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 Main subject - nov/dec 2004
The processes require more and more fine adjustments
Dr. Orlando Lucca Filho
Prof. UFPel
luccaf@ufpel.edu.br
The technological progress observed in the last decades in the production and seed technology area, allowed a significant increase in productivity of most crops. These new production techniques demand the observance of small details, previously considered insignificant. It is known today, for instance, that using 50 kg of seeds per hectare is not enough; these seeds must have high physical, genetic, physiologic and sanitary quality, besides that, they must be evenly distributed in the soil. Providing fertilizer is not enough; macro and micronutrients must be present with the right king and amount for adequate plant nutrition. It is now known that the seeds, after reaching physiologic maturity, enter in decline and, if harvested with high moisture and dried, they will have better quality than those that remain in the field for an extended period of time.
Therefore, in seed analysis laboratories, progresses in the procedures of quality evaluation also appeared, demanding the observance of improved methods for its execution.
The quality of a seed lot is determined by a group of physical, genetic, physiological and health attributes. There are rules that should be followed for the evaluation of each one of these attributes. These rules are very known by those who work in the laboratories. The methodology that should be followed for the evaluation of a certain characteristic of the seed lot is found in the Rules for testing seeds. In these rules, the basic principles to do an evaluation of a certain analysis are described. However many times, the fundamental information is not clear, especially those related with the observance of small details that can exert significant effect in the precision and accuracy of the results.
These terms, precision and accuracy, are assuming greater importance, as time goes by, not particularly, but especially in the agricultural section, where the reduction of the production costs and optimization of the applied resources is a constant objective of the modern agro business. Inside of this context it is observed that the competitiveness of the sector is resulting in the incorporation of new techniques and production principles. Thereby, treacing procedures, quality certificates of production systems and of products, precision agriculture, among others, are being searched by several companies, with the intention of providing to the consumer a differentiated product.
It should be considered as integral part of a quality system in seed analysis, the control of the variables that can interfere upon the precision and accuracy of the results. It is recognized as accuracy the agreement degree between the result of the analysis and the true value of the measurement, while precision refers to a data that represents unequivocally the real value of the measured object. Therefore, let's imagine that in a scale certain mass of 1 kg will be weighted. When we put it in the scale and it indicates 1000g, we can say that it is precise, but if it indicates 999,5g instead of 1 kg, the scale will be considered exact.
Everything that we do in life should be based on precision, because in case we look for the accuracy we are predestined to frustrations. It should be the same with seed lots analyses. We should always look for the precision, although a lot of times we only reach accuracy.
Among the main variables that compete to hinder to obtain precise results there are the environmental ones (temperature, moisture and light), the ones inherent to the test itself (equipment, additional treatments, evaluation period, interpretation of the results), the biotic ones (asepsis) and the ones inherent to the appraiser (aptitude, training and responsibility).
Temperature and moisture
Among the environmental factors that compete to hinder to obtain precise results, for sure are temperature and moisture. While moisture has the main function to activate metabolic processes, such as seed germination, the temperature acts as a decisive factor on the speed which these reactions will occur. These two factors have significant effect on the enzymatic activity and use of storage materials. For those reasons, as much temperature as moisture should be strictly observed when conducting tests where the development of seedlings are evaluated. The description of a certain methodology to conduct the germination test, for instance, has clearly that the temperature should be used during the incubation period of the seeds and seedlings. This temperature should be strictly observed, therefore lower temperature that the proper one, will provoke the prolongation of the test, so that all seeds have opportunity to germinate and to form normal seedlings. The prolongation of the test can provoke the appearance of secondary infections, whether caused by fungus or bacteria, which will hinder the interpretation and leading to not precise results.
We must not forget that all the variables of a standardized methodology and of international use, were exhaustively studied before indicated, being, therefore, this methodology adequately worked to supply compatible results with the real value of the parameter to be measured. As a result, the precision of that is directly related to the observance of all techniques and suitable procedures for the test in use.
Moisture disposal, in appropriate amount for a fast, uniform and complete germination of the seeds, is also important. Substrata too wet will allow a fast absorption of water by the seeds. In case they present low moisture levels, irreversible damages will occur in their organelles, which will cause the appearance of dead seeds or of abnormal seedlings, giving misleading results with the true qualitative potential of the seed lot. Another factor that occurs due to the excess of moisture of the substrata is the restriction to the absorption of oxygen, when there is a formation of a film of water around the seed. This restriction of oxygen can also delay the germination and induct the appearance of abnormal seedlings.
On the other hand, the moisture deficit is also a factor to restrict precise results. We know that seeds need to absorb a certain amount of water, so that the processes associated to the germination can begin. In substrata with low moisture, seeds will need more time to reach the critical level for germination. This will add time to the test, so that all seeds can germinate, if there is enough amount of water. Not having enough moisture, the seed will absorb less water, than necessary to germinate. In this situation, usually, seeds end up being aken by microorganisms, not germinating or producing abnormal seedlings.
The seed moisture content assumes fundamental importance in the precision of vigor tests, such as the accelerated aging and the electric conductivity, where the initial seed moisture content can turn them more or less susceptible to high temperatures, or to interfere in the leaching of electrolytes. Therefore ISTA recommends, in its vigor test, that the seed moisture content should be determined before conducting the tests (electric conductivity, accelerated aging and controlled deterioration).
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