Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Uneven or poor stands may be caused by reduced seed vigor in a specific lot of sweet corn seed. This results in reduced yields in fresh market sweet corn and may cause losses in processing corn if differences in growth are significant.
By its nature, sweet corn has lower stored food reserves compared to field corn. With the advent of different endosperm types than the traditional sugary (su), such as homozygous sugary enhanced (se), shrunken supersweets (sh2), and the more recent augmented shrunken types, vigor became even more of an issue. In general, vigor of sweet corn rated from highest to lowest is: normal sugary su > se heterozygous > se homozygous > sh2 augmented > shrunken sh2. Synergistic sweet gene varieties may have seed with vigor characteristics of a se or a su sweet corn depending on the specific genetics (check with your sweet corn seed company for specifics on the vigor of these hybrids). Supersweet hybrids (shrunken sh2) are noted for having inherently low seed vigor due to reduced food reserves and it has been a standard recommendation to plant these varieties only when soil temperatures are above 60°F.
This inherent lower vigor of sweet corn becomes magnified when a specific seed lot has problems. Sweet corn seed vigor will be affected by seed growing conditions, seed conditioning practices, storage conditions, and length of storage. Seed companies spend significant resources evaluating sweet corn seed for quality (viability and vigor) prior to release and suspect lots are removed from sale. However, seed lots can decline between testing and sales. An acceptable seed lot can become problematic over time.
Sweet corn fields planted with reduced vigor seed will often have uneven stands with healthy plants next to smaller stunted plants. This pattern will often be present across the field. Low vigor plants are less productive and can actually act as weeds in a field, taking resources away from healthy plants and reducing the potential for compensation (producing bigger ears or multiple ears as occurs with remaining plants in fields with stand losses). Reduced vigor seed will produce these field patterns in warm weather but the effects are most severe in cold soils.
Stand reductions are also common with reduced vigor seed. Often, when digging up the seedlings and examining the seed remnants and mesocotyls of stunted plants, the kernels will be disintegrated and there will be darkening at the mesocotyl attachment. This means that the seeds deteriorated prematurely and the full content of the food reserves in the seed were not available for seedling development leading to the stand and vigor issues.
Seed viability is measured with a germination test which is done under optimum temperature, moisture, and light conditions. However, germination tests do not directly measure seed vigor, and seed vigor declines before germination is reduced. Therefore, it is possible to have seed that will germinate in a field but be of low enough vigor that sweet corn plants do not grow properly.
If a seed lot is suspected of having low vigor, then seed vigor tests are recommended. Testing for vigor is also very important for carryover seeds or seeds stored for long periods in unfavorable conditions. Seed vigor testing is also useful when troubleshooting fields where seed vigor issues are suspected (testing left-over seed).
Tests that are used to evaluate seed vigor that are available from different state and private seed laboratories include:
The Cold Test – Seeds are germinated using a specific cold, moist treatment regime. This will be useful in selecting those lots that will perform the best under early cold soil conditions.
Seedling Vigor Classification Test (SVCT) – In this test seedlings from a normal germination test are rated visually according to vigor (strong or weak). Visual ratings are based on if the seedlings have normal developmental characteristics in all seedling plant parts. With sweet corn this would be the roots, the mesocotyl, and the coleoptile. In low vigor seed one or more of these parts will be abnormal. This is a good test to troubleshoot suspect low vigor seed lots.
Tetrazolium (TZ) Test – This is a quick biochemical test that essentially stains living tissue in a seed a red color. The more red staining, the more viable the seed. This test is good for spotting lots with significant differences in vigor between seeds.
Accelerated Aging Test (AAT) – In this test, seed is put under a high temperature and humidity regime for a period of time and then is evaluated using a standard germination test. This is often used to check the storability of seeds under less than ideal conditions but also will do a good job of evaluating seed vigor. Modifications to the Accelerated Aging Test have been made to do a better job of evaluating sweet corn types such as shrunken sh2 varieties.
Electric Conductivity Test – This test measures cell membrane integrity which is correlates well with seed vigor and sweet corn seed emergence. As seeds age and cell membranes deteriorate, cell contents leak, the more leakage, the higher the electrical conductivity and the lower the seed vigor. This is most useful in comparing different lots after extended storage.
Most seed companies also grow out sweet corn lots in field tests prior to sales (commonly in winter nurseries) to confirm results from germination and vigor tests that have been performed.