Optimization of in vitro germination and cryopreservation conditions for preserving date palm pollen in the USDA National Plant Germplasm System

Date palm (Phoenix dactylifera L.) is a staple crop across arid and semi-arid regions around the world. Both genebanks and date palm breeders benefit by having clearly defined methods for the long-term preservation of pollen. Methods were developed to cryopreserve pollen from date palm trees in the USDA-ARS National Plant Germplasm System. First, in vitro pollen germination conditions were optimized using five cultivars. Then, date palm pollen equilibrated over saturated salt solutions of Ca(NO3)2 or MgCl2 at 23 °C (46% and 33% RH, respectively) or in a room set at 23% RH and 5 °C. Pollen was kept at these conditions for one to 56 days and then placed into liquid nitrogen vapor (LNV). Viability was assessed for fresh, moisture-equilibrated (non-LNV and LNV-exposed), and pollen stored in LNV for 9 months. The highest level of in vitro pollen germination was observed for date palm cultivar Thoory BC3 (60 ± 2%) when it was rehydrated for 2 h and then incubated at 23 °C on Marquard medium with 15% sucrose. Viability remained stable for pollen that was stored in LNV for 9 months. In addition, pollen also remained viable if it was cooled to and warmed from LNV for up to 60 cycles. These data suggest that date palm pollen can be successfully placed in genebank cryostorage. Using optimized in vitro germination conditions, date palm pollen can survive at least 9 months in LNV and can be cooled to and warmed from LNV at least 60 times.