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 .