F. G. Anderssen
1929 Plant Physiology  
The sap in the tracheae of plants has long been known to contain various substances derived from the soil or from the surrounding tissues. Most of our knowledge of its composition has been obtained by study of the exudate (bleeding sap) from wounds during late winter (bleeding season) (CLARK, 4; SCHROEDER, 23; HORNBERGER, 11; MOREAU and VINET, 16) . This has limited the study to a relatively short portion of the annual cycle and to comparatively few plants. Qualitative studies such as those of
more » ... ISCHER (9, 10) have shown that certain constituents, reducing sugars, occur in the tracheae at various times of the year. DIXON and ATKINS (6, 7) carried out the most comprehensive study on tracheal sap yet reported. They obtained sap from tracheae by centrifuging wood from various parts of trees at different times of the year. They were able to follow the trend of changes in total electrolytes and sugars throughout the year and to estimate the concentrations of the sugars. MAcDOUGAL (13) has also reported a few observations on the sugar content of the sap of pines. The principal obstacle to the study of tracheal sap has been the difficulty in obtaining the large quantities needed for quantitative work with such a dilute solution. The sap has an electrolyte content approximating that of tap water, and organic substances are usually equally low. The only methods of obtaining the sap outside of the bleeding season have been to centrifuge portions of wood as done by DIXON and ATKINS (6) or to wash out the contents of the tracheae with water as done by MAcDOUGAL (13) and MASON and MASKELL (15). Neither of these methods is suited to obtaining large quantities of sap, and both are liable to give sap which has been altered by contamination. An improved method has been described by BENNETT, ANDERSSEN, and MILAD (2) which reduces the liability of contamination and allows sap to be obtained from woody plants in quantities adequate for quantitative work. It seems desirable to extend our knowledge of the composition and changes in the tracheal contents. The sap in the tracheae amounts to several per cent. of the weight of a woody stem. Our only accurate knowledge of it is limited to the bleeding season, during which period the tree is relatively inactive so far as the major activities of growth, absorption, trans-1I take pleasure in expressing my grateful appreciation to Dr. J. P. BENNETT for his interest and valuable advice throughout this work. 459 on July 20, 2018 -Published by Downloaded from PLANT PHYSIOLOGY piration, food manufacture, and translocation are concerned. Exact knowledge of the composition of, and changes occurring in, the tracheal sap, may aid materially in understanding these activities. The work reported here is a preliminary study of tracheal sap from pear and apricot trees. The sap was obtained by the gas displacement method (2) from the main branches of three-year-old Bartlett pear and Royal apricot trees growing in a heavy clay-loam soil. Each branch extracted consisted of two-and threeyear-old wood. Extraction was always done within two hours after cutting the branches, it having been found that appreciable changes occurred in the sap if the branches were cut several hours before use. The branches were cut the same time of the day in order to avoid possible diurnal changes in the sap. Appreciable diurnal variation was noted by HORNBERGER (11) in the titratable acidity of sap from bleeding Birch trees. The sap obtained in the present work was colorless and clear or slightly cloudy, and of relatively uniform conductivity throughout the length of each branch. Expressed sap, used for comparison with tracheal sap in part of the work, was obtained from portions of the same branches used for extraction of tracheal sap by grinding the wood after removal of the bark and pressing the ground tissue under a pressure of 400 kg. per sq. cm. The expressed sap thus obtained was clear but highly colored. Buffer value and reaction of sap Titration curves shown in figure 1 were prepared for tracheal and expressed sap of the Bartlett pear. Fresh 5-ml. portions extracted May 16 were titrated with 0.02 N H2SO4. The reaction of tracheal sap was determined colorimetrically, that of expressed sap electrometrically. From the curves it may be seen that the buffer value of expressed sap was about 25 times that of tracheal sap at the middle of 'May. The buffer value of both would doubtless vary during the year. The low buffering of tracheal sap shows that extreme care must be exercised in determining its reaction. Contamination during extraction with very small amounts of vacuolar sap or other cell contents, or losses of dissolved gases might markedly affect it. Especial care was taken that all contamination from the cut surface of the wood through which the sap emerged was washed away before samples were retained for use. This was accomplished by determining the conductivity of succeeding ml.-portions of the sap as it was collected and discarding these until the conductivity became uniform. In the gas displacement method the sap is collected in a partially evacuated vessel, causing a loss of part of the dissolved gases. The gas of chief concern in relation to the reaction of the sap is carbon dioxide. This is known to be present in considerable amounts in the gases in the tracheae 460 on July 20, 2018 -Published by Downloaded from / 0 / 2 3 ..4 "ti of spS f rnc . %Na Nh FIG. 1. Titration curves of sap of pear branches at the middle of spring. (MACDOUGAL, 13; CLARK, 4) and tracheal sap would consequently be rich in it. It was not feasible in the present work to attempt to collect the sap with its full carbon dioxide content, and the losses during extraction would be variable. In order to make the samples comparable they were allowed to come into equilibrium with the air. The reaction of the sap as determined was then not necessarily the same as it was within the tracheae; but the variations in reaction found were due to substances other than carbon dioxide. The expressed sap with its relatively high buffer value presented no especial difficulties in preparation. The reaction of tracheal and of expressed sap from pear and apricot branches was determined at monthly intervals throughout a year. For each reported value sap was extracted separately from six branches, each from a different tree, the reactions determined separately and averaged. The results are shown in figures 2 and 3. The outstanding features of the curves for both apricot and pear is their convergence in the late winter and wide divergence later. For the pear, fig. 2 , the period of approach lasts through spring, summer, and fall. During this period the curves are almost parallel. For the apricot the divergence of the curves began in midsummer and the period of close approach is relatively short. During one-half of the year in the apricot and one-third in the pear the tracheal sap was con-461 on July 20, 2018 -Published by Downloaded from ANDERSSEN: PEAR AND APRICOT TREES siderably more alkaline than the expressed sap. The maximum difference in reaction was in both cases about three-fourths of one pH. The total fluctuation in reaction was about 0.6 pH in the expressed sap of the pear and 0.7 pH in the tracheal sap, while in the apricot the expressed sap changed only about 0.4 pH and the tracheal sap about 1.0 pH. The winter divergence in the reaction was then due about equally to changes in expressed and tracheal sap in the pear, while in the apricot it was due more largely to changes in the tracheal sap. The trends of the curves suggest that during the active growing season the reaction of the tracheal sap is strongly influenced by that of the surrounding tissues, while during the dormant season this influence is absent. It seems probable that the increased acidity in early spring is due to an increase in organic acids in the sap. During the period of renewed growth respiration is very active and may result in a general increase in the concentrations of organic acids present in the tissues. HORNBERGER (11) found that the titratable acid in "bleeding" sap increased during the blossoming period and later decreased. Inorganic constituents It has long been known that the tracheal sap obtained from bleeding plants contained salts. SACHS (22) detected the presence of K, Ca, PO, and SO4. HORNBERGER (11) studied quantitatively the exuded sap of the Birch and Hornbeam and determined the amounts of Na, Ca, and Mg present. PFEFFER (18) pointed out that the composition of the sap is not constant. The study of bleeding sap, of course, gives a very incomplete picture, leaving the composition and changes in the sap during most of the year entirely unknown. The most comprehensive investigation of tracheal sap is that of DIXON and ATKINS (6, 7) mentioned previously. The contamination of the sap by materials from the injured cells at the cut surface of the branch during the extraction of tracheal sap by the centrifuge method makes it seem probable that the results of DIXON and ATKINS are somewhat too high. TOTAL ELECTROLYTES The specific resistance of the sap was determined at monthly intervals throughout the year. The sap used was of the same lots as used in the determinations of reaction. Each value reported was the average of six determinations, each on sap from a single branch. The results are presented in figures 4 and 5. Both kinds of trees showed a minimum concentration of electrolytes during the dormant season, the pears at about midwinter and the apricots at the time of leaf fall. From very early spring there was a very rapid increase in concentration, reaching a maximum at or shortly after full bloom; after this maximum there was again a gradual decrease in 463 on July 20, 2018 -Published by Downloaded from
doi:10.1104/pp.4.4.459 pmid:16652628 fatcat:upakcvcphrcu5czzpi2fmfomau