Background Drought is a limiting factor to the low productivity and even death of Populus plantations in the three-north region of China. However, the eco-physiological responses and adaptation mechanism of Populus under drought and re-watering are still not very clear.

Methods Potted Populus simonii cuttings were used as materials and soil water contents were controlled by weighting. Three soil water levels were set: control (75%±5% of field capacity), moderate drought (50%±5%) and severe drought (25%±5%). After three months, moderate drought and severe drought treatments were re-watered to 75%±5% of field capacity. Under drought and re-watering conditions, changes of leaf photosynthesis and chlorophyll fluorescence parameters, leaf morphology and anatomy, organ biomass and non-structural carbohydrate (NSC) accumulation and distribution were investigated.

Results Under drought conditions, root biomass allocation and soluble sugar content significantly increased, which could help to promote root water absorption, while the leaves became smaller and the aboveground growth decreased to reduce water consumption. The increase of soluble sugar and total NSC reserves in stem may contribute to the improvement of stem embolization repair ability. Under moderate drought, chlorophyll content decreased and NPQ increased, which could help to protect the photosynthetic system. And light energy utilization was increased by thickening sponge tissue. However, under severe drought, G_s and T_r were reduced by stomatal closure to maintain water status but the expense of P_n and carbon fixation. After re-watering of severe drought treatment, the P_n significantly increased and showed positive compensatory effects, which may due to the higher chlorophyll content and palisade-spongy tissue ratio than the controls under severe drought.

Conculsions Populus simonii showed different adaption strategies to soil drought degrees by regulating organ carbon investment, leaf anatomical structure and chlorophyll content. Populus simonii showed shows obvious compensatory effects under re-watering after severe drought. Soluble sugar content in stem and roots increased under droughts, which could be helpful to embolization repair and water uptake.