(1) Background: Down-regulation of photosynthesis has been commonly reported in elevated CO₂ (eCO₂) experiments and is accompanied by a reduction of leaf nitrogen (N) concentration. Decreased N concentrations in plant tissues under eCO₂ can be attributed to an increase in nonstructural carbohydrate (NSC) and are possibly related to N availability. (2) Methods: To examine whether the reduction of leaf N concentration under eCO₂ is related to N availability, we investigated understory Fraxinus rhynchophylla seedlings grown under three different CO₂ conditions (ambient, 400 ppm [aCO₂]; ambient × 1.4, 560 ppm [eCO₂1.4]; and ambient × 1.8, 720 ppm [eCO₂1.8]) and three different N concentrations for 2 years. (3) Results: Leaf and stem biomass did not change under eCO₂ conditions, whereas leaf production and stem and branch biomass were increased by N fertilization. Unlike biomass, the light-saturated photosynthetic rate and photosynthetic N-use efficiency (PNUE) increased under eCO₂ conditions. However, leaf N, Rubisco, and chlorophyll decreased under eCO₂ conditions in both N-fertilized and unfertilized treatments. Contrary to the previous studies, leaf NSC decreased under eCO₂ conditions. Unlike leaf N concentration, N concentration of the stem under eCO₂ conditions was higher than that under ambient CO₂ (4). Conclusions: Leaf N concentration was not reduced by NSC under eCO₂ conditions in the understory, and unlike other organs, leaf N concentration might be reduced due to increased PNUE.

Waterlogging induces oxidative damage by accumulation of reactive oxygen species due to stomatal closure. Plants alter their physiological and molecular mechanisms to reduce and adapt to oxidative stress. This mechanism of adaptation to stress, known as hardening, can support future stress tolerance. Pinus densiflora seedlings were grown under waterlogging treatment for three years and then transplanted to another site to identify the waterlogging hardening effect on transplanting. Transcriptome analysis was conducted before and after transplanting, and physiological factors were measured after transplanting. After transplanting, wounding stress is the main cause of transplant stress, and 13 genes related to phenylpropanoid were upregulated for the recovery of wounded roots in waterlogged hardened seedlings. The leaf starch and soluble sugar content of the waterlogged hardened seedlings were 50.3% and 40.5% lower due to the formation of cell walls. However, auxin-related genes were downregulated in waterlogging hardened seedlings, resulting in a lower tendency for height growth in hardened waterlogged seedlings. Waterlogging hardening mitigated transplant stress by wounding more than non-hardening, whereas waterlogging hardening may negatively affect seedling height. Our study provides evidence for the hardening effect of long-term waterlogging on transplanted P. densiflora seedlings.

Acorn size is of great importance, both ecologically and economically. However, the long lifespan is an obstacle to improvement in breeding. Keumsura1ho is a recently bred cultivar of oak (Quercus acutissima Carruth) selected after eight years of selection due to its larger acorns and higher weight compared to the control. In the present study, we investigated the transcriptional mechanisms underlying the outstanding morphological characteristics of Keumsura1ho. For this purpose, one- and two-year-old acorns from the control and Keumsura1ho groups were used for comparative transcriptome analyses of their seeds. Among morphological characteristics, the Keumsura1ho seeds were 18.3% longer than those of the control. Seed width and depth were 33.9% greater than those of the control. In the transcriptome analysis, genes related to seed size were further enriched, with biennial Keumsura1ho showing higher enrichment in comparison to control genes. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that the upregulated genes of the two-year-old Keumsura1ho seeds showed higher enrichment in phenylpropanoid metabolism compared to the control, with OMT1 and PRXs exhibiting high expression levels. This pathway has a significant impact on seed coat lignification and size in Keumsura1ho. The results of this study will benefit the development of breeding techniques by identifying marker genes for seed size.

Poplar (Populus spp.) is an economically and ecologically important temperate tree species known for its rapid growth. Clonal propagation has facilitated genetic advancements, but it remains challenging due to substantial variations in rooting capacity among poplar species and clones. Poplar clones were divided into two groups based on their rooting ability (high or low), and their transcriptome was analyzed for 3 weeks following stem-cutting propagation to investigate the rooting mechanisms of a hybrid of two fast-growing poplar species (Populus alba × P. tomentiglandulosa). The root length and area of the high-rooting group were 668.7% and 198.4% greater than those of the low-rooting ability group, respectively (maximum p < 0.001). Compared to week 0, genes involved in auxin signaling, cell wall organization, and secondary metabolite biosynthesis were consistently upregulated at 1, 2, and 3 weeks after planting, respectively. The expression of genes associated with cell wall differentiation and flavonoid biosynthesis was greater in the high- than in the low-rooting group at week 2. MYB and AP2/ERF transcription factors, which regulate flavonoid biosynthesis, as well as chalcone isomerase, a key enzyme in early flavonoid biosynthesis and root formation, were upregulated in the high-rooting group. The flavonoid biosynthesis pathway is important in rooting after stem cutting of Populus alba × P. tomentiglandulosa hybrids.