Multiple ears per node: Iowa 2006 situation & hypothesis
I. INTRODUCTION: SCOPE OF PROBLEM
Corn hybrids from several companies have developed more than one ear at a single node across Iowa, Illinois, and Indiana this year. Multiple ears are not unexpected but they typically occur at different nodes, not on the same node. This trait has been expressed in different ways in Iowa. In the most extreme cases, up to eight ears have occurred at a single node. Some have called this expression, 'bouquets' or 'banana trees' (Photo 1). In cases where there are numerous ears, each is usually barren. In one case, a field with bouquet ears yielded 50 bu/acre. In other locations, only two ear shoots are visible at a node; we'll call these 'double ears'. Often one of the double ears is smaller, consisting of a few short husks surrounding a tiny cob with silks (Photo 2a). The other ear appears to be full size and not hampered by the double ear. The smaller of the double ears occasionally sets some grain (Photo 2b). A susceptible hybrid may show bouquet ears in one location and double ears at another site.
Photo 1. 'Bouquet' of many ears at the same node. Iowa, Sept. 2006
Photo 2a. Double ears. Note smaller, barren ear. Northeast Research and Demonstration Farm, Iowa State University. Nashua, IA. Lori Abendroth. August 2006.
Photo 2b. Double ears at one node. Note smaller, nubbin ear with a few kernels. Northeast Research and Demonstration Farm, Iowa State University. Nashua, IA. Lori Abendroth. August 2006.
The expression of the multiple ear trait is unusual for hybrids grown in the Corn-Belt (which are typically non-prolific). Prolific hybrids, however, are available in the Midwest; these hybrids are specifically bred to have multiple ears on a plant. The key difference though is that prolific hybrids have multiple ears develop at different nodes.
We saw differential expression of this double-ear trait in a susceptible hybrid in a plant population experiment. The other three hybrids included in the trial did not show this expression. Refer to Table 1. The double ear trait was expressed more often with lower seeding rates due to less interplant competition.
|Table 1. Effect of plant population on the percentage of double ears on one hybrid. ISU Northeast Research and Demonstration Farm. Nashua, IA. September 2006.|
|Seeding rate (seeds/acre)||Total plants with double ears||Plants with some grain on both ears|
|21,100||86% a||18% a|
|31,300||52% b||2% b|
|40,800||55% b||0% b|
Incidence reports from fields expressing the bouquet trait range from 20% of the plants to 100%. Incidence of double ears is similar, with a range from 5 to 10% in affected hybrids to 100%. It does not appear that there is one single cause for the 2006 reports of this phenomenon. Different hybrids from different seed companies were affected, several different herbicides (pre and post) were used, some were affected by early-season drought, some occurred with mid-season fungicide applications and some without.
Upon more detailed analysis, we found that double ears developed at the first node below the primary ear node (Photo 3). The uppermost ear shoot never developed. We were able to locate the uppermost ear shoot above the double-eared node, on several plants. In instances where the shoot could no longer be seen, we could often see the internode 'groove' where the ear shoot would have been. Ear heights were similar among most plants within a field and therefore we can assume that the primary ear was aborted on most, if not all plants, even though the traces of the initial ear shoot were not evident on every plant.
Photo 3. Double ears developed at the first node below the primary ear node. Note the aborted ear shoot at the primary ear node. Story County, IA. Roger Elmore. October 2006.
Is this multiple ear trait expression new? No. Agronomists have seen both of these trait expressions in other years as well. They often concluded they were a chemical application error of some type or inherent to the hybrid. Others report that the trait is common in sweet corn. We propose that these are genetic responses to particular environmental and/or management stresses.
II. OTHER'S COMMENTS IN '06
Bob Nielsen, Extension Corn Specialist, Purdue University, recently discussed the oddity of multiple ears per node (see article: http://www.agry.purdue.edu/ext/corn/news/articles.06/Bouquets-0912.html). He reported that multiple ears originated from individual nodes on a single ear shank. The following are the main points of his article:
First Comment: "...The ear shank is essentially a replica of the main stalk of the plant. Additional ear shoots can develop from individual nodes of the ear shank like additional ear
shoots that develop from individual nodes of the main stalk."
Second Comment: "Normally the ear shank does not initiate these secondary ears or ears initiate but eventually cease development likely due to apical dominance from the apical ear."
Third Comment: "...in many cases none of the multiple ears successfully pollinate and set kernels...In some situations, as much as 30% to 50% of the plants are affected. Yield loss in these severe situations will be dramatic."
Fourth Comment: "The cause of this 'bouquet' effect of multiple ears is not known...To my knowledge, there has yet been no single common thread...what I suspect is that a) some hybrids are genetically prone...and b) more than one external 'trigger' enables the development of multiple ears to occur on these hybrids."
A difference between these comments and the plants we have examined and dissected is that the double ears come from the same node and not the same shank (Photo 4). In these images, you can see that the ear shanks split at the very beginning and form ears. We have not been able to dissect a field with 'bouquet' ears; they also may originate from one shank.
Photo 4. Cross section of double ear showing that the two ear shoots arise from the same node. Northeast Research and Demonstration Farm, Iowa State University. Nashua, IA. Lori Abendroth. August 2006.
III. SCIENTIFIC LITERATURE
Two articles in the scientific literature help us understand the dynamics of genetics, apical dominance, and ear formation. Apical dominance within the plant typically causes a growth inhibition of the secondary and tertiary ears, etc.; the upper-most ear shoot has dominance. Under most circumstances in the Corn-belt, a non-prolific hybrid will develop only one ear on the upper-most ear node because of apical dominance. This upper ear becomes the 'sink' for resources from the plant; the lower ear shoots rarely develop.
III.A. TWIN EAR RESEARCH (HALLAUER)
A 'twin-ear' trait was noted in corn as early as 1973 (Hallauer 1984). Dr. Hallauer reported that "two independent sources of the twin-ear expression were identified...an S2 progeny of BS10 (FR)C2-2388-10 and an S5 progeny of BSAAo2, an opaque-2 strain of BSAA." He described the twin eat trait as having "...two separate ears with separate shanks attached to the same node..." and that "...the twin ears must depend on some specific timing during the ontogeny of the plant." After more research Frank and Hallauer (1997) found that the twin-ear trait was recessive and that the non-twin-ear trait (single ear per node) was almost completely dominant over the twin-ear trait. The trait fit the definition of a threshold trait. A threshold trait is one that is inherited quantitatively but expressed qualitatively. It is inherited as a quantitative trait, but it occurs as an either/or trait. They could not stabilize the trait (Hallauer, 1988; Hallauer, Iowa State Univ., personal communication, 2006). Their observance of the twin ears originating from the same node but having separate ear shanks matches what we have seen in many fields.
III.B. EAR INITIATION (LEJEUNE & BERNIER)
Researchers in Belgium found that inbred B22 (AGPM, France) frequently aborted the primary ear (uppermost ear) which then became a sterile leaf-like structure at the primary ear node. A secondary ear (of relatively normal size and appearance) would then develop at the next lower node along with a smaller ear growing from the same node. The smaller 'ear' did not produce seed. Because their observations of primary ear abortion were dependent on year, location, and planting date, they suggested that an environmental factor was the trigger for the expression. They designed growth chamber research projects to study the effects of various environmental factors.
Leaf and ear initiation and development:
- The primary ear was initiated leaf stage v6 to v7 (6 or 7 visible collars) shortly (0.3 to 0.8 day) after the tassel was initiated and at about the same time the last leaf was initiated.
- Even under normal conditions, B22 produced ears on the primary node that stopped growing soon after initiation in 35% of the plants.
- Length of light exposure was important. Abortion of the upper most ears was three times more frequent with a longer photoperiod (16 hour versus 12 hour). Reducing the photoperiod near v3 to only 12 hours nearly eliminated the plants' abortion of the primary ear.
- Temperature was important. Seedlings exposed to cold temperatures (50o F) for 3, 5, or 7 days beginning at the 6th leaf stage increased primary ear abortion. If the temperature occurred earlier in development (at v3), no ear abortion occurred.
- Intensity of light was important. While plants were exposed to a 7-day cold period, the severity of ear abortion was reduced if light irradiance was reduced at v6.
- Water was important. Flooding by itself did not cause ear abortion. It only increased ear abortion if the plant was subjected to cold temperatures at the same time.
- Environmental stress may alter the balance of growth regulators that affect apical dominance.
- A genetic factor was obvious as inbred responses to chilling were dramatically different.
IV. SUMMARY OF OBSERVATIONS AND RESEARCH
It is possible that the double ears reported in 2006 are from genetic materials that carry the twin-ear trait described by Hallauer. Indeed Hallauer (1988) stated that the twin-ear trait he described usually expressed itself at the top-ear node but also occurred less frequently at the 2nd and 3rd nodes below the top-ear node. We believe that this year, most of the double ears occurred at nodes below the primary ear node. Both of the traits are specific to particular genetic combinations. We would need to know the genetic background of all hybrids today to be able to completely differentiate and determine why some hybrids expressed this trait and others did not.
Based on our observations and the comments and findings of Lejeune and Bernier (1996) we propose that apical dominance of the primary ear node was lost. This allowed multiple ear formation on lower nodes. We suggest this is true for both of the multiple ear traits (bouquet and double eared) observed.
It is highly doubtful that a single management or environmental factor induced the multiple-ear trait expression from Iowa to Indiana in 2006. As mentioned above we saw this expression occur on several hybrids, from several companies, and with different in-field management. We agree with Dr. Nielsen (2006) that the cause of the multiple ears is not definitively known. We believe that we are dealing with a threshold genetic trait that is triggered by particular stress events that occur during primary ear formation. We may never determine exactly what the specific stress was in any given situation. It would be possible to conduct experiments with susceptible hybrids subjected to various stresses during ear initiation stages to better understand this unique genetic-environment-management phenomenon; yet we expect the results to be similar to Lejeune and Bernier (1996). In terms of yield impact, we expect that plants that had ears on different shanks to not have a substantial yield loss because the larger ear appeared fairly normal in size and development. Yet in instances where a 'bouquet' of ears occurred, we expect a greater yield loss because in those cases, often no kernels were produced on the ears. Therefore, the differential expression of this multiple ear trait will cause substantially different yield responses.
Frank, T.E. and A.R. Hallauer. 1997. Generation Means Analysis of the Twin-Ear Trait in Maize. Journal of Heredity. 88:469-474.
Hallauer, A.R. 1984. Twin-ear expression. Maize Genetics Cooperation Newsletter apr 30 1984 58:21-22
Hallauer, A.R. 1988. Penetrance and expressivity of twin ears. Maize Genetics Cooperation Newsletter 1988 62:2-3
Lejeune, P. and G. Bernier. 1996. Effect of environment on the early steps of ear initiation in maze (Zea mays L.) Plant, Cell and Environment. 19:217-224.
Nielsen, R.L. 2006. A problem with "Bouquets." Agronomy Tips. Purdue University. 22 Sept. 2006.
Text written by Roger W. Elmore and Lori J. Abendroth, Extension Corn Production, Iowa State University. November 2006