The Research On Mechanism Of Pinus Yunnanensis Franch. In Central Yunnan Province In A Low Phosphorus Soil Environment

Phosphorus is an important element controlling the process of plant life, especially during its growth phase. Since it was found that the P availability in rhizosphere of crop adaptive to low phosphorus environment increased significantly in the 1970s, the research on relations between plant and its low-phosphorus environment has been attached importance, and how plant adapts to low-phosphorus environment and makes use of soil phosphorus effectively has become a hot research field throughout the world nowadays. Many research results have indicated that low-phosphorus in the soil usually leads to adaptive changes to root, root/shoot ratio, organic acid exudation and phosphatase release in plant rhizosphere. Different species or varieties have different P-efficiency genotype and different P-activation mechanism. It is realistically significant for economy and environmental protection to unveil the countermeasure of plant response to low-phosphorus environment and to explore the plant potential to make use of phosphorus in soil effectively.Yunnan Plateau, being its origin and distribution range, Pinus yunnanensis Franch., which is abundant in genetic resources and various in geographical provenance, reflects great capability of adapting to low-phosphorus red soil. Therefore, study on such mechanism of its adaptation towards low-phosphorus becomes an important reason to understand basic resource utilization.Present study was to analyze the response of Pinus yunnanensis Franch. seedlings under various phosphorus supplies and Pinus yunnanensis Franch. forests in Xiushan Forest Garden, Tonghai county, Yunnan Province to low phosphorus. The research results were as follows:l) The testing results on total P, organic P, inorganic P and available P in rhizosphere and non-rhizosphere soil of Pinus yunnanensis Franch. forest showed:Pinus yunnanensis had strong tolerance to low soil P; The pH value of rhizosphere was significantly lower than that of non-rhizosphere. The difference of total P and organic P between rhizosphere and non-rhizosphere of Pinus yunnanensis forest was not significant, but that of available-P was significant. Averagely, available P in rhizosphere soil of Pinus yunnanensis was more than twice of that in non-rhizosphere, and this difference was higher than that of some other plants, which indicated that the capability of Pinus yunnanensis to activate phosphorus at rhizosphere was much higher than that of some other plants. Although inorganic P in both rhizosphere and non-rhizosphere soil of Pinus yunnanensis accounted for 70-90% of total P and organic P only 18% averagely, positive correlation was found between available P and organic P, which was different from some other plants.2) The pH in rhizosphere and non- rhizosphere of Pinus yunnanensis Franch. forest with different ages and that in rhizosphere of Pinus yunnanensis Franch. seedlings under different P supplies at the laboratory had been studied to unveil their change discipline. The results showed that the pH in both rhizosphere and non-rhizosphere of Pinus yunnanensis Franch. forest was very low, ranking from 3.49 to 3.79. The pH in rhizosphere of Pinus yunnanensis Franch. was significantly lower than that in non-rhizosphere and their differences were very obvious, but not obvious between different age groups of Pinus yunnanensis Franch. forest. The test on pH in rhizosphere of Pinus yunnanensis seedlings under different P supplies indicated that the roots of Pinus yunnanensis seedlings could decrease the pH value of root surrounding and the ability of Pinus yunnanensis seedlings to decrease pH was strengthened while P supply decreased. Different from that on some other plants, the test results on Pinus yunnanensis showed that acidification in rhizosphere was one of active responses of Pinus yunnanensis Franch. to low-P environment.3) A study was carried out on soil phosphatase and available P of the rhizosphere and non-rhizosphere of Pinus yunnanensis with different ages and of different distance from P. yunnanensis seedlings under different P supplies at the laboratory. Results showed that large quantity of phosphatase distributed at the rhizosphere and non-rhizosphere of P. yunnanensis, among which acid phosphatase was the major one and followed by alkaline phosphatase. The activities of either acid, or medium or alkaline phosphatase were higher in rhizosphere than those in non-rhizosphere. Result from the cultivation of seedlings showed that, anywhere of the distance from the seedling roots, the less the supply of P, the higher the amount of acid and alkaline phosphatase, which implied that phosphatase can be induced by P starvation. The relationship between changes of phosphatase and changes of available P was not significant. So the increase of phosphatase was not a unique adoption mechanisms of P. yunnanensis under P starvation, it may be a symptom only.4) Positive correlation was found between available P and total P, organic P, pH and alkaline phosphatase concentration, which suggested that the activation of Pinus yunnanensis forest was resulted from multiple factors. Difference existed in the capability to activate phosphorus in rhizosphere among different age groups.5) The study analyzed root response of seedlings(Pinus yunnanensis Franch.) to various phosphorus levels looking at the root morphological characters, like biomass and root/shoot ratio. Results showed that both the length of taproots and the root/shoot ratio increased as phosphorus supplies decreased to 0.5mmol/L. However, no particular change happened on the number of lateral roots and the biomass, and the root biomass remained at a relatively stable level. Further analysis made it clear that Pinus yunnanensis Franch. seedlings ensured the priority of dry matter allocation to root under phosphorus deficiency. Conclusively, such an adaptation by the seedlings does not depend on its number of lateral roots, rather on the increment of taproot's length.6) The test on Pinus yunnanensis'seedlings under phosphorus deficiency showed that there were significant differences in total biomass, root biomass and plant height between treatments of Pinus yunnanensis' seedlings under different phosphorus supplies. As phosphorus supplies decreased, total biomass, root biomass, plant height as well as the number of bud of Pinus yunnanensis'seedlings decreased. A significant positive correlation and a linear regression were borne between total biomass and shoot biomass. The total biomass decreased in accordance with the phosphorus supplies mainly coming from shoot biomass. The study showed that no obvious decrease happened on root biomass as phosphorus supplies decreased, and so the root biomass maintained at a relatively higher level, and Pinus yunnanensis seedlings under phosphorus deficiency ensured their livelihood by increasing root/shoot ratio and maintaining root biomass at a relatively higher level at a cost of decreasing shoot biomass. This also made it clear that there may be a critical content of phosphorus around or from 0.00781 to 0.03125mmol·L-1 for Pinus yunnanensis seedlings to tolerate phosphorus deficiency.7) The result showed that there were oxalic acid, tartaric acid and malic acid among the secretion of Pinus yunnanensis root. However, organic acid exudation was small and the speed of secreting was low. These three organic acids were not mainly responsible for the change of pH. Positive correlation was found between the content of oxalic acid and malic acid in root and the concentration of phosphorus supplies, and negative correlation was found between the content of dehydromalic acid and the concentration of phosphorus supplies.8) The effect of different phosphorus supplies on the content of phosphorus in seedling of Pinus yunnanensis was evaluated. The results showed that the total content of phosphorus significantly increased with the increment of phosphorus supplies. By its own mechanism, Pinus yunnanensis seedling could adapt to different phosphorous level to absorb and utilize phosphorous. It ensured the need of phosphorus of shoot through reducing phosphorus in root as possible as it could. The PTPC (Physiological threshold P content) of Pinus yunnanensis seedling was very low,0.67～1.1 mg.g-19) The effect of different phosphate supplies on seedling of Pinus yunnanensis was investigated. The results showed that no obvious changing rule existed in plant height, taproot length, shoot biomass and total biomass under different phosphate supplies. It was easier to absorb KH2PO4 than other phosphates. Under whatever phosphate supplies, shoot of Pinus yunnanensis seedling contained more phosphorus than root.