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Alfalfa Osmotic Potential: A Comparison of the Water-release Curve and Frozen-tissue Methods¹

Osmoadjustment is an important response to water deficits of some plants. The purpose of our study was to examine the suitability of two methods of osmotic potential () measurement for researching osmotic adjustment of alfalfa (Medicago saliva L. ‘Saranac‘). Water release curves for the top fully-developed alfalfa leaves from different stems were generated from a series of paired leaf mass and leaf water potential () measurements using a pressure chamber. Using leaves from the same alfalfa stem, the determined from the water release curves were compared with psychrometer measurements on sap expressed from frozen and thawed tissue of the next three to four lower leaves on each stem. Osmotic potential of sap expressed from the thawed tissue was 2.1 to 8.9 bars lower than obtained from water-release curves. The of the top leaf used for water-release curves typically was 1 to 2 bars higher than that of the next three to four lower leaves used in the frozen-tissue method; however, this between-leaf difference could account for less than 50 % of the difference between methods. The remaining difference is believed to result from production of solutes in thawed tissue by enzymatic hydrolysis of nonstructural carbohydrates because rapid changes in were observed in the tissue following thawing. This appears to invalidate routine measurements on alfalfa using the frozen tissue method. Unless the changes observed in frozen and thawed tissue can be circumvented, we recommend all be determined from alfalfa leaf water release curves.

Key Words: Medicago sativa L. • Plant-water relations • Pressure chamber • Vapor pressure psychrometer

² Research assistant and professor, Dep. of Soil Science, Univ. of Wisconsin, Madison, WI53706. Senior author is presently senior research biologist, Monsanto Co., St. Louis, MO 63166.

Received for publication January 5, 1982.

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