Freezing tolerance in hydrated lettuce (Lactuca sativa) seeds is dependent on cooling rate but not imbibition temperature

Abstract

The effect of cold acclimation and cooling rate conferring viability of fully hydrated Lactuca sativa cv. Iceberg seeds were studied using differential scanning calorimetry (DSC) and controlled freezing experiments, with an intention to gain insights into the mechanisms seeds use in the natural environment to survive low temperatures. Rapidly cooled (60 °C h⁻¹) cold acclimated or non-acclimated seeds although showed a high-temperature exotherm (HTE) and a low-temperature exotherm (LTE), the mortality correlated only with the LTE noted predominantly around −16 °C and below which no seed survived. In contrast, cooling cold acclimated and non-acclimated seeds at a slower rate (3 °C h⁻¹), generated only one exotherm above −13 °C and seed viability was unaffected below this exotherm. However, noticeable seed mortality occurred in slow-cooled seeds during decrease in temperature and at below −40 °C no seeds survived, indicating that there is a lower limit to this mechanism. Excessive desiccation of the embryo water and/or membrane damage is proposed to be a likely factor contributing to seed mortality at lower temperatures. We conclude that cold acclimation is neither an essential nor a satisfying requirement for low-temperature survival of L. sativa seeds. The mechanism is rather determined by the rate at which the seeds are cooled.