Survival of Phoenix Pollen Grains under Cryogenic Conditions1

Abstract

Date palm (Phoenix dactylifera L.) pollen may retain viability for a single growing season. Pollen self‐life may be extended through low temperature storage but viability progressively declines. As an alternative, Phoenix pollen was subjected to various cryogenic treatments in order to develop a new method to preserve palm pollen for prolonged periods. A simple means to store date palm pollen in liquid nitrogen (LN) is presented. Phoenix pollen grains may be cryogenically stored by immersion of aluminum foil packages into LN. Grains are rapidly thawed by removal from LN and then plunging into germination medium until bubbling subsides. Germination tests conducted on a commercial date palm pollen blend revealed no difference in survival of unfrozen (79.9%)and frozen grains (78.9%). Pollen germinated at 28 C for 24 hours on a modified Brewbaker and Kwack medium containing 15% sucrose. Multistage freezing of Phoenix grains, either dry or in the presence of various concentrations of a cryoprotective mixture consisting of polyethylene glycol, glucose, and dimethylsulfoxide (PGD) in a temperature controlled freezer, was less effective than the direct freezing technique for retaining pollen viability. Only 44.2% pollen germinated following multistage freezing and germination was further reduced by addition of 20%/16%/20%, 10%/8%/10% or 5%/4%/5%, w/v PGD to 17.6, 15.3, and 16.8% germination, respectively. Pollen derived from the Crane cultivar frozen in LN for 7 and 435 days germinated at 58.7 and 59.2%, respectively, compared to the unfrozen control of 56.2%. A commercial pollen blend was found to germinate originally at 68.5%. Pollen stored in LN germinated at 66.4 and 73.5% after 7 and 435 days, respectively. The germination rate was compared for eleven sources of Phoenix pollen before and after exposure to LN. In all cases no significant difference in germination rates were observed.