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 Essay - mar/apr 2004
Seed quality is not always the problem
James C. Delouche
Professor Emeritus Mississippi State University
When I was young and much more confident than I should have been, I delivered a very emotional talk on poor quality seeds as the cause of crop establishment problems before one of large farm associations. I must confess that in my enthusiasm I blamed most of the problems in crop production from emergence failures to low yields on low quality seeds. During the subsequent recess in the sessions I was confronted by several officers of seed companies who admonished me for failing to mention the many factors other than seed quality that can and do have profound effects on the establishment of crop stands. I replied that I appreciated their concern and regretted that my presentation was somewhat unbalanced. After a few days reflection, I came to recognize that my presentation was more than just somewhat unbalanced, it was grossly unbalanced and, most important, it unfairly pointed to seeds, and by implication to seed producers, as the main villains in crop establishment problems. And, I resolved to develop broader, fairer and more balanced views relating to the very complex process of crop establishment.
Crop emergence failures are sometimes the result of poor quality of the seeds planted and sometimes the result of environmental stresses in the seedbed that are of such magnitude that even the highest quality seeds fail. Most often, however, they are the result of the interaction of seed quality and environmental stresses. Seed technologists, seed company representatives and farmers need to be knowledgeable about the factors other than poor quality seeds that can contribute to or cause emergence failures so that they can investigate complaints in an informed manner and defend seeds against unjust claims about their role in crop establishment problems. The factors other than seed quality that influence the performance of planted seeds make up the microenvironment of the seedbed They are identified and characterized in the following sections.
Temperature
Temperature has a great influence on the rate and percentage of seed germination and emergence. Three cardinal temperatures are recognized: the minimum, optimum and maximum. For each kind of seed there is a range of temperature within which germination and emergence proceed to completion if moisture supply is adequate and other stresses are minimal. The temperature range for germination is bound on the cooler side by the minimum temperature, the temperature below which seeds do not germinate in a reasonable time period, and on the warmer side by the maximum temperature, the temperature above which seeds usually die in a few days. The optimum temperature is that segment of the range within which the maximum germination/emergence percentage is attained most rapidly, i.e., in minimal time. For most kinds of seeds it is closer to the maximum temperature than the minimum.
As the temperature increases above the optimum, germination/emergence slows down and the "weaker" seeds succumb to high temperature stress, i.e., the percentage germination/emergence decreases. The rate of germination/emergence also decreases as the temperature decreases below the optimum although the percentage often remains high but very non-uniform if enough time is allowed. Any delay or slowing down of the germination process caused by stress levels of temperature, too high or too low, increases the opportunity and probability of attack of the seeds by soil microorganisms with the result that emergence is reduced. This is the area in which temperature stress and seed quality interact. The well-known and widely used cold test for maize seeds evaluates the vigor or emergence potential of the seeds in the laboratory under simulated seedbed conditions of near minimal temperature and excessive moisture, conditions that are frequently encountered in the field in the temperate maize growing regions. The significant variables affecting emergence in the cold test are inheritance, mechanical damage and physiological quality of the seeds. Seeds with poor inheritance, mechanically damaged and/or low in vigor emerge poorly or not at all in the cold test. Thus, in the cold test and especially in the field where conditions are variable rather than constant, emergence is determined by the interaction of temperature and moisture stresses and "seed quality."
The cool germination test for cottonseed is another example of the how knowledge of the interacting effects of seed quality and temperature on germination/emergence has been used to develop a seed vigor test. It has been well established that germination/emergence of cotton seed, especially mechanically damaged and low vigor seeds, proceeds slowly and with reduced percentages under cool conditions. During cotton planting season the soil temperatures are broadcast daily on the local radio stations by the extension service and farmers are advised not to plant until the minumum soil temperature is 18 degrees C or warmer for at least 4 days. The cool germination test determines the percentage of cotton seeds that germinate and produce seedlings 4 cm or longer in 7 days at 18 degrees C. There is excellent correlation of seed performance in the laboratory test and in the field under low temperature stress, i.e., the interacting effects of seed quality and low temperature stress on germination/emergence. The accelerating aging test is still another example of a vigor test that is essentially based on the interaction of seed quality and temperature stress.
Soil Moisture
Seeds have to re-hydrate to the critical moisture content for germination which varies among the different kinds of seeds. (Interestingly, the critical seed moisture content for germination appears to be the same as the moisture content of seeds when physiological maturity is attained.) Seeds absorb moisture even in relatively "dry" seedbeds but very slowly and often they do not attain the critical moisture level for germination. Unless and until the seedbed moisture is increased by rain or irrigation, the seeds are in effect "stored" in the soil at a relatively high and increasing moisture content in a microenvironment that can be warm or even hot, i.e., like accelerated aging. The performance of seeds under these conditions is determined by the interaction of the environmental stresses and seed quality, i.e., vigor. Prolonged rains after planting combined with slow or poor soil drainage produce excessive or saturated levels of soil moisture and poor aeration with reduced oxygen supply that can quickly lead to rotting of even the highest quality seeds. Under less stressful moisture regimes and aeration, emergence is largely determined by the interaction of the stress levels and seed quality. Knowledge of this interaction was used in the 1930s by a plant pathologist at Iowa State University (one of the inventors of the cold test for maize) as an alternate test for determining the vigor of maize seeds and evaluating the need for treatment with a fungicide. Replicates of 100 maize seeds were placed in about 500 ml of water with several grams of top-soil from a maize field and incubated at a warm temperature (about 40 degrees) for several days. The seeds were then removed and subjected to a standard germination test. Seeds that germinated poorly after the soak in warm water were considered to be low in quality and most likely to be benefited by a seed fungicide.
Emergence failures, among others factors, can lead to low yields
Soil Crusts
Heavy rains during planting can break down the soil aggregates and cause them to form a "seal." As the soil dries a "crust" is formed that depending on the soil type can mechanically impede the emergence of seedlings. In some cases the seedlings break through the soil but are themselves broken or damaged, i.e., crippled in the process. In other cases they simply cannot exert enough force to break through the soil crust. Soil crusting is a significant cause of emergence failures in regions where the soils are heavy, i.e., high in clay, and there are frequent hard rains during the planting period. Information and data on the interaction of the resistance imposed by soil crusts and seed quality or vigor on emergence are scant and inconclusive. One of the first vigor tests was the Hiltner or so-called brick grit or gravel test originally designed (circa 1911) to evaluate the effects of Fusarium infection on emergence of cereal seeds and later to evaluate several other seed/seedling "defects" such as injury from hot water treatment, mechanical damage and so on. While the brick grit does impose some level of mechanical resistance to emergence its magnitude is much less than that of a well-established soil crust. The most significant variables in the emergence of seeds through soil crusts appear to be the species, e.g., type of germination (hypogeal or epigeal), inheritance, i.e., the variety, and seed size. Soybean varieties with small average seed size emerged more rapidly and to a higher percentage than those of larger average seed size in heavy soils susceptible to crusting.
The next installment of this essay will continue with the review of some of the important factors other than seed quality that can be and are involved in crop emergence problems, e.g., microorganisms, insects, other animals, applied chemicals, condition of the seedbed, etc., their interactions, and some of the strategies for minimizing or evading the microenvironmental stresses in the seedbed.
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