Abstract: [Objective] In this study, effects of different phosphorus levels on growth and root morphological changes of the variety ‘Guinong 5’ seedlings of Rosa roxbunghii Tratt., as well as their interactions with endogenous hormones were studied in order to understand the adaptation strategies of this species under low phosphorus stress. The study would provide scientific basis for phosphorus nutrient management and growth regulation of R. roxbunghii on low phosphorus soil of Karst area.[Method] Five different available phosphorus levels (5,25,45,65 and 85 mg·L^-1) were applied to the culture matrix for growing the seedlings. The growth and root morphological indexes and the endogenous hormones contents in leaves and roots were were determined. The relationship between root morphology and endogenous hormone contents at different phosphorus supplying levels was analyzed.[Result] The seedlings treated with 45 mg·L^-1 phosphorus performed best compared with other treatments, with the fastest growth rate, the greatest biomass of whole plant, the longest main root length, the greatest amount of total root tips, total root length, total volume, total surface area, average diameters, lateral root length and number, and density of first order lateral root. The content of GA₁₊₃ in roots and leaves of seedlings treated with 45 mg·L^-1 phosphorus was the highest and the content of ABA was the lowest. The plant growth was inhibited when the supplying level of phosphorus was lower or more than 45 mg·L^-1, with a significantly reduced biomass, a decreased content of GA₁₊₃ and an increase of ABA as well as decreases in indexes of root morphology mentioned above. The root-shoot ratio, the density of secondary and tertiary lateral roots and the root hair density and length of R. roxbunghii seedlings reached the maximum at 5 mg·L^-1 phosphorus level. The contents of IAA and ZR in the roots and leaves of R. roxbunghii increased with the decrease of phosphorus level. A significant quadratic nonlinear regression relationship was found between the growth indexes, biomass of the plant at different phosphorus supply levels and the contents of IAA, ZR in roots and leaves, and a significant positive correlation and negative correlation with the content of GA₁₊₃ and ABA, respectively. The inhibitory effects of low or high phosphorus supply on plant growth were the results of synergistic effects of changes in IAA, ZR, GA₁₊₃, and ABA content. The inhibition of root growth at low phosphorus level was related to the increase of IAA, ZR and ABA contents and the decrease of GA₁₊₃ in leaves and roots. High concentration of GA₁₊₃ and low concentration of ABA promoted the elongation of main roots and the formation of lateral roots, and high concentration of IAA and ZR promoted the formation, elongation of root hairs and the density of root hairs.[Conclusion] Seedlings treated with 45 mg·L^-1 phosphorus performed best according to growth and root development, in main root length, total amount of root tips, total root length, total volume, total surface area, mean diameters, length and number of lateral roots, the density of first order lateral roots. The plant growth was inhibited when the phosphorus levels were lower or higher than 45 mg·L^-1, and the root morphological indexes mentioned above were decreased. The root-shoot ratio of Rosa roxbunghii seedlings increased, and the density of secondary and tertiary lateral root, root hair and length increased significantly under low phosphorus stress. The content of IAA, ZR, GA₁₊₃, and ABA in leaves and roots of the plant changed due to the changes of phosphorus supplying levels, which co-regulated the growth and root morphology of R. roxbunghii seedlings. The increase of root-shoot ratio, root density, root hair density and the length of second and third order lateral roots was the adaptation strategy of R. roxbunghii seedlings to low phosphorus stress, in which high level of ZR and IAA in roots and leaves played an important promoting and regulating role.

Key words: phosphorus, Rosa roxburghii, seedling, growth, root morphology, endogenous hormone