The Role Of Lignified Cell Wall Water Content In Transport Processes In Woody Stems

The water in the xylem cell wall of woody plants not only forms the apoplastic space,but also plays an important role in the radial transport of plants.The present study was conducted on the campus of Northwest A&F University in Yangling,Shanxi,China.Four species of gymnosperm were selected from campus,they were Metasequoia glyptostroboides Hu&W.C.Cheng,Ginkgo biloba Linnaeus,Podocarpus macrophyllus(Thunb.)D.Don,and Cunninghamia lanceolata(Lamb.)Hook.Centrifugetechnology,xylem anatomical method and other methods were used to measure cell wall water content in the xylem of gymnosperm and to estimate how large molecules can pass through lignified cell wall.The primary objective of this study was to reveal the importance of cell wall water content in transport processes in woody stems.The main results were as follows:1.There were obvious difference on xylem anatomical traits among the four species of gymnosperm:(1)The ray cells were very clear in the cross section of stem in Ginkgo and Cunninghamia compare to that in Metasequoia and Podocarpus under the microscope,in the latter two species,that were not easy to distinguish.(2)For Metasequoia and Ginkgo,the pith volume fraction to whole xylem volume were small,the average values were 3.61±0.91%and 7.97±1.11%,respectively.However,for Podocarpus and Cunninghamia,the pith volume fraction was much larger,the average values were 11.84±2.83%,17.01±0.96%,respectively.(3)The diameter of tracheids and ray cell were smaller in Metasequioa compare to Ginkgo.The average values of the tracheids diameter were 12.93±0.31 μm,14.35±0.19 μm,respectively(P<0.001),and the average values of the ray cell diameter were 9.86±0.32 pm,13.28±0.27 μm,respectively(P<0.001).2.The tensions(negative pressure)that inducing more than 99%embolism of stems in Metasequoia,Ginkgo,Podocarpus and Cunninghamia were different.The average values were-3.98±0.10 Mpa,-5.37±0.07 Mpa,-7.70±0.29 Mpa and-6.33±0.06 Mpa,respectively.The volume fraction of different cell types to the woody volume was also different,the sequence was cell wall volume fraction>tracheids lunmen volume fraction>pith volume fraction>ray cell volume fraction.3.Cell wall water content per stem dry weight were 49.83± 1.15%,40.60 ± 0.67%,42.27±0.94%,37.40±0.38%;the water content per cell wall volume(= cell wall water weight/cell wall volume)were 36.33 ± 1.36%,35.94±2.07%,34.64±3.40%,32.27±1.13%;and cell wall water content per stem wood volume(= total water per unit volume of wood-ray cells water volume per volume of wood)were 22.13±0.87%,20.51±0.95%,21.13±1.32%,15.85±0.31%respectively in Metasequioa,Ginkgo,Podocarpus and Cunninghamia.4.There was significant difference in time that for outputting same 10 Brix concentration solution among three molecular weight sugars,the sequence from long to short time was raffinose(594.5 g/mol)>sucrose(342.3 g/mol)>glucose(198.2 g/mol).In addition,the trend of outputing concentration change with time in glucose and sucrose was consistent:in the beginning,the concentration fast increase then gradually become stable.However,the output concentration change with time in raffinose was slower compare to glucose and sucrose.5.The small molecular weight PEG 200 accumulated in the cell wall was more than PEG 800 and PEG 1540 that with large molecule weight,however,the PEG 800 and PEG 1540 have not significant difference(P=0.99).Moreover,the percentage of free space for dissolving PEG 200 in lignified cell walls were largest,PEG 800 and PEG 1540 were significant smaller than PEG 200(P<0.001),but without difference(P=0.98).