Abstract: Roots consist of different orders. However, it is unclear whether their anatomical structures are consistent with their response to aridification of habitat. In this study, roots of Kobresia humilis were collected from six plots along a gradient of habitat aridification in an alpine meadow of the Dongda Mountains, southeast Tibet. The paraffin method, one-way ANOVA and principal component analysis were used to examine the difference of anatomical structures and the strength of plasticity for the first and second-order roots from habitats with different moistures. There was no difference of anatomical structures between the first and second-order roots. The epidermal cells were nearly rectangular and closely arranged, and area of single cell was large (87.6-126.0 μm²). The anatomical structures of the first and second-order roots were not consistent in response to habitat aridification. Area and thickness of the epidermal cell and cortex thickness (including thick-walled tissues and parenchyma) in the first-order roots had significant negative correlation with soil moisture, while those in the second-order roots had significant positive correlation (P<0.05). The ratios of epidermis to diameter of first-order roots were significantly different along the soil moisture gradient, while other anatomical structures had no significant difference. The ratios of anatomical structures (except cortical thick-walled tissues) of second-order roots were significantly different along soil moisture gradient. In the first-order roots, the thickness of cortical parenchyma had strongest plasticity, while the thickness of cortical thick-walled tissue had weakest plasticity; for the second-order roots, the strongest was transverse section area and the weakest was vascular cylinder area. Collectively, the anatomical structure of root epidermis and cortex had strong plasticity to the changes of soil moisture. However, anatomical structure of different root orders had different response mechanisms to aridification, showing an improved potential capability to store or absorb water. Therefore, root order method provides a new idea for understanding the adaptation of roots to changing habitats.

Key words: herbaceous plant, anatomical structure, root order, plasticity, water gradient