Seed Purity

Introduction

The components examined in purity analyses are pure seed, seeds of other species and inert matter. The quality of seed lot is judged by the relative percent age of various components. The quality is considered superior, if pure seed percentage is above 98, and the other species seeds and inert matter percentage as low as possible. The percentage of seeds of other species should be almost negligible, or nil.

The following structures: Shrivelled, diseased or germinated are regarded as pure seed, unless transformed into fungal sclerotia, smut balls or nematode galls. Seeds in the botanical sense; achenes and similar fruits, schizocarps and mericarps with or without perianth, and regardless of whether they contain a true seed or not, unless it is readily apparent that not true seed is present. In case of Graminae the florets with an obvious caryopsis containing endosperm and free caryopses. Pieces of seed units larger than one-half the original size

Genetic purity of seeds refers to the trueness to type. If the seed possesses all the genetic qualities that breeder has placed in the variety, it is said to be genetically pure. The genetic purity has direct effect on ultimate yields. If there is any deterioration in the genetic makes up of the variety during seed multiplication and distribution cycle, there would definitely be proportionate decrease in its performance e.g. yield, disease resistance etc.

Genetic purity of a variety can deteriorate due to several factors during production cycles. The important factors are as follows:

1. Development variations

When the seed crops are grown in difficult environment, under different soil and fertility conditions, or different climatic conditions, or under different photoperiods, or at different elevations for several consecutive generations, the development variations may arise sometimes as differential growth response. To minimise the opportunity for such shifts to occur in varieties it is advisable to grow them in their areas of adaptation and growing seasons.

2. Mechanical mixtures

This is the most important source of variety deterioration during seed production. Mechanical mixtures may often take place at the time of sowing, if more than one variety is sown with same seed drill; through volunteer plants of the same crop in the seed field; or through different varieties grown in adjacent fields. Two varieties growing alongside each other in the field are often mixed somewhat during harvesting and threshing operations. Often the seed produce of all the varieties are kept on same threshing floor, resulting in considerable varietal mixtures. To avoid this sort of mechanical contamination it would be necessary to rogue the seed fields, and practise the utmost care during seed production, harvesting, threshing and further handling.

3. Mutations

This is not a serious factor of varietal deterioration. In the majority of the cases it is difficult to identify or detect minor mutations. In the case of vegetatively propagated crops, periodic increase of true to type stock would eliminate the deteriorating effects of mutations.

4. Natural crossing

In sexually propagated crops, natural crossing is another most important source of varietal deterioration due to introgression to genes from unrelated stocks, which can only be solved by revention.

5. Minor genetic variations

Minor genetic variations may still exist even in the varieties appearing phenotypically uniform and homogeneous at the time of their release. During later production cycle some of these variations may be lost because of selective elimination by the environment. To overcome these, the yield trials of lines propagated from plants of breeders seed in the maintenance of self-fertilised crop varieties.

6. Selective influence of diseases

New crop varieties often become susceptible to new races of diseases often caused by obligate parasites and are out of seed programmes. Similarly the vegetatively propagated stocks deteriorate fast if infected by viral, fungal and bacterial diseases. During seed production it is, therefore, very important to produce disease-free seeds/stocks.

7. Techniques of plant breeder

In certain instances, serious instabilities may occur in varieties due to cytohenetical irregularities not properly assessed in the new varieties prior to their release. Premature release of varieties, still segregating for resistance and susceptibility to diseases or other factor may also be important in the deterioration of varieties. Other factors, such as, break down in male sterility, certain environmental conditions, and other heritable variations may considerably lower the genetic purity.

• Providing adequate isolation to prevent contamination by natural crossing or mechanical mixtures.

• Roguing of seed fields prior to the stage at which they could contaminate the seed crop.

• Periodic testing of varieties for genetic purity.
• Avoiding genetic shifts by growing crops in areas of their adaptation only.
• Certification of seed crops to maintain genetic purity and quality of seed.
• Adopting the generation system.
• Grow out tests.

The other crop seeds are the seeds of plants, which are grown as crops, other than main crop seeds. The weed seeds are the seeds of plants of those species recognized as weeds by Laws, official regulations, or by general usage. With respect to classification as other crop seeds/weed seeds or inert matter, the distinguishing characteristic described under pure seed shall also be applicable.

Inert matter includes seed units and all other matter and structures that are not defined as pure seed, other crop seed or weed seed as follows.

1. Seed units in which it is readily apparent that no true seed is apparent.
2. Pieces of broke nor damaged seed units half or less than half the original size.
3. Those appendages not classed as being part of pure seed in the pure seed definitions must be removed and included in the inert matter.
4. Seeds of Leguminosae, Cruciferae, Cupressaceae, Pinaceae and Taxodiaceae with the seed coat entirely removed.
5. Unattached sterile florets, empty glumes, lemmas, paleas, chaff, stems, leaves, cone scales, wings, bark, flowers, nematode galls, fugues bodies, such as ergot, sclerotia and smut balls, soil, sand stones and all other non-seed matter.
6. All material left in the light fraction when the separation is made by the uniform blowing method except other seeds.

Legume seeds usually, break along the line of cleavage between the cotyledons. Rye and barley seeds frequently break transversely. The fragment does into have to contain an embryo in order to be regarded as pure seed.

Grasses with thick, horny lemma and paleas are difficult to classify as one cannot see through them and must resort either to reflected light or to some form of pressure. Standard blowing procedures and schedules have been adopted for Canada bluegrass and Kentucky bluegrass seed.

The filled and empty fruits do not separate in blowing and it would be impractical to open each fruit to determine its content. Only broken fruits found to contain no true seeds or one-half seed or less are to be regarded as inert matter.

Crop seeds in the legume and crucifer families with the seed coat entirely removed are classed as inert.

If one-half or more than one-half of the seed has been consumed by the insect it is considered inert matter. With experience, most analysts will be able to determine insect-infested seeds by visual examination alone.

In some instance nematodes have replaced most or all of the contents of these seed, forming nematode galls. In other instances mycelia of fungi have developed to the extent that they replace the conetens of the seed, forming sclerotia. If nematodes are suspected a positive test can be made by crushing the seed like structure, placing it in water for a few hours and then examining it under a microscope.

Ag.
Technologies
(Seeds)