd presented in the nuclear and chloroplast fractions, mitochondria fractions, especially exposure to high Cd concentrations,55.00% Cd were bound in the soluble fractions. Deposited Cd black particles were observed in the cell walls, chloroplasts, nuclei, and vacuoles through electrical microscope slice. This fact indicated that the cell walls of soybean leaves were the first barrier protecting organelles from Cd toxicity, and the cell walls and soluble fractions were the main place for Cd storage. Due to Cd accumulated in the organelles, the intercellular space was enlarged and the subcellular structure was damaged, especially for the chloroplasts. It might be an internal reason of high Cd concentrations inhibiting the growth of plant seedlings.5. The pot experiments showed that Cd contents in soybean leaves increased significantly with increasing Cd stress concentrations, and showed linear function correlation (y=8.76x+4.55,R2=0.987; n=7, R20.01=0.766, R20.05=0.569). Cd accumulated in the seedling leaves showed stimulating effects at low Cd concentrations and inhibitory effects at high Cd concentrations. With increasing Cd accumulated in the seedling leaves, the chlorophyll contents of seedlings decreased slightly, and showed linear function correlation (y=-0.008x+3.300, R2=0.657, n=7). Cd accumulated in the leaves decreased SOD activities. POD activities increased at first and then decreased with increase of Cd accumulated in the leaves, and this correlation could be expressed as a parabola function (y=-0.045x2+5.65x+204, R2=0.578, n=7). With increasing Cd accumulated in the soybean seedlings, MDA contents decreased at first, then increased, decreased slowly finally, and the regression equation was as y=-0.000001x3+0.0001x2-0.003x+0.131 (R2=0.804, n=7). There were significant or very significant correlations between Cd contents in the leaves and the height, biomass, chlorophyll content, POD activity, MDA contents in soybean plants, which could be used as pre-warning indexes for Cd accumulation in the seedling leaves.6. The responses of main physiological and growth indexes, such as biomass, height, SOD, POD activities, and chlorophyll contents, at the flowering-poding stage between two varieties of soybean plants, Wu Yue Wang and Ri Bn Qing, were very different. The most significant stimulating effects of Cd concentrations on the heights of Wu Yue Wang and Ri Ben Qing at flowering-poding stage were 0.50 and 0.25 mg·kg-1, respectively. Those on chlorophyll contents of Wu Yue Wang and Ri Bn Qing at early flowering-poding stage were also 0.50 and 0.25 mg·kg-1, respectively. The most significant stimulating effects of Cd concentrations on SOD activity of Wu Yue Wang and Ri Ben Qing at early flowering-poding stage were 2.50 and 1.00 mg·kg-1, respectively. At early flowering-poding stage, below 0.50 mg·kg-1 of Cd, SOD activity of Ri Ben Qing rose slowly with the increase of Cd concentrations added in soil, but SOD activity of Wu Yue Wang basically changed quite slightly. All the facts above showed that Ri Ben Qing was more sensitive to Cd stress at flowering-poding stage.