Title
The rise of sap in tall grapevines
Document Type
Article
Publication Date
1955
Volume Number
30
Source Publication
Plant Physiology
Abstract
The common northern grapevine (Vitis labrusca L.) grows rather abundantly in the wooded area around Woods Hole, Massachusetts. It frequently grows as a liana, climbing high in various trees, and attaining heights of 17 to 18 meters. In the winter the vessels of the stem and branches are filled with gas and usually contain no sap. With the warm weather in April and MIay, and before the leaves are outs the vines become full of sap which drips out from the slightest cut in the wood of stem or twigs. The vessels are so wide and so closely packed together that one can see the light through a piece of vine 2.5 cm long, and one can easily blow air through a meter- long section. The sap will flow easily back and forth in a section when it is slightly tilted. This material seemed suitable for measurements of sap pressure and sap flow. As vines could be found that were taller than ten meters, such measurements possibly could give some direct information on how sap reaches the top of tall trees. The cohesion theory of Dixon and Joly (14) and Askenasy (4) (cf. Dixon, 13) is now rather generally accepted as the explanation of this phenomenon and it seems well documented as a factor in sap transport in low plants. In its application to tall trees, how- ever, the theory is still disturbingly void of direct evi- dence. Excellent reviews of sap transport are to be found in the monographs by Crafts, Currier and Stocking (11), and Preston (32). We shall only briefly mention those observations which bear most directly on our subject. EV IDENCE FOR COHESION IN VESSELS: That a slight tension can exist in the vessels of living plants has long been known. When a twig is cut under water and then sealed air-tight to the top of a water- filled mercury manometer, transpiration will some- times pull the mercury a few decimeters above baro- metric level (8, 28, 44, 45, 47). A twig forced to take in water from a potometer via an artificially produced resistance will often pull the water through at a rate much higher than a vacuum pump can pull it through the same resist- ance. From this, the pressure drop across the resist- ance can be calculated to reach as much as 10 to 20 1 Received June 14, 1954. 2 Contribution No. 709 from the Woods Hole Oceano- graphic Institution, Woods Hole, Massachusetts. 3 Present address: The Johnson Foundation, Univer- sity of Pennsylvania, Philadelphia, Pennsylvania. atmospheres, or even more (6, 18, 20, 21, 27, 29, 30, 35, 38). Various other potometer experiments have been performed on undetached branches or stems of trees and bushes (20, 30, 36, 38), which sometimes gave a tension of several atmospheres. As will be shown, however, these experiments did not necessarily mean that the vessels were involved. Kramer (22) found that the transpiration rate in intact tomato and sunflower plants was more than 20 times greater than the rate at which a vacuum pump could pull sap up through the stump. Bode (6) found that, when he wilted shoots of Syringa by closing the cut end with wax, the decrease in diameter of the twig and indi- vidual vessels was greater than could be effected by a vacuum pump. Crafts (10) produced a break in the sap in the intact vessels of wilted Ribes plants by lightly tapping the vessels. The ends of the broken sap columns re- tracted rapidly, indicating cohesion before the break, provided no air entered. A study of the injection rate of dye solutions into the vessels of various trees led Preston (32) to estimate that cohesion of up to three atmospheres may exist in at least some vessels. Arcichovskij and his collaborators (1, 2, 3) deter- mined the concentration of sucrose which would neither gain nor lose volume when in contact with the exposed cambium. This was done by a potometer or by ob- serving the absence of optical streaking (Schlieren). The results reflected beautifully the diurnal transpira- tion cycle and showed also a correlation with height. It was assumed that the osmotic pressure of the test solution equalled the sap pressure in the xylem. A birch tree had, accordingly, sap tensions as high as almost -37 atmospheres and a desert tree gave the staggering figure of - 143 atmospheres. The pressure gradient in a birch tree was as high as 5 atmospheres per meter and in the desert tree no less than 44 atmospheres per meter. Whatever these very high figures mean, they are hardly conclusive as to the sap pressure in the vessels, for they were also obtainecl on excised material where the vessels were open. Dixon (12) estimated that a pressure gradient of the order of 0.2 atmosphere per meter would suffice for normal sap ascent. Several observations have been used in support of the cohesion theory, which fit it but do not prove its validity. The fact that the cohesion of sap or water in glass tubes or fern annuli (12, 37, 46) may be great enough to overcome both the weight, and the esti- atmospheres, or even more (6, 18, 20, 21, 27, 29, 30, 35, 38). Various other potometer experiments have been performed on undetached branches or stems of trees and bushes (20, 30, 36, 38), which sometimes gave a tension of several atmospheres. As will be shown, however, these experiments did not necessarily mean that the vessels were involved. Kramer (22) found that the transpiration rate in intact tomato and sunflower plants was more than 20 times greater than the rate at which a vacuum pump could pull sap up through the stump. Bode (6) found that, when he wilted shoots of Syringa by closing the cut end with wax, the decrease in diameter of the twig and indi- vidual vessels was greater than could be effected by a vacuum pump. Crafts (10) produced a break in the sap in the intact vessels of wilted Ribes plants by lightly tapping the vessels. The ends of the broken sap columns re- tracted rapidly, indicating cohesion before the break, provided no air entered. A study of the injection rate of dye solutions into the vessels of various trees led Preston (32) to estimate that cohesion of up to three atmospheres may exist in at least some vessels. Arcichovskij and his collaborators (1, 2, 3) deter- mined the concentration of sucrose which would neither gain nor lose volume when in contact with the exposed cambium. This was done by a potometer or by ob- serving the absence of optical streaking (Schlieren). The results reflected beautifully the diurnal transpira- tion cycle and showed also a correlation with height. It was assumed that the osmotic pressure of the test solution equalled the sap pressure in the xylem.