Leaves facing different directions (north, south, east, and west) receive differing levels of illumination, resulting in spatial differences in photosynthesis P_N in the crowns of mature trees. We measured diurnal trends in P_N for a semi-solitary European ash (Fraxinus excelsior) over spring, summer, and autumn and compared these data with leaf biometric traits and leaf area distribution. The highest light-saturated P_N (P_Nmax) was to the south and west, and the lowest to the north. Likewise, intrinsic water use efficiency, defined as the ratio (P_N:g_S) of photosynthetic rate (P_N) and stomatal conductance (g_S), was also lowest to the north. The thickest leaves were found on the northern face and the thinnest in the south, suggesting differences in leaf anatomy may have contributed to differences in P_N. The greatest leaf area was recorded in the southern crown quadrant, which contributed more than 50% of the tree’s accumulated P_N. Our research emphasises the importance of choosing representative leaves for gas exchange measurements. In-depth studies into the spatial distribution of leaves and their traits will be necessary for accurate upscaling of leaf-level photosynthesis to whole tree and canopy levels. Full article

Atmospheric CO₂ levels have been increasing, and these changes may result in differential adaptive responses in both genera and species and highlight the need to increase carbon sequestration. Ecophysiological and morphological responses of four early-successional deciduous species were examined under ambient CO₂ (aCO₂, 400 ppm) and elevated CO₂ (eCO₂, 800 ppm) treatments. The four species, all of which are used in restoration, were Alnus viridis subsp. crispa (Ait.) Turrill (green alder), A. incana subsp. rugosa (Du Roi) R.T. Clausen (speckled alder), Betula populifolia (Marshall) (gray birch), and B. papyrifera (Marshall) (white birch); all are from the same phylogenetic family, Betulaceae. We examined biochemical efficiencies, gas exchange, chlorophyll fluorescence, chlorophyll concentrations, foliar nitrogen (N), and growth traits. A general linear model, analysis of variance, was used to analyze the functional carbon efficiency and growth differences, if any, among genera, species, and provenances (only for growth traits). The alders had greater biochemical efficiency traits than birches, and alders upregulated these traits, whereas birches mostly downregulated these traits in response to eCO₂. In response to eCO₂, assimilation either remained the same or was upregulated for alders but downregulated for birches. Stomatal conductance was downregulated for all four species in response to eCO₂. Intrinsic water use efficiency was greater for alders than for birches. Alders exhibited a consistent upregulation of stem dry mass and height growth, whereas birches were somewhat lower in height and stem dry mass in response to eCO₂. Foliar N played an important role in relation to ecophysiological traits and had significant effects relative to genus (alders > birches) and CO₂ (aCO₂ > eCO₂), and a significant genus × CO₂ interaction, with alders downregulating foliar N less than did birches. Covariate analysis examining carbon efficiency traits in relation to foliar N showed clear functional responses. Both species in both genera were consistent in their ecophysiological and morphological responses to CO₂ treatments. There was supporting evidence that assimilation was sink-driven, which is related to a plant organ’s ability to continue to grow and incorporate assimilates. The alders used in this study are actinorhizal, and the additional available foliar N, paired with increased stem dry mass sink activity, appeared to be driving upregulation of the carbon efficiencies and growth in response to eCO₂. Alders’ greater carbon efficiencies and carbon sequestration in impoverished soils demonstrate that alders, as opposed to birches, should be used to accelerate ecological restoration in a world of increasing atmospheric CO₂. Full article

This study aimed to establish the effects of elevated CO₂ concentration and temperature on the photosystem II (PSII) performance and photosynthetic characteristics of Populus simonii × P. nigra ‘1307’ leaves. Different CO₂ concentrations (400 and 800 µmol·mol⁻¹) and temperatures (room temperature and room temperature +2 °C) were set in artificial climate change simulation and control chambers, and the rapid chlorophyll fluorescence induction kinetics curve (OJIP curve) of Populus simonii × P. nigra ‘1307’ was determined. The generated OJIP curve was used to analyze the change characteristics in photosynthetic performance. The results revealed that under elevated temperature conditions, the concentrations of chlorophyll a, b in Populus simonii × P. nigra ‘1307’ leaves were significantly increased. At the same time, there were no significant changes in the chlorophyll concentration under the superimposed effect of elevated CO₂ concentration and temperature. The PSII comprehensive performance index (PI_ABS) of Populus simonii × P. nigra ‘1307’ was significantly inhibited under elevated temperatures due to the increased closure degree (V_j) of the PSII reaction center and the damage of the receptor side. This reduced the electron transfer capacity per unit reaction center (ET_o/RC) and unit cross-sectional area, which decreased the quantum yield of the electron transfer. Under the elevated CO₂ concentration, ET_o/RC was also inhibited. Still, PI_ABS was enhanced owing to the increased number of active PSII per unit area and the low reduction rate of the primary quinone receptor (Q_A). Under the superimposed effect of the two factors, the electron transfer performance of the donor and receptor sides of PSII was improved compared to the treatments only subjected to elevated temperature; thus, PI_ABS was not significantly reduced compared to the control. Therefore, the continuous increase in temperature by 2 °C significantly inhibits the electron transfer capacity of the photosynthetic system of Populus simonii × P. nigra ‘1307’ leaves. On the other hand, an increase in CO₂ concentration expands the PSII reaction center, while enhancing the electron transfer capacity of the donor and receptor sides, which alleviates the photosynthetic inhibition caused by the elevated temperature. Full article

To understand the response of hazelnut to the increased concentration of carbon dioxide (CO₂) under cadmium (Cd) pollution stress, this paper used an artificial open top chamber to control the CO₂ concentration (at 370 and 750 μmol·mol⁻¹) and to study the effects of an elevated CO₂ concentration on the growth and photosynthetic capacity of hazelnut leaves under different levels of Cd stress. The results showed that the increase in atmospheric CO₂ concentration has a tendency to alleviate the inhibition of plant growth caused by Cd. The net photosynthetic rate rose significantly, although the transpiration rate and stomatal conductance of hazelnut leaves decreased slightly with the rise in CO₂ concentration. The rise in CO₂ concentration had no significant effect on the activity of the photosystem Ⅱ (PSII) reaction center in hazelnut leaves. Under Cd stress conditions, the rise in CO₂ concentration significantly enhanced the PSII hazelnut leaves’ photochemical activity, which promotes the PSII receptor’s electron transfer capacity side and alleviates the degree of damage to the oxygen-evolving complex and the thylakoid membrane of the PSII donor side. The number of active reaction centers per unit area of hazelnut leaves, and the proportion of energy absorbed by PSII that is used for photosynthetic electron transfer, increased under severe stress conditions, which in turn reduced the energy proportion that was used for heat dissipation, providing CO₂’s effective fixation energy in the dark reaction. In conclusion, the rise in the CO₂ concentration enhances hazelnut’s heavy metal resistance by improving the PSII function under Cd stress conditions. Full article

Using a static chamber-gas chromatography method, we investigate the characteristics of soil CO₂, CH₄, and N₂O fluxes and their relationships with environmental factors during the growing season in four typical Larix gmelinii forests (moss–Larix gmelinii forest, Ledum palustre–Larix gmelinii forest, herbage–Larix gmelinii forest, and Rhododendron dauricum–Larix gmelinii forest) in the Greater Khingan Mountains. Our results show that all four forest types are sources of CO₂ emissions, with similar average emission fluxes (146.71 mg·m⁻² h⁻¹–211.81 mg·m⁻² h⁻¹) and no significant differences. The soil in the moss–Larix gmelinii forest emitted CH₄ (43.78 μg·m⁻² h⁻¹), while all other forest types acted as CH₄ sinks (−56.02 μg·m⁻² h⁻¹–−28.07 μg·m⁻² h⁻¹). Although all forest types showed N₂O uptake at the beginning of the growing season, the N₂O fluxes (4.03 μg·m⁻² h⁻¹–5.74 μg·m⁻² h⁻¹) did not differ significantly among the four forest types for the entire growing season, and all acted as sources of N₂O emissions. The fluxes of CO₂, CH₄, and N₂O were significantly correlated with soil temperature and soil pH for all four forest types. Multiple regression analysis shows that considering the interactive effects of soil temperature and moisture could better explain the changes in greenhouse gas emissions among different forest types. The average Q₁₀ value (8.81) of the moss–Larix gmelinii forest is significantly higher than that of the other three forest types (3.16–3.54) (p < 0.05), indicating that the soil respiration in this forest type is more sensitive to temperature changes. Full article

Drought is the most important environmental factor inhibiting plant photosynthesis. In this study, the morphological characteristics, biomass allocation, and physiological and biochemical characteristics of four potted mulberry plants under drought stress were analyzed. The study revealed the drought tolerance differences of four mulberry potted seedlings in semi-arid sandy areas of China. Combined with the results of two-way ANOVA, under normal growth conditions, Shensang No. 1 and Ji’an grew well and produced higher benefits, which was attributed to their larger leaf areas, biomass, and total Chl contents, and there were significant differences between their other traits (p < 0.05). Drought stress led to a decrease in the photosynthetic capacity of the mulberry leaves, and the drought resistance capabilities of the four mulberry trees were different. Among the trees, Aerxiang and Fujia were less affected by drought, and their cultivation in a naturally arid environment was able to achieve certain drought resistance effects. The branch length, total leaf area, and specific leaf area were significantly differently correlated with the biomass components’ dry leaf weight, dry branch weight, dry root weight, total biomass, and root-to-crown ratio (p < 0.05), and there was also a significant positive correlation with the photosynthetic fluorescence parameters G_S, PI_ABS, ABS/RC, and TR_o/RC and the biochemical parameters NSC (p < 0.05). Studies have shown that plant biomass and physiological and biochemical characteristics jointly affect plant growth. Our research results will help in the screening of mulberry trees, providing data support for the strategic planning of subsequent breeding, and maximizing the quality and resource benefits of mulberry trees. Full article

In order to investigate the effect of Ca on the biomass allocation strategies of tree species with different growth rates under drought conditions, we treated poplar (Populus canadensis cv) cuttings and mulberry (Morus alba) seedlings with two soil moisture levels (40 ± 5% and 80 ± 5% maximum water holding capacity) and two soil Ca levels (0 and 200 mg·kg⁻¹ Ca²⁺) in a greenhouse experiment, and then measured the Ca uptake, growth, gas exchange parameters, biomass allocation, and leaf traits. Drought induced a reduction in biomass accumulation of poplar cuttings and mulberry seedlings, and the cuttings and seedlings exhibited different biomass allocation patterns in response to drought stress. Under Ca0 treatment, poplar cuttings allocated more biomass to leaves and less biomass to stems under drought conditions, leading to an increased leaf/stem (L/St) ratio and higher SLA than under moist conditions in order to maintain higher Pn, and had enhanced WUE to cope with drought stress. Under the same treatment, mulberry seedlings allocated more biomass to roots and less biomass to stems, leading to an increased root/shoot (R/S) ratio and lower SLA, to improve drought resistance. Ca200 treatment decreased the growth of poplar cuttings and mulberry seedlings, whereas it enhanced the WUE, root growth, and R/S ratio of poplar cuttings and the WUE of mulberry seedlings, and alleviated drought stress in both species. Full article

Increasing soil N not only aggravates calcium (Ca) stress by stimulating Ca leaching from the soil but also impacts the sensitivity of plants to Ca stress. However, how increasing N influences the soil Ca demand of seedlings is largely unknown. We studied the influence of different concentrations of exogenous Ca (i.e., 0, 200, 400, 600, and 800 mg⋅kg⁻¹ Ca²⁺) on the growth, photosynthesis, Ca absorption, and intrinsic water use efficiency (iWUE) of mulberry (Morus alba) seedlings under two N levels (i.e., 200 and 600 mg⋅kg⁻¹ NH₄NO₃). We found that there was an optimal concentration of soil Ca for the growth and net photosynthetic rate (Pn) of mulberry seedlings; the optimal Ca concentration was 200 mg⋅kg⁻¹ under low N conditions and 400 mg⋅kg⁻¹ under high N conditions. Therefore, the application of N fertilizer increased the optimal Ca concentration. Different from the unimodal relationship between biomass and Ca levels, the iWUE of mulberry was significantly and positively correlated with soil Ca levels. At the same time, except under the 800 mg⋅kg⁻¹ Ca treatment, the soil Ca levels were reflected by foliar Ca concentrations. The N deposition, large-scale N fertilizer application, and drought increase Ca demand in plants, thus causing the application of Ca fertilizer to be necessary in low-Ca soil while alleviating Ca stress in high-Ca soil. The balance between the optimal Ca level needed for growth and drought resistance should be considered when determining the amount of Ca fertilizer required. Full article

Calcium (Ca), a secondary messenger, plays an essential role in improving drought resistance. We used the Fast Chlorophyll Fluorescence Induction Dynamics technique to investigate the effects of exogenous calcium on electron transport and energy fluxes in an 8-year-old Mongolian pine to investigate the mechanism of action of Ca in regulating drought adaptation in Pinus sylvestris var. mongolica. We found water stress significantly decreased Pn and Gs, but exogenous calcium significantly improved photosynthesis under water stress. The chlorophyll a fluorescence transient (OJIP) analysis revealed that water stress increased Fo and decreased Fm, inactivating reaction centers. Water stress reduced V_I and V_J while increasing Mo, destroying the electron transport chain. Exogenous calcium increased Sm while decreasing V_I and Mo under water stress, enhancing electron transport from QA to QB. Furthermore, 5 mM Ca²⁺ increased I-P phase and ψPo, δRo, and φRo, decreasing the drought-induced reduction in electron accepters of PSⅠ. The increase in ABS/RC, TRo/RC, ETo/RC, and DIo/RC caused by 5 mM Ca²⁺ demonstrated that calcium can regulate photoprotection to promote photosynthetic activity. Thus, exogenous calcium alleviated drought-induced reductions in photosynthetic activity by regulating photoprotection and boosting the electron transport efficiency at the acceptor side of PSⅡ and PSⅠ. Full article

In the present study, using the BLUP-GGE approach, southern-type (ST) Populus deltoides genotypes were analyzed and evaluated, and variations in growth traits, seedling height (H), and ground diameter (GD) were studied in various climatic regions, which could facilitate the increase of the breeding range of ST. The test materials were 119 one-year-old ST genotypes, and the test sites were Ningyang (NY) and Hainan (HN). A linear mixed-effects model was constructed, and the BLUP values of H and GD were obtained using the linear unbiased prediction (BLUP) method. GGE-Biplots were generated. The H variation was greater than the GD variation. The effects of environment, block, and genotype–environment interaction (G×E) were highly significant. The goodness of fit of the GGE-Biplots obtained by extracting the BLUP values was >95%. According to the GGE-biplot results, the performance of each genotype varied considerably. The genotype No. 13 had the highest average GD and the highest average H. In NY, the genotypes No. 93 and 115 had the highest H and GD, and in HN, the genotype No. 9 had the highest H and GD. ST had a better second-year survival rate in NY than in HN. The hybridization of tropical Populus deltoides can be performed using the No. 13 and 117 genotypes, which grow rapidly and have high yields. Full article