--J. Bennetzen and P. San Miguel
Due to the severe drought conditions and a low soil moisture content at the time of our latest planting this summer, many of the seed in our nursery did not germinate until the middle of July. This necessitated some very late pollination dates, from late August through mid-September. In particular, potential mutants from one acre of a screen for a Mutator-associated inactivation of Rf*-c were not pollinated until September 15. Since early October frosts are common in the Lafayette area, we were not certain whether any of these ears would survive. Rather than panic (or, more correctly, in addition to attaining a reasonable level of panic), we decided to take advantage of this situation to test various approaches for the rescue of late field pollinations.
We had constructed an Rf*-c/Rf*-c cms-C Mutator stock (in a mixed B73/B77/B78/Q66/Q67 background) and crossed this as a female to male Mo17 (rf*-c/rf*-c cms-N). Over 200 ears of this material were generated, and each ear was individually shucked and shelled. In the summer of 1988, about one-third of this material was machine planted, on an ear-by-ear basis. As the plants matured, we walked down each row and detasseled all male fertile plants with a machete. Apparent male sterile plants were observed at a frequency of about one in 300. Most or all of these apparent Rf*-c mutants were probably escapes due to physiological events causing poor male fertility. These male steriles were all hand pollinated by Mo17 on September 15. The rest of September and the first two weeks of October were unusually cool, with daytime highs in the 51 F-90 F range (averaging 71.2 F) and lows in the 21 F-69 F range (averaging 45.9 F). Normal high and low temperature averages in our area for this time of year are 74.2 F and 46.6 F, respectively. In all cases, temperatures were recorded at the National Weather Service Station at the Purdue Agronomy Farm, 1.2 miles from our nursery. We also received above average rainfall during this period. Subfreezing temperatures of 31 F, 29 F, and 28 F were recorded on October 6, 7, and 12, respectively. Our first hard frost was predicted for the early morning hours of October 13, when the temperature dropped to 21 F. The fertilized materials in the Rf*-c mutant screen were subdivided into five categories. One group of plants was left in the field to check for resistance to this temperature; the plants in a second group were each covered on the afternoon of October 12 with thirty- gallon black trash bags; ears from plants in the third group were harvested and shucked on October 12, while (following the suggestion of Dr. Janice Clark) ears from a fourth group were harvested but not shucked; and, in the fifth group, whole plants were uprooted. In the latter three groups, all ears or plants were left in open pollination bags in a well ventilated environment at approximately 70 F. On October 13, the trash bags were removed from the covered plants after the air temperature increased to 30 F. Between October 13 and October 24, conditions in the field remained unusually cool and damp, although the temperature did not drop below 32 F. All ears were harvested, shucked, and dried on October 24. Forty seed off each ear were soaked overnight in tap water and germinated on wet towels in a humidified incubator. As shown in Table 1, major differences were observed in the germination competence of seed from each experimental group.
|Harvest date||Plant or ear harvested||Bagged in field||Shuck date||Range||Avg.|
The most effective approach for rescuing these late pollinations was removal of the whole ear from the field and allowing the unshucked ear to further mature in a warm environment. Harvesting the entire plant carrying the fertilized ear, and allowing a similar maturation indoors, was not significantly superior to harvesting only the ear. Removing the husk from a harvested, immature ear was a major negative factor, decreasing seed viability from 82% to 8%. Leaving these young ears in the field was also less successful, whether or not the plants were covered during the hard frost. Many of these ears had no germinating seed, while a few had reasonable percentages of survivors.
On ears that gave low germination frequencies of 5% to 30% (both those samples that were left in the field during and after the hard frost and those immature ears that were both harvested and shucked on October 12), the seed in the middle of the ear was most likely to germinate (Table 2). It is not known whether this reflects a faster maturation rate, a greater resistance to freezing and/or desiccation, and/or a more efficient post- harvest translocation of nutrients to these mid-ear kernels.
|Kernel position in row||Germination from ears left in field until 10/25||Germination from ears harvested and shucked on 10/12|
|1 (base of ear)||2/28*||1/12**|
|18 (toward tip of ear)||1/28||0/12|
In an adjacent field, plants in slow-maturing opaque backgrounds were crossed between late August and early September. Crosses performed less than four weeks prior to harvesting demonstrated little or no decrease in germination efficiency if stored as unshucked ears at room temperature for an additional 23 days (Table 3).
|Harvest date||Shuck date||Cross date||# of ears||Range||Avg.||Kernels scored|
Taken together, these results indicate that seed which has matured less
than four weeks post-pollination can be rescued by harvesting the unshucked
ear and allowing it to mature indoors. We did not determine to what degree
the 21 F hard frost of October 13 decreased germination efficiency compared
to any possible negative effects of the prolonged retention of the ears
in a cool, damp field environment. All things being equal, it might be
appropriate to rescue vital immature ears from the field even in the absence
of an imminent freeze if the ensuing field conditions are likely to be
continuously cool and damp until the next hard frost.
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