NEGATIVE TRANSPORT & RESISTANCE TO WATER FLOW THROUGH PLANTS1"2 R. DUANE JENSEN, STERLING A. TAYLOR, & H. H. WIEBE3 UTAH AGRICULTURAL EXPERIMENT STATION, LOGAN

INTRODUCTION I. Some investigators have considered the entire soil-plant-atmosphere system (1, 6, 19, 24). They Negative transport is the downward conduction applied an analogue of Ohm's law and showed that of water in the plant. This phenomenon has been water transport is controlled by the potential dif- studied by several investigators, yet oonsiderable con- ference across the section and the resistance within troversy about several aspects of the problem still the segment. This theory also proposes the im- exists. portant consideration that the rate of movement is The portion of the leaf through which water en- governed by the point or region of greatest resistance ters is obscure. Meidner (16) suggested that spe- in the system. Those who have studied this theory cialized epidermal cells of the plant, Chaetachme agree that the greatest resistance under natural con- aristata were involved in the phenomenon. Gessner ditions is usually located at the leaf-atmosphere inter- (8) decided that most of the water was absorbed face where the water is converted from liquid to directly through the cuticle. Most investigators (4, vapor. Most of these studies seem to be based more 23) have considered that no water enters through the upon theoretical arguments than direct experimental stomates (except perhaps a small amount of water results. vapor). II. Other scientists have investigated the move- Breazeale, McGeorge, and Breazeale (2, 3) in- ment of water in plants by studying some particular vestigated the absorption of water by leaves and its part of the system, such as the flow of water in the subsequent transport through the plant to the soil roots, leaves, or stem. Resistance to water flow in surrounding the roots. They concluded that tomato the conducting tissue of the stem is generally con- plants could grow to maturity, flower, and set fruit sidered to be small as compared to other parts of the with no other source of water than that absorbed plant (5, 13, 15, 17). Some researchers have found through the leaves from an atmosphere of 100 % the resistance in the roots is much larger than in the humidity. They demonstrated that tomato plants stems (12, 13, 14). Others have observed that the can absorb water from a saturated atmosphere, trans- resistance in leaves is larger than in stems and roots port it to the roots, and build up the soil moisture to (26). The resistance in the vascular elements can or above the field capacity. Other investigators re- become larger when very small diameters are en- peated the experiments of Breazeale but could get no countered (7, 27). It has also been indicated that evidence of actual water secretion by roots (9, 10, 25). the resistance to water flow is uniform through the Stone, Shachori, and Stanley (22) concluded that cell walls, membranes, and vacuoles of plant tissues negative transport occurs only when the tempera- (1, 19). The experimental evidence to support these ture is allowed to fluctuate and is caused by vapor concepts is meager and inconclusive. pressure gradients and not by any active secretive Experimental measurements of the relative magni- force within the plant itself. Slatyer (20, 21), who tude of the resistance of the stem, leaves, and roots reviewed these studies, stated that the main reason to water flow in the absence of a water phase change for lack of transport into soil is lack of an adequate have been made. This gives evidence of the relative gradient. contribution of the several plant parts to water flow The movement of water in plants has been studied resistance without the complicating factor of vapor- from two different approaches: ization. Once this contribution to water flow resist- ance is known then studies can be made to combine the vaporization and vapor diffusion resistance as well as the soil resistance to water flow to the ab- 'Received Feb. 6, 1961. sorbing root surface. These experiments have also 2Work reported here was done in cooperation with the twelve western states and the Agricultural Research produced some information about negative transport. Service, U.S.D.A. through Western Regional Research Project W-67. Published with approval of the director, Utah Agricultural Experiment Station as Journal Paper MATERIALS & METHODS 172. 3 Research assistant, professor of agronomy (soil A schematical drawing of the apparatus is shown physics), and associate professor of botany, Utah State in figure 1. The apparatus consisted of two cylin- University, Logan. drical lucite chambers ( A & B) each 12 inches long 633 634 PLANT PHYSIOLOGY The amount of water flowing through the plant was determined by the movement of mercury droplets in the water filled capillary tubes. To make sure D---- that water was moving from the one chamber through the plant into the other chamber, the mercury droplets had to show movements in both tubes before the flow was recorded. The system was designed so that the roots or leaves or both, could be cut from the plant with a razor blade through the stopper fill- ed openings (H, H', H", & H"') in the cylinders without removing the plant. This made it possible FIG. 1. Suction apparatus for measurement of water to see how the rate of water flow differed when either flow through plants. the roots or leaves or both were removed. The leaves were severed at the point where they joined the petioles; the roots were cut off just above the up- and 5 inclhes in diameter. A removable lucite parti- per roots. All experiments were performed with the tion (C) containing a 2-inch diameter hole was placed entire system below atmospheric pressure, conse- between the two chambers. Calibrated capillary quently the term suction is used rather than pressure. tubes (1 mm diameter) (E & E'), 100 cm long, Suction differences of 15, 25, 35, and 45 cm of were fastened to the ends of the cylindrical chambers. mercury and 20 C were used. Mercury manometers (D & D') constructed from 1 The apparatus was tested by sealing a glass rod cm diameter glass tubing, were joined to the capillary in place of the plant stem, the system was then placed tubes. The apparatus was suspended in a constant under suction to be sure that there were no leaks in temperature water bath after the plants were properly the system. With no leaks in the system any trans- placed in the chambers. The temperature could be fer of water from one chamber to the other would be quickly changed and maintained at any desired value. through the plant. A coil of %4 inch copper tubing was placed inside each chamber through which the bath water was cir- culated, thus reducing the temperature lag inside the RESULTS & DISCUSSION chambers. Tomato (Lycopersicont esculentum Mill.) and sunflower (Helianthus annuus L.) plants, grown Water flowed equally well through plants in both in Hoagland and Arnon's number two nutrient solu- the normal and negative directions. This observa- tion (11) until they were between 10 and 14 inches tion was further confirmed by the use of dyes. A high, were used for the experiment. The plant stem water solution of Red Shilling U.S. Certified Food was placed through the hole of the partition (C) and Color (McCormick & Co., Inc., Baltimore Md.) was sealed with Armstrong's Adhesive A-1 (Armstrong used. The color solution was tested to see that it did Products Co., Warsaw, Ind.) a short distance above not move through the plant tissue to any considerable the roots. When the adhesive had hardened suffi- extent unless it moved with the water when a proper ciently, the partition was placed between the chambers gradient was established, (table I). With both sun- so that the roots protruded into one chamber and the flower and tomato leaves the food color solution leaves into the other. The chambers were then bolt- streamed from the leaf veins which extended to the ed together and filled with air-free distilled water to leaf edge. The coloring appeared as small streams or give a continuous water system from each chamber rivers flowing into the water filled cylinders. The through the capillary tubes to the mercury columns solution apparently was escaping through the hyda- of the manometers. At the beginning of each run, thodes of the leaves. In not a single instance was the suctions of equal magnitude were applied to both solution observed to escape from any other part of the chambers simultaneously to remove the air from the tissue. When the flow was in the negative direction, plant tissue. Preliminary studies showed that unless the solution escaped near the tips of the roots much this air was removed, the water measurements were the same as described for the leaves, except that the jumpy and unpredictable. dye streams were considerably smaller.

FIG. 2. The relationship between the quantity of water flowing through plant tissue and time at a series of suction differences for a whole sunflower plant No. S-14. FIG. 3. The relationship between the quantity of water flowing through plant tissue and time at a series of suction differences for sunflower stem plus leaves No. S-14. FIG. 4. The relationship between the quantity of water flowing through plant tissue and time at a series of suction differences for sunflower stem No. S-14. FIG. 5. The water flux as a function of the suction difference for sunflower tissues at 20 C. JENSEN ET AL.-NEGATIVE TRANSPORT & RESISTANCE TO WATER FLOW 635

TIME (min)

-18 I 2 4 0.14 5 - 161 2

141_ 0.12

- E i2 0.10 4

E 10 U- *E 0.08

UJ E 8 D 0.06 -J U- 6 I 45 cm Hg 2 35 cm Hg Q04_ 41 3 25 cm Hg 4 IS cm Hg 0.02- 2 f

I . I I a I I . -- . . 2 4 6 8 10 12 1DIC 0% 1c, TIME (min) SUCTION DIFFERENCE 636 PLANT PHYSIOLOGY

TABLE I TIME REQUIRED FOR FOOD COLOR SOLUTIONS TO MOVE THROUGH PLANT TISSUES IN NORMAL & NEGATIVE DIRECTIONS UNDER DIFFERENT SUCTION DIFFERENCES SUNFLOWER TOMATO PLANT DIRECTION OF SUCTION TISSUE TISSUE PART TIME SUCTION TIME FLOW DIFFERENCE LENGTH (min ) DIFFERENCE LENGTH (cm Hg) (cm) (cm Hg) (cm) (min) Nor. 47 6.6 46 5.6 Whole Neg. 50 6.0 52 4.8 Stem Nor. 46 16 3.8 & Roots Neg. 54 13 3.4 46 16 3.4 Stem Nor. 49 20 2.8 50 15 1.9 & Leaves Neg. 47 21 3.2 Nor. 54 26 0.3 42 20 0.3 Stem Neg. 45 21 0.4 43 15 0.25

No reports from the literature have been found ably faster than through the roots. The differences that mention the possibility of the hydathodes func- between the magnitudes of the resistances in the vari- tioning in the movement of water into and out of the ous sunflower tissues and the tomato tissues may leaf except in the process of guttation. It has also have resulted from the difference in the age and size been observed that the drops of guttation water are of the plants and the structural differences in the sometimes reabsorbed through these pores (18). conducting tissue of the two plant species. This evidence, coupled with the observation that the The sum of the resistances for the stem, leaves, water moved into and through the hydathodes of the and roots does not equal the resistance obtained for leaf equally well in both the normal and negative the whole plant. The total of the resistances of the directions, indicates that these pores might be in- several parts is somewhat smaller. It was impos- volved in water absorption through the leaf. sible to obtain measurements on the leaves or roots The resistance to water movement in the various alone. The plant stem was included and had to be tissues was calculated by A. directly measuring the subtracted to obtain the resistance in the roots or water flow as a function of time at a series of suc- leaves. If there was an error which increased the tion differences; B. evaluating the slope of the plot stem resistance, the error would have a double ef- water flow versus time to obtain the flux; C. obtain- fect. The stem resistance could easily have been in- ing the apparent conductivity from an evaluation of creased if some of the vessel elements were crushed or the slope of the graph water flux versus the suction plugged when the roots and leaves were cut from the difference, and D. applying the fact that resistance stem. and conductivity are reciprocals. The resistance in a system is usually proportional The plot of amount of water moved through the to the length of the conductor and inversely propor- plant versus time for the several plant tissues at a tional to its cross section. This raises the logical series of suction differences shows a linear relation- question: Is the resistance in plant tissue the same ship (typical examples in figs 2, 3, & 4). The water per unit length of tissue? If so, the larger resist- flux versus the suction difference relationships also ances in the leaves and roots could be caused by the are linear, (typical samples of plots in fig 5). longer channels involved in these tissues. Even The resistance to water flow was 67 % greater in though the length and cross section of the conduct- whole sunflower plants than in tomatoes when no ing tissues could not be determined, the resistance water phase change was involved. The resistance to per unit of root and leaf tissue was calculated from fluid transfer was 66 % greater in sunflower roots the root volume and leaf surface area measurements and 92 % greater in tomato roots than in leaves of that were obtained from the plants used. The resist- the respective species. The resistance in the stem ance was 0.14 millibar seconds per gram of water plus leaves, stem plus roots, and the whole sunflower per cubic centimeter of sunflower leaf tissue. The plant was 101, 234, and 438 % larger than the stem values were 0.075 and 0.0011 for tomato roots and resistance. The same comparisons with the tomato leaves, respectively. These results confirm that the plant yielded percentages of 75, 238, and 483, thus resistance per unit is greater for root than for leaf confirming that the conducting vessels of the stem of- tissue. No phase change was involved in water fer little resistance to water transfer. The rate of movement out of the leaf. Any phase change would liquid water movement through the leaves is consider- undoubtedly (lecrease the flow rate considerably. JENSEN ET AL.-NEGATIVE TRANSPORT & RESISTANCE TO WATER FLOW 637 TABLE II AVERAGE CONDUCTANCE & RESISTANCE TN VARIOUS PLANT TISSUES CONDUCTANCE RESISTANCE PLANT PART cm3 H2 (cm Hg) (min) (mb) (sec)a (cm Hg) (min) cm3 H)O g H20 Sunflower plant Whole plant 5.2±0.2 1,928 2.4 Stem & roots 8.4±0.3 1,195 1.5 Stem & leaves 14.0±0.3 719 0.88 Stem 28.1±0.4 358 0.44 Tomato plant Whole plant 8.7±0.3 1,150 1.4 Stem & roots 15.1±0.3 666 0.81 Stem & leaves 29.0±0.3 345 0.42 Stem 51.2±0.3 197 0.24 a mb represents millibar.

In moving from the root surface to the xylem, the movement of water through the roots and leaves water must traverse the protoplasm or move along of the same plant, at the same time, and under identi- the walls of the several layers of cells comprising the cal conditions. This appeared to be the best method epidermis, cortex, endodermis, and pericycle. It is of comparing the rate of water movement in the likely that these tissues account for most of the resist- leaves with the flow in the roots. ance in the root, and that the resistance per unit The average water flow per minute (the slope of length along the xylem of the roots is no greater than the plot water flow versus time) through the roots the resistance in the xylem of stems or leaves. In and leaves of each plant are recorded in table TII. the leaves the water moved out through the hyda- The movement of water through the sunflower leaves thodes, in which the cells are loosely arranged, and was 64 and 65 % greater than in the roots for suction which apparently presented much less resistance than differences of 25 and 35 cm of mercury, respectively. the root tissues. The same comparisons with tomato plants yielded Water transfer through the roots and leaves was percentage values of 63 and 69. Thus, the flow of further studied by altering the experimental ap- water was always considerably greater through the paratus so that the roots, a small section of the stem, leaves than through the roots of both sunflower and and the leaves of intact plants were sealed in separate tomato plants. These results confirm the conclusion adjoining chambers. To measure the water flow, a that the resistance to water movement in the roots is mercury manometer was fastened to the central considerably larger than that in the leaf tissue. chamber, and the plant stem in this chamber was severed. The amount of water moving into and through the roots was measured with the capillary SUMMARY & CONCLUSIONS tube fastened to the root chamber. The other capil- lary tube showed the water flow through the leaf tis- Experiments, conducted under conditions which sue. This system provided a method of measuring eliminated the leaf-atmosphere interface and sub- stituted a leaf-water interface, confirmed that water can move into and through plants equally well in both the normal and negative directions when the proper TABLE III gradient is established. Water flows through the aerial parts of the plant more than AVERAGE WATER FLUXa THROUGH SUNFLOWER & TOMATO easily through the ROOTS & LEAVES AT SUCTION DIFFERENCES OF 25 & 35 root tissue and appears to escape through hydathodes CENTIMETERS OF MERCURY of the leaves. This might explain the lower resist- ance observed in the leaf tissue since the water needs PLANT SUNFLOWER TOMATO to pass through only a few layers of loosely arranged TISSUE 25 cm Hg 35 cm Hg 25 cm Hg 35 cm Hg cells in order to escape from the leaf. On entering the root, in comparison, the water probably encounters Leaves 0.0222 0.0340 0.0302 0.0466 most of the resistance on traversing the epidermis, Roots 0.0134 0.0207 0.0181 0.0271 cortex, endodermis, and pericycle before reaching the % Greater 64 65 64 69 in leaves xylem. The greatest resistance was found in the roots, followed by the leaves, with resistance to water a cm3/min. flow very much lower in the stem. 638 PLANT PHYSIOLOGY LITERATURE CITED 15. KRAMER, P. J. 1949. Plant & Soil Water Relation- ships. McGraw-Hill Book Co., New York. 1. BONNER, J. 1959. Water transport. Science 129: 16. MEIDNER, H. 1954. Measurement of water intake 447-450. from the atmosphere by leaves. New Phytol. 53: 2. BREAZEALE, E. L., W. T. MCGEORGE, & J. F. BREAZEALE. 1950. Moisture adsorption by plants 423-426. 17. MER, C. L. 1940. The factors the re- growing in an atmosphere of high humidity. Plant determining Physiol. 25: 413-419. sistance to the movement of water in the leaf. 3. BREAZEALE, E. L., W. T. MCGEORGE, & J. F. Ann. Botan. 4: 397-401. BREAZEALE. 1951. Water absorption & transpira- 18. MEYER, B. S. & D. B. ANDERSON. 1959. Plant tion by leaves. Soil Sci. 72: 239-244. Physiology. D. Van Nostrand Co., Princeton, 4. CRAFTS, A. S. 1933. Sulfuric acid as a penetrating N. J. agent in arsenical sprays for weed control. Hil- 19. PHILIP, J. R. 1958. Osmosis & diffusion in tissue: gardia 8: 125-147. Half-times & internal gradients. Plant Physiol. 5. CRAFTS, A. 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Citrus leaf stomata: Struc- leaves in an atmosphere of high humidity. J. Expt. ture, composition, & pore size in relation to pene- Botan. 3: 95-98. tration of liquids. Botan. Gaz. 108: 476-483. 10. HAINES, F. M. 1953. The absorption of water by 24. VAN DEN HONERT, T. H. 1948. Water transport in leaves in fogged air. J. Expt. Botan. 4: 106-107. plants as a catenary process. Disc. Far, Soc. No. 11. HOAGLAND, D. R. & D. I. ARNON. 1950. The 3: 146-153. water-culture method for growing plants without 25. WIERSMA, D. & F. J. VEIHMEYER. 1954. Absence soil. Cal. Agric. Expt. Sta. Circ. 347. of water exudation from roots of plants grown in 12. KRAMER, P. J. 1933. The intake of water through an atmosphere of high humidity. Soil Sci. 78: 33- dead root systems & its relation to the problem of adsorption by transpiring plants. Am. J. Botan. 36. 20: 481-492. 26. WILSON, J. D. & B. E. LIVINGSTON. 1937. Lag in 13. KRAMER, P. J. 1938. Root resistance as a cauise of water absorption by plants in water culture with the absorption lag. Am. J. Botan. 25: 110-113. respect to changes in wind. Plant Physiol. 12: 14. KRAMER, P. J. 1940. Root resistance as a cause of 135-150. decreased water adsorption by plants at low tem- 27. WIND, G. P. 1955. Flow of water through plant peratures. Plant Physiol. 15: 63-79. roots. Neth. J. Agric. Sci. 3: 259-264.