Pelecyphora vivipara. Freezing and the recovery of activity

Material from Mercury Valley

In Plants of the World Online, Pelecyphora vivipara (Nutt.) D.Aquino & Dan.Sánchez includes the historical name Coryphantha vivipara var. deserti (Engelm.) W.T.Marshall. Nobel used this name in 1981 to describe plants from Mercury Valley in Nevada, at 1110 m elevation. The current species has a broader range, from southern Canada to northern Mexico. The measurements come from the selected site and its material. [2-WWW][Nobel 1981, POWO, 11 October 2026; Nobel, pp. 194–195]

In the field, the author compared four plants during a freezing night in February 1979. With a minimum air temperature of −9,4°C measured at 1 m height, the surface of two plants beneath a four-centimetre snow layer reached a minimum of −7,8°C. In two similar plants after snow removal, surface minimum was −11,5°C. All four survived. In this set of four plants, stem temperature differed from air temperature and depended on the presence of snow cover. [Nobel 1981, pp. 194–195]

Nine hours of freezing and the survival criterion

Plants for laboratory studies were transplanted with local soil. They were usually maintained at an air temperature of 10°C around the clock, under a twelve-hour day and about 350 µmol photons/m²/s. The stem surface warmed to about 23°C during the day. Watering was weekly. [Nobel 1981, pp. 194–195, Materials and methods]

Exposure to subzero temperature lasted nine hours during the night. In the final four hours, stem temperature was within 0,6°C of air temperature. Five plants were assessed at each freezing level. After a trial, they were allowed to regain a positive nocturnal CO₂ balance before another exposure at a lower temperature. The same plants passed through successive trials after a period of activity recovery. [Nobel 1981, p. 195, Materials and methods]

At −12°C, five of five plants survived, at −15°C three of five, and at −20°C none. Survival was defined as recovery of positive nocturnal CO₂ uptake within two months. This criterion linked plant survival with recovery of one function. Small sample size and a defined exposure duration limit treating these points as a precise tolerance boundary for the entire species. [Nobel 1981, pp. 195–196, Results]

The surviving stem gradually regains function

The curve for a single plant after a night at −10°C showed about a 70% decline in maximum nocturnal CO₂ uptake rate the following night. The temperature at which the greatest flux was reached fell from 10 to 6°C. By the second night, activity had reached about 90% of its previous level, and by the fourth it had recovered that level. [Nobel 1981, pp. 195–196, Fig. 1]

After −15°C, functional recovery took much longer. In plants that ultimately survived, no positive CO₂ uptake was found during the first ten nights. By the fourteenth night, it reached about 23% of the initial value. After two months it remained at about half its previous activity. Survival was therefore compatible with a prolonged restriction of carbon balance. [Nobel 1981, p. 196, Fig. 2]

Supercooling and ice formation

Cell sap had an osmotic potential of about −1,05 MPa, corresponding to a freezing-point depression of about 0,9°C. Survival of nine hours at −12°C additionally involved responses to ice formation and cellular dehydration. During cooling at about 2°C per hour, tissue remained supercooled to about −6°C. Its temperature then rose towards −2°C through heat released during ice formation. [Nobel 1981, pp. 196–197, Fig. 3]

Nobel interpreted this transition as ice formation outside the protoplasts, the living contents of cells. From analysis of the released heat, he estimated that about 10% of stem water initially froze. Protoplasts lost water to spaces containing ice and contracted. At −10°C, contraction was observed in about 68% of protoplasts, and at −15°C in all those assessed. The observations thus linked freezing with dehydration of the cell interior. [Nobel 1981, pp. 196–197]

The author adopted a model in which a contracting protoplast increases the concentration of its own osmotically active substances. On this basis, he estimated that at −15°C, cell-sap osmotic potential would have to reach about −17 MPa, requiring loss of about 94% of cellular water in the adopted model. Water loss was calculated from the assumed relationship between hydration and cellular osmotic potential. Assessment of the mechanism also relied on observations of plasmolysis and the capacity for vital staining after exposure. [Nobel 1981, pp. 197–198, Discussion]

Hardening measured at the cellular level

Viability was assessed through neutral red staining and the ability to induce plasmolysis, separation of the living cell contents from the wall under an osmotic solution. After −15°C, staining ability declined by about 70%, and after −20°C it disappeared. About 450 cells from one plant were assessed per temperature. [Nobel 1981, pp. 195–197, Fig. 4]

In a hardening trial, plants were maintained for successive weeks at 30, 20, 10, 0 and −10°C. The temperature causing a 50% decline in cellular vital-staining capacity shifted from about −10 to −17°C. Gradual changes in conditions thus increased tolerance measured by this indicator. The value of −17°C refers to cells and the adopted response threshold, separate from survival of the whole stem and subsequent growth. [Nobel 1981, pp. 196–198]

Exposure duration also mattered. An additional four hours at −12°C worsened staining and the plasmolytic response by about 12%. Results connect temperature with freezing duration, the plant's previous condition and a specific measurement method. Cellular tolerance and recovery of stem activity after laboratory exposures were measured. The cooling course, freezing duration and hydration state are among the conditions of these results. [Nobel 1981, pp. 196–198]