IntroductionTop

Determination of germination potential is one of the most important measurement when the quality of seeds is evaluated, and it represents the germination when seeds are exposed to appropriate conditions, expressed as the speed rate with which the germina­tion process occurs (germination rate GR). For inter­national seed trade, it is very useful to have a common standard for evaluating germination potential. Gonzá­lez & O­roz­co (1996) and Ranal & García (2006) presen­ted reviews in which they accurately describe the diffe­rent me­thods that have been used for the calculation of seed germination parameters, where GR and germination percentage (GP) represent the most commonly used.

The reliability of calculation of a germination parameter is necessarily linked to the number of seeds evaluated. Increased numbers produce more accurate results; however, if the number is too high, there may be an unnecessary increase in the cost of the research and the time required to complete it.

Based on the above, the International Seed Testing Association (ISTA) developed criteria to define minimum sample sizes in the germination tests of different species, mainly from the temperate zone. Ac­cor­ding to this organization, the evaluation of com­mercial seeds should be based on a quantity of 400 seeds divided into four replicates of 100 units each, although less seeds could be used depending on their size and collection difficulties (ISTA, 2012).

While GP represents seed viability, GR is a parameter that reflects the vigor of the seed and varies depending on the time required for the germination process to occur. Groups of seeds with similar GP may differ significantly in their GR.

A formula widely used for the calculation of GR in seed studies was developed by Maguire (1962), who adapted it from the original of Throneberry & Smith (1955). Another valuable equation, developed by Cza­bator (1962), and known as germination value, is an index that varies directly and proportionally with the GR, GP, or both, although this index has been much less used than Maguire's formula.

Despite the rationale presented by Brown & Mayer (1988) against Maguire's formula, the method has been shown to be suitable for seeds of woody perennials, a group of plants characterized by slow germination rates (Hartmann et al., 2002). The method is a popu­lar manner of assessing germination rates of different plant species and has been cited 4709 times according to Google Scholar, with 9895 downloads up to March 2019, according to the ACSESS Digital Library (https://dl.sciencesocieties.org/publications/cs/abstracts/2/2/CS0020020176).

Germination rate, as proposed by Maguire (1962), must be calculated as the sum of the values obtained when dividing the percentage of partial germination in each count by the time elapsed since the beginning of the test. It is literally expressed as follows:

In the tests, it is inferred that Maguire used 100 seeds, but in the numerator of his formula he placed "number of seedlings" instead of "percentage of seedlings", as it should be according to the concept correctly indicated by him. However, in the text, he made clear that his calculations referred to 100 seeds. Obviously, in his case, the "number of seedlings" was exactly the same as the percentage of seedlings.

The use of number of seedlings instead of percen­tage of seedlings in many of the subsequent seed studies where the sample size differed from 100 seeds has led to serious inconsistencies in the reported results and the impossibility of direct comparison with those of other studies.

In a review of the pertinent bibliography of articles of recent decades referring to seed germination, it can be observed that different formulas have been used to calculate the seed germination rate. Since in several cases errors have been detected in the application of these formulas, the present study reviewed an important number of publications related to seed germination in order to determine the frequency of occurrence of inaccuracies in the application of the aforementioned formula. The aim of this study is to present statistical information regarding the inconsistencies found in the­se publications and the errors that these imply when in­ter­preting the vigor of the seeds of many botanical materials.

Material and methodsTop

A search was conducted through Google Scholar and the databases of the Web of Science and Scopus, utilizing a filter that included only articles from the last 25 years (1994 to 2018). This produced a total of 216 articles that involved the word germination in the title and, of these, 152 articles were selected that cited the article by Maguire (1962) in their references. This literature included parameters described as germina­tion speed, germination rate index or germination rate. The papers were obtained in PDF format from Proquest, Springer, Scielo, or directly from open access journals.

Articles that did not present numerical results or in which the methodology was not clearly indicated were excluded, along with those that, despite having determined germination parameters, used different re­pre­sentations of the germination rate or had incorrectly transcribed Maguire's formula. This filtering process left a total of 124 articles, which were those ultimately used in this study.

We evaluated the articles in which there would be some form of imprecision when applying the formula developed by Maguire or that had not followed the recommendation of ISTA (2012) in relation to the number of seeds to be used in the tests. Special atten­tion was paid to articles in which the sample size was lower than 100 seeds without conversion to percentages, and that clearly indicated that the number of seeds was used for the purposes of the calculations. In order to determine whether the number of cases has varied over time, the results were classified into periods of the year and, to determine whether they occur more frequently in certain agricultural species or certain geographical areas, they were classified by plant species or country of origin of the paper. The results are presented comparatively in graphs showing the total number of papers and the number of these in which Maguire’s formula would have been incorrectly applied.

A linear regression analysis was carried out between the number of seeds used in each paper and the mean value reported for the germination rate. When a paper stated that measurements had been converted to percentages, the independent variable was considered as 100, regardless of the actual number of seeds tested. A total of 107 papers were used for this analysis, with 17 papers discarded because their results were unclear or highly variable. The regression was developed by means of the SAS v.9.1 program (Cary, NC, USA).