| Global climate changes pose new challenges to the survival and adaptability of warm-temperate plants,which are expected to change the frequency,severity and duration of the precipitation events in many regions.Precipitation changes will limit the growth and productivity of warm-temperate plants,and may even threaten their survival.And these events thus could disturb the inter-relationships among plants,affecting the safety and stability of warm-temperate forest ecosystems.Hence,it is critical to understand the water use strategies,drought-recovery responses,drought mortality mechanisms,and stress memory of plants.This knowledge is essential for predicting the fate of forest ecosystems,and protecting the stability of forest ecosystem under future climatic conditions.The related research is a hot spot in the study of plant physiological ecology,but there are still many problems that need to be resolved.For example,there is still no perfect definition of the isohydric-anisohydric response strategy of plants.The dynamics of hydraulic and nonstructural carbohydrates(NSC)in the process of drought response are under controversy.And more researches are needed to verify whether the hydraulic structure can recover completely after rewatering.The hypotheses about drought mortality mechanisms are still very controversial.And there are still considerable conflicts about stress memory.Based on the issues mentioned above,the greenhouse experiments and field experiments were conducted to explore drought stress responses and recovery,drought mortality mechanisms,drought stress memory,and water use strategies of plants.Firstly,the seedlings of Quercus acutissima,Robinia pseudoacacia,Vitex negundo var.heterophylla,and Amorpha fruticosa were used to explore their responses to drought and subsequent rehydration.In this experiment,leaf gas exchanges,stem hydraulics,and NSCs were measured,in which leaf stomatal conductance was measured daily as an indicating parameter.Secondly,the seedlings of Q.acutissima and R.pseudoacacia were used to explore the mortality mechanisms,according to their hydraulic conductivity loss and NSC depletion.Thirdly,the seedlings of Q.acutissima were used to explore their adaptability and plasticity to soil water content fluctuations by calculating the mean values of the relative distance plasticity index(RDPI)of avoidance traits and tolerance traits,whereafter their growth and NSC transfer dynamics were measured to detect drought stress memory.Finally,Besides,field sampling was conducted on 20 deciduous woody plants to clarify their water use strategy along an isohydry-anisohydry continuum according to their hydroscape areas.The key traits relating to plant water use strategies were combined with tradeoffs to explore the strategies adopted in resource acquisition and allocation.The main results and conclusions are as follows:A controlled experiment was conducted on seedlings of Quercus acutissima,Robinia pseudoacacia,Vitex negundo var.heterophylla,and Amorpha fruticosa to explore the water use strategies under drought stress and rewatering.The results showed that under mild drought,the stomatal conductance(gs)of Q.acutissima and V.negundo var.heterophylla seedlings declined slower than R.pseudoacacia and A.fruticosa seedlings.And Q.acutissima seedlings could maintain the midday water potential(?md)relatively stable,while the ?md of V.negundo var.heterophylla seedlings fluctuated along soil drought.According to this,the Q.acutissima seedlings were defined as isohydric behavior,and the V.negundo var.heterophylla seedlings were isohydric in their stomatal behavior but anisohydric in their water potential.The R.pseudoacacia and A.fruticosa seedlings showed a larger slope of gs descending.And A.fruticosa seedlings could maintain the ?md relatively stable,while the ?md of R.pseudoacacia seedlings fluctuated.That is,R.pseudoacacia exhibited an anisohydric behavior while A.fruticosa were anisohydric in their stomatal behavior but isohydrodynamic in their water potential.It can be seen that leaf stomatal behavior could decouple with water potential response,indicating that the stress response strategies are a complex combination of functional traits among different organs.Under severe drought stress,the water potential of the four species declined to a lower level,and the xylem hydraulic conductivity also decreased significantly.It indicated that once the stomatal control on water loss was surpassed,the stem hydraulic conductivity eventually declined,that is,hydraulic integrity breaking.After reirrigation,the seedlings of four species were able to recover from the mild drought stress at a faster rate(within one day),while the recovery from severe drought stress took longer(up to 11 days),indicating that the recovery time mainly depended on the stress severity.Based on the results mentioned above,the seedlings of Q.acutissima and R.pseudoacacia were chosen to investigate the drought mortality mechanisms under different drought intensity.A complete loss of hydraulic conductivity,but a partial depletion of total nonstructural carbohydrates(TNC)were detected,in the dead plants.The TNC contents of dead R.pseudoacacia seedlings were 69%and 34%of the control seedlings under intense drought and mild drought,respectively,which were 85%and 81%in dead Q.acutissima seedlings(not significantly different under intense drought).Therefore,loss of hydraulic conductivity and carbon limitation both contribute to drought-induced death,and the stem hydraulic failure could be associated with the loss of tissue carbohydrates required for osmotic adjustment and the refilling of embolized conduit.It can be proposed that carbon runout with drought mortality is not ubiquitous.Based on the drought-response performance of Q.acutissima seedlings in the above experiments,another experiment was conducted on Q.acutissima seedlings to elucidate the mechanisms responding to soil water fluctuations and its priming effects on growth and carbon dynamics in a recurrent successional drought.The results showed that Q.acutissima seedlings responded to soil water stress by drought avoidance and drought tolerance,but the mean relative distance plasticity index(RDPI)of the avoidance traits(0.23)was significantly higher than that of the tolerance traits(0.16),according to which,the Q.acutissima seedlings were classified as drought-avoiding species.This study also found that the seedlings of Q.acutissima tended to maintain a relatively stable amount of total nonstructural carbohydrates(about 5%of the biomass)when subjected to soil water stress,while reduce the structural growth rate to near-zero or even negative.The ability to "sense" the size of the carbon storage and maintain its stability reflects a tradeoff between increasing stress resistance and growth maintenance.In addition,among the training methods adopted in this study,being droughted and then well-watered helped Q.acutissima seedlings to perform better in response to a recurrent drought stress.A field experiment was conducted on 20 deciduous woody species to explore their water use strategies.The results showed that the hydroscape areas differed significantly among these 20 species,with the values ranging from 27.8 MPa2(Albizia julibrissin)to 0.24 MPa2(Rhus typhina).Accordingly,all the species were arranged along an isohydric-anisohydric continuous spectrum based on their hydroscape areas,and the isohydry degree of a given species was relative.The linear regression of the hydroscape areas of all species and other measured traits showed that the hydroscape areas were significantly related to the turgor loss point and wood density.Specifically,the hydroscape areas were negatively related to the turgor loss point,but positively related to wood density.These two traits could be used as key traits to reflect plant water use strategies.Correlation matrix analysis of all traits showed that the turgor loss point was positively correlated with hydraulic conductivity and the percentage loss of conductivity,and wood density was negatively correlated with hydraulic conductivity,the percentage loss of conductivity,and midday water potential.It can be concluded that the species near to the anisohydric response were close to the slow economic strategy,with higher wood density and lower turgor loss point,as well as lower resource acquisition and utilization rate,and higher stress tolerance.While the species near to the isohydric response were close to the fast economic strategy.Therefore,it is speculated that although isohydric species can plunder resources to occupy a competitive advantage when the environmental resources are abundant,the anisohydric species have a higher tolerance to low-resource environments and may have greater survival chance in extreme-droughted conditions.The functional traits measured in this study covered water transport and utilization,and carbon assimilation and consumption.Based on the whole-plant scale,it showed that the leaf stomatal response decoupled from the stem water potential fluctuations,and the process of carbohydrates assimilation and remobilization coupled with the hydraulic function,which enriched the research results of the water-carbon coupling process in the whole-plant level.This study used the plasticity of plant functional traits to classify stress tolerance/avoidance strategies,proposing a new quantitative classification method.This study ranked 20 species along the"isohydric-anisohydric" spectrum,and their relative position along the "fast-slow" economic spectrum,aiming to providing a new perspective on the tradeoffs study considering multiple continuous spectrums.The results of this study will enrich the researches of plant stress response strategies,drought mortality mechanisms,plant stress memory,and hydraulic physiology and ecology.The study will provide theoretical support for accurate vegetation dynamic prediction in the context of climate change.The study can also guide the forestry practices,such as vegetation restoration and reconstruction,forest management,and forest quality improvement. |
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