Unexpectedly extended pollen viability in California
--Greg Barnes and Virginia Walbot
Anecdotal reports suggest that maize pollen remains functional for 30 minutes or less under a variety of climatic conditions. Heat and humidity are both thought to contribute to pollen aging. Our field site has particularly low humidity (20-40% RH) and mild temperatures (mid-day highs of 25-28 C are typical) through the pollinating season. Even though pollen is rarely shed prior to 9 am, we have routinely bagged tassels the preceding night to avoid contamination on the assumption that all pollen viability would be lost overnight.
To determine viability, pollen was collected from a bz2 tester
in a W23/K55 hybrid background at approximately 9:30 am, fractionated in
a glassine envelope to remove anthers and other debris, and the "purified"
pollen was kept in a glassine bag in our field lab under ambient conditions
of light and temperature. Generous aliquots of this pollen were delivered
to cut-back ears of the same bz2 tester in a set of detasseled rows
flanked on three sides by Bz2 material. Most contaminating pollen
should yield purple kernels, but no purple contaminants were detected in
the 15 ears examined:
|Pollen storage (hours)||Kernels per ear||Average K/ear|
|1||415, 351, 288||351|
|2||274, 117, 104||165|
|4||232, 68, 55||118|
Averaging the yield of the 0 and 1 hour time samples we set the expected yield at 310 kernels. Seed set decreases about 50% by the 2 hour point and is progressively lower at later time points. It is striking, however, that yield is still substantial (29%) at 8 hours and 23 hours (15%) after pollen collection. We cannot completely rule out contaminating pollen from a neighboring bz2 tester on one side of the detasseled area, however, the absence of purple kernels means that contamination is probably low.
To confirm the result a second experiment involved recipient bz2
R-g W23/K55 ears crossed by stored Bz2 r-g W23 pollen; the recipient
ears were in a large bz2 tester plot; none of the recipient plants
were detasseled, allowing self-pollination to occur. In this test purple
kernels are from the stored pollen and bronze kernels represent contaminants:
|Pollen storage (hours)||Kernels per ear||Average K/ear||# bz2 K|
|1||278, two smutty ears||278||1|
|4||36, 8, 0||15||0|
|8||0, 0, 0||0||2|
|24||21, 8, 0||10||0|
|32||4, 0, 1||2||0|
|48||0, 0, 0||0||0|
A low level of contamination occurred in this experiment, however, it is clear that considerable functional pollen persists for two hours, and that small amounts of viable pollen are present in samples taken at 4-32 hours. That any pollen survives for 24 hr or more is surprising.
The phenomenon of unexpectedly long-lived pollen will be pursued to determine the contributions of pollen genotype, prevailing environment and storage conditions to pollen viability. This summer we will repeat these experiments with additional genotypes, concentrating on time points between 2 and 24 hours. At the initial time points, there is a considerable difference in the viability of bz2 pollen vs. the Bz2 r-g pollen. We will also titrate pollen amounts, obtain pollen samples from tassels of different ages and vary the storage conditions.
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