Seed Technology

                                                                                                           Professor R. S. Sengar

INTRODUCTION:

Seed technology is a science that deals with the techniques of seed production, seed processing, seed storage, seed testing and certification, seed marketing and distribution, and the related research on these aspects. Here, the term seed refers to seed or any other propagating plant part used for raising a crop. Seed, in this context, may include not only the plant part botanically known as seed but also some other plant parts used for vegetative or asexual propagation such as tubers, stolons, stem cuttings, etc. The importance of seed technology to plant breeding or agriculture lies in the fact that howsoever good soil, agronomic conditions and environment are provided, a plant can at most achieve but cannot exceed the maximum ceiling or limit of character expression or yield performance that is permitted and governed by its inbuilt genetic architecture as conditioned by genes supplied to it from parents through the seed. It is the seed which gives rise to plant on which economic yield is born due to combined efforts of plant genotype and the environment. Thus, abiotic and biotic environmental factors such as soil fertility, temperature, moisture, diseases, insects and weeds, etc. can cause any impact on economic yield, only indirectly, through their influence on plant or, more precisely, on plant genotype that carries the biological responsibility of pro producing economic yield. Since the plant raised from the seed is key to crop production, the genetic and physical characteristics of seed have immense value in modern agriculture. Genetic charcteristics of seed relate to genetic constitution or sum total of genes contained in its cells and determine the genetic potential of plant to manifest economic yield while physical characteristics signify the seed quality in relation to its plant stand producing potential as conditioned by properties like purity, vigour, health, viability, germination percentage, etc. of seed. Genetic characteristics of seeds are chiefly specific to a variety and are shaped and manipulated mainly during the varietal development process, but once attained in a variety, the responsibility of their maintenance in unambiguous form falls primarily on seed technology. The physical poperties of seed have greater involvement, therefore of more relevance, in seed technology because these characteristics are concerned more to particular seed lots rather than to specific varieties, and their control, manipulation and maintenance could be done mainly through the techniques falling under jurisdiction of seed technology. In other words, seed technology can be defined as the science comprising of principles and techniques involved in production, maintenance, preservation and distribution of quality seed possessing desirable genetic and physical characteristics.

Seeds with poor genetic characteristics, i.e., seeds of inferior varietyproduce plants that lack the capability to give high economic yield even under optimum or best agronomic conditions for reasons explained earlier. Seeds with poor physical characteristics may give rise to weaker, less vigorous, diseased or unviable plants and poor plant stands that lead to low economic yield even when the seeds possess desirable genetic characteristics and seedlings are subjected to ideal cropping conditions. Consequently, the use of good quality seed possessing desirable genetic and physical characteristics is crucial for sustaining high economic yields in commercial crop production.Furthermore, a high yielding variety developed through rigorous breeding efforts would be of little value unless its seeds possessing genetical purity and other desired qualities such as high germination percentage, high physical purity and sound health, reach the farmers. For instance, advent and spread of hybrid cultivars in some corps like maize revolutionized maize cultivation by bringing quantum jumps in productivity levels in U.S.A., but hybrids of maize with much higher yield potential than existing varieties failed to make any significant impact, and to get popularity among farmers when introduced in India. One of the main reasons attributed to failure of hybrids in India, was the lack of efficient seed industry and distribution system because of which farmers were unable to procure hybrid seed in required quantities, and even when available it was too costly to afford for poor or marginal farmers. Shortcomings of seed industry cause more damage to utility of hybrid varieties, especially the single and double cross hybrids, because they have to be raised using fresh seed in every season or more frequently to sustain heterosis for manifestation of high yield.

The above discussion emphatically underlines the significance of seed technology to crop improvement and so also to agriculture. Keeping this in view, important experimental techniques of seed technology are dealt in this Chapter.

ISOLATION DISTANCE

The isolation of seed crops from various sources of contamination is necessary for maintaining genetic purity and quality. Contamination may occur due to natural crossing with other varieties of same species or related species grown in neighbouring fields as well as due to off-types present in the seed crop field itself. The disease infection from nearby fields may not only damage the crop resulting in the poor seed yield, but may also reduce the seed vigour and quality, especially in case of infection due to seed borne diseases. Therefore, it is essential to protect the seed crop from being subjected to pollen and disease inoculum coming from nearby fields through wind, insects and other vectors. The contamination from off-types plants present in seed crop field can be checked by identifying and subsequently roguing them out before pollination occurs. The problem of contamination from other fields can be solved only by maintaining isolation of seed crop from others either in time or space. The isolation of seed crop utilizing the time factor, can achieved by adjusting the sowing time in such a way that its blooming period does not coincide with the same crop present in the fields around upto a distance from where pollen may reach. Early or late sowing of a seed crop as compared to normal sowing period in adjoining area may separate the period of growth and reproductive phases of seed crop from neighbouring ones to the extent that when external supply of viable pollen and disease inoculum are available from nearby fields the seed crop does not have stages receptive to them to enable the escape from contamination through pollination and disease infection. However, use of time isolation is usually difficult and practically limited because suitable adjustment of sowing time requires great care and has to be determined after taking into account the vegetative and reproductive phases of all varieties of same crop or related species crossable with it, falling inside the area of influence. It is often difficult to obtain the information about sowing time and crop duration of numerous varieties or strains growing in nearby fields whether the seed production is done at research station or farmer's field. Secondly, one cannot be sure that a time isolation schedule based on complete and careful survey of required area will always hold true because environmental fluctuations sometimes bring drastic alterations in growth phases of varieties. Moreover, the excessive early or late growing of a crop may cause too much decline in its performance to make it unprofitable and unacceptable for seed production.

The isolation by distancing the seed crop far enough from the fields having same crop so that no possibility of contamination remains, is referred to as isolation distance. Isolation distance is used more commonly in seed production programmes than adjustment of sowing or growth periods. In this, no other field having other varieties or strains of same crop species or crossable relatives, should fall within the area around the seed crop from where pollen or disease infection can reach to seed crop. The determination of minimum isolation distance for a crop depends upon several factors such as mode of pollination, pollen attributes, environmental factors and purity requirements which vary from crop to crop. In case of self-pollinated crops like wheat, rice, etc., it is usually sufficient to isolate seed crop fields with a strip of three metres all around which is planted with a non-cereal crop, or left uncropped. This isolation is enough to avoid mechanical mixture and chance cross-pollination in such strictly self-pollinated crops. On the other hand, isolation distance of 200 metres to 400 metres is required for cross-pollinated crops like maize, jowar and bajra. In crops like maize, the minimum isolation distance may be considerably reduced by planting border rows of pollinator parent and by using a larger field for seed production. But in several other crops such as jowar and bajra, such modifications are not permitted. However, when seed crop is susceptible to one or more diseases and it is essential to prevent external infection for them, the isolation distance should be modified as per requirements for avoiding disease inoculum. The seed crop should be separated from any source of inoculum of the disease in question by a distance which cannot be covered by discase spores dispersing from source of inoculum. For example, isolation distance for loose smut of wheat is 150 metres because the spores from infected plants cannot dispersé beyond this distance. In seed production programmes, generally, more emphasis is given on disease and insect control through chemical and cultural applications for checking the disease infection rather than maintaining isolation for each of several diseases affecting a crop. As it is difficult to determine and maintain proper isolation distances against several diseases, usually isolation for only one or few important diseases, especially for seed borne diseases, is maintained. The isolation distance for maintaining genetic purity of a crop may be worked out by estimating the distance from which pollen from contaminating source are unable to reach the seed crop. In other words, isolation distance should be more than the maximum distance that can be travelled by pollen grains through winds, insects, etc. after dispersing from plants. Similarly, isolation for a disease can be determined by estimating the migration range of its spores. The method of determination of isolation distance of a crop is illustrated below.

Writer: Professor R. S. Sengar, Director Training and Placement, Sardar Vallabhbhai Patel University of Agriculture and Technology, Modipuram, Meerut.