Effect Of Different Environmental Factors On Rhizobium-alfalfa Symbiotic Association

Alfalfa, which distributes widely in the world and has high economy value in agriculture, is a perennial legume herb and known as 'king of herb'. Rhizobia-alfalfa association is a high efficient N-fixing system, has been widely applied on the agricultural production. The N-fixing system can reduce application of chemistry fertilizer, and make for protecting the environment and sustainable development of agriculture. However, with the development of human civilization, the destruction, which the environment suffered from, was without precedent in history. Consequently, environmental factors become an important reason for developing potential fully on the agricultural production.Soil heavy metal pollution, which was one of environment problems concerned by the people all over the world, was getting more and more serious. In addition, Purple soil is an important soil resource in China, and distributes widely in the western China. However, the home and oversea reports about effect of these environmental factors on rhizobia-alfalfa symbiotic association were lack. In this paper, effect of Cd and Pb on seed germination, growth and nitrogen fixation of alfalfa (Medicago Stiva L.) was studied by laboratory incubation and pot experiment, and indigenous alfalfa rhizobia distribution, growth and symbiosis nitrogen fixation of alfalfa that grew in different purple soils were investigated with comparative experiment in the field. This paper dealt with stress resistance and adaptability of rhizobia-alfalfa symbiotic association, and offer reference for the bioremediation of contaminated soils, constructing the symbiotic association with high efficiency and resistance, and finding high efficient symbiotic association adapted to different purple soils and using the purple soil resource completely to develop the stockbreeding. The results are as follows:1 Effect of cadmium on seed germination and seedling growth of alfalfaThe germination rate of seeds had no obvious change when Cd²⁺ concentration was in the range of Omg·kg^-110mg·kg^-1 While Cd²⁺ concentration was 30mg·kg^-1, the germination of seed was evidently inhibited, and the germinating energy and germination rate were decreased significantly. After germinating, the stress resistance of the plant was lower than the seed, especially in treatments with low Cd²⁺ concentrations. Cd²⁺ with different concentrations all could inhibit the growth plumule and root significantly, and exhibited more inhibition on root than plumule, and affected the index of seed vigor significantly. Basically, with increase of Cd²⁺ concentration, its inhibition was enforced, and the index of seed vigor was decreased. The growth of seedling was almost inhibited completely while Cd²⁺ concentration was 50mg·kg^-1.2 Effect of lead on seed germination and seedling growth of alfalfaWhen Pb²⁺ concentration was in the range of 0mg·kg^{-1 1}00mg·kg^-1, effects of Pb²⁺ on seed germination of alfalfa had no significant difference, but seedling growth of all the treatments were obviously inhibited. While Pb²⁺ concentration was 250mg·kg^-1, it inhibited evidently seed germination and seedling growth of alfalfa. Otherwise, the results showed that the stress resistance of the plant was lower than the seed, and inhibition of Pb on radicle was more significant than plumule.3 Effect of cadmium on growth, nodulation and nitrogen fixation of alfalfaThere was little effect of cadmium on aboveground biomass, height, and chlorophyll content of alfalfa in treatments with low cadmium addition. After cadmium addition in soil was up to 3.0 mg·kg^-1, these indexes decreased with continuative increase of cadmium addition. The distribution of cadmium in alfalfa could be sequenced as: Root>aboveground part, and cadmium content of different parts of alfalfa and available cadmium content in the soil had a significant positive relation with cadmium addition in the soil. The weight of root nodule in treatments declined along with increase of cadmium addition in the soil, and nitrogenase activity in the treatment began to be restrained when 5.0 mg·kg^-1 of cadmium was added into the soil. When 3.0 mg·kg^-1 of cadmium was added into the soil, the growth of alfalfa was enhanced to a certain extent, and biomass and chlorophyll content were increased, and nitrogenase activity also got back.4 Effect of lead on growth, nodulation and nitrogen fixation of alfalfaEffect of lead with lower concentration on growth of alfalfa was not obvious. When lead addition was 1500mg·kg-1, lead could obviously inhibit growth of plant. With the increase of lead concentration, lead content of alfalfa increased continually, a majority of lead in plant was enriched in root, and lead content of different parts of alfalfa and available lead content in the soil had a significant positive relation with lead addition in the soil. Chlorophyll synthesis of leaf was inhibited in treatments with high concentration, and content of chlorophyll basically decreased with the increase of lead concentration in treatments. When lead addition in soil was the range of 0mg·kg^-1 100mg·kg^-1, nodulation and nodule growth of alfalfa were effected obviously. When lead addition was 250 mg·kg^-1, nodulation and nodule growth began to be inhibited obviously. Lead endurance of nodule nirogenase was low, and basic current was that restraint was enhanced with the increase of lead addition in soil.