The reasons for assessing the quality of a seed lot are numerous and include determining the value, purity and moisture content. The different aspects of
quality as defined in the section above are used to evaluate the sowing value of a specific seed lot, i.e. its true value to farmers and growers. The criteria may also be used to determine the commercial value, i.e. the price paid to the con- tractor who produced the seeds or vendor in a subsequent commercial transac- tion, and ultimately to protect the final purchaser of the seed who uses it to produce a vegetable crop. Different evaluations may be done in the pathway of the seeds from original producer through to the grower who sows them, but each test or evaluation generally follows an internationally agreed and accepted procedure, and this is essential for evaluations subject to legislation.
The criteria for the assessment of seed quality are referred to as attributes while the factors, or causes, that may predispose the seed lot to any of the attributes are known as determinates. It should be noted that there can be several determinants of both pre-harvest and postharvest origin that may affect an attribute, these are summarized at the end of the chapter.
The seed quality attributes are:
● Germination and vigour.
● Genetic purity (trueness to type).
● Mechanical purity.
● Seed health.
● Moisture content.
Genetic quality: this may be referred to as ‘trueness to type’, cultivar purity or varietal purity. The strict control of seed generations or categories coupled with implementation and monitoring of seed certification procedures (or other monitoring schemes where they apply) provide ways of assisting the authenti- city of a seed lot.
Physical purity: the range of components of a seed lot can include seed of the stated species (i.e. pure seed), seed of other species (i.e. including other crop species and weeds) and contaminants. The contaminants may be derived from a range of sources including materials from seeds, parts of plants, living organisms (but not of plant origin) and other materials such as soil. The pure seed definitions of a wide range of crop species, including vegetables, have been published by ISTA (2009a).
Seed health: this may be described as the extent to which seed-borne patho- gens and/or pests are present. ISTA has published a list of seed-borne diseases with annotations regarding bibliographies and treatments (Richardson, 1990).
Viability: this can be referred to as the potential germination and subse- quent production of a seedling of the stated cultivar. Although the viability of an individual seed can be determined, it is more usual to refer to the germin- ation potential of a seed lot. In this context, a seed lot is taken to be from a specific population, i.e. it is composed of a homogeneous population derived from the same stock, from the same production location at the same time and therefore having the same reference number; it is also assumed that all seeds in the seed lot have been treated in the same way regarding processing and storage. It is essential that methods used for determining the germination of a sample taken from a seed lot are both repeatable and reproducible; this is especially important from the aspects of legislation and international trade.
The seed testing procedures described by ISTA (2009a) provide an inter- national standard. The seed testing rules set by AOSA (2008) are used in the USA and Canada, although there is very close cooperation between AOSA and ISTA.
Vigour: defined by ISTA (1995a) as ‘the sum total of those properties of the seed which determine the level of activity and performance of the seed or seed lot during germination and seedling emergence’. The tests that are recognized are described or listed by ISTA (1995b) and AOSA (2008).
Moisture content: this is the percentage of moisture of the seed lot. Although there are quick electronic systems in use, moisture content is usually deter- mined by an oven drying method as specified by ISTA (2009a).
The International Seed Testing Association (ISTA)
The main purpose of ISTA is the promotion of uniformity in seed testing pro- cedures. To achieve this it produces the International Rules for Seed Testing that embrace the advice of its various technical committees. The rules prescribe principles and definitions while the ‘Annexes’ describe methodology; both of these are published as a supplement to the Association’s journal Seed Science and Technology.
The Association’s secondary purpose includes the promotion of research in the subject areas of seed science and technology and the encouragement of cultivar certification. These objectives are achieved by organizing and partici- pating in conferences and training courses.
ISTA has developed a system for the quality assurance of seed testing through its system of accredited laboratories. This embraces international standards and audit visits to ensure that requirements such as staff training, operating procedures, test records and laboratory independence.
In 2009, there were more than 70 member countries and over 1500 sub- scribers. The Association produces an information pamphlet, publishes the ISTA News Bulletin three times a year and organizes a triennial World Seed Conference.
Role of ISTA in evaluating seed lots for GM material
● Produce additional seed testing rules for the detection, identification and quantification of GMO material in conventional seed lots.
● Organize proficiency testing for the detection of GMO material in conven- tional seed lots.
● Set up a platform for the exchange of information between seed testing laboratories.
● Additional objectives of the task force include identification of stacked genes and publication of performance test results and the availability of documentation relating to testing for specific traits.
The work and future programmes of the task force have been described by Haldeman (2008).
The Association of Official Seed Analysts (AOSA)
In North America, the Association of Official Seed Analysts (AOSA, 2008) plays a similar role to ISTA. The Association’s constitution and by-laws were published in 1993 (AOSA, 1993). The history, legal framework, technical achievements and possible future developments of AOSA have been reviewed by Bradnock (1998b). AOSA publishes a journal Seed Technology and Seed Technology News. AOSA and ISTA work in close collaboration on technical matters.
Seed legislation
The general objectives of legislation related to seed are to ensure that seed that is offered for sale to farmers conforms to minimum required standards of qual- ity, i.e. cultivar purity, health, vigorous germination and freedom from adulter- ating materials including weed or other crop seeds. Coincidental to these objectives are the protection of the vegetable grower from unscrupulous ven- dors. In countries where the seed industry is less developed, the latter point may be more important but once a seed industry is established, it is the moni- toring of the different facets of seed quality that gain importance, especially when certification schemes are introduced. This subject is discussed further by Kelly (1994), national and international examples are detailed in Kelly and George (1998b).