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 Essay - sept/oct 2005
Thoughts and reflections on seed storage III
James C. Delouche
Professor Emeritus Mississippi State University
Seeds in storage are the main recurring assets of seed companies. They represent the only means for them to recover the costs incurred in producing seeds and to earn a reasonable return on substantial investments of management, labor and resources. Yet, only in rather recent times have they taken all or at least most of the steps long available to ensure that their seed assets do not depreciate as a result of losses in quality due to deterioration, insects and rodent depredation. As emphasized in the previous parts of this essay, the steps taken to assure that the quality of seeds is maintained through marketing and planting must begin long before they enter the packaged seed storehouse and distribution channels, preferably they should with the selection and contracting of seed producers. It is not sufficient, however, that the storage program begin at the proper time, it must be based on an adequate knowledge base of the genetic, physical, environmental and physiological factors that determine seed longevity.
In the mid1960s my colleagues and I realized that while the essential information for an adequate seed storage knowledge base was available to us and other researchers and technologists in academia, it was neither readily available nor well understood by workers and supervisors in the seed industry responsible for quality assurance and control. Much of the information on seed storage was widely scattered in scientific journals, very specific, overly complex, too "scientific", and often lacking in practical applications and implications. We began, therefore, to gather information on seed storage we felt should be available to seed technicians and managers and assemble it in a summarized but well documented, practically oriented and understandable format. The result was a package of basic knowledge on seed longevity and storage presented in the form of "Precepts of Seed Storage." The original package presented in the late 1960s consisted of 10 precepts with extensive documentation and commentary but a subsequent and final revision in 1973 combined two of them to reduce the number to 9.
The preface to the Precepts reviewed the reasons for storing seeds, the principal causes of seed storage problems, the "storage period" and its sequential segments all of which were considered in the previous parts of this essay on seed storage in my usual long-winded, personalized and anecdotal way. The preface concluded with the characterization of seed deterioration, the major process involved in seed quality losses during the storage, in terms significant to workers in the seed industry: seed deterioration is an inexorable or inevitable process because seeds are alive and all living things eventually die - the rate of deterioration, however, can be controlled; seed deterioration is an irreversible process - quality losses cannot be restored; and the rate of deterioration varies among seed species, varieties, lots of the same variety and individual seeds within a lot as a consequence of the interaction of genetic, physiological and environmental factors. The Precepts accompanied by substantial documentation, commentary and explanation were well received by seed workers in the U.S. and other countries and copied in many quality control manuals. Today, they seem rather simplistic and obvious but I feel they are still an important informational package that needs to be in the knowledge base of persons involved in producing and supplying seeds to farmers and gardeners. The Precepts are presented below with very brief commentary as the final part of this essay on seed storage.
I. Longevity is a characteristic of the species and/or variety.
Longevity is an inherent characteristic. Soybean seed are inherently shorter lived than rice seed.
II. High quality seeds store better than low quality seeds.
The most effective means of preventing storage problems is to produce and package high quality seeds. Even the best of storage conditions cannot restore losses in quality.
III. Seed moisture content and temperature are the most important factors influencing the longevity of specific kinds of seeds in storage.
The life span of seeds of a genotype is largely determined by their moisture content and temperature. The rate of deteriorative processes increases as seed moisture content and temperature increase. Within "normal" ranges the activity of other agents of destruction of seeds, e.g., insects and fungi, also increases as seed moisture content and temperature increase.
IV. Seed moisture content is a function of relative humidity and to a lesser extent of temperature.
Seeds are hygroscopic. They absorb moisture from the atmosphere or lose moisture to it until an equilibrium is reached. The moisture content of a specific kind of seed at a specific level of relative humidity and temperature is termed the equilibrium moisture content. Storage problems are more common and severe in the humid tropics and sub-tropics where relative humidity and temperature are relatively high.
V. Seed moisture content is more important than temperature.
Seed moisture content and temperature are the two most important factors in seed storage. Of the two, moisture content has the greater influence on seed longevity. Well-dried seeds store quite well at temperatures as high as 40°C., a fact that led to the development of sealed storage of seeds. On the other hand, relatively high moisture content seeds will deteriorate even at temperatures of 10°C. or less. One of Harrington's well known "rules" for seed storage takes into account both relative humidity and temperature: viz., good seed storage is achieved when the percent relative humidity in the storage environment and the temperature (°F) add to 100, e.g. 50% & 50°F (10°C), 60% & 40°F (4°C), 20% & 80°F (26°C). These combinations would provide much more than adequate conditions for storage of seeds of most kinds of grain, grain legume, oil seed, and fiber crop for the usual 6- 8 months storage period.
VI. A one (1) percent decrase in moisture content or a 10°F (5.5°C) in temperature nearly doubles the storage life of seed.
These two "rules" for seed storage by Harrington dramatize and bring into sharp focus the critical importance of seed moisture content and temperature in seed storage and longevity. When a lot of soybean seed tested 14% moisture content which exceeded the maximum 13% moisture content certification standard and the seed producer argued that 1% didn't matter, we quoted Harrington's 1% rule.
VII. Dry, cool conditions are best for seed storage.
This general prescription for seed storage raises the questions: how dry and how cool? The answers depend on the kind of seed, physiological quality of the seeds, and the length of the packaged or bulk seed segment of the storage period. The focus, however, should always be on both dry and cool. Many good lots of valuable seeds were lost during the 1960s and 1970s when interest in conditioned storage, i.e., climatically controlled storage, for seeds became widespread. Some seed companies rented storage space in cold storage warehouses designed for fresh vegetables, potatoes, etc., with relative humidity above 85% and temperature 5 - 10°C. When the seeds were taken out of storage after 6 months or so for marketing seed moisture content was often 18% or higher which resulted in heating of the seeds and rapid losses in germination under the prevailing ambient temperatures during distribution and marketing.
VIII. Effective sealed storage of seeds requires that seed moisture content be substantially lower than for open or non-sealed storage.
The relative humidity and equilibrium moisture content in sealed storage remain constant, while in open storage they fluctuate and are often reduced.
IX. Sanitation and cleanliness are essential for successful storage.
Storage insects and rodents must be kept out of the seed storehouse.
The Precepts are rather bare and overly simplistic without the extensive documentation, explanations, implications and applications that accompanied them. However, I have long found that a simple restatement of matters or items of information that seem obvious is very useful in refreshing the core base of knowledge about seeds that I want to maintain. Perhaps, some of you have the same experience.
My thoughts and reflections about seed storage are now exhausted. They will turn to other seed matters in the next essay.
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