Pecan pollen viability has been the subject of research for about 85 years. Early studies indicated that pecan pollen was short-lived and viable for only a few days (Smith and Romberg, 1932; Stuckey, 1916; Wetzstein and Sparks, 1985; Woodroof, 1930). Studies later proved that 'Desirable' pecan pollen stored for nearly 2 years at -112 oF (ultra cold freezer) and -321 oF (liquid nitrogen) retained germination capacity equal to freshly collected pollen (Yates and Sparks, 1989). Shortly thereafter, 'Desirable' pollen stored at -112 or -321 oF for up to 3 years was demonstrated to effect fruit set (Yates and Sparks, 1990). Dried 'Stuart' pollen could be stored in moisture proof bags for at least 2 years at -10 oF, and at least 2 months a 41 oF, without losing viability (Yates et al., 1991), thus facilitating economical storage of large amounts of pollen for supplemental pollination in commercial orchards.
A viability decay curve derived by mathematical modeling (Yates and Sparks, 1989) and viability following ultra-cold temperature storage (Yates and Sparks, 1989, 1990) suggest cryopreservation (storage at extremely low temperatures) of pollen might be a means for long-term storage of pecan pollen. The current research was designed to evaluate the merits of pecan pollen cryogenics by determining if pecan pollen: 1) could be stored for longer than 3 years, 2) viability decreased with increasing years of storage, and 3) viability was dependent on the year of collection.
Pollen was collected from four or more 'Desirable' pecan trees growing near Athens, Ga. during Spring 1986, 1987, 1988, and 1989. The trees were randomly selected each year. For each collection year, pollen was bulked and stored in cryotubes at -321 oF in liquid nitrogen (Yates and Sparks, 1990) for up to 13 years. Viability of pollen was determined by in vitro germination, using the methods of Yates and Sparks (1989).
Germination
of pecan pollen stored for up to 13 years in liquid nitrogen was significantly
higher than that
of freshly collected pollen (Table 1) and pollen viability
did not decline with time in storage up to 13 years. The year of collection
did not affect pollen germination following cryogenic storage. Pollen stored
for 11 years in liquid nitrogen produced normal pollen tubes (Fig. 1).
Germination of 1986 and 1987 fresh pollen was determined before our discovery that rehydration of pecan pollen prior to suspension in germination medium is essential for repeatable results (Yates and Sparks, 1989). The lower germination for 1986 fresh pollen (12%) than for 1987 (46%) may be due to more arid environmental conditions existing during pollen collection in 1986 (a major drought year) than in 1987. The results for germination of fresh pollen in 1986 and 1987 support the necessity for rehydration of fresh pollen prior to in vitro germination (Yates and Sparks, 1989).
Germination of pollen collected in 1988 and 1989 was lower for fresh than cryogenically stored pollen. Speculatively, less fit pollen grains (e.g., those with weak or malformed cell walls) do not withstand the rigors of liquid nitrogen freezing and instead deteriorate. Only pollen grains with intact cell walls, not pollen fragments, were assessed for the presence or absence of a germ tube. Deterioration of weak grains would account for higher germination of cryogenically stored pollen. However, disintegration of pollen grains on exposure to cryogenic conditions would not explain the decreased viability of 1986 pollen stored for 1 year compared to 13 years. A justification for these results is not readily apparent based on current knowledge of pecan pollen biology.
In summary, the results of germination tests (Table 1) show that pecan pollen stored in liquid nitrogen for at least 13 years is viable, and that pollen vigor does not decline with time in storage, and that pollen viability is independent of collection year. Thus, pecan pollen can be established as a long-lived species, rather than short-lived pollen as concluded by earlier workers. In addition to providing for short-term storage for breeding purposes, cryogenic pollen storage may also be an effective means of long term germplasm preservation for pecan.
Literature Cited
Smith, C. L. and L. D. Romberg. 1932. Pollination control, period of receptivity and pollen viability in the pecan. Proc. Texas Pecan Growers Assn. 12:23-34.
Stuckey, H. P. 1916. The two groups of varieties of the Hicora pecan and their relation to self-sterility. Georgia Expt. Sta. Bul. 124.
Wetzstein, H. Y. and D. Sparks. 1985. Structure and in vitro germination of the pollen of pecan. J. Amer. Soc. Hort. Sci. 110:778-781.
Woodroof, J. G. 1930. Studies of the staminate inflorescence and pollen of Hicoria pecan. J. Agr. Res. 40:1059-1104.
Yates, I. E. and D. Sparks. 1989. Hydration and temperature influence in vitro germination of pecan pollen. J. Amer. Soc. Hort. Sci. 114:599-605.
Yates, I. E. and D. Sparks. 1990. Three-year-old pecan pollen retains fertility. J. Amer. Soc. Hort. Sci. 115:359-363.
Yates, I. E., D. Sparks, K. Connor, and L. Towill. 1991. Reducing pollen moisture simplifies long term-storage of pecan pollen. J. Amer. Soc. Hort. Sci. 116:430-434.
Table 1. In vitro germination of fresh and stored 'Desirable' pecan pollen.
z Pollen germination as percentage of intact grains forming germ tubes. Means followed by the same letter are not statistically different from each other.
Y Fresh pollen examined without rehydration.
x Fresh pollen examined following rehydration.