| Forests,as the main part of terrestrial ecosystems,play a crucial role in improving the Earth’s ecosystem and regulating the global climate.In recent years,global forest ecosystems have been seriously threatened by factors such as global climate change and human activities.Therefore,promoting natural forest regeneration and improving the success rate of artificial forest restoration are currently the key concerns of sustainable forest management.The growth and development stage of seedlings is a crucial stage affecting the success of forest regeneration or vegetation restoration and is affected by the interaction of biological characteristics and environmental factors.At present,research on the impact of environmental factors on forest seedlings only considers single or double factors of a certain environment,lacking physiological and ecological responses of seedlings to multiple environmental factors and their interactions.In addition,previous studies have mostly focused on the change of seedling functional traits under a certain environmental intensity,and rarely involved the functional traits of seedlings under multiple intensities,especially how the functional traits related to growth and defense respond and adapt,and their regulatory mechanisms,which greatly limits our understanding of the mechanism of plant response to environmental changes.Quercus variabilis Bl.as the main vegetation species in the warm temperate deciduous broad-leaved forest zone of China,has been recognized for its unique biomass distribution characteristics of"one year grow roots and two years grow seedlings"in the practice of Quercus sowing and afforestation.However,there have been no reports on the physiological and ecological strategies of seedlings adapting to environmental changes from the perspective of functional plasticity.Therefore,this study focuses on Q.variabilis seedlings and assumes that their functional traits are influenced by environmental factors（light,nitrogen addition and water stress）,which in turn affect biomass allocation patterns and carbon storage strategies,thereby affecting seedling growth and adaptability.From the perspective of plasticity of seedling functional traits,this study aims to control the growth physiological characteristics,biomass allocation characteristics,accumulation,transformation,and distribution patterns of non-structural carbohydrates（NSC）,carbon,and nitrogen in various organs of Q.variabilis seedlings under different shading（100%,75%,50%,and 25%of normal lighting）,nitrogen addition(0 kg N·ha-1·a-1、50 kg N·ha-1·a-1和100 kg N·ha-1·a-1),and drought stress（70-75%、35-40%and 20-25%of field water capacity）levels through controlled experiments.The study aims to analyze the effects of different environmental factors on Q.variabilis seedlings and the key factors limiting their plasticity of functional traits,revealing the growth defense balance strategy of Q.variabilis seedlings in regulating growth and adapting to external environmental changes.The main research findings are as follows:（1）Light and water are the main environmental factors limiting the height and diameter growth of Q.variabilis seedlings.Specifically,shading inhibits plant height growth,while ground diameter is affected by the combined effects of shading and drought stress.Moderate shading can alleviate the negative impact of drought stress on seedling growth.The coupling effect of shading and drought stress,as well as the coupling effect of three factors,significantly affects the morphological characteristics of Q.variabilis seedlings,such as specific leaf area,leaf size,and root tissue density,diameter,volume,and length of roots.Q.variabilis seedlings mainly adapt to shading and drought stress by adjusting the above morphological traits to maintain normal growth and development.The biomass accumulation and allocation of Q.variabilis seedlings are significantly influenced by nitrogen addition,drought stress,and their interactions.Water is the key factor directly affecting biomass accumulation.Under sufficient water conditions,Q.variabilis seedlings tend to prioritize allocating biomass to the aboveground parts to improve photosynthetic capacity,while under drought stress,they invest more in root growth to improve water use efficiency and survival ability.（2）Q.variabilis seedlings mainly responded and adapted by adjusting their antioxidant capacity（e.g.,POD,MDA,NR and superoxide anion production rate,etc.）,photosynthetic pigment content（e.g.,chlorophyll b）and net photosynthesis efficiency under shading,nitrogen addition,and drought stress and their interactions.For example,under drought stress,seedling leaves reduce dark respiration rate to reduce energy consumption.Under shading and drought stress,leaves increase their chlorophyll b content and peroxidase activity to maintain metabolic balance and adapt to environmental changes,as well as maintain a balance between water and carbon acquisition by adjusting stomatal size.In addition,Q.variabilis seedlings also avoid damage by regulating the content of malondialdehyde and nitrate reductase in the aboveground（leaves）and underground（roots）and rely on the superoxide system to stimulate physiological response mechanisms,thereby enhancing their ability to resist the environment.（3）The effect of shading on the anatomical characteristics of Q.variabilis seedlings leaves is extremely significant.Compared to the control,the thickness of the upper epidermis of the leaves decreases with increasing treatment intensity under shading intensity not less than 50%of normal light,and significantly decreases under normal light intensity of 50%.However,it shows an increasing trend when the intensity is not higher than 25%of normal light.The thickness of the lower epidermis increases with the increase of shading intensity,and significantly increases under 50%normal light conditions.The thickness of palisade tissue,sponge tissue,palisade tissue/sponge tissue,and leaf thickness decreased with increasing shading intensity,and all significantly decreased under 50%normal light conditions;The effect of nitrogen addition on the anatomical characteristics of Q.variabilis leaves is not significant;The effect of drought stress on the thickness of the upper epidermis and the palisade/sponge tissue of leaves is extremely significant,and it significantly increases the thickness of the upper epidermis under field water holding conditions of 10-15%.Except for the thickness of the stem cortex and xylem,the anatomical characteristics of the stem of Q.variabilis seedlings are strongly influenced by shading.The thickness of the periderm,phloem,and cambium,as well as the diameter of the pith,decrease with increasing shading intensity.The thickness of the periderm,phloem,and cambium significantly decreases under 50%of normal light intensity;The addition of nitrogen only has a significant effect on the thickness of the stem cortex,causing it to decrease with increasing nitrogen content,and it is significantly reduced at high nitrogen levels(100 kg N ha-1 a-1);Drought stress significantly reduced the thickness of the periderm and xylem of the stem.The interaction between shading,nitrogen addition,and drought stress has a significant effect on the thickness of the upper and lower epidermis,sponge tissue,and leaf thickness of Q.variabilis seedlings;The dual factor interaction of three factors has a significant impact on the thickness and pith diameter of the periderm,phloem,cambium,and xylem of the stem;The interaction between shading,nitrogen addition,and drought stress has a significant impact on the anatomical characteristics of leaves and stems in Q.variabilis seedlings.（4）Under shading,nitrogen addition,and drought stress conditions,annual and 2a Q.variabilis seedlings and their various organs（leaves,stems,coarse roots,and fine roots）exhibit different physiological response patterns.Specifically,the starch content and NSC of coarse and fine roots in 2a seedlings are higher than those in annual seedlings,while the starch content and NSC of leaves and stems are lower than those in annual seedlings.This change partly suggests that the ability of its fine roots to store carbohydrates increases with the increase of seedling age.The reactions of leaves and stems are usually related to the regulation of photosynthesis and water transport,while the reactions of roots are concentrated on the ability to store and absorb resources.（5）The carbon storage of annual Q.variabilis leaves is relatively high,and they have strong defense ability.They tend to be preferentially allocated to the stem for growth during the seedling stage;The carbon storage of 2a seedlings under normal light conditions of 75%is significantly higher than that of the other three shading treatments.Compared with normal water treatment（W0）,the carbon storage in the coarse roots of annual seedlings is lower than that of moderate drought treatment（W1）,indicating that they activate defense mechanisms by reducing the growth rate of coarse roots when facing drought stress.The carbon storage of leaves,stems and coarse roots of annual seedlings under the co-stress of three factors was higher than that of 2a,but the storage of fine roots was less than that of 2a,indicating that with the extension of environmental factor stress,the seedlings allocated more carbon to the underground fine roots to obtain more nutrients.Under different nitrogen addition treatments,the leaves of annual Q.variabilis always maintained the highest nitrogen storage capacity among the four organs,which promoted growth investment.Under the interaction of three environmental factors,2a Q.variabilis stores more N in fine roots,which is beneficial for root growth and development.Investment tends to be defensive and conservative strategies.Under the interaction of three environmental factors,the C:N ratio of fine roots is lower,and the turnover rate of fine roots is higher in annual and 2a Q.variabilis seedlings,which is beneficial for obtaining the nutrients required for plant growth.The C:N of the fine roots of 1a and 2a cork oak seedlings was lower and significantly different under the interaction of the three factors.Q.variabilis 1a and 2a had a 6.7%and 10%reduction in medium nitrogen levels（N1）compared to C:N without nitrogen addition（N0）in the control group,respectively.（6）Q.variabilis seedlings can adapt to complex and changing environments by adjusting their growth and defense strategies under shading,nitrogen addition and drought stress conditions.These adaptation strategies exhibit significant heterogeneity in the seedlings and their different tissues（leaves,stems,and roots）.Under environmental stress,seedlings achieve resource redistribution by regulating carbon and nitrogen balance,improving antioxidant enzyme activity,and accumulating defense substances.The root growth and defense strategies of Q.variabilis seedlings exhibit a direct impact on resource regulation,indicating a trade-off between plant growth and defense.The physiological response of the stem reflects that under limited resource conditions,seedlings tend to enhance storage function to improve survival rate and growth capacity.The growth and defense response of leaves indicate the rapid adaptability of Q.variabilis to environmental changes,such as optimizing light energy utilization and resource allocation by regulating photosynthesis efficiency and material accumulation. |
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