Freeze Desiccation: a Second Mechanism for the Survival of Hydrated Lettuce (Lactuca sativa L.) Seed at Sub-zero Temperatures

• Published in: Annals of botany 1981-05, Vol.47 (5), p.635-645
• Main Authors: KEEFE, P. D., MOORE, K. G.
• Format: Article
• Language: English
• Subjects: Achenes; Cooling; Cooling rate; differential thermal analysis; Embryos; Endosperm; Freezing; freezing avoidance; Ice; Lactuca sativa L; lettuce; Low temperature; Nucleation; Pericarp; seed; Seeds; supercooling; water migration
• ISSN: 0305-7364; 1095-8290
• DOI: 10.1093/oxfordjournals.aob.a086060

Differential Thermal Analysis of hydrated lettuce cv. Great Lakes achenes using a rapid cooling rate (20 °C h−1) produced two exotherms per achene. Both exotherms represented the freezing of supercooled water. The high temperature exotherm occurred at −9∓3 °C and was produced by freezing of water inside the pericarp but exterior to the endosperm. The temperature at which it occurred could be altered by the addition of nucleating agents. The low temperature exotherm produced by freezing of the embryo occurred at −16∓2 °C and marked the death of the seed. Its temperature was not changed by the addition of nucleating agents but its occurrence required the structural integrity of the endosperm. At low cooling rates (1 and 2 °C h−1) low temperature exotherms were not recorded and samples removed at −25 °C had high viability. Slow cooling caused a redistribution of water within the seed whereby ice forming outside the endosperm caused desiccation of the embryo and prevented its freezing. A mechanism is proposed, in terms of established supercooling and nucleation theory, to explain the observed results and the value of freeze tolerance to the species in its natural habitat is discussed.