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Osmosis

and

the Water Potential of Potato Tissue

by

Robert J. Kosinski
Clemson University

Last Update: October 2013


Osmosis has been noticed by biologists since the middle 1700s, and by the 1870s, careful quantitative observations were being made of it (Baumgarten and Feher, 1998). However, while we can predict it exactly, the cause of osmosis is still in dispute (Baumgarten and Feher, 1998; Weiss, 1996, p. 216).

Theories on the Cause of Osmosis

The OMP mentioned three theories on the cause of osmosis. A simple and appealing explanation for osmosis is the concentration of water explanation--water in pure water is simply more concentrated than water in solutions because the solute has to take up some room in the solution. According to this idea, water diffuses into a hyperosmotic solution because it is diffusing down its concentration gradient. However, a detailed examination reveals problems with this theory. As Weiss (1996) points out, this predicts water movement in the right direction, but not of the right magnitude. Water movement in osmosis is faster than diffusion, and seems to be more like mass water movement caused by a pressure difference (Weiss, 1996, p. 218). Also, as Salisbury and Ross (1992, p. 39) point out, adding solutes to a solution decreases the concentration of water in most cases, but in some cases solutions have a higher concentration of water . The Handbook of Chemistry and Physics has a large section on solutions of common solutes, and it discloses that a 0.2 M solution of NaCl has a markedly higher water concentration (995 g/L) than a 0.2 M solution of sucrose (955 g/L) (Wolf et al., 1982, pp. D261 and D270). Yet our past expereiments have shown that a potato core loses water to a 0.2 M solution of NaCl but it gains water from a 0.2 M solution of sucrose. Also, if we compare 0.2 M solutions of sucrose and glucose, 0.2 M glucose has a higher water concentration than 0.2 M sucrose (976 g/L vs. 955 g/L) (Wolf et al, 1982, p. D239) . This makes sense because the smaller glucose molecules take up less room in the solution. However, these two solutions have exactly the same osmotic potential.

A slightly more complex theory that is often found in general biology books (including your text, p. 117) is the “bound water” explanation. This says that any hydrophilic solute (like sucrose or NaCl) will bind up hydrating water and prevent it from moving freely. Therefore, the side of a semipermeable membrane with pure water has a higher “free” water concentration than the side with the solute molecules. According to this explanation, “free” water moves into hypertonic solutions simply because it is diffusing down its concentration gradient. Although it is popular in introductory texts, this theory is not even mentioned in several reviews (Baumgarten and Feher, 1998; Weiss, 1996, pp. 216-222). If the bound water explanation were true, we would expect that a greater mass of hydrophilic solute would bind more water. Whether a certain mass of solute is present in a few large molecules or in many small ones shouldn’t matter. Also, when predicting osmosis, we would have to carefully consider how hydrophilic the solute is (that is, how many water molecules it binds per molecule). In fact, the number of molecules present does affect osmosis, and we can predict osmosis without considering how hydrophilic the solute molecules are.

Another explanation is van't Hoff's Law, or the “number of particles” explanation. Jacobus H. van’t Hoff (1887) gathered much data on the osmotic potential of many kinds of solutions. As long as the solution was relatively dilute and the temperature was constant, he found that the osmotic potential was proportional to the concentration of solute particles. The size or nature of the solute particles didn’t matter. So, for example, a small sodium ion would have the same osmotic effect as a large sucrose molecule, and both of these would be equivalent to a very large starch molecule (Baumgarten and Feher, 1998). This also means that ionizing substances like NaCl should have a greater osmotic effect than nonionizing substances like sucrose because when they ionize, they generate more particles. A measure of concentration called “osmolality” (the concentration of solute particles in moles/L) was invented to express this. A 1 molal solution of sucrose would have an osmolality of 1 osmol/L because each mole of sucrose also produces one mole of solute particles. However, a completely dissociated 1 molal concentration of NaCl would have an osmolality of 2 osmol/L because each mole of NaCl ionizes into two moles of particles. A completely dissociated 1 molal solution of CaCl2 would have an osmolality of 3 osmol/L because each mole of CaCl2 ionizes into three moles of particles .

Van't Hoff's Law is an empirical relationship, not a physical description of why osmosis occurs. Why the number of particles should matter remains unclear. Van't Hoff himself was only interested in predicting osmosis, and he expressed frustration with those who wanted him to explain why his law worked (van't Hoff, 1892; quoted in Weiss, 1996, p. 185). Since even modern experts can't agree why osmosis occurs, we'll have to follow van't Hoff's example and satisfy ourselves with prediction of osmosis.

In this exercise, we tested two predictions of Van't Hoff's Law:

The no-weight-change osmolality (number of particles per L) will be the same for experiments conducted with glucose, sucrose, and NaCl;

Van't Hoff's Law will estimate the same water potential for potato cores when using glucose, sucrose, and NaCl.

The Water Potential of White Potatoes

White potatoes are important food crops in the cooler regions of the world. The tuber, the edible part of the white potato is a very short and thick, starchy stem, with the "eyes" being the buds on the stem (Burton, 1989, Chapter 2). White potatoes have firm tissue and convenient size, so they are favorite subjects for the teaching laboratory determination of the water potential of plant tissue. 

The "change in weight" method we use in our lab was published in 1935 by Meyer and Anderson as a modification of a method published by Ursprung (1923). Meyer and Wallace (1941) employed this method and found that white potato tuber water potential was between -7.7 and -8.3 bars, with the variance due to the length of time the sample was tested. Ashby and Wolf (1947) later confirmed these estimates. Epstein and Grant (1973) used gravimetric methods using sucrose solutions to test two varieties of white potatoes and found water potentials ranging from -2 to -8 bars. Ehlenfeldt (1992) used a technique very similar to ours (except that he used sorbitol as a solute rather than sucrose) and found that the solution with the same water potential as white potatoes ranged from 0.24 to 0.31 M, corresponding to about -7 bars. Instead of using a wide range of solutions, as we did, he focused on the point where the line crosses the x axis by using solutions that ranged from only 0.20 to 0.35 M. Using an entirely different technique, Gandar and Tanner (1976) used a pressure chamber to determine water potential in potato tuber cores. Here pressure was applied to the test specimen until sap just began to wet of the xylem traces at the ends of the cut sample. They calculated water potential of white potato tubers to be equal to -6 to -7 bars. Again, extreme variance (up to 5 bars) resulted, due to the moisture of the sample. Ros Barcelo and Calderon (1994) cited a value of -6.7 bars for white potato tuber tissue.

Despite the strong agreement in potato water potentials determined by these various methods, lengthy discussion has centered around the potential sources of error associated with each method. Meyer and Wallace (1941) suggest that weight change may not be due to the osmotic movement of water but rather due to the gain or loss of solutes to or from the soak medium. Bland and Tanner (1985) did a critical comparison of three methods of determining water potential and found that while two methods might agree in a certain range of water potentials, in another range they might diverge widely. Boyer (1969) reviewed numerous methods of water potential determination.

Growth conditions (especially soil water) cause significant differences in tuber water potential. As one would expect, drier soils produced drier white potatoes with more negative water potentials. For example, Burton (1944) found that the water content of some white potatoes in England increased from 73% to 77% as the rain increased from 30 mm/month to 70 mm/month. Win et al. (1991) found that white potato tubers in dry soil in New Jersey increased from a water potential of -4 bars before a rainstorm to -0.8 bars after the storm. Bland and Tanner (1985) measured the water potential of white potatoes and found a very wide range, from -0.1 to -0.9 MPa (-1 to -9 bars), although some stored white potatoes went as low as -15 bars. Bland and Tanner (1986) traced the drying of stored white potatoes, and found that their water potentials declined from -3 bars to -5 or -6 bars over the first 5 to 7 weeks of storage. After 25 weeks of storage, they had gotten down to -7 bars. We have no idea how long our potatoes were stored. Also, different parts of a tuber might have very different water potentials. Meissner (1997) found that beet storage organ tissue had a water potential 5.6 bars lower than the tissue close to the water-conducting vessels (the xylem). Shibairo et al. (2002) found that weight loss in stored carrots was usually associated with changes in water potential or osmotic potential under common storage conditions.

White potatoes can acclimate to very dry conditions. Leone et al. (1996) found that the growth of white potato cells in tissue culture would be completely inhibited by sudden transfer to a solution with an osmotic potential of -23 bars, but the cells could continue to grow in this solution if they were gradually acclimated to it. Part of the acclimation was changing the fatty acid composition of the potato cell membranes, mediated by activation of different genes than in the unstressed cells (Leone et al., 1996 and 1994). Liu et al. (2006) found that white potato leaf water potentials declined from -5.3 bars to - 8.5 bars during an experiment testing the effect of water stress. Vos and Oyarzun (1987) studied the decline in photosynthesis as potato leaves declined from -6 bars to -11 bars.

The same general conclusions also apply to sweet potatoes, although all the sweet potato water potentials tend to be lower. Sung (1985) found that sweet potato leaves became wilted when the leaf water potential dropped to about -16 bars. Ghuman and Lal (1983) found that sweet potato leaves in Nigeria had an average water potential of -9.6 bars. Despite the tough appearance of the tuber, sweet potato is sensitive to water stress, and the tubers increase their dry matter as water stress gets worse (Ekanayake and Collins, 2004).


Literature Cited

Ashby, E. and R. Wolf. 1947. A critical examination of the gravimetric method of determining suction force. Annals of Botany (London) (N.S.) 11(42): 261-268.

Baumgarten, C. M. and J. I. Feher. 1998. Osmosis and the regulation of cell volume. Pp. 253-292 in N. Sperelakis (Ed.), Cell Physiology Source Book, 2nd Ed. Academic Press, San Diego, 1095 pp.

Bland, W. L. and C. P. Tanner. 1985. Measurement of the water potential of stored potato-tubers. Plant Physiology 79: 891-895.

Bland, W. L. and C. B. Tanner. 1986. Potato-tuber water potential components during storage. American Potato Journal 63(11): 649-653.

Boyer, J. S. 1969. Water status measurements in plants. Annual Reviews of Plant Physiology 20: 351-364.

Burton, W. G. 1944. The characteristics of certain varieties of potato, with special reference to their suitability for drying. Annals of Applied Biology 31: 89-96.

Burton, W. G. 1989. The Potato, 3rd Ed. Longman Scientific and Technical Publishers, Essex, UK, 742 pages.

Ehlenfeldt, M. K. 1992. Evaluation of differential tuber tissue expansion and plant transpiration as methods for early hollow heart screening. American Potato Journal 69(9): 537-546.

Ekanayake, I. J. and W. Collins. 2004. Effect of irrigation on sweet potato root carbohydrates and nitrogenous compounds. Journal of Food, Agriculture and Environment 2(1): 243-248.

Epstein, E. and W. J. Grant. 1973. Water stress relations of the potato plant under field conditions. Agronomy Journal 65: 400-404.

Gandar, P. W. and C. B. Tanner. 1976. Potato leaf and tuber water potential measurements with a pressure chamber. American Potato Journal 53: 1-14.

Ghuman, B. S. and R. Lal. 1983. Mulch and irrigation effects on plant-water relations and performance of cassava and sweet potato. Field Crop Research 7(1): 13-29.

Leone, A., A. Costa, S. Grillo, M. Tucci, I. Horvath and L. Vigh. 1996. Acclimation to low water potential determines changes in membrane fatty acid composition and fluidity in potato cells. Plant, Cell & Environment 19(9): 1103-1109.

Leone A., A. Costa, M. Tucci and S. Grillo. 1994. Comparative analysis of short- and long-term changes in gene expression caused by low water potential in potato (Solanum tuberosum) cell-suspension cultures. Plant Physiology 106(2): 703-712.

Liu, F., A. Shahnazari, M. N. Andersen, S. E. Jacobsen and C. R. Jensen. 2006. Effects of deficit irrigation (DI) and partial root drying (PRD) on gas exchange, biomass partitioning, and water use efficiency in potato. Scientia Horticulturae 109(2): 113-117.

Meissner, S. T. 1997. Water potential measurements of the storage parenchyma tissue and xylem solution of red beets (Beta vulgaris) suggests apoplastic discontinuity between the conducting xylem and sink areas. The American Journal of Botany 84(6): S150.

Meyer, B. S. and D. B. Anderson. 1935. Laboratory Plant Physiology. 107 pp. Edwards Bros., Ann Arbor.

Meyer, B. S. and A. M. Wallace. 1941. A comparison of two methods of determining the diffusion pressure deficit of potato tuber tissues. American Journal of Botany 28: 838-843.

Ros Barcelo, A. and A. A. Calderon. 1994. Measuring water conductivity coefficients in plant tissues. Journal of Biological Education 28(2): 83-85.

Salisbury, F. B. and C. W. Ross. 1992. Plant Physiology, 4th Ed. Wadsworth Publishing Co., Belmont, CA. 682 pp.

Shibairo, S.I., M.K. Upadhyaya, and P.M.A. Toivonen. 2002. Changes in water potential, osmotic potential, and tissue electrolyte leakage during mass loss in carrots stored under different conditions. Scientia Horticulturae 95(1): 13-22.

Sung, J.-M. 1985. Studies on physiological response to water stress in sweetpotato. II. Osmotic adjustment in sweetpotato leaves. Journal of the Agricultural Association of China 129: 50-55.

Ursprung, A. 1923. Zur Kenntnis der Saugkraft. VII. Eine neue vereinfachte Methode zur Messung der Saugkraft. Ber. Deutsch Bot. Ges. 41: 338-343.

van't Hoff, J. H. 1887. Die Rolle des osmotischen Drukes in der Analogie zwischen Losungen und Gasen. Z. Physik. Chemie 1: 481-493.

van't Hoff, J. H. 1892. Zur Theorie der Losungen. Z. Physik. Chemie 9: 477.

Vos, J. and P. J. Oyarzun. 1987. Photosynthesis and stomatal conductance of potato leaves--effects of leaf age, irradiance, and leaf water potential. Photosynthesis Research 11: 253-264.

Weiss, T. F. 1996. Cellular Biophysics, Vol. 1: Transport. MIT Press, Cambridge, MA. 693 pp.

Win, K., G. A. Berkowitz and M. Henninger. 1991. Antitranspirant-induced increases in leaf water potential increase tuber calcium and decrease tuber necrosis in water-stressed potato plants. Plant Physiology 96: 116-120.

Wolf, A. V., M. G. Brown and P. G. Prentiss. 1982. Concentrative properties of aqueous solutions: Conversion tables. Pp. D227-D276 in R. C. Weast and M. J. Astle, CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, Florida.


Coursewide Data

These data were based on 64 water potential values estimated with NaCl, 47 with sucrose, and 47 with glucose. The experiment was done during the week of September 30, 2013.
 

Table 1. Average percent change in mass of white potato cores during a 1.5-hour incubation in several NaCl, sucrose, and glucose solutions.
Molality
NaCl
Sucrose
Glucose
0.0 +12.34%

+14.38%

+13.03%
0.1 +5.86% +10.14% +7.73%
0.2 -3.70% +6.56% +4.88%
0.3 -13.34% -0.50% +0.81%
0.4 -20.43% -5.73% -6.73%
0.5 -24.53% -10.77% -13.68%


You should use these data in order to make an Excel graph (Figure 1) in your report. This graph should plot the percent weight change (y axis) vs. osmolality of the soak solution (x axis). Each graph should have a line for the 2013 coursewide NaCl, glucose, and sucrose averages above plus a fourth line for your lab group's data (either NaCl, glucose, or sucrose), as was illustrated in the OMP. These three lines should be distinctive (e.g., points with different shapes and dotted vs. solid lines). Use the techniques you practiced in the data presentation exercise the second week of lab. This implies that graphs will not have color, will use only circles, squares and triangles (black or white) for the points, etc.

All of these x axis variables can be derived from the molality data above. Remember, osmolality = molality for sucrose and glucose (because sugars do not ionize), and osmolality = 1.8x molality for NaCl (because NaCl ionizes into two particles). The molecular mass of NaCl is 58.4; the molecular mass of sucrose is 342.3; the molecular mass of glucose is 180.2.

Then you will need a table. Remember, the different solutes are the treatments in this case, and we are testing to see if the treatments are the same. This single table should summarize the chi-square results below for the following questions:

Are the potato water potentials estimated from the NaCl, glucose, and sucrose data the same?

Do potato cores have no weight change in the same osmolalities of NaCl, glucose, and sucrose?

You will need to summarize these data in a table that shows the means of each no-weight-change variable for NaCl, glucose, and sucrose, the chi-square for the test of equality of NaCl, glucose, and sucrose for each variable, and the P-value for each variable's test. Note that you do not have to include the above- and below-median data in the tables below. It might be a good idea to devote one line of your table to each variable.

Table 2. Potato water potentials (bars) estimated by van't Hoff's Law for NaCl, sucrose and glucose, and the results of a chi-square median test comparison of the water potential estimates by the three solutes.


Means

Below Median

Above Median

NaCl -6.65
30
34
Sucrose -6.73 22
25
Glucose -6.82 27
20

Chi-square = 1.48; P value = 0.476

In interpreting the table above, remember that -6.65 bars is above -6.82 bars.

Table 3. Osmolalities (moles of solute particles/kg of water) of NaCl, sucrose, and glucose that produced zero weight change in potato cores, and the results of a chi-square median comparison of the no-weight-change osmolality for the three solutes.


Means

Below Median

Above Median

NaCl 0.261
33.5
27.5
Sucrose 0.270 24.0
22.0
Glucose 0.273 19.5
26.5

Chi-square = 1.742; P value = 0.418

 


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