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The stress and strain (surface tension and fractional change in area) in the plasma membrane of protoplasts isolated from rye leaves (Secale cereale L. cv Puma) were measured during osmotic expansions from isotonic into a range of more dilute solutions. The membrane surface tension increases rapidly to a maximum and then decreases slowly with some protoplasts lysing in all phases of the expansion. The maximum surface tension is greater for rapid expansions, and protoplasts lyse earlier during rapid expansion. Over the range of expansion rates investigated, the area at which lysis occurs is not strongly dependent on expansion rate. The value of the maximum tension is determined by the expansion rate and the rate at which new material is incorporated into the membrane. During osmotic expansion, protoplasts isolated from cold-acclimated plants incorporate material faster than do those from nonacclimated plants and thus incur lower membrane tensions.

The plasma membrane of protoplasts isolated from rye leaves (Secale cereale L. cv. Puma) can withstand a maximum elastic stretching of about 2%. Larger area expansions involve the incorporation of new material into the membrane. The dynamics of this process during expansion from isotonic solutions and the probable frequency of lysis have been measured as a function of membrane tension in populations of protoplasts isolated from both cold-acclimated and nonacclimated plants. To a first approximation, both increase exponentially with tension. An analytical solution is reported for the membrane tension as a function of time during an arbitrary expansion in area.