Phenotypic Plasticity Of Whole Plant And Leaf Traits In Quercus Acutissima And Robinia Pseudoacacia In Relation To Different Habitats

Because of global climate changes and frequently anthropization by which the effects will reach the threshold of carrying capacity of the earth system,forest ecosystems suffer from severe destroy going with serious degradation of ecological functions.The main effects on plants are causing the changes of environmental gradients to which the plants are used to adapt.It includes the changes of spatial or temporal environmental gradients.In recent years,more and more researches are focused on the severe drought caused by extreme climate events and light heterogeneity in habitat fragmentation.The traits of whole plant and organisms respond to environmental variation on various timescales.Phenotypic plasticity enables a given genotype to produce a range of trait values across environmental gradients and within the lifetime of an individual organism.By contrast,variation in selection pressures along environmental gradients gives rise to heritable differences between populations or species,through the intergenerational process of evolution. There is often strong similarity between the plastic and allometry responses of quantitative traits to environmental gradients.It has became a hot point in modern plant ecophysiological researches.In this study,we chose Quercus acutissima Carr. and Robinia pseudoacacia L.as the research objects.The two plant species are normal deciduous broadleaf species and usually form the mixed forests in north China.We used modern ecophysiological equipments and measurements to study whole plant growth,architectures,biomass accumulation and allocation,leaf morphology,leaf movements,photosynthesis,biophysiology and biochemistry in the seedlings of these two species under different light and water conditions.The results will enrich the research contents of global climate changes,plant hydro,stress and photosynthetic ecophysiology,help to understand the mechanisms of adaptation and niche differentiation of coexisting species,and correlate the relationships between climate and plant tolerance.It may also provide evidences for building the database of vegetation and climate,supervising the reforestation and establishing the blueprints for local ecological environment and sustainable development. Different responses of deep and shallow root system of Q.acutissima and R. pseudoacacia to a precipitation pulse event after a prolong drought period in summer was studied to analyze the photosynthetic sensitivity of these two species.The results indicated that the deep root system species could tolerate more serious drought stress than the shallow root system one when facing to the summer drought. When compared with R.pseudoacacia in the simulated forest edge,the photosynthetic rate,stomatal conductance and effective quantum yield of PSII became less depressed for Q.acutissima in the simulated forest edge.Besides the lower degree of noon inhibition,these parameters could recover to a better extent in the evening.And the transpiration rate maintained higher values during the whole day.All parameters had a certain recovery after the precipitation,and the degrees of recovery were time lag for the seedlings in the simulated forest edge because of the higher press levels.The increased extents of photosynthesis were larger for R. pseudoacacia in the simulated understory.Midday depressions of photosynthesis were still obvious for Q.acutissima.The deep root system species have a higher drought tolerance.They can maintain the photosynthesis during the drought periods but can recover slowly after the rainfall.The shallow root system species are just diametrically opposite.A water gradient experiment with four different water supply levels was conducted by artificial water control in the rainout shelters to study the plasticity of morphological architecture,photosynthetic characters,biomass accumulation and allocation in Q.acutissima and R.pseudoacacia seedlings in response to diverse water stress.The results showed that morphological variables of height,stem diameter,total leaf area,crown area,leaf area index and total leaf number decreased with increased in water stress.Consistent with the increasing leaf water saturation deficit in water stress,net photosynthetic rate and transpiration rate also decreased. The diurnal course changed from single-peak to two-peaks patterns.Factors that limited photosynthesis shifted from stomatal limitation to non-stomatal limitation. Reduced light use efficiency,as well as weak photoprotective effect in photosystem, caused more serious photoinhibition.Water use efficiency can be improved under moderate water stress.Biomass accumulations to each organ were restricted by the deficit of soil water content.Under water stress,more photosynthetic products were transferred to belowground biomass,especially to the lateral roots.As a result, carbon allocation patterns were altered by increased in root mass ratio at the expense of decreased leaf mass ratio,and it therefore led to higher root to shoot ratio,lower leaf area ratio and lower specific leaf area.The contents of chlorophyll were not significantly different under water stress.Allometric trajectories of morphology were changes in different water treatments.Plasticity index analysis indicated that the seedlings could adapt to diverse water stress through plastic responses in morphology and photosynthesis,whereas the differences of biomass accumulation and allocation might be partly affected by the allometry.The effects of water stress on seedlings of Q.acutissima and R.pseudoacacia showed some differences for the different grow patterns.The former was sensitive in photosynthesis,but the latter was affected in morphology and biomass of aboveground.A light control treatment with three light gradients was conducted in shade shelters covered by plastic films or woven black nylon nets to study the plasticity of morphological architecture,photosynthetic characters,biomass accumulation and allocation in Q.acutissima and R.pseudoacacia seedlings in response to diverse gradients of light.The results showed that morphological variables of height,stem diameter,total leaf area,crown area,leaf area index and total leaf number were totally inhibited under serious shade conditions.Whilst,growth of seedlings had some advantages under moderate shade treatments.Effects of light on allometric trajectories of morphology were smaller in compared with the water effects,and the changes in R.pseudoacacia were more obvious than Q.acutissima.The diurnal course of photosynthetic rate changed from two-peaks to single-peak patterns in Q. acutissima with the light decreasing,and photosynthetic parameters and biomass accumulation were also decreased.But R.pseudoacacia could maintain the curves for its leaf movement,and the maximum of photosynthetic parameters and biomass accumulation occurred under moderate shade treatments.Higher leaf area index and lower root to shoot ratio could increase the capacity of light capture.Meanwhile,the contents of chlorophyll were increased and Chl a/b were decreased.The different strategies of light utilization can reveal the status of tree species in succession to a certain extent.Leaf inclination,physiological and biochemical parameters of Q.acutissima and R.pseudoacacia seedlings were studied under different stress conditions to find out the features of leaf movement and its protective functions.The results showed there was no significant leaf movement in Q.acutissima.Co-operation of midrib and petiole angles caused significant leaflet movement in R.pseudoacacia under different water and light conditions.Leaf inclination was affected mainly by light;a low level of irradiance caused leaves to be arranged horizontally.Diurnal rhythmicity was lost after the long-term stress,but resumed,in part,in the recovery period.It reflected the hysteresis in the light-dark transformation and rapid recovery in the dark-light transformation.Drought stress caused leaves to tilt more obviously during the whole periods and decreased damage to the photosystem.It could be reflected by changes of chlorophyll fluorescence and chlorophyll content.Significant physiological changes occurred under different conditions of light.Increased energy dissipation and light capture were the main responses to high and low level of irradiance,respectively.Sun tracking movement in a single leaf and sun avoiding movement in the whole plant coexisted,which reflected the rapid leaf protective movements and the tardive adaptation in the whole canopy harmoniously guaranteed that the seedlings could show better response to the stress and normally growth.Habitat effects on leaf morphology and the underlying regulations were studied by a field measurement and simulated experiments with interactive treatments and different gradients of water and light availability.The morphological parameters we investigated include leaf size,shape and venation pattern which can be easily measured in the field.The results indicated that leaf morphology variations occurred over most of the parameters,and the causes were consistent between the field study and lab experiments.Light was the main factor inducing leaf morphological plasticity.The variations caused by drought were due mainly to the allometry.The leaf size,including leaf area,leaf mass,leaf length and width,became smaller with a short supply of resources.Leaf length and width were only affected by leaf area using analysis of covariance.The leaf petiole did not lengthen under shade stress, suggesting a trade-off relationship between functional tissues and support structures in leaf lamina and leaf petiole.Meanwhile,trade-offs between investments in support and functional structures also optimized the venation pattern of major and minor veins.Leaf shape became narrower in drought and broader in the shade,as reflected in changes in leaf elongation and three leaf fractions of Q.acutissima in supplement.Leaf elongation and fractions of the lamina area altered to enhance resources acquisition and conservation.Higher vein density played a part in enhancement of mechanical support and water supply under stress conditions. Leaves with more teeth show more active photosynthesis for better survival and growth,but are disadvantageous in xeric environments because of higher transpiration.Leaflets of R.pseudoacacia partially played a role such as leaf teeth, for they are not only individual units,but also a part of the compound leaf.The variations of leaflet morphology should consider the benefits of whole compound leaf.In conclusion,this research shows that the seedlings of Q.acutissima and R. pseudoacacia under different habitats can respond and adapt to environmental changes via phenotypic plasticity and allometry of whole plant and leaf traits.It reflects that there are many adaptive mechanisms,by which these two species assure their seedlings can normally grow and survive under different habitats,and may adapt to more fluctuant climate changes in the future.All of these may be important reasons for Q.acutissima and R.pseudoacacia to be the ideal species for ecological reconstruction in the center of Shandong mountain areas.