| Calcium is an important lake ecosystem nutrient. Information of Ca2+ uptake and exudation in aquatic macrophytes might play an important role in calcium transferring in water, which caused calcium redistribution in the system. Water and plants are the important mediums in which calcium performs its geochemical characteristics of lake ecosystem. Meanwhile, in the water during the formation of calcium carbonate, co-precipitation of calcium carbonate and phosphate occurs. As a result, the migration of calcium has a significant impact on phosphorate cycle.In order to study the effects of submerged macrophyte Potamogeton crispus on the phosphorus cycle in water while calcium existing, the experiments of Ca2+ uptake and exudation kinetics of P. crispus were done in two-container system, with different calcium concentration in root-containers. By comparing the parameters of Ca2+ uptake and release kinetics of P. crispus under different Ca2+ concentration, keeping on the study of the influence of Ca2+ concentration on Ca content in roots and leaves of P. crispus, studing the effects of low temperature on Ca transferring, we analyzed the migration rules of Ca in P. crispus. In addition, by keeping on the study of the characteristics of plasma membrane Ca2+-ATPase from P. crispus root and leaf, and the effects of Ca2+ concentration on activity of plasma membrane Ca2+-ATPase from P. crispus root and leaf, we studied the mechanism of Ca2+ uptake and exudation in P. crispus. The primary conclusions are as follows:(1) By studing the process of calcium transferring on the interefaces of P. crispus and solution, we found that when calcium concentration of the solution was 10 mg/L in the root chamber, P. crispus had the best absorption ability to Ca2+. The Ca uptake rate of P. crispus roots decreased as Ca2+ concentration of the solution went higher. For Ca2+ concentration of 15 mg/L treatment in root-container, the content of Ca exudation from the stems and leaves was lower than that of 10 mg/L treatment, while the content of Ca uptake from roots of 10 mg/L treatment was significantly less than that of 15 mg/L treatment.(2) The Ca2+ uptake kinetics of P. crispus was investigated by using the Lineweaver-Burke double reciprocal method (referred to as the LB method). At the same time, the parameters of Ca2+ uptake kinetics were fitted by LB method. The results showed that with the prolongation of absorption time, the maximum uptake rate (Vmax) overally decreased, and the corresponding Michaelis-Meten constant (Km) also gradually decreased. As a comprehensive reflection of the root absorption ability, a got highest in the beginning absorbing process, it means that Ca2+ uptake rate was highest. But a got lower as time went on, it means the root absorption ability gradually got worse.(3) The first order kinetics model (ln(qo-q)/qo=A-kt) can be used to fit the calcium release kinetics characteristics of the stems and leaves. Reaction rate constant k increased as calcium concentration increased in the root chamber when the concentration of Ca2+ was lower than 10 mg/L. When calcium concentration of the solution was 10 mg/L in the root chamber, P. crispus had the best exudation ability to the ion.(4) Calcium concentration in root-containers and culture time affected Ca2+ uptake rate in roots, Ca2+ exudation rate in the leaves and Ca content in the roots and leaves of P. crispus. During the experiment, the calcium content in roots which were in the nutrient solution with 10mg/L Ca was always higher than that of other three treatments (Omg/L, 5mg/L,15mg/L), meanwhile Ca2+ uptake rate from P. crispus roots of 10mg/L treatment is the highest. In the nutrient solution with no Ca, the calcium content in roots increased, which showed calcium in overground plants transport down to roots.(5) By several hydroponic experiments, characteristics of Ca2+ uptake were investigated in P. crispus. The results illustrate that when the Ca2+ concentration in nutrient solutions wan in the range of 0-10mg/L, the Ca2+ uptake rate enhanced with increasing Ca2+ concentration in the root-container. However, the Ca2+ uptake rate decreased when the Ca2+ concentration in nutrient solutions increased to 15mg/L. It suggested the Ca2+ uptake ability of P. crispus roots was not entirely dependent on Ca2+ concentration in nutrient solutions. LB model was used to simulate these absorbing processes. In this way, calcium uptake kinetics of P. crispus was simulated well and the difference of treatments was significant. The absorption vs. time basically followed the Michaelis-Menten model. Low temperature (10℃) treatment significantly inhibited Ca2+ uptake in P. crispus. These results suggest that Ca2+ uptake is an active process in plants of P. crispus.(6) The properties of plasma membrane Ca2+-ATPases from root and leaf of P. crispus were compared. The activity of root plasma membrane Ca2+-ATPase reached the maximum at pH 6.0 and at an optimum reaction temperature of 40℃, while the leaf enzyme activated in a broad range of pH at an optimum reaction temperature of 45℃. An optimum ATP concentration of root plasma membrane Ca2+-ATPase was 3mmol/L. In contrast, that of leaf plasma membrane Ca2+-ATPase was 4mmol/L. The root and the leaf plasma membrane Ca2+-ATPase were activated by Ca2+ in cells. (7) Application of CaCl2 in nutrient solution improved the Ca2+-ATPase activites of root and leaf plasma membrane of P. crispus. There was difference for the activity of root and leaf plasma membrane Ca+-ATPase in different Ca+ concentration. In addition, the activity of root and leaf plasma membrane Ca2+-ATPase varied at different times. From the same time point of view, when the Ca2+ concentration in nutrient solution was 10mg/L, Ca2+-ATPase activity of root and leaf plasma membrane reached the highest. |
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