The Growth and Non-Structural Carbohydrate Response Patterns of Siberian Elm (Ulmus pumila) under Salt Stress with Different Intensities and Durations

(1) Background: Soil salinity is one of the major abiotic stresses that limits plant growth and production. However, the response patterns of plant growth and carbon metabolism to salt stress are still unclear. (2) Methods: We measured the relative growth rate, non-structural carbohydrate (NSC) concentrations and pool size across organs, the leaf mass area (LMA), root-to-shoot ratio, midday leaf water potential (Ψmd), and photosynthetic characteristics of elm seedlings planted in the field under different salt stress intensities and durations. (3) Results: Salt stress can reduce the photosynthesis rate, stomatal conductance, and Ψmd and inhibit the growth of elm species. LMA increased with the degree and duration of salt stress, indicating an increase in leaf carbon investment to resist salt stress. The root-to-shoot ratio decreased under salt stress to reduce salt absorption by the roots. In the early stage of stress, the concentrations of starch and total NSCs in all organs increased to improve stress resistance and the survival of plants. In the late stage of stress, the concentration of NSCs in the root decreased, which could restrict root growth and water uptake. The relationships between NSC concentration and growth in different organs were contrasting. Meanwhile, the pool size of NSCs had a more significant impact on growth than their concentration. Moreover, the pool size of NSCs in below-ground organs is more closely related to growth than that of above-ground organs. (4) Conclusions: Our research elucidates the carbon allocation mechanism across organs under different salt stress intensities and durations, providing theoretical support for understanding the relationship between tree growth and carbon storage under salt stress.