Dispersal by stem segments
Easily separated segments enable vegetative recruitment. They can be moved by animals, gravity or water. The species’ distribution therefore depends both on conditions for survival and on the effectiveness of propagule transport and competition with other vegetation. [Loik and Nobel 1993b]
Current POWO treatment gives a range from Canada to the northern, western and central USA. This broad climatic range allowed comparison of populations from places with different winter temperatures. In Loik and Nobel’s study, this material was transferred to uniform experimental conditions. [2-WWW] [Loik and Nobel 1993b]
Cold acclimation
Twenty populations were compared after a shift from day/night temperatures of 25/15°C to 5/−5°C. After fourteen days, a marked increase in freezing tolerance was found, with its magnitude differing among populations. Tolerance and acclimation capacity correlated with the climate of the source locations, including minimum winter temperatures. [Loik and Nobel 1993b]
During acclimation, segments lost on average about a third of their water and became thinner. Dehydration also affected the photosynthetic parenchyma. Osmotic pressure increased to an extent largely consistent with concentration of the dissolved substances already present. The results did not indicate a uniform, large increase in their total pool. [Loik and Nobel 1993b]
Mucilage and variation in mechanisms
In the experiment, mucilage content in O. fragilis segments declined during acclimation. This distinguished its response from some previously studied opuntias. The authors considered, among other possibilities, the role of mucilage in water movement and the use of its components in metabolic processes. A single measurement did not resolve these alternatives. [Loik and Nobel 1993b]
How to compare frost-hardiness results
Laboratory values concern a specified acclimation duration, freezing rate and duration, and criterion for cellular damage. The properties of material from Virden in Manitoba do not automatically represent every population. Detailed experimental results must be read together with these conditions and the plants’ provenance. [Loik and Nobel 1993b]
Results of overwintering experiments
In the comparison of twenty populations after 14 days at day/night temperatures of 5/−5°C, LT₅₀ ranged from −15,6°C for Princeton in Colorado to −48,9°C for Virden in Manitoba. Five segments were used per population, and samples remained at the test temperature for an hour. The assessment measured living cells’ ability to take up stain, rather than an absence of damage to the whole plant. All results are included in the table of 20 populations. [Loik and Nobel 1993b, Table 2]
In material from Saskatchewan, winter segments had an LT₀ of −50°C in a controlled freezing test with prior ice initiation. Damage was assessed after four weeks, using at least five segments per temperature and three replicates. New June growth died after freezing at only −3,5°C. Winter tolerance therefore does not describe the tolerance of fresh growth. [Ishikawa and Gusta 1996]
Materials from the author's archive
The species also appears in Konrad Szychowski's private material archive. [Bydgoski Klub Miłośników Kaktusów 2002, file p. 10] [Anonim b.d.r, file p. 121] [Sukulenty b.d.b, file p. 2, 3] [Kaktusy i Inne 2005a, file p. 7] [Modrakowski 2005, file p. 46, 47]