Interclonal Differences In Phosphorus Efficiency Of Poplar And Its Mechanism

The genotype dependent on nutrient efficiency was found diffenent from specieces to specieces (even from variety to variety), and could be compared by the growth responses to specific nutrient stress. As one of the most stressed nutrient in infertile soils, phosphorus is the critical factor to tree's growth. The exploring of high phosphorus efficiency tree varieties for timber production would help to avoid the problems associated with fertilization such as energy consumption , environmental pollution.The differences of phosphorus efficiency among hybrid poplar clones of S17 (Popidus deltoides×P. cathayana), S19(Popnlus deltoides×P. cathayana), 105 (Poptdus deltoides×P. simonii), 106 (Populus deltoides × P. simonii) , I-69 (Populus deltoides Bartr. cv. Lux), 797(Populus deltoides×P. euramerieana) , 1388(Populus deltoides×P. euramerieana), and 3244(Populus deltoides) and its mechanism were studied by the methods of soil culture and solution culture of seedlings in this study.As showed by the above —ground biomass defferences, the phosphorus effi-ciences varied among the treated poplar clones under P deficiency stress. Clone S17, S19 and 105 were efficient phosphorus absorbers because of their large biomasses, 55. 15,48. 28, and 49. 25 g/seedling, respectively; in soil culture tests, and comparative small biomass decrements (less than 30%) in minus P treatments. They were ranked in Grade A of high phosphorus efficiency. Similarly, the phosphorus efficienies of clone 106, 797, and 1-69 were considered as Grade C, as associated with their quite lower biomasses, of 35. 45, 31. 45, and 28. 35g/seedling, respectively. The biomasses of clone 1388 and 3244 were the lowest, only 23.05 and 23. 94g/seeding, therefore, the most inefficient P absorbers of Grade D. The results of solution culture tests were in good conformity with that in soil culture treatments , as the further proof to confirm the efficiency patterns. None of the poplar clones was found in Grade B.One of the reasons for interclonal differences in phosphorus efficiency was the intensity of rhizosphere acidification which is generally accepted as an index of phosphorus activation ability of plant in P deficiency stress. High phosphorus efficiency clones, such as S17, S19 and 105, could strongly decrease their pH values in rhizosphere, the most decrement of pH among them even accounted to 1. 32 pH u-nits. The ratioes of the decrements were over 10% in comparison with the pH values in non—rhizosphere, indicating great abilities to acidify thier rhizosphere environments. Whereas low phosphorous efficiency clones, such as 106, 797, I -69, 1388 and 3244, the pH values in rhizosphere were decreased slightly, all decrements were less than 0. 21 pH units and 2. 5% of the decrease ratio. It was believed that high phosphorous efficiency clones reacted by acidificatioa with P deficiency in a feedback compensation pattern, that was they enforced the acidification with the intensity of P deficiency in environment. This specific mechanism was observed in high phosphorous efficiency clones because the roots increased the induced excretion of H+ when in the condition of P deficiency.As a consequence of rhizosphere acidification, the P availabilities were apparently higher in the rhizopheres of high P efficiency clones than that of low P efficiency clones when they were in P deficiency stressed. In minus P treatments, the amounts of available P in the rhizospheres of clone S17, S19, and 105 reached 2. 64, 3. 27, and 3. 28mg·kg-1, respectively, obviously larger than those of the other five low phosphorous clones, which were all below 2. 00mg·kg-1. The soil used in this study was plentiful of Ca-P, approximately account for 50% of the total P, it seemed that Ca-P in soil was easy to be activated by H+ excreted from roots, also the effects of P activation by the root excretive chelating agents was not excluded. The compliance of larger accumulation of available P with the intensive acidification in the rhizosphere of high phosphorous efficiency clones strongly support the hypothesis that induced excretion of H+ was the possible path of the plants to improve the phosphorus nutritient supply.Regression analysis showed that the increment of available P in rhizosphere was closely correlated with the decrement of pH valve in rhizosphere in P deficiency stress, reflecting the effect of rhizosphere acidification on availability of P in rhizosphere.The interclonal differences in phosphorus efficiency were also found to be wellcorrelated with the root/shoot ratioes. Soil culture test showed that the root/shoot ratioes of high phosphorus efficiency clones, within a range of 0. 31 ~0. 32, were larger than that of low ones, within a range of 0. 24~0. 27. In comparison with the Plus P treatments, the increments of root/shoot ratioes were more than 20% for high phosphorus efficien clones, but less than 4 % for low phosphorous efficiency clones in minus P treatments. The results in solution culture were basically consistent with those in soil culture except for clone 1-69. This indicated that high phosphorus efficiency clones could actively regulate their root developments and thus acquire adequate P to meet the growth demands when P deficiency took place. Low phosphorus efficiency clones possessed little abilites in improving root developments and grew worse because of insufficient P supply. The interclonal differences of P uptake amounts by seedlings admited the conclusion in another way.Another important mechanism for the interclonal differences in phosphorus efficiency was that clones were aparently different in kinetic characteristics of root absorption to H2PO4- discribed by kinetic parameters of Vmax, the maximum rate of influx; Km, the Michaelis constant, which is the concentration where influx is one — half of Vmax; Cmin, the critical concentration, the ion concentration in solution below which net influx of the ion ceases. The values of Km and Cmin of clone S17, S19 and 105 were much smaller than those of the other five clones, indicating that high phosphorus efficiency clones were much stronger than low ones in the affinity of root system to H2PO4- and in tolerating P deficiency. In comparison with in P sufficiency; the values of Km were decreased from 13. 8, 13. 4 and 14. 0μM to 10. 1, 10. 7, and 11. 2μM,and the affinities (1/km) of clone S17, S19, and 105 were increased over 20% ,in the environment of P deficienty stress, respectively, but the affinities of low ones were increased slightly, below 5%. As showed by the indexes of Cmin, the tolerance to P deficiency raised more than 85% for high phosphorus efficiency clones but less than 15% for low ones. The values of Vmax of high phosphorus efficiency clones were not larger than those of low ones, some ever smaller. This couclusion was inconsistent with some other reports that high phosphorus efficiency plants should have a larger Vmax, indicating that further studies were needed to explain the difference of phosphorus efficiency by the index of Vmax.Besides the absorption efficiencies, the interclonal differences in phosphorus efficiency were also probably controlled by the efficiency of phosphorus utilization,which is regarded as the reciprocal of P concentration in plants. Both in soil culture and solution culture, it was observed that efficiencies of phosphorus utilization in high phosphorus efficiency clones were larger than those in low phosphorus efficiency clones when facing P deficiency stress. The adapting abilities to P deficiency stress,showed by the ratio of efficiency of phosphorus utilization in minus P treatment to that is plus treatment, were inereased over 60% for high phosphorus efficiency clones, but below 20% for low ones in the soil cuture.The former were more than 510. 0kg/kg, and the adapting abilities were increased over 60%; but in low phosphorous efficiency clones, most of them were less than 400. 0kg/kg and below 20% , rerpectively.Conclusively, this study revealed that the phosphorus nutritional efficiencies of different poplar clones in deficient stress were associated with the absorption efficiencies on one side, such as the induced acidification for P activation, the stimulated root increments for larger absorptoin surfaces, and strenghtened affinities of P ions in environmerts. On the other side, they also recreated to the interior cycling efficiencies, e. g. the plant would enhance the utilization efficiencies when deficient stressed. Clone S17,clone S19 and Clone 105 demonstrated their high adapting abilities on the both sides and were recomended as the potential poplar clones for the plantations on phosphorus deficient soils.