(1) Tree peony (Paeonia suffruticosa Andrews) is a woody ornamental plant originating from China, and beloved by people worldwide. Non-structural carbohydrates (NSCs) play a crucial role in regulating the flowering quality of tree peonies in both field and potted conditions. However, the effects of NSCs accumulation and allocation in various organs during the vegetative growth stage on the flowering quality of tree peony under forcing culture remains unclear. (2) Two-year-old grafted seedlings of tree peony cv. ‘Luoyanghong’ were subjected to orthogonal treatments to investigate the role of NSCs accumulation in plants’ developmental process. We measured leaf photosynthetic capacity, NSCs accumulation in the organs of seedlings, observed key ornamental characteristics of flowering quality under forcing culture conditions, and evaluated the qualities of seedlings and flowers using the seedling index (SI) and flowering index (FI), respectively. (3) There was a significant positive correlation between leaf photosynthetic capacity and NSCs accumulation in both the whole plant and roots of potted tree peony. Roots were identified as the primary organs for NSCs accumulation in potted tree peonies. Sufficient NSCs accumulation in the plant, particularly in the roots during the defoliation period, was essential not only for enhancing the seedling quality of potted tree peonies but also for improving the flowering quality under forcing culture conditions. Both the seedling index (SI) and flowering index (FI) exhibited a significant dose-response with increasing root NSCs accumulation at defoliation. The T3 group, which involved slight root pruning (by 25%), combined with a high-concentration rooting agent (750 mg·L⁻¹) and Metarhizium anisopliae (20 million U·mL⁻¹), resulted in the highest photosynthetic capacity, SI, FI and NSCs accumulation status (NSCAR), making it the optimal treatment combination. (4) This finding indicates that increasing NSCs accumulation in the roots of potted tree peonies is a crucial biological foundation for producing high-quality potted flowers under forcing culture conditions, which provide new insights into the importance of NSCs in tree peony flowering and may improve the production technology for high-quality potted tree peony flowers under forcing culture conditions.

Potted cultivation serves as a vital strategy for industrialized production of standardized tree peonies, engineering seedlings capable of year-round and off-site transplantation. However, the limited root zone in potted conditions restricts root development, resulting in suboptimal seedling quality and hindering commercial-scale production. This study aimed to investigate the relationship between the accumulation characteristics of non-structural carbohydrates (NSCs) and growth performance in potted tree peonies, while also optimizing the transplantation technologies for potted cultivation. Using two-year-old grafted seedlings of ‘Luoyanghong’ as experimental material, the effects of root pruning, rooting agent, and Metarhizium anisopliae application on morphological development and NSCs accumulation in potted tree peony seedlings were investigated. The results showed that old roots serve as the primary storage organs for NSCs in the potted tree peony. Slight root pruning (25%) was beneficial for fibrous root growth, whereas excessive root pruning (50%) resulted in reduced biomass and NSCs accumulation. The application of a high concentration of rooting agents effectively promoted root growth and mitigated the adverse effects of root pruning. Furthermore, Metarhizium anisopliae significantly increased the stem number in potted tree peonies. The optimal protocol identified through range analysis involved 25% root pruning, followed by irrigation with a solution containing 750 mg·L⁻¹ rooting agent and 20 million spores·mL⁻¹ of Metarhizium anisopliae. The rational distribution of NSCs and coordinated growth across different organs enhanced NSCs accumulation in potted tree peonies. These results demonstrate that combining root pruning with the application of rooting agent and Metarhizium anisopliae can effectively increase NSCs accumulation, optimize plant morphology, and ultimately improve the quality of potted tree peony seedlings.

Tree peony (Paeonia sect. Moutan) is one of the most important traditional ornamental woody flowers in China. However, its cultivation is often challenged by intense light and high temperatures during summer, leading to severe photo-oxidative damage and leaf senescence. In this study, we investigated the protective effects of polyolefin (PO) film on potted tree peony leaves under summer intense light and high temperature stress. Using tree peony ‘Luoyanghong’ as experimental material, we systematically compared the effects of two controls under natural light (CK1 and CK2, housed in separate greenhouses), single-layer PO film (PO1), and double-layer PO film (PO2) treatments. Microenvironment monitoring showed that single-layer and double-layer PO films reduced light intensity by 21% and 49%, respectively, while having limited effects on temperature. Morphological and physiological analyses indicated that PO film treatments effectively alleviated leaf yellowing and withering, maintained normal physiological morphology, and increased chlorophyll (Chl) and carotenoid (Car) content. The net photosynthetic rates of PO1 and PO2 plants were 18–36% higher than those of the control groups (CK1 and CK2). Evans blue and NBT staining revealed that PO film reduced cellular damage and reactive oxygen species (ROS) accumulation, while decreasing malondialdehyde (MDA) levels and increasing catalase (CAT) activity. Furthermore, qRT-PCR results showed that stress-responsive genes (Hsp70, Hsp90, and ATG5) and photosynthetic genes (RbcS and RbcL) were upregulated under PO film treatment. Principal component analysis (PCA) indicated that high light intensity, rather than temperature, was the primary factor causing leaf damage in potted tree peonies. The results show that PO film is an effective and low-cost agronomic measure, which can alleviate the intense light stress under high temperature conditions, relieve photo-oxidative damage, maintain photosynthetic performance, and increase the survival rate of potted tree peonies in summer.