| Soil alkalization, caused by Na2CO3 and NaHCO3 based alkaline, is currently difficult to cope with in worldwide. Alkaline soil, due to its high pH characteristics, affects the absorption of the plants on the soil mineral elements. Phosphorus is an essential nutrient for plant growth, development and reproduction. However, in alkaline soil, its plant utilization efficiency is greatly reduced.Alfalfa is a world widely grown perennial legume, and also the ideal material for alkaline soil modification. In this paper, two alfalfa varieties (Medicago sativacv. Zhongmu No.1 and Medicago sativacv.Sanditi) were selected as materials to research the physiological regulation mechanisms of phosphorus efficiency and carbon metabolism under low phosphorus and alkaline conditions. In the experiment, low phosphorus (100 μmol/L KH2PO4) stress and alkali stress (0, 30,60,90 mmol/L NaHCO3) were performed, and the phosphorus mobilization mechanisms in alfalfa under the stress conditions were ultimately revealled.The results are as follows:Alfalfa showed a certain tolerance to phosphorus deficiency and alkaline stress with concentration lower than 90mmol/L. Under phosphorus deficiency, the parameters of the whole plant dry weight, proline, malondialdehyde and root activity in both Zhongmu No.1 and Sanditi showed little difference between treatment and control.Under phosphorus deficiency plus alkaline stress,30-P treatment did not cause significant changes in relevant indicators; 60-P,90-P treatment significantly increased the contents of proline and malondialdehyde, and these two indicators of Sanditi were much higher than that in Zhongmu No.1 in 90-P-treatment. Low phosphorus plus appropriate concentration of alkali treatment (30,60 mmol/L NaHCO3) facilitated phosphorus absorption of alfalfa from the external environment, while 90 mmol/L NaHCO3 was harmful to plants and plant phosphorus content decreased. There was great difference in the phosphorus metabolism between Zhongmu No.1 and Sanditi. Parameters of root to shoot ratio, root activity, phosphorus content of the whole plant, root morphology, content of organic acid in root, and rhizosphere acidification in Zhongmu No.1 were consistant higher and more obvious than that in Sanditi, which indicating Zhongmu No.1 was a phosphorus efficient type alfalfa.On one hand, high phosphorus efficiency existed in changing of plant morphology. Under phosphorus deficiency, two species were consistent with the performance of root elongation and root surface area increase. Changes in root morphology were conducive to plant phosphorus uptake. On the other hand, High P uptake efficiency mechanism might also exist in the change of plant organic acid metabolism. The highest content of total organic acids were found in 60-P treatment in both of Zhongmu No.1 and Sanditi, however, with the increas of alkaline concentration, the content of total organic acid decreased. Under low phosphorus and saline stress, malic acid change did the greatest contribution to total organic acids change, while other organic acids concentration changed a little; Sanditi, malic acid contributed to the increase of total organic acids, citric acid, oxalic acid content also increased significantly as well. This may be induced by the different secretion amount and type of organic acids to environment, and further experiment was needed to improve the Hypothesis.Under phosphorus deficiency, the activity of Acid phosphatase increased both in Zhongmu No.l and Sanditi. Trends were not consistant under phosphorus deficiency plus alkaline stress, however, they were consistant with trends of phosphuros content of whole plant. The results indicated that there was a liner relationship between the activity of acid phosphatase and the phosphuros content of whole plant. |
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