%0 Journal Article %1 McCullyetal_98 %A Mccully, M. E. %A Huang, C. X. %A Ling, L. E. C. %D 1998 %J New Phytologist %K bibtex-import, citeulikeExport cryo, droughtbolism, grass, techniques %P 327--342 %T Daily embolism and refilling of xylem vessels in the roots of field-grown maize %V 138 %X Embolisms in the vessels of maize axile roots of di?erent types were observed directly after rapid freezing of intact, functioning roots in the ?eld, by cryo-scanning electron microscopy. Quanti?cation of the degree of embolization in each root was made by counting empty and full vessels of both the late and early metaxylem (LMX & EMX), and expressed as percent embolized vessels of the LMX, and \% EMX poles containing embolized vessels. Contents of the connecting xylem (CX) at branch root junctions, and of xylem in branch roots were observed also, but not systematically quanti?ed. Records of \% embolized vessels were made from dawn to dusk on summer days in Ottawa under moderate irradiance, and in Canberra under high irradiance. Measurements in Canberra were supported by estimates of irradiance, of stomatal conductance, and of chamber balance pressure of bagged and unbagged leaves. Soon after sunrise embolisms appeared in all types of vessel, at balance pressures c. 300?400 kPa, and increased rapidly with increasing irradiance. During the middle of the day \% embolized vessels reached a maximum (LMX " 70 \% in Ottawa, and " 80 \% in Canberra). At all times the EMX vessels were less embolized. The midday maximum was brief in Ottawa, and \% embolized vessels fell to a low value during the afternoon. In Canberra the maximum was prolonged into late afternoon. By dusk nearly all vessels were once again ?lled with sap. The balance pressures measured during vessel re?lling in Canberra ranged from 500 kPa to 1200 kPa. At all times of the day sap was seen entering some embolized vessels. Almost all were re?lling by mid- to late-afternoon. Such re?lling was especially frequent at junctions of branch roots with the axile roots. X-ray microanalysis of the sap entering the vessels, and of the liquid ?lling or partly ?lling vessels, showed the concentration of mineral solutes present in the sap was below the threshold of detection (" 12 m?). These results are discussed in relation to current opinions about embolisms and vessel re?lling.

@article{McCullyetal_98, abstract = {{Embolisms in the vessels of maize axile roots of di?erent types were observed directly after rapid freezing of intact, functioning roots in the ?eld, by cryo-scanning electron microscopy. Quanti?cation of the degree of embolization in each root was made by counting empty and full vessels of both the late and early metaxylem (LMX \& EMX), and expressed as percent embolized vessels of the LMX, and \% EMX poles containing embolized vessels. Contents of the connecting xylem (CX) at branch root junctions, and of xylem in branch roots were observed also, but not systematically quanti?ed. Records of \% embolized vessels were made from dawn to dusk on summer days in Ottawa under moderate irradiance, and in Canberra under high irradiance. Measurements in Canberra were supported by estimates of irradiance, of stomatal conductance, and of chamber balance pressure of bagged and unbagged leaves. Soon after sunrise embolisms appeared in all types of vessel, at balance pressures c. 300?400 kPa, and increased rapidly with increasing irradiance. During the middle of the day \% embolized vessels reached a maximum (LMX " 70 \% in Ottawa, and " 80 \% in Canberra). At all times the EMX vessels were less embolized. The midday maximum was brief in Ottawa, and \% embolized vessels fell to a low value during the afternoon. In Canberra the maximum was prolonged into late afternoon. By dusk nearly all vessels were once again ?lled with sap. The balance pressures measured during vessel re?lling in Canberra ranged from 500 kPa to 1200 kPa. At all times of the day sap was seen entering some embolized vessels. Almost all were re?lling by mid- to late-afternoon. Such re?lling was especially frequent at junctions of branch roots with the axile roots. X-ray microanalysis of the sap entering the vessels, and of the liquid ?lling or partly ?lling vessels, showed the concentration of mineral solutes present in the sap was below the threshold of detection (" 12 m?). These results are discussed in relation to current opinions about embolisms and vessel re?lling.}}, added-at = {2019-03-31T01:14:40.000+0100}, author = {Mccully, M. E. and Huang, C. X. and Ling, L. E. C.}, biburl = {https://www.bibsonomy.org/bibtex/2ac6e19dc339a3eafa798e6aa5c2a8361/dianella}, citeulike-article-id = {1523845}, comment = {(private-note)Grass appears to droughtbolise readily refilling occurred under local positive pressures... (ie embolised vessels are either filled with water vapor or air which has positive pressure) but also under active transpiration Roots are the least likely part of a plant in which to ?nd xylem embolisms, because (as required by the cohesion theory), in any transpiring plant there is a gradient of increasing tension in the xylem from the roots to the top of the shoot. Recently, Canny (1997) has demonstrated the great advantage of using cryo-scanning electron microscopy to observe embolisms in situ in leaf tissue directly. We have used this method too Results: Especially in the LMX vessels, the liquid appears to have come into the vessel through pits on the side facing the endodermis, although un?ooded pits also occur in this position (Fig. 4 h). In some cases the liquid drops have formed on the inside of pits that overlie living xylem parenchyma cells (Fig. 4 l). But frequently when liquid appeared to be entering the vessels through the pits in the lateral walls, it was in the vicinity of a branch root junction. This ob- servation led us to investigate these regions in more depth. The CX vessels, when they are full of sap, are also easily distinguished from the surrounding living cells in the cryo-preparations by their much darker lumens, which are relatively free of solute separation lines The liquid entering re?lling vessels contained mini- mal amounts of solute. The one element present in signi?cant amounts was potassium. The direction of refilling is transverse, not longitudinal Read Canny 1997: sunflower leaves refilling Whatever is refilling these vessels, it isn't root pressure in the normal sense, nor is it osmotic pressure of the refilling sap. Possibly the paradox may be resolved along the lines suggested a century ago by Pfe?er (1900), i.e. a non- osmotic, ? active ? secretion of water into the vessels by the adjacent living cells, or by the similar proposal elaborated by Canny (1997), that the balance press- ure measured in the pressure chamber is not, in fact, a measure of the tension in the xylem, but of the tissue pressure that forces water out of cells by reverse osmosis and into the empty vessels.}, interhash = {355de3204e4b8a1ac21f1ba5c79cede1}, intrahash = {ac6e19dc339a3eafa798e6aa5c2a8361}, journal = {New Phytologist}, keywords = {bibtex-import, citeulikeExport cryo, droughtbolism, grass, techniques}, pages = {327--342}, posted-at = {2007-07-31 06:39:44}, priority = {2}, timestamp = {2019-03-31T01:16:26.000+0100}, title = {{Daily embolism and refilling of xylem vessels in the roots of field-grown maize}}, volume = 138, year = 1998 }