New determinants for tolerance of coffee (Coffea arabica L.) seeds to liquid nitrogen exposure

The present work establishes for the first time that tolerance of coffee seeds to liquid nitrogen (LN) exposure depends on the initial quality of the seedlot and on the rewarming regime employed. Seedlot quality was estimated by the parameters of a quantal response model of desiccation sensitivity developed previously. The percentage of seedlings recovered from cryopreserved seeds was very well correlated with the relative humidity (RH) at which 90 percent of the initial viability was retained, RH90, as estimated by the model. Whatever the cooling regime employed, rewarming the seeds slowly by exposing them to ambient air was highly detrimental. Slow rewarming-induced viability loss was not due to imbibitional damage since seeds pre-heated at 37 degree C after slow rewarming to 0 degree C exhibited a survival percentage lower than seeds thawed rapidly to 0 degree C before sowing. The optimal hydration status for coffee seed cryopreservation was also re-examined. Drying seeds in 81 percent RH provided survival percentages considerably higher than those obtained using the drying RH always employed until now, i.e. 78 percent. A new procedure for slowly precooling the seeds prior to immersion in LN was also established. It consisted of placing the vials containing the seeds in a dry ice-bath for 25 min. Using this procedure in combination with seed drying in 81 percent RH and rapid rewarming in a 37 degree C water-bath for 30 min ensured the highest survival percentages ever obtained with coffee seeds, i.e. 89 percent, a value which was not significantly different from the initial viability percentage.