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Annals of Botany 51: 373-383, 1983
© 1983 Annals of Botany Company

RESEARCH-ARTICLE

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

P. D. KEEFE and K. G. MOORE

Plant Biology Group, School of Biological Sciences, University of Bath Claverton Down, Bath BA2 7AY

Accepted: 27 June 1982   

Low temperature tolerance was investigated in the imbibed seed of 15 seed lots compnsmg seven cultivars of Lactuca sativa L. During rapid cooling (20 °C h–1) some seeds of all seed lots survived to –16 °C but none to –20 °C. The majority of seed lots retained over 50 per cent viability above –14 °C due to isolation of the embryo from external ice by the endosperm, and subsequent embryo super-cooling. Certain seed lots, including all three seed lots of cv. Tom Thumb, showed high mortality at temperatures above –10 °C. Correlation of mortality with the formation of external ice suggested that the endosperm is not an effective nucleation barrier in these seed lots.

Survival to –20 °C was increased at slower cooling rates (6 to 1 °C h–1) due to freeze desiccation of the embryo, but seed lots varied considerably in their tolerance of specific cooling rates. A model to explain this variation was developed incorporatmg (1) seed lot super-cooling limit temperature, (2) the rate at which freeze dehydration of the supercooled embryo took place, (3) the moisture content at which nucleation (at –20 °C) was no longer certain and (4) the.initial equilibrium moisture content of the fully imbibed seed. Factors (1), (2) and (3) were found to be relatively constant, but low (or artificially reduced) seed moisture content was closely correlated with high survival at natural cooling rates. Seed size fractions of similar moisture content from a single cultivar showed that more small seeds survive cooling at 3 °C h–1 to –20 °C than larger seed.

Seed with pierced endosperms or ineffective nucleation barriers were capable of surviving to at least –10 °C if cooled slowly (1 °C h–1) but were killed by rapid (20 °C h–1) cooling.

Lactuca sativa L, lettuce, seed germination freezing tolerance, super-cooling


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