Low temperature tolerance was investigated in the imbibed seed of 15 seed lots of Lactuca sativa, 6 seed lots of Lactuca virosa and 8 seed lots of Lactuca serriola. During rapid cooling (20°Ch-1) some seed of all seed lots survived to -16°C but none to -20°C. The majority of seed lots retained over 50% viability above -14°C and investigations with L. sativa employing differential thermal analysis indicated that this was due to isolation of the embryo from external ice by the endosperm, and subsequent embryo supercooling. Certain seed lots showed high mortality at temperatures above -10°C and correlation of mortality with the formation of extracellular ice suggested that the endosperm in these seed lots was not an effective nucleation barrier. At slower cooling rates survival to -20°C was increased due to freeze desiccation of the embryo, and variation between species and seed lots was revealed. The maximum cooling rate for 50% survival to -20°C varied from 1.5 to 3°Ch-1 (L. sativa seed lots), from 4 to 6°Ch-1 (L. virosa seed lots) and from 3 to 10°Ch-1 (L. serriola seed lots). A model to explain variation in cooling rate tolerance was developed incorporating (1) the initial equilibrium moisture content of the fully imbibed seed, (2) the rate at which freeze desiccation of the supercooled embryo took place and (3) the seed moisture content at which nucleation (at -20°C) was no longer certain. In L. sativa, variation between seed lots appeared to be governed by factor (l); low seed moisture content was closely correlated with high survival at slow cooling rates. In L. serriola, variation between the two seed lots examined appeared to be due to differences in factor (2). Organic solvent and warm water treatment of L. sativa seed suggested that saccharides rather than lipids were responsible for the barrier action of the endosperm. Attempts to store fully imbibed seed at -8+/-2°C for prolonged periods were unsuccessful. The ecological and cryobiological significance of the results was discussed.
|Date of Award||1984|