Freezing Tolerance in Hydrated Lactuca sativa (L) Seed: A Model to Explain Observed Variation Between Seed Lots

Low temperature tolerance was investigated in the imbibed seedof 15 seed lots compnsmg seven cultivars of Lactuca sativa L.During rapid cooling (20 °C h^–1) some seeds of allseed lots survived to –16 °C but none to –20°C. The majority of seed lots retained over 50 per centviability above –14 °C due to isolation of the embryofrom external ice by the endosperm, and subsequent embryo super-cooling.Certain seed lots, including all three seed lots of cv. TomThumb, showed high mortality at temperatures above –10°C. Correlation of mortality with the formation of externalice suggested that the endosperm is not an effective nucleationbarrier in these seed lots. Survival to –20 °C was increased at slower coolingrates (6 to 1 °C h^–1) due to freeze desiccation ofthe embryo, but seed lots varied considerably in their toleranceof specific cooling rates. A model to explain this variationwas developed incorporatmg (1) seed lot super-cooling limittemperature, (2) the rate at which freeze dehydration of thesupercooled embryo took place, (3) the moisture content at whichnucleation (at –20 °C) was no longer certain and (4)the.initial equilibrium moisture content of the fully imbibedseed. Factors (1), (2) and (3) were found to be relatively constant,but low (or artificially reduced) seed moisture content wasclosely correlated with high survival at natural cooling rates.Seed size fractions of similar moisture content from a singlecultivar showed that more small seeds survive cooling at 3 °Ch^–1 to –20 °C than larger seed. Seed with pierced endosperms or ineffective nucleation barrierswere capable of surviving to at least –10 °C if cooledslowly (1 °C h^–1) but were killed by rapid (20 °Ch^–1) cooling. Lactuca sativa L, lettuce, seed germination freezing tolerance, super-cooling