Responses of Planting Modes to Photosynthetic Characteristics and Fluorescence Parameters of Fokienia hodginsii Seedlings in a Heterogeneous Nutrient Environment

: Fokienia hodginsii seedlings tend to vary signiﬁcantly in response to differences in the surrounding environment, especially when the nutrient environment is heterogeneous and neighboring plants are in competition. Plant physiological differences occur not only in the root system, but also in the photosynthetic characteristics and ﬂuorescence parameters of the leaves. Therefore, in this experiment, three planting modes (single planting, pure planting of F . hodginsii and mixed planting of F . hodginsii


Introduction
Soil is the most important carrier of plant nutrients.As a result of leaching, fixation and soil biochemical process, the texture, water content and microbial activity of differ-Forests 2023, 14, 984 2 of 15 ent forest soils display significant differences, resulting in a patchy spatial distribution of soil nutrients with high heterogeneity [1][2][3].Compared with a homogeneous nutrient environment, plants can obtain more nutrients from a heterogeneous environment by changing their morphology and physiology [4][5][6].To acquire more nutrients, the belowground part of a plant mainly manifests root proliferation and root architecture changes, whereas the aboveground part exhibits changes in photosynthesis and enzyme activity to improve physiological functions [7,8].Previous studies have shown that throughout their evolution plants have formed various mechanisms, including morphological and physiological plasticity, to adapt to a high spatial heterogeneity of soil nutrients and obtain soil resources to the maximum extent possible.This has led to considerable differences in growth, development and relative competitiveness when plants face a heterogeneous nutrient environment, which further affects the growth, development and interspecific competition of trees, as well as the structure and expansion of tree communities [9,10].Whereas differences in physiological plasticity often determine the adaptive capacity of plants in the face of different heterogeneous nutrient environments, Huante et al. [11] showed that fast-growing species have better adaptability and search capacity in heterogeneous nutrient environments than slow-growing species, and their physiological indicators have higher thresholds of change and plasticity than slow-growing species.Fransen et al. [12] found that the photosynthetic intensity, fluorescence parameters and root nutrient uptake capacity of Festuca rubra and Anthoxanthum odoratum were essentially the same when grown alone in homogeneous nutrient environments, while the photosynthetic intensity and root nutrient uptake capacity of Anthoxanthum odoratum increased significantly in heterogeneous nutrient environments.It can be seen that changes in the physiological plasticity of plants in the face of heterogeneous nutrient environments are not only limited to the root system, but their leaf photosynthetic properties also have a significant trend of change.Currently, more studies have focused on the comparison of differences in root morphology and nutrient uptake efficiency of plants in heterogeneous nutrient environments [13,14] and less on the response of leaf photosynthetic properties of aboveground parts of plants to heterogeneous nutrient environments.
In the natural environment, plants continuously engage in intra-or inter-specific competition for environmental resources [15].When the planting density is high, the plant senses the competition intensity of neighboring plants mainly through the change of the ratio of red light (R, wavelength 655-665 nm) to far-red light (FR, wavelength 725-735 nm) in the leaves [16,17].Li et al. [18] found that the photosynthetic rate, transpiration rate and stomatal conductance of the dominant plants in light competition were significantly higher than those of the associated species but lower than those in the non-competitive environment.Under co-existence of Flavia bidentis and Chenopodium album, the net photosynthetic rate of both plants is also significantly lower than that under the single planting mode, indicating effective inhibition of the spread and growth of Flavia bidentis [19].Woody vines can also significantly inhibit the photosynthetic capacity of light-demanding plants by changing their phenotypic characteristics, thereby enhancing their competitiveness [20].Furthermore, it has been shown [21] that the plasticity response of plants in neighboring plant competition plays an important role in the efficient use of nutrient resources and productivity.Plants with higher morphological plasticity and physiological plasticity can occupy resources quickly and obtain more water and fertilizer resources, which in turn promote plant growth and gain competitive advantage.A study by Hodge et al. [22] illustrated that fast-growing plants can absorb or deplete most of the nutrient and light resources in the nutrient patches, inhibiting the growth of neighboring plants.The dominant species in the competitive system can separate ecological niches through morphoplastic adjustment, thus allowing different individual plants to use nutrient resources from different ecological regions, reducing competition for resources and achieving coexistence among species.Most soils show nutrient heterogeneity.Under such conditions, plants show different responsive behaviors to the competition intensity of neighboring plants, and the competitive relationship with neighboring plants is also more complex [23,24].Does the competitive behavior Forests 2023, 14, 984 3 of 15 of plants in nutrient heterogeneous environments vary according to differences in planting patterns or regulation by two factors?This is currently a research hotspot [25].
Fokienia hodginsii (Dunn) Henry et Thomas is an evergreen tall tree species of the genus Fokienia in the Cupressaceae family.It is a unique single-species genus plant and a national second-level key protected wild plant in China.F. hodginsii is light-loving, shade-tolerant at seeding age, suitable for slightly acidic to acidic yellow and yellow-brown soils, shallowrooted, drought and infertile, with developed lateral roots and no obvious main roots.The species is often used in mixed forest plantations and is mainly distributed in Vietnam and in forests in Southwest, South, and East China at elevations of 350-700 m [26].In recent years, studies on F. hodginsii mainly focused on cultivation technology, plantation management and seedling growth.Fewer attempts have been made to reveal the changes in the photosynthetic efficiency and enzyme activity of F. hodginsii in response to competition by neighboring plants under different nutrient heterogeneity patterns from the perspective of soil nutrient heterogeneity and interspecific competition.
In this study, 1-year-old seedlings of F. hodginsii were cultivated under different nutrient and competition conditions, and the differences in their photosynthetic capacity and enzyme activity were measured.The results provide a scientific basis for the cultivation of F. hodginsii seedlings.

Overview of the Experimental Conditions
The experiments were conducted in the greenhouse (119 • 13 51.18E, 26 • 05 4.35 N) of the College of Landscape Architecture and Art, Fujian Agriculture and Forestry University.This is an experimental (educational) greenhouse with ventilation, equipped with spray cooling devices and a sunshade net.During the experimental period, the average temperature of the greenhouse ranged between 27.8 • C and 28.9 • C, with sufficient sunshine hours.

Experimental Materials
The 1-year-old clones of F. hodginsii seedlings, cultivated by the Fengtian state-owned forest farm in Anxi County, Quanzhou City, Fujian Province, were selected for the experiment.To ensure a consistent initial state, 180 F. hodginsii seedlings were selected, with an average in-ground diameter of 2.65 ± 0.86 mm and an average height of 21.47 ± 4.12 cm.Pot experiments were carried out in four nutrient environments, namely homogeneous nitrogen (N), phosphorus (P) and potassium (K), and one homogenous environment.The pot medium was sourced from the barren acid red soil in the greenhouse of the College of Landscape Architecture and Art, Fujian Agricultural and Forestry University, and had the following composition: organic matter 5.91 g kg −1 , total N and total P 0.43 and 0.40 g kg −1 , respectively, hydrolyzed N, available K, and available P 29.08, 238.68 and 6.15 mg kg −1 , respectively, and pH 4.97.

Experimental Design
The experiment was started in early March 2022, using a 4 × 3 two-factor analysis factor design (four nutrient environments and three planting methods).Polyethylene pots with an inner diameter of 22.3 cm at the upper end, 15.5 cm at the lower end and a height of 20 cm were used to plant the F. hodginsii seedlings.The barren acid red soil was sterilized with 0.5% potassium permanganate, covered with plastic film, sealed, exposed and dried for 1 week.Subsequently, the soil was mixed with perlite at a mass ratio of 3:1 to establish a heterogeneous/homogeneous nutrient environment.Each pot contained approximately 4.5 kg of soil, which was added to the upper part of the pot (4 cm from the brim), whereas the lower part was divided into three sections, namely the nutrient-rich patch (Zone A), the nutrient-poor patch (Zone B) and the planting area in the middle part (as shown in Figure 1).Zones A and B had the same area.The nutrient patches and the planting area were separated by non-woven fabric coated with ager.The soil was filled with film bags in the lower part of the pot to prevent nutrient cross flow and loss, and to ensure the smooth penetration of seedling roots, facilitating root growth observation in the later stages.According to Mou et al. [10], Urea (N 46%), calcium superphosphate (P 2 O 5 16%) and potassium chloride (K 2 O 60%) were homogenously mixed in different nutrient patches on both sides of the container.For the homogeneous nutrient patches, 0.1087 g of urea, 1.0081 g of calcium superphosphate and 0.1433 g of potassium chloride were added to each kg of the above-mentioned substrate, the concentrations of N, P and K in the nutrient patches on both sides were 50, 125 and 75 mg.kg−1 , and thus a homogeneous nutrient environment was constructed.Enriched soil was used to ensure that the N, P and K heterogeneous patches had twice the corresponding element content of the homogeneous patches, while the nutrient-poor patches did not contain the corresponding nutrient elements.This was to ensure that the total amount of N, P and K nutrients added to the heterogeneous and homogeneous nutrient environments was the same.The specific nutrient application scheme is shown in Table 1.In the treatment of different heterogeneous nutrient environments, the A side of the potted plant represents the nutrient elements corresponding to the heterogeneous environment.A side of the pot plant represents the side that is richer in nutrient elements and the B side represents the side that is poor in the corresponding nutrient elements, and the content of each nutrient element is the same on both sides of A and B in the pots with homogeneous nutrient environments (Table 1).establish a heterogeneous/homogeneous nutrient environment.Each pot conta proximately 4.5 kg of soil, which was added to the upper part of the pot (4 cm brim), whereas the lower part was divided into three sections, namely the nutr patch (Zone A), the nutrient-poor patch (Zone B) and the planting area in the mid (as shown in Figure 1).Zones A and B had the same area.The nutrient patches planting area were separated by non-woven fabric coated with ager.The soil w with film bags in the lower part of the pot to prevent nutrient cross flow and los ensure the smooth penetration of seedling roots, facilitating root growth obser the later stages.According to Mou et al. [10], Urea (N 46%), calcium superphosph 16%) and potassium chloride (K2O 60%) were homogenously mixed in different patches on both sides of the container.For the homogeneous nutrient patches, 0. urea, 1.0081 g of calcium superphosphate and 0.1433 g of potassium chloride we to each kg of the above-mentioned substrate, the concentrations of N, P and K in trient patches on both sides were 50, 125 and 75 mg.kg - , and thus a homogeneous environment was constructed.Enriched soil was used to ensure that the N, P an erogeneous patches had twice the corresponding element content of the homo patches, while the nutrient-poor patches did not contain the corresponding nut ments.This was to ensure that the total amount of N, P and K nutrients adde heterogeneous and homogeneous nutrient environments was the same.The spe trient application scheme is shown in Table 1.In the treatment of different hetero nutrient environments, the A side of the potted plant represents the nutrient corresponding to the heterogeneous environment.A side of the pot plant repre side that is richer in nutrient elements and the B side represents the side that is po corresponding nutrient elements, and the content of each nutrient element is the both sides of A and B in the pots with homogeneous nutrient environments (Tab  Under both homogeneous and heterogeneous nutrient environments, three modes were used: single planting, pure planting of F. hodginsii and mixed plan  Under both homogeneous and heterogeneous nutrient environments, three planting modes were used: single planting, pure planting of F. hodginsii and mixed planting of F. hodginsii and Cunninghamia lanceolata (Chinese fir).In the single planting, one F. hodginsii was planted in the middle of each pot; in the pure planting of F. hodginsii, two F. hodginsii were planted at 5 cm depth on either side of the center line of the pot; in the mixed planting, one F. hodginsii seedling and one Chinese fir seedling were planted on each side of the center line of the pot.For each of the three planting methods, plants were planted at a depth of about 5 cm and cultivated in a heterogeneous and homogeneous environment for N, P and K nutrients.Plants were watered daily with 100 mL of purified water in the afternoon.Due to the long growth period, and to ensure the maintenance of the nutrient Forests 2023, 14, 984 5 of 15 patch environment, the second round of fertilization was performed in September 2022 with the same nutrient composition as in the first round.

Index Measurement 2.4.1. Determination of Photosynthetic Intensity
At the beginning of October 2022, the photosynthetic intensity of seedlings of F. hodginsii in the different nutrient patches was measured.Photosynthesis was measured in the functional leaves receiving uniform light in the middle and upper parts of the plant.Measurements were performed using a Li-6400 XT portable photosynthesis system (LI-COR Biosciences, Torrance, CA, USA) at 8:00-12:00 on a sunny day.Average daily temperature was 26-35 • C, relative humidity was 62%-90%, leaf temperature was 25 • C, CO 2 concentration was 400 µmol/mol, air flow rate was 0.5 L.min −1 and photosynthetic effective radiation was set as 1000 µmol −2 .m.s −1 .Three measurements were conducted for each leaf.The measured parameters included net photosynthetic rate (P n ), stomatal conductance (G s ), intercellular CO 2 concentration (C i ) and transpiration rate (T r ).

Determination of Chlorophyll Fluorescence Parameters
In the middle of November 2022, measurements were performed on leaves of plants subjected to different treatments using the OS5p portable pulse-modulated chlorophyll fluorescence instrument (OPTI science, Hudson, NH, USA) at 8:00-12:00 on a sunny day.Five seedlings with similar growth were selected from each treatment, and three functional leaves were selected from the middle of each plant for the dark adaptation treatment with shading clips for around 20 min.Leaves were measured in F v/ F m mode and, subsequently, the shading clips were removed and the marked leaves were measured using the yield and kinetic modes.The fluorescence parameters measured by the yield and kinetic modes included the actual photochemical efficiency (yield) and the photosynthetic electron transfer rate (ETR) of PSII.The initial fluorescence value (F o ), the maximum fluorescence value (F m ), the maximum photochemical quantum yield (F v /F m ) and the potential activity (F v /F o ) of PSII were measured in F v /F m mode.

Data Analysis
The data were analyzed using the software SPSS 22.0.One-way analysis of variance (ANOVA) and least significant difference (LSD) were used for the significance test of difference (α = 0.05).Two-way ANOVA was used to determine whether interaction occurred between the two environmental factors and the photosynthetic characteristics of F. hodginsii seedlings.Correlation analysis of photosynthetic characteristics was conducted and fluorescence parameters of F. hodginsii seedlings' leaves were determined using correlation analysis.Principal components analysis (PCA) was employed to identify the dominant factors and comprehensively rank the different environmental factor combinations.Figures 2 and 3 were generated using the Origin 2022 software.

Effects of Competition on the Photosynthesis of F. hodginsii in Different Heterogeneous Environments
Regarding the influence of planting modes on the photosynthetic characteristics of F. hodginsii leaves, competition had an impact on all four characteristics, with most of the differences being statistically significant (p < 0.05).The average P n , T r and G s of values of F. hodginsii seedlings in the single planting mode were significantly higher than those in the pure and mixed planting modes, in which P n in the single planting treatment was 28.8% and 22.3% higher than that in the pure and mixed planting modes, irrespective of the nutrient environment (A).Both T r and G s were lowest in the pure planting mode, with the overall mean of T r being 27.2% and 5.5% lower, the overall mean of G s being 36.5% and 14.7% lower than that of single planting and mixed planting, respectively (B,D).In the competition treatment, C i showed an opposite trend compared to the other photosynthetic indicators.Mixed and pure planting generated higher C i values than single planting Forests 2023, 14, 984 6 of 15 (C), although this difference was not statistically significant (p > 0.05).Regarding the influence of the heterogeneous nutrient environment on the photosynthetic characteristics of F. hodginsii leaves, P n , T r and G s in the N and P patches were significantly higher than those in the homogeneous patches (p < 0.05).Both P n and T r displayed the highest values in the N patches, with 24.1% and 68.8% higher than in homogeneous environments, respectively.The G s in the P patches showed the highest average value of 36.5% higher than the homogeneous environment, but the values were similar to those observed in the N patches (p > 0.05).On the contrary, C i in the N patches was significantly lower than that in the other nutrient patches.The average values of P n and C i in the K patches were slightly higher than those in the homogeneous patches, whereas T r was significantly lower than in the homogeneous patches by 31.7%.The G s values differed significantly among the different planting methods.
the pure and mixed planting modes, in which Pn in the single planting treatment wa and 22.3% higher than that in the pure and mixed planting modes, irrespective of th trient environment (A).Both Tr and Gs were lowest in the pure planting mode, wit overall mean of Tr being 27.2% and 5.5% lower, the overall mean of Gs being 36.5% 14.7% lower than that of single planting and mixed planting, respectively (B,D).I competition treatment, Ci showed an opposite trend compared to the other photosynt indicators.Mixed and pure planting generated higher Ci values than single planting although this difference was not statistically significant (p > 0.05).Regarding the influ of the heterogeneous nutrient environment on the photosynthetic characteristics hodginsii leaves, Pn, Tr and Gs in the N and P patches were significantly higher than in the homogeneous patches (p < 0.05).Both Pn and Tr displayed the highest values i N patches, with 24.1% and 68.8% higher than in homogeneous environments, re tively.The Gs in the P patches showed the highest average value of 36.5% higher tha homogeneous environment, but the values were similar to those observed in the N pa (p > 0.05).On the contrary, Ci in the N patches was significantly lower than that i other nutrient patches.The average values of Pn and Ci in the K patches were sli higher than those in the homogeneous patches, whereas Tr was significantly lower in the homogeneous patches by 31.7%.The Gs values differed significantly among th ferent planting methods.The results of the two-way ANOVA (Table 2) from planting modes and nutrient heterogeneity indicate a significant interaction between planting modes and heterogeneous patches in terms of photosynthetic characteristics such as P n and G s (p < 0.05).Single factors had a significant impact on P n , T r and G s (p < 0.01).Nutrient patches had a significant impact on C i , but the effect of planting methods on the C i of F. hodginsii leaves was low and not significant (p > 0.05), and the two factors showed no significant interaction effect on C i and T r .

Effects of Neighboring Competition on Chlorophyll Fluorescence Parameters of F. hodginsii under Different Heterogeneity Conditions
Regarding the impact of single factors on the chlorophyll fluorescence parameters of F. hodginsii leaves, the impact of competition was large and the overall impacts on F o , F v /F m and qP were low (A,B,E).The average values of these indicators in each nutrient patch under single planting mode were higher than those under pure and mixed planting, but most of the differences were not statistically significant (p > 0.05).The difference in ETR between single and pure planting was not significant (D), but the values for these two modes were significantly higher than that of mixed planting (36.2% and 28.3% higher, respectively; p < 0.05).The NPQ and Yield values in the single planting mode were significantly higher than those for the pure and mixed planting modes, 54.8% and 14.6% higher than pure planting and 39.9% and 16.4% higher than mixed planting, respectively (C,F).In general, the chlorophyll fluorescence parameters of the single planting mode were mostly higher than those of the pure and mixed planting modes.
Nutrient heterogeneity generated significant effects on the chlorophyll fluorescence parameters of F. hodginsii leaves.The parameters under the N and P patches were significantly higher than those under the homogeneous patches, and F v /F m peaked in the N patches with significantly higher values than those in the other nutrient patches, at 34.2% higher than homogeneous plaques.Yield and ETR showed the highest mean values in the P patches, but no significant difference to the values observed in the N patches was found (p > 0.05).The qP and NPQ values in the N and P patches differed due to the different planting modes, but the overall difference was not significant.The F o value showed no obvious change pattern.Most of the fluorescence parameters in the K patches were lower than those in the homogeneous patches, indicating that the N and P patches had significantly higher fluorescence parameters of F. hodginsii leaves, whereas in the K patches, fluorescence was inhibited.
The results of the two-way ANOVA (Table 3) indicate that both planting modes and nutrient patches had a significant impact on F v /F m , yield, ETR and NPQ (p < 0.01).Planting mode had a highly significant impact on F o , whereas nutrient patches had no significant impact on this factor.The qP was only significantly impacted by the nutrient patch (p < 0.05).These two environmental factors had a significant interaction effect on F o , F v /F m , yield, NPQ and ETR but not on yield and qP.The results of the two-way ANOVA (Table 3) indicate that both planting modes and nutrient patches had a significant impact on Fv/Fm, yield, ETR and NPQ (p < 0.01).Planting mode had a highly significant impact on Fo, whereas nutrient patches had no significant impact on this factor.The qP was only significantly impacted by the nutrient patch (p < 0.05).These two environmental factors had a significant interaction effect on Fo, Fv/Fm, yield, NPQ and ETR but not on yield and qP.

Correlation Analysis of Photosynthetic Fluorescence Parameters of F. hodginsii
Photosynthesis intensity is generally affected by numerous factors.Any changes in environmental factors lead to differences in plant photosynthetic fluorescence parameters, along with interaction effects among the different parameters.As seen in Table 4, P n was significantly positively correlated with Gs and Tr and significantly negatively correlated with Ci.In contrast, Gs was significantly negatively correlated with Tr and significantly positively correlated with Ci, indicating that stomatal opening and closing affect plant photosynthesis and transpiration and, thus, plant growth.In addition, P n and Gs were significantly positively correlated with Fv/Fm, suggesting that photosynthesis was also correlated with fluorescence parameters.Yield showed a significant positive correlation with Fv/Fm and ETR.

Comprehensive Evaluation of Photosynthetic Fluorescence Parameters of F. hodginsii
Principal components analysis was performed on 10 photosynthetic fluorescence parameters (Table 5).The cumulative variance contribution rate of the first two principal components was 82.69%, which can largely explain all the variation of the data.Therefore, the first two principal components were used as comprehensive evaluation indicators.The absolute values of the P n , G s , T r , F v /F m and NPQ coefficients in the first principal component were relatively large, indicating that these are the most important physiological indicators reflecting the photosynthetic intensity and fluorescence parameters of F. hodginsii seedlings in different planting modes and under different heterogeneity conditions.The comprehensive evaluation score (Table 6) was calculated according to the characteristic values of the PCA (Table 6).The modes with a higher comprehensive evaluation score were single planting × N patches, single planting × P patches, F. hodginsii pure planting × N patches and mixed planting × N patches.This indicates that F. hodginsii showed a higher photosynthetic intensity and better fluorescence parameter indicators in these planting modes.

Discussion
The spatial distribution of nutrients in forest soil is on a gradient and patchy.Differences in nutrient heterogeneity not only affect the nutrient absorption and development of roots but also, albeit to various degrees, the photosynthesis and fluorescence parameters of plants.These responsive behaviors determine the photosynthesis and competitiveness of trees in different nutrient or light environments, and the plastic response to light significantly affects the productivity of trees [27,28].In this study, the P n , G s and T r values of F. hodginsii seedlings in N and P heterogeneous patches were significantly higher than those in homogeneous patches and K patches, indicating that the net photosynthetic rate, stomatal conductance and transpiration rate of F. hodginsii leaves can be significantly improved when N and P nutrients are heterogeneous in the soil, compared with homogeneous conditions.Similarly, Yan et al. [29] showed that heterogeneous nitrogen supply can signifi-cantly promote the root growth and chlorophyll synthesis of Pinus massoniana.Xu et al. [30] studied clonal ramet growth and photosynthesis differences under heterogeneous nutrient conditions in Zoysia japonica and showed that increasing soil nutrient environmental heterogeneity within a certain range can improve the P n , G s and T r values, which is similar to the results of our study.This phenomenon may be explained by the hypothesis that the fine roots on one side of the plant root system can sense N deficiency, leading to the induction and promotion of the growth of root primordium and primary roots to acquire more N.The N-rich patches can provide sufficient N to accelerate such root elongation.To meet the increased growth needs of the plant itself, F. hodginsii may obtain more energy and material by increasing the intensity of photosynthesis in the heterogeneous environment, which is mainly reflected in the increase in stomatal conductance.This thereby increases respiration intensity, transpiration rate and material exchange intensity with the outside environment, resulting in an increased net photosynthetic rate.However, Mistelle et al. [31] showed that the P n and G s values of maize (Zea mays) in the seedling stage decreased significantly in an environment with a low N heterogeneity, which is different from the results of our experiment.The main reason for the discrepancy may be that F. hodginsii is a species with strong tolerance to barren conditions, whereas maize requires a richer nutrient environment.Therefore, in the early and medium stages of growth, plants that are not tolerant to barren conditions preferentially use nutrient substances for fine root growth to obtain more nutrients, resulting in the reduction of overall photosynthetic efficiency.In addition, the P n and G s values of F. hodginsii in the K patches were lower than those in the homogeneous patches, indicating that substantial K heterogeneity reduces the photosynthetic rate of F. Hodginsii seedlings, which is similar to the results of Jin et al. [32] and Guo et al. [33] These findings can be explained by the high potassium ion content in the plant during the seedling stage, which inhibits N absorption and leads to slow growth and reduced photosynthetic efficiency.The increase in stomatal conductance results in increased amounts of CO 2 entering the leaf cells, leading to the reduction of intercellular CO 2 ; thus, the C i of F. hodginsii leaves in the N and P patches was significantly lower than that that in the K patches.
Competition may occur between different individuals of the same species or different plant species, namely inter-or intra-specific competition.In this study, the leaf P n , G s and Tr values of F. hodginsii seedlings under single planting were significantly higher than those under pure and mixed planting.In the competition treatment, C i showed an opposite trend with other photosynthetic indicators.Under mixed and pure planting, C i was higher than that under single planting, but its overall increment was not significant, which is similar to the results of Niu [34] and Zhang [35].This can be attributed to the fact that if a plant encounters competition during its growth, it can control the concentration of CO 2 and water in its leaves by adjusting its stomatal conductance, thereby regulating photosynthetic carbon fixation and, thus, controlling photosynthetic and transpiration rates and maintaining its competitive position [36].The change in stomatal conductance is a strategy for plants to perceive whether competitors exist [37].Some hypotheses have also been put forward [38] to suggest that when plants encounter competitive relationships during the seedling stage, they tend to regulate the allocation pattern of nutrient and water resources according to the type of competitive relationship encountered.For example, in the case of sufficient light, plants will preferentially allocate resources to the growth of the root system in the face of heterogeneous and competitive nutrient environments, balancing the relationship between photosynthetic intensity and root growth.This hypothesis has some relevance to the results of this study.The pure and mixed planting patterns of F. hodginsii seedlings will preferentially allocate nutrient resources to the development of the root system and balance their own photosynthetic intensity, which also makes the photosynthetic intensity of pure and mixed planting patterns at seedling stage lower than that of monoculture.In addition, the P n and G s values under the mixed planting mode were higher than those under the pure planting mode.This may be because the same plant species occupy the same niche during the early growth stages, and competition for resources is therefore intense.In the heterogeneous nutrient environment without shading treatment, the light resources for F. hodginsii seedlings were sufficient.The seedlings therefore altered their photosynthetic carbon fixation by adjusting their stomatal conductance, reducing the photosynthetic intensity and transpiration to a certain extent and transferring more nutrients to the root growth.
Chlorophyll fluorescence parameters are the reflection of internal processes, such as absorption, transmission, distribution and dissipation of light energy during photosynthesis [39].The F v /F m ratio is an important indicator of the potential maximum photosynthetic capacity and photochemical conversion rate, mainly reflecting plants' adaptation to the current environmental factors.A lower F v /F m value indicates that the plant is subjected to a higher degree of photoinhibition [40].In this study, F. hodginsii seedlings showed F v /F m values in N heterogeneous patches, followed by P patches, with higher levels than those found in the homogeneous patches.The F v /F m ratio of F. hodginsii seedlings in the K patches was not significantly different from that in the homogeneous patches, indicating that the growth of seedlings in N and P heterogeneous environments was less inhibited, and the seedlings had a higher ability to adapt to the heterogeneous nutrient environment; in addition, the K heterogeneous environment did not significantly promote the F v /F m ratio of F. hodginsii leaves.This result is similar to the findings of Zhang et al. [40] and may be explained by the high tolerance of F. hodginsii to barren soil; this species can maintain a high F v /F m ratio in N-P heterogeneity, or low N/low P environments, which increase its ability to capture light energy and allow the acquisition of more sufficient light energy for photosynthesis.This is also the photosynthetic mechanism employed by barren soil-tolerant tree species to adapt to heterogeneous environments.In addition, yield is the main indicator of the actual photosynthetic efficiency and the relative rate of electron transfer of plants, whereas ETR can reflect the speed of the photosynthetic electron transfer efficiency outside the leaves under the actual light intensity [41].The change trends of yield and ETR of F. hodginsii showed positive correlations for each patch, with patches N and P showing high levels of yield and ETR, suggesting that N-P heterogeneity can improve the photosynthetic efficiency of F. hodginsii, accelerate the electronic transfer efficiency and enhance the adaptability to the environment.The qP is an indicator reflecting the opening degree of the PSII reaction center and the change in photosynthetic activity [42].In the N and P patches, the qP of F. hodginsii leaves was higher than that of the homogeneous patches, indicating that N-P heterogeneity can increase the opening degree of the PSII reaction center of F. hodginsii, accelerate the photosynthetic electron transfer rate, increase the proportion of captured light energy used for photochemical reaction and enhance photochemical activity.In addition, the planting mode also affected the fluorescence parameters of F. hodginsii.The most significant difference was found in NPQ, with the plants using the excessive energy consumed by heat dissipation to protect themselves from damage.The NPQ under single planting was significantly higher than that under pure and mixed planting, which may be due to the high photosynthetic capacity of F. hodginsii under the single planting mode, in which level of qP is improved, the rate of photosynthetic electron transfer is accelerated, the light energy capture ability is enhanced and the residual light is increased, resulting in increased NPQ, decreased light inhibition and enhanced light adaptation ability [43].
Based on the results of two-way ANOVA and PCA, planting mode and nutrient heterogeneity interacted with most of the photosynthetic indicators and fluorescence parameters.Among the photosynthetic indicators, the interaction produced by the two environmental factors on P n and G s reached a significant level.This indicates that the two factors have a significant influence on the photosynthetic intensity of F. hodginsii seedlings.In the PCA, the coefficients of variance of the two indicators were significantly higher than those of the other indicators, indicating that net photosynthetic rate and stomatal conductance, the most important photosynthesis indicators, are sensitive to changes in environmental factors.According to the comprehensive evaluation score calculated by the characteristic values of PCA, single planting × N patches, single planting × P patches, pure planting × N patches and mixed planting × N patches were the four planting and nutrient modes that generated desired photosynthetic characteristics and fluorescence parameters.

Conclusions
In the heterogeneous environment with N and P nutrients, the photosynthetic intensity of F. hodginsii seedlings was increased and their fluorescence parameters were better, while the photosynthetic intensity and fluorescence parameter levels of F. hodginsii seedlings were reduced to some extent in the heterogeneous environment of K elements, compared with the homogeneous environment.The photosynthetic intensity and fluorescence parameters of F. hodginsii seedlings were better in single planting than in the competition model, and mixed planting of F. hodginsii and Cunninghamia lanceolata had higher photosynthesis than pure planting of F. hodginsii.Photosynthetic characteristics and fluorescence parameters of F. hodginsii seedlings in N and P patches in the monoculture planting pattern were highly evaluated in the calculation of eigenvalues by principal component analysis.In this study, the photosynthetic characteristics and fluorescence parameters of the seedlings were compared and evaluated in different planting patterns in different heterogeneous nutrient environments.Due to the long experimental period, studies on biomass and root morphology and functional traits were not conducted at this stage, and the molecular mechanisms controlling the foraging ability and competition ability of the neighboring plants of F. hodginsii are not yet clear, which led to the imperfect completeness of the experiment.At the next stage, we will add root morphology and functional traits to the study of photosynthesis and fluorescence parameters in response to nutrient patches and planting patterns, and reveal the molecular mechanisms of response to heterogeneous nutrient environments and competition patterns of F. hodginsii seedlings.

Figure 1 .
Figure 1.Front view of the pot container.

Table 1 .
Concentrations of N, P and K in the heterogeneous and homogeneous nutrient p

Figure 1 .
Figure 1.Front view of the pot container.

Table 1 .
Concentrations of N, P and K in the heterogeneous and homogeneous nutrient patches.

Figure 2 .
Figure 2. Differences in photosynthesis of F. hodginsii seedlings under different treatments.capital letters indicate significant differences in leaf index values of F. hodginsii seedlings unde ferent planting patterns in the same nutrient patch (p < 0.05); lowercase letters indicate signi differences in leaf index values of Fokienia hodginsii under different nutrient patches in the

Figure 2 .
Figure 2. Differences in photosynthesis of F. hodginsii seedlings under different treatments.Note: capital letters indicate significant differences in leaf index values of F. hodginsii seedlings under different planting patterns in the same nutrient patch (p < 0.05); lowercase letters indicate significant differences in leaf index values of Fokienia hodginsii under different nutrient patches in the same planting patterns (p < 0.05).(A) represents the comparison figure of P n ; (B) represents the comparison figure of G s ; (C) represents the comparison figure of C i ; (D) represents the comparison figure of T r .F−SP: single planting; F-PP: pure planting of F. hodginsii; F-MP: mixed planting of F. hodginsii and Cunninghamia lanceolata.The error bars describe standard error.

Figure 3 .
Figure 3. Differences in chlorophyll fluorescence parameters of F. hodginsii seedlings under different treatments.Note: capital letters indicate significant differences in leaf index values of F. hodginsii seedlings under different planting patterns in the same nutrient patch (p < 0.05); lowercase letters indicate significant differences in leaf index values of Fokienia hodginsii under different nutrient patches in the same planting patterns (p < 0.05).(A) represents the comparison figure of Fo; (B) represents the comparison figure of Fv/Fm; (C) represents the comparison figure of Yield; (D) represents the comparison figure of ETR; (E) represents the comparison figure of qP; (F) represents the comparison figure of NPQ.F−SP: single planting; F-PP: pure planting of F. hodginsii; F-MP: mixed planting of F. hodginsii and Cunninghamia lanceolata.The error bars describe standard error.

Figure 3 .
Figure 3. Differences in chlorophyll fluorescence parameters of F. hodginsii seedlings under different treatments.Note: capital letters indicate significant differences in leaf index values of F. hodginsii seedlings under different planting patterns in the same nutrient patch (p < 0.05); lowercase letters indicate significant differences in leaf index values of Fokienia hodginsii under different nutrient patches in the same planting patterns (p < 0.05).(A) represents the comparison figure of F o ; (B) represents the comparison figure of F v /F m ; (C) represents the comparison figure of Yield; (D) represents the comparison figure of ETR; (E) represents the comparison figure of qP; (F) represents the comparison figure of NPQ.F−SP: single planting; F-PP: pure planting of F. hodginsii; F-MP: mixed planting of F. hodginsii and Cunninghamia lanceolata.The error bars describe standard error.

Table 2 .
Effects of planting pattern and nutrient heterogeneous patches on photosynthetic characteristics of F. hodginsii seedlings.

Table 3 .
Effects of planting pattern and nutrient heterogeneity on chlorophyll fluorescence parameters of F. hodginsii seedlings.

Table 4 .
Correlation analysis of photosynthetic fluorescence parameters of F. hodginsii.

Table 5 .
Principal components analysis of photosynthetic fluorescence parameters of F. hodginsii seedlings.

Table 6 .
Comprehensive evaluation of treatment effect of photosynthetic fluorescence parameters for F. hodginsii seedlings.