By Bob Nielsen
The occurrence of severe photosynthetic stress (severe drought, extreme heat, severe nutrient deficiency, severe foliar disease) during or shortly after pollination in corn often results in poorly filled ears due to incomplete pollination or abortion of young kernels. Often such poor kernel set occurs primarily on the tip end of a cob, but sometimes the problem manifests itself down one side of the cob, affecting several rows of potential kernels.
Such a pattern of poor kernel set is euphemistically referred to as a "zipper ear."
The absence of kernels on the tips of ears as a result of stress can be explained physiologically from the standpoint that tip silks are usually the last to emerge from the husk during pollination and, thus, are usually the last to capture pollen if pollen is still available.
If pollen is no longer available when the tip silks emerge, then the tip ovules are never fertilized and no kernels develop (i.e., blank cob tips or "tip back"). If pollen capture by the later emerging tip silks is successful and the tip ovules are fertilized, then the tip kernels are younger relative to the others on the cob.
Younger kernels at the tips of ears are more vulnerable to abortion when severe photosynthetic stress occurs early in the grain fill process, resulting in the usual pattern of poor kernel set at the tips of ears.
A less common pattern of poor kernel set is one that is often described as the zipper pattern wherein 1 or more entire rows of kernels along one side of a cob are absent due to some combination of pollination failure and kernel abortion.
A subsequent symptom that often develops on such zipper ears is a noticeable curvature of the cob, sometimes to the extent that folks describe it as a banana ear. These curved ears are a consequence of the absence of kernels on one side of the cob coupled with the continued development of kernels on the other side that force the cob to bend or curve.
While most recognize that the absence of kernels down one side of the ear is the result of severe photosynthetic stress, it is less obvious why the problem occurred along one side of the ear rather than being localized at the tip of the ear. The cause(s) of the zipper pattern of kernel abortion is not well understood nor well documented. Following are a couple of ideas.
Silks "Shading" Other Silks?
In a number of fields where I have observed the zipper pattern, the side of the ear with the kernel set problem is the same side over which the silks draped during the pollen shed period. This leads me to speculate that perhaps the draping of the silks resulted in the underlying silks being "shaded" from initial contact with pollen, causing those silks to either a) never capture pollen and the connected ovules never fertilized or b) capture pollen later than the rest, resulting in delayed kernel development and, thus, being more vulnerable to abortion under subsequent stress.
The connection between silk draping and zipper ears would be more likely in situations where silk emergence occurs several days prior to pollen shed, allowing for silk elongation and draping to occur.
Note: Some modern drought-tolerant hybrids have an aggressive silking habit that resists the usual delay in silk emergence under extreme drought. When soil moisture is not limiting, these hybrids often silk 3 to 4 days prior to the beginning of pollen shed. In that period of time, the first emerging silks can elongate upwards of 4 to 8 inches before pollen shed begins.
Differential Heating Around Circumference of Ears?
Another possible contributing factor was suggested by an acquaintance of mine at DuPont Pioneer who indicated that drought researchers within that company reportedly documented that cob/ovule/kernel temperature can vary around the circumference of a developing ear of corn, with upper side of the ear potentially warmer than the lower side of the ear. If true, I presume such a difference would be caused by more exposure to sunlight on the upper side of the ears during daylight hours.
That observation reminded me of a research article published in 2001 that looked at the effects of differential heating on silk timing and kernel survival in corn. In that research, the authors applied electrical heating strips at the tips or along one side of ears for a 14-day interval from about 2 days before silk emergence to about 12 days after silk emergence. The heating strips were designed in such a way as to raise the temperature of the affected area of the ear by about 9 degrees with respect to the ambient air temperature.
Neither of the heating treatments affected silk emergence timing or the number of emerged silks, probably because the heating treatments were imposed after silk elongation had already begun within the husks. Heating the tips of ears also had no effect on final ovule numbers.
However, heating one side of the ears resulted in significant kernel abortion on the opposite, non-heated side. The authors speculated that the heating treatment may have enhanced kernel metabolic activity and increased the partitioning of photosynthetic assimilate to the developing kernels on the heated side of the ear, at the expense of lesser photosynthetic assimilate available to the developing kernels on the non-heated side of the ear.
The results of this research, coupled with the observations from DuPont Pioneer researchers that the upper sides of ears are often warmer than lower sides, would certainly offer a possible explanation of the zipper pattern of kernel abortion in years where the crop experiences not only excessive heat but also drought stress during or shortly after pollination.
In addition to delayed metabolic rates and restricted photosynthetic assimilates, the development rate of the cooler kernels would be slower and, thus, the kernels somewhat younger and more vulnerable to the effects of severe stress.
As I have looked more closely at zippered ears in recent years, though, the zipper pattern does not always occur on the lower (and possibly cooler) sides of the ear. However, the concept of differential heating of ears around their circumference certainly seems plausible as a factor contributing to the zipper pattern of kernel abortion.