Objective  This research was carried out to study the effect of different soil water potentials on seedling growth rhythm, morphology and physiology of Populus tomentosa seedlings, so as to provide a scientific basis for precision irrigation strategy and parameters.

  Method  One-year-old Populus tomentosa seedlings were used as materials and treated with three water potential conditions (−20, −40, −60 kPa of soil water potential threshold) and conventional irrigation was used as control (initial irrigation threshold < −80 kPa), then Logistic equation was adopted to establish seedling growth model and divide the growth stage, and the changes of seedling growth rhythm, biomass allocation and nutrient accumulation under different soil water potentials were studied.

  Result  (1) Compared with CK, seedling height and ground diameter increased by 29.33% and 24.12% on average, while biomass was significantly increased by 176.17% under −20 kPa treatment, shoot to root ratio of seedlings under −20 kPa treatment was superior to −40 and −60 kPa treatments. (2) Under different soil water potential treatments, seedling height and ground diameter presented a curve changed like “S” and significantly fitted with Logistic equation, which can therefore be used to predict the seedling growth. Stem and root biomass were significantly correlated with seedling height and ground diameter, respectively, so seedling biomass can be predicted referring to seedling height and ground diameter. (3) The growth phase according to one-year-old P. tomentosa seedling height growth can be divided into 4 stages: establishment phase, from transplanting to 15 d after transplanting; early growth phase, 16 to 53 d; rapid growth phase, 54 to 138 d; and hardening phase: 139 to 220 d. (4) Nutrient concentration of P. tomentosa seedling shoot showed no significant trend, but root concentration showed a tendency of dropping with the increase of soil water potential. The highest stem and root nutrient concentration was found in CK, which was 15.14%−46.43% higher than other treatments; while nutrient content had the opposite pattern, that is, under −20 kPa treatment, the nutrient content was the highest, 100.08%−237.51% higher than CK. (5) P. tomentosa seedlings had the best seedling integrated evaluation results and seedling quality under −20 kPa treatment.

  Conclusion  Soil water potential has significant effects on seedling growth and biomass allocation of Populus tomentosa seedlings. The seedlings have the highest height, ground diameter, biomass and nutrient content when −20 kPa is chosen as irrigation threshold of 10 cm from the surface, the seedling height, ground diameter, biomass and nutrient mass per plant are the largest.