Effect Of Cadmium Stress On Leaf Water Relationship Of Five Typical Tree Species In Southern China

The Pearl River Delta of China has experienced substantial increase of heavy metal pollution in the soil,with accelerated urbanization and industrialization over recent decades.Numerous studies have shown that cadmium,one of heavy metals,enrichment can inhibit plant growth and development,but the underlying mechanisms remain unclear.It is known that plant water relation is an important physiological process of plants,and it is very sensitive to environmental conditions,such as heavy metal pollution in the soil.In this thesis,we investigated the response of typical tree species in Southern China to cadmium addition,in term of plant water relations,aiming to understand the adaptation mechanism of plants to cadmium stress,and provide theoretical basis for selecting suitable greening tree species in southern China,where cadmium pollution is severe.This study was located in the South China Botanical Garden,Chinese Academy of Sciences in Guangdong Province.Five dominant tree species of evergreen broad-leaved forest and artificial forest in Southern China were selected,including Acacia auriculaeformis,Cinnamomum camphora,Syzygium hainanense,Liquidambar formosana and Castanopsis carlesii.Leaf hydraulic conductance,and anatomical structure were measured systematically for cadmium treated(treatment concentration is 30g ha^-1)and the control plants.In addition,the expression of cell membrane aquaporin genes was determined using Quantitative real-time polymerase chain reaction?qRT-PCR?.The results showed that leaf hydraulic conductance decreased significantly under cadmium stress except for Acacia auriculaeformis.Meanwhile,there was no significant changes in leaf anatomical structure such as stomata size,stomata density,upper and lower epidermis thickness,palisade tissue thickness,spongy tissue thickness and leaf thickness,indicating that the leaf anatomical traits were not major determinants responsible for the decreases of leaf hydraulic conductance under cadmium stress.Next,we selected Acacia auriculaeformis and Cinnamomum camphora among the five tree species to quantify the expression of leaf cell membrane aquaporin genes at the transcriptional level using qRT-PCR.Consistent with the results of changes in leaf hydraulic conductance under cadmium stress,we found no significant differences in the expression level of aquaporin genes of Acacia auriculaeformis,whereas the expression of two aquaporin isoforms of Cinnamomum camphora?CcPIP1-3 and CcPIP2-1?was significantly down-regulated under cadmium treatment.The results demonstrate that cadmium stress can inhibit leaf water transport,and plants may maintain water balance via regulating the expression of cell membrane aquaporin.Among the five study species,Acacia auriculaeformis,which showed strong resistance to cadmium stress in term of plant water relations,can be a candidate of greening tree species in regions of southern China with severe cadmium pollution.