Varietal Deterioration: Causes and Maintenance
Authors: *Kana Ram Kumawat, Ravi Kumar and Ravi Kumawat
Department of Plant Breeding and Genetics, S.K.N. Agriculture University
Jobner-303329, Jaipur, Rajasthan, India
The genetic purity of a variety or trueness to its type deteriorates due to several factors during the production cycles.
Causes of deterioration of varieties
Following are the main causes of varietal deterioration:
• Developmental variations
• Mechanical mixtures
• Natural crossing
• Minor genetic variations
• Selected influence of pest and diseases
• The techniques of the plant breeder
1. Developmental variations: When seed crops are grown under environments with differing soil fertility, climate, photoperiods, or at different elevations for several consecutive generation's developmental variations may set in as differential growth responses. It is therefore, preferred to grow the varieties of crops in the areas of their natural adaptation to minimize developmental shifts.
2. Mechanical mixtures: Mechanical mixtures, the most important reason for varietal deterioration, often take place at the time of sowing if more than one variety is sown with the 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 next to each other field is usually mixed during harvesting and threshing operations. The threshing equipment is often contaminated with seeds of other varieties. Similarly, the gunny bags, seed bins and elevators are also often contaminated. Roguing the seed fields critically and using utmost care during seed production and processing are necessary to avoid such mechanical contamination.
3. Mutations: Mutations do not seriously deteriorate varieties. It is often difficult to identify or detect minor mutations occurring naturally.
4. Natural crossing: The deterioration sets in due to natural crossing with undesirable types, diseased plants or off types. Natural crossing can be an important source of varietal deterioration in sexually propagated crops. In self-fertilized crops, natural crossing is not a serious source of contamination unless variety is male sterile and is grown in close proximity with other varieties. The natural crossing, however, can be major source of contamination due to natural crossing are the breeding system of the species, isolation distance, varietal mass and pollinating agent.
5. Minor genetic variations: Minor genetic variations can occur even in varieties appearing phenotypically uniform and homogenous when released. The variations may lost during later production cycles owing to selective elimination by the nature. The yield trials of lines propagated from plants of breeder's seed to maintain the purity of self-pollinated crop varieties can overcome these minor variations.
6. Selected influence of pest and diseases: New crop varieties often are susceptible to newer races of pests and diseases caused by obligate parasites and thus selectively influence deterioration. The vegetatively propagated stock also can deteriorate quickly if infected by virus, fungi or bacteria.
7. The techniques of the plant breeder: Serious instabilities may occur in varieties owing to cytogenetic irregularities in the form of improper assessments in the release of new varieties. Premature release of varieties, still segregating for resistance and susceptibility to diseases or other factors can cause significant deterioration of varieties. This failure can be attributed to the variety testing programme. In addition to these factors, other heritable variations due to recombination's and polyploidisation may also take place in varieties during seed production, which can be avoided by periodical selection during maintenance of the seed stock.
Maintenance of genetic purity during seed production
The various steps suggested to maintain varietal purity, are as follows:
• Use of approved seed only in seed multiplication.
• Inspection and approval of fields prior to planting.
• Field inspection and approval of growing crops at critical stages for verification of genetic purity, detection of mixtures, weeds, and for freedom from noxious weeds and seed borne diseases etc.
• Sampling and sealing of cleaned lots
• Growing of samples of potentially approved stocks for comparison with authentic stocks.
The various steps suggested for maintaining genetic purity are as follows:
• Providing adequate isolation to prevent contamination by natural crossing or mechanical mixtures
• Rouging 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 in their adaptation only.
• Certification of seed crops to maintain genetic purity and quality of seed.
• Adopting the generation system.
• Grow out tests.
1. Sen, S. and Ghosh, N. (2012). Seed Science and Technology. 2nd Revised and Enlarged Edition, Kalyani Publishers, New Delhi.
2. Singh, B.D. (2015) Plant Breeding Principles and Methods. 10th Revised Edition, Kalyani Publishers, New Delhi.
3. Internet (IASRI)
About Author / Additional Info:
Post-Graduated in subject of Plant Breeding and Genetics from SKNAU, Jobner