Genetic Variation In Root Architecture And Phosphorus Efficiency In Response To Heterogeneous Phosphorus Deficiency In Pinus Massoniana

Masson pine (Pinus massoniana) is a native evergreen conifer with high timber yield andexcellent timber quality. It is widely distributed in the southern part of China, and the area ofmasson pine plantations has expanded rapidly with an estimated area of5.7million hectares.Masson pine plantations are distributed mainly in the tropical and subtropical area in China,where the red soils is widely distributed and the available P can easily be bound by Ca, Al, andFe through chemical precipitation or physical adsorption. The limited availability of P in forestsoils is thus one of the most important factors causing a decline in productivity of masson pineplantations. Genetic improvement on phosphorus efficiency of masson pine has been focused.However, most of reported researches were conducted on the assumption that verticallydistribution of P is homogeneous. The adaptive mechanism in response to verticallyheterogeneous low P condition has not been regarded. Moreover, the influence of environmentfactors such as light conditions and interspecific competition on phosphorus efficiency ofmasson pine has not been studied. Therefore, we design pot experiments to disclose theadaptive mechanisms and P efficiency of masson pine in response to different types of low Pconditions. The objectives of this study were to provide insight for selecting and breeding offavorite genotypes with higher P efficiency. Through our researches, we have demonstratedthat:1. Large genotypic variations in P efficiency were observed under different types oflow-P conditions. The adaptive mechanism for P deficiency was different under heterogeneousand homogeneous low-P conditions. Under homogeneous low-P conditions, adaptive responsesof root growth parameters were present in genotypes with high-P efficiency such as '3201' and'1217', while it was not observed in genotypes with low-P efficiency. Under heterogeneouslow-P conditions, root architecture was found to closely related to P efficiency. Genotypes withshallow root architecture had optimal root parameters including root length, surface area andbiomass in the top layer of soil, thus having the greater ability for P absorption and having higher P efficiency and biomass. The heritability for the root biomass and the proportion ofroot in the top soil layer was0.88and0.72, respectively. A significant interac-tive effectbetween patterns of low-P conditions and P efficiency was observed. Given that P. massonianahas great genetic potential for adaptation to low-P soils, the selection of high-P efficiencygenotypes with optimal root architecture may significantly increase wood production of P.massoniana under low-P conditions.2. Four provenances of Pinus massoniana were used to investigate the genotypicvariations and adaptive mechanism in response to heterogeneous and homogeneous lowphosphorus (P) depress. Large variations in seedling height,ground diameter and dry matteraccumulation among provenances of Pinus massoniana were demonstrated in both types oflow P depress (p<0.01). Guangdong Xinyi and Fujian Wuping provenances exhibited higher Pefficiency and dry matter accumulation in heterogeneous low phosphorus treatment. The higheradaptable ability of root architecture and increased root parameters in the top layer of soil,which leading to higher phosphorus absorption efficiency, should be responsible for higher Pefficiency and larger dry matter accumulation. Under heterogeneous low P depress, significantpositive correlation between root architecture and dry matter accumulation were detected withcorrelation coefficient>0.95. Under homogeneous low P deress, there were no significantcorrelation between root parametres and P efficiency,whereas in common or in top layer.Difference in the adaptive mechanism for heteroneneous and homogeneous low P efficiencywere demonstrated, implying different strategies should be considered in the slection ofhigh-P-efficiency varieties for different forest stands.3. Pot experiment comprising treatments of high level and low level of phosphorus wasconducted using9open-pollinated families randomly selected from Guangdong provenece.Most of the tested families exhibit relatively high PE and better growth performance incomparison with open-pollinated families that are randomly selected from the first-generationseed orchard. Moreover, significant genetic variations in growth traits and PE are detectedamong the nine tested families, indicating the potential for selecting families with better growth performance and higher PE from provenance with high PE. Familiesof'185,''337','33'and'67'exhibit better growth performances and higher PEs than those ofother families under the low P condition. Families with higher PE have greater root parametersthan those of the families with lower PE, and significant correlations with correlationcoefficients of0.700-0.999are found between PE and root parameters. These results indicatethat the genetic variations in root parameters should responsible for the family variations in PEand biomass accumulation. The results of the retrospective analysis indicate that there arepositive correlations between growth traits in1-year pot experiment and growth traits in5-year-age progeny trials. Based on the above results, the pot experiment can be used as aneffective approach for early selection for genotypes with high phosphorus efficiency andfavorite growth performance.4. Large genotypic variations in P efficiency were observed among differentthird-generation full-sib families under different types of low-P conditions. The adaptiveexudation of APase activity in response to P deficiency was observed under both heterogeneousand homogeneous low-P conditions. Under homogeneous low-P conditions, larger degree ofAPase activity was detected as compared with heterogeneous low P conditions, indicating largecontribution of root APase activity to P efficiency. There were significant interaction effect ofN*P treatment. Under homogeneous low-P conditions, root growth traits were not sensitive toN supply. In comparison, under heterogeneous low-P conditions, sensitive growth response ofroot to N supply was observed, as major root parameters were enhanced with N supply. As aresult, major growth traits as well as P efficiency components were elevated by N treatment.Given that N concentration would be largely increased by nitrogen deposition from air, theselection of high-P efficiency genotypes with large sensitivity to rhizosphere N maysignificantly increase wood production of P. massoniana under low-P conditions. Most of thetested third-generation full-sib families exhibit relatively high PE and better growthperformance in comparison with the first-generation of open-pollinated families, indicating thegrowth potential and productivity of improved genetic materials of masson pine was large. 5. Under heterogeneous low P conditions, P.massoniana exhibited larger P efficiencyand better growth performance as compared with S superb. Further analysis revealed that thelarger proportion of root allocated in the top soil layer with high P availability should beresponsible for the larger P efficiency of P.massoniana. The influence of interspecificcompetition on growth traits and P efficiency of P.massoniana was related to P level. Underheterogeneous low P conditions, P.massoniana exhibited large competition ability, as it havelarger P efficiency and better growth performance as compared with S superb. The largerproportion of root in the top soil layer and greater P acquisition ability should be responsiblefor the larger competition ability of P.massoniana. In contrast, under homogeneous low Pconditions, S superb exhibited larger competition ability and larger P efficiency thanP.massoniana, owing to its larger degree of root proliferation under homogeneous low Pconditions. Light conditions had significant effect on growth and development of P.massoniana,whereas the influence on S superb was relatively small. Under shading conditions, growthtraits, dry matter accumulation, phosphorus uptake efficiency and root parameters ofP.massoniana was significantly decreased. In contrastion, the influence of light contions ongrowth traits and P efficiency of S superb was not large.