Evolution Of Hydraulic Structure In Basal Angiosperms And Its Relation To Photosynthesis

The dominant role of angiosperms in the land ecosystems were proposed to be linked with the efficient water transportation and photosynthetic rate.However,early angiosperms were primitive in the hydraulic structure and their photosynthetic rate was low,limited in the understory of tropical forest.Previous studies suggested that the rise of angiosperms,is related to the evolution of hydraulic structure.Efficient water transport contributed to adequate water supply to the leaves,thus angiosperms could achiieve high photosynthetic rate,consequently be competitive over ferns and gymnosperms.In this study,we explored the evolution of hydraulic structure in basal angiosperms and its correlation to photosynthesis,based on five case studies about leaf hydraulics,photosynthesis and xylem hydraulic structure.(1)We measured the leaf hydraulic structure and function in woody and non-woody species on Piperles.We found that the leaf design is similar in different growth farms,that there is no significant difference b:etween non-woody and woody species in most of the hydraulic and photosynthetic traits,while woody species has significantly lower turgor loss point and higher leaf density.Leaf photosynthesis are related to leaf hydraulic structure and function.The results indicate that the leaf design of woody species is associated to high resource input and low hydraulic risk,while the non-woody species tends to be with low resource input and high hydraulic risk.(2)We compared the leaf structure and function between plants under forest gap and understory in two understory plants,Illicium ternstroemioides and Camellia cuspidate in Fanjingshan Mountain.We found that despite there are difference in leaf structure of plants in two light conditions,the similar high efficiency to low light and tolerance to high light indicates for a convergent adaption to the local light environment.As a basal angiosperm,Illicium ternstroemioides have a moderate photosynthetic rate,while its stomatal density and vein density are low,consequently lower stomatal adjustment rate,limited its growth under gap.Our result indicates that the restriction of the basal angiosperms to the understory may due to the hydraulic risk from the slow stomatal response,not photoinhibition from high light.(3)We collected six key hydraulic traits of global woody plant species,and reconstructed the ancestral state to assess the historical performance and its evolution of these traits,based on the phylogeny.We found that the hydraulic structure-function evolution happened independently and frequently in most of the clades of angiosperms,and be higher in efficiency and higher variation and moderate hydraulic safety than gymnosperms and ferms,which lead angiosperms be competitive in hydraulic traits.There is difference among ANITA,magnoliids and eudicots.The occur of clades with extreme drought tolerance may related to the climatic drought in nearly 70 Ma.(4)We compiled leaf stomatal density and vein density data of 520 land vascular species,to assess the coordination of leaf water supply-demand balance and its evolution.We found a global coordination between leaf stomatal density and vein density among these species,though the lack of this coordination in the gymnosperm and angiosperm species with tracheids as the only conducting elements.The evolution of xylem conduit elements enabled the increase of stomatal and vein densities in modern angiosperms,that high stomatal and vein density consequently promoted the stomatal conductance.Our results indicate there are two major events associated with the surges in xylem hydraulic evolution of angiosperms,the origin of vessels and the emergence of vessels with a simple perforation plate,which diminished the physical limitation to stomatal conductance.The present results reveal that the evolution of xylem conduit elements and leaf shape and venation influence the leaf water supply and demand.(5)At the end of one extreme cold event,we assessed the cold tolerance of 101 warm-climate plants,and the correlation between plant cold tolerance and its northern latitude limit.We found that the distribution of plants is in fluenced to its cold tolerance to cold temperature,that it's poleward movement might be impeded by extreme cold events.Although there are no significant difference in cold tolerance between basal angiosperms and eudicots,our results indicates that the limited early angiosperms may be cold sensitive,the expansion of angiosperms may be related to the increase of global temperature.The quick measurement of Fv/Fm is useful for assessing the cold tolerance of plants,providing valuable information for modelling species range shifts under changing climate conditions and species selection for horticulturalmanagement and urban landscape design.In conclusion,the clearly evolutinonary trend of hydraulic structure evolution make angiosperms achieve more derived physological traits.The transision of the traits,played important roles in the specification,expansion of angiosperms and outcomplete gymnosperms and ferns.The hydraulic structure and function determine the photosynthesis capacity in basal angiosperms.