A Probe Into The Mechanisms Of Canopy Dew Uptake By Photosynthetic Organs Of Desert Trees

Water,as the main abiotic limiting factor in a desert ecosystem,determines the combination and variation of other biotic and abiotic factors.Under the condition of low level of groundwater and deficiency of soil water,the water absorption strategy of the aerial organs(photosynthetic organs)of desert plants was developed.However,the most of the study is mainly concerned the coverage of this phenomenon or to explore the mechanism at a certain level,there is a lack of an integrative study which contains variety of methods from different levels.The existing research is based on the herbaceous with shallow roots and soil biological crusts,and there is a lack of systematic research for desert woody plants with the developed roots and xylem water migration system.Desert woody plants are the dominant species in the desert ecosystem,and the research regarding the intrinsic mechanism of canopy dew absorption of woody plants photosynthetic organs in the desert ecosystem is conducive to the understanding of positive effect of dew in arid land and adaptation of desert plants to drought in the context of global change.In this study,the photosynthetic organs of Populus euphratica and Haloxylon ammodendron were used to study the mechanisms of foliar uptake water intercepted by canopy.The ecophysiology measurements,in situ experiments and laboratory experiments using quantitative RT-PCR,paraffin sections,fluorescence detection,isotope signature and water relationships of shoots was combined to reveal the mechanisms of foliar uptake water intercepted by canopy from multiple scales.The research results indicated that:1)Under typical conditions,the transpiration in daytime and the root water pressure at night were the main regulators of the water relationships between photosynthetic organs and secondary branches of Populus euphratica and Haloxylon ammodendron.In the high humidity conditions,assimilating branches of Haloxylon ammodendron actively absorbed canopy dew and transfer the canopy dew down to the secondary shoots through the reverse water potential gradient between photosynthetic organs and secondary branches(?Photosynthetic organs>?Secondary branches).After an extreme rainfall event,assimilating branches of Haloxylon ammodendron actively absorbed the rainwater intercepted by the canopy and transport the water from the assimilation branches to the secondary branches through the reverse water potential gradient(?Photosynthetic organs>?Secondary branches).The signature of 18 O isotope tracer(the phenomenon of the enrichment of 18 O in the photosynthetic organs,secondary branches,and trunk xylem)showed that the photosynthetic organs of desert woody plants are able to absorb canopy dew and transfer it to the trunk xylem;2)There is a layer of dense cuticles on the surface of Populus euphratica and Haloxylon ammodendron photosynthetic organs,cuticles contacts with air moisture firstly.In addition to,there are hydrophilic polysaccharide compounds on the cell wall of epidermal cells,crystalline cells,and vascular bundle cells in the photosynthetic organs of Populus euphratica and Haloxylon ammodendron.These polysaccharide compounds in the photosynthetic organs connected into an apoplast water migration pathway,which accept the water absorbed by cuticles and transported it to the xylem.The microstructure on the surface of photosynthetic organs and hydrophilic compounds inside photosynthetic organs is the material basis for photosynthetic organs to absorb canopy dew;3)The PIP2-1-like(plasma membrabe intinsic aquaporins regulatory genes)and TIP3-1-like genes(tonoplast membrabe intinsic aquaporins regulatory genes)in photosynthetic organs of Populus euphratica and Haloxylon ammodendron had relatively high relative expression levels under simulated canopy dew conditions.More importanltly,this phenomenon was still obvious under the condition of without root water pressure.It is suggested that aquaporins plays an important role in the process of transporting dew through symplast pathways.The change in the transcription level of PIP2-1 and TIP3-1 which regulated the expression of aquaporins is the molecular mechanism of Photosynthetic organs absorbing canopy dew.4)Along with the water potential of photosynthetic organs increases up to a certain extent,the reverse water potential gradient between ?Photosynthetic organs and ?Secondary branches may be established,and the photosynthetic organs can transport the excess dew to the trunk stem via reverse water potential gradient,which is conducive to the continuous absorption and utilization for canopy dew.The reverse water potential gradient is the energy structure of Haloxylon ammodendron photosynthetic organs absorbing canopy dew.Compared with the Haloxylon ammodendron photosynthetic organs,the transport velocity and amount of dew are low in Populus euphratica,as free diffusion is the dominant way of Populus euphratica photosynthetic organs to migrate water to trunk stem.