In vitro viability and germination of Carya illinoinensis pollen under different storage conditions

Pecan (Carya ininoinensis) is a crossing-pollinated nut tree with male and female flowering separated and matured at different time. This dichogamy pattern requires appropriate pollen storage strategy making hybridization successful in the pecan breeding program. In this study, we used pecan cultivar 'Pawnee' pollen to optimize germination medium composition on basis of four published formulas. Consequently, an agar-solidified medium containing 15 g sucrose, 0.005 g boric acid, and 0.075 g calcium nitrate per 100 mL at pH 7 was optimal to the pecan pollen germination. Pollens of 14 pecan cultivars and 1 breeding line (7 protandrous and 8 protogynous genotypes) were collected and examined for germination rates on the optimized medium in order to determine germination variation among genotypes. 'Pawnee' pollen was investigated for germination pattern at four storage temperatures with different time intervals. As results, pollen germination rates varied among cultivars, possibly due to the difference in flowering pattern and catkin maturity level at the time of harvest. `Pawnee' pollen remained viable after storage at -20 degrees C for six weeks (58.5 % germination rate) or 4 degrees C for four weeks (57.5 %), and still maintained 51.8 % viability when stored at -80 degrees C for one year. Pollen remains viable when removed from ultralow temperature and acclimated at 4 degrees C for 8-10 h and at RT for additional hour without typical rehydration before pollination to female flowers. These results provide useful information for short-term storage of pollen source in the pecan crossbreeding program.