Pollen grains can be stored for several years in liquid nitrogen having a temperature of

Storing pollen grains in $-{196}^{\mathrm{o}}\mathrm{C}$ in liquid nitrogen is called cryopreservation.
It is a process where organelles, cells, tissues, extracellular matrix, organs or any other biological constructs susceptible to damage caused by unregulated chemical kinetics are preserved by cooling to very low temperatures.