Adaptability response of root architecture of Cotinus coggygria seedlings to soil nutrient stress

ObjectiveThe study was conducted to explore the adaptability strategy of root architecture of Cotinus coggygria seedlings to soil nutrient stress environment.

MethodTaking one-year-old C. coggygria seedlings as research object and carrying out pot-culturing simulation control test, five soil nutrient gradients were set: nutrient-rich condition (sand proportion 0, CK), slight nutrient stress condition (sand proportion 30%, N₁), medium nutrient stress condition (sand proportion 50%, N₂), severe nutrient stress condition (sand proportion 70%, N₃) and extreme nutrient stress condition (sand proportion 100%, N₄). Seedlings were sampled at 31th, 38th, 45th, 52th and 59th day after transplanting to explore the changing rules of branching pattern and geometric characteristics of seedling root, fine root morphology at varied diameter classes under the above nutrient conditions.

Result(1) Topological indices (TI or DBI) under CK were higher than the other treatments, but the number of root longest link path (a), the number of root external link (μ) and the total number of all root link paths (P_e) were lowest. Seedlings in the N₁ treatment showed the lowest TI and DBI while the highest a, μ and P_e , which were higher than those in CK by 53.2%, 131.6% and 194.7%, respectively. (2) In the N₁, N₂, N₃ and N₄ treatments, specific root length (SRL), specific root surface area (SRA) and root branching index (RBI) increased gradually with the nutrient stress degree rising. Among which, SRL, SRA and SRA of seedlings under N₄ treatment were higher than those in CK by 67.7%−157.4%, 52.3%−120.7% and 14.7%−42.1%, respectively.The number of root tip (RT), the number of root link (RLN), root link total length (RLTL) and the branching number of root (RF) decreased with the nutrient stress degree rising. All these four parameters in N₁ treatment were highest, and higher than those in CK by 95.0%−279.6%, 104.3%−247.4%, 77.4%−193.5% and 102.6%−235.0%, respectively. Root link average length (RLAL), root link average diameter (RLAD) and root tissue density (RTID) had a reducing trend with the nutrient stress degree rising, and RLAL, RLAD and RTID in N₄ were lower than those in CK by 15.2%−22.7%, 9.3%−21.4% and 32.4%−42.7%, respectively. (3) The positive correlation coefficient (0.951) between SRL and SRA was highest under CK environment, and the positive correlation coefficients between RF and RLN (0.989) and between RLAL and RLAD (0.904) were highest under N₄ environment. The negative correlation coefficients under N₁, N₂ and N₃ treatments were − 0.915, − 0.889 and − 0.893, respectively. (4) The average length proportion, average surface area proportion, average volume proportion and the average tip number proportion of fine roots in 0−0.50 mm were all higher than the other treatments, and they were 86.3% and 86.1%, 67.6% and 66.7%, 40.2% and 38.1%, 98.6% and 98.4% in N3 and N4, respectively. While these parameters under CK were lowest, which were 80.9%, 59.4%, 32.2% and 97.2%, respectively.

Conclusion(1) The root system of C. coggygria in soil nutrient-rich environment is closest to the herringbone pattern with few branches and a relatively simple structure. It also has an extending tendency towards deeper layers of soil. In contrast, C. coggygria develops increased branches and a high degree of secondary root overlap in slight nutrient stress soil. Under medium, severe and extreme nutrient stress conditions, the relatively simple root structure strategy is adopted, and the root system strengthens the ability of local nutrient utilization by forming short, thin and dense lateral branches (mainly fine roots). (2) The adjustment of coordination and trade-off relationships between geometric characteristic parameters of root architecture for economizing soil resource utilization is also an important aspect for C. coggygria to cope with different soil nutrient environments. (3) The differentiation of fine roots in 0−0.50 mm is markedly enhanced from soil nutrient-rich to extreme nutrient stress environment. Fine roots in 0−0.50 mm diameter are the significantly active organs for C. coggygria to absorb nutrients in severe stress environment. When the soil is extremely poor, a certain number of fine roots of 0.50−2.00 mm diameter are promoted production to reduce the carbon consumption caused by fine root turnover, and then to maintain the root resource utilization efficiency.