Available Online: http://www.ejpau.media.pl/volume18/issue4/art-05.html
MASS AND DENSITY AS SEPARATION CRITERIA OF PEDUNCULATE OAK (QUERCUS ROBUR L.) SEEDS
Paweł Tylek1, Zdzisław Kaliniewicz2, Paweł Kiełbasa3, Tomasz Zagrobelny1
1 Department of Forest Work Mechanisation, University of Agriculture in Krakow, Poland
2 Department of Heavy Duty Machines and Research Methodology, University of Warmia and Mazury, Poland
3 Institute of Machinery Management, Ergonomics and Production Processes, University of Agriculture in Krakow, Poland
The growth of seedlings in the juvenile phase and the distribution
of biomass in oak seedlings (Quercus robur L.) are significantly affected
by seed weight. For the purposes of separation, what is important is the density
of seeds, used in practice as the separative feature. The aim of the study is
to determine the properties of oak acorns, such as mass, volume and density,
depending on their vitality. A hypothesis was made that the degree of development
of the embryo and the mummification of acorns affect the above physical characteristics.
In order to measure the mass and determine the density, an analytical balance
was used with a set applied to determine the density of solids; vitality was
determined by slicing, performing a macroscopic examination of embryonic development,
cotyledon tissue condition, the root ovary and the internal damage caused by
fungi and insects. It was shown that the density of the acorns, and to a limited
extent also their mass, depend on vitality; therefore they may be regarded as
distributive characteristics. There is, however, no possibility to separate only
the healthy seeds; healthy seeds together with half-spoiled seeds can be separated
from the remaining, spoiled ones.
The growth of seedlings in the juvenile phase and the distribution of biomass in oak seedlings (Quercus robur L.) are significantly affected by seed weight. For the purposes of separation, what is important is the density of seeds, used in practice as the separative feature. The aim of the study is to determine the properties of oak acorns, such as mass, volume and density, depending on their vitality. A hypothesis was made that the degree of development of the embryo and the mummification of acorns affect the above physical characteristics. In order to measure the mass and determine the density, an analytical balance was used with a set applied to determine the density of solids; vitality was determined by slicing, performing a macroscopic examination of embryonic development, cotyledon tissue condition, the root ovary and the internal damage caused by fungi and insects. It was shown that the density of the acorns, and to a limited extent also their mass, depend on vitality; therefore they may be regarded as distributive characteristics. There is, however, no possibility to separate only the healthy seeds; healthy seeds together with half-spoiled seeds can be separated from the remaining, spoiled ones.
Key words: germination capacity, pedunculate oak, seed density, seed weight, seed quality, separation.
Oak is present almost all over Europe: from the Iberian Peninsula in the west to the foothills of the Ural Mountains in the east and from the Scandinavian Peninsula in the north to the Balkan and the Apennine Peninsulas in the south. In Poland, it is one of the main forest-forming species, the so-called transitive one, i.e. not achieving the limit of its natural range [1, 21]. Decades-old data concerning oak acorn harvesting show that an abundant crop of acorns occurs in Poland usually every 5–7 years, while good seed years are sometimes more frequent (every 3 years), and sometimes less frequent (9 years) [15, 24]. In the mild climate (Western Europe) good seed years occur more often (every 2–5 years). Both in Poland and in Europe the artificial regeneration of oak is applied, using seedlings grown in nurseries. Natural regeneration is confined to certain areas and only to the years of the most abundant harvest . Direct sowing of acorns is rarely used . Nationally, the main source of seeds are managed seed oak stands. In the case of a large harvest, the amount of seeds needed is about 600 000 kg, which is a large logistic problem concerning the preparation of such a large quantity of seeds. Oak seeds fall into the "recalcitrant" category; that is why in order to retain their vitality they require constant maintenance of their natural moisture, and cannot be frozen for the purpose of long-term storage. Because of the rapid loss of vitality, their storage is practically limited to 2–3 years. Seeds age the faster, the worse the quality at the beginning of storage. This is the reason for the importance of the issue of seed separation, the purpose of which is seed cleaning and separating the solid, well-formed seeds from the empty or damaged ones. The collection of acorns with larger dimensions is a selection process itself. Nursery production, in turn, promotes faster growing seedlings, which means that the slower growing seedlings fail in the competition or are eliminated when sorting young plants .
Modern technologies and techniques of nursery production require healthy seeds with a high germination ability so as to be able to cultivate plants with particular morphological parameters. The factor which determines the application of precision seeders is the use of seeds characterized by high purity and vitality as well as small variation of physical characteristics and a high degree of looseness . Supreme germination ability is expected of seeds used for sowing under controlled conditions, especially in container nurseries, where only one seed should be planted in each cell of a container. This is related to the economic balance of the production of seedlings , especially those vaccinated with mycorrhizal bioproducts . Increasing the efficiency of sowing is particularly important in the case of oak, since this species has a very uneven emergence. The first seedlings begin to appear after 2–3 weeks from seeding, and the last ones even after 16–17 weeks. This results in diverse growth of seedlings and enhances competition between them. Seedlings that appear later are already under the cover of the larger seedlings with well-developed leaves, which effectively restricts the access of light and water. The non-uniformity of emergence and growth affects the decrease of sowing effectiveness . However, the criterion of germination (and therefore of yield) does not alone guarantee obtaining seedlings of the appropriate morphological parameters. The growth of seedlings in the juvenile phase is significantly affected by the weight of seeds. Seedlings grown from the larger ones are characterized by a higher ratio of the length of the roots to the shoot length, thereby suggesting their better production quality . The relationship between the seed size and the distribution of biomass is particularly evident in oak seedlings [4, 31]. It must be noted, however, that trees growing in the same stand can produce seeds very diverse in terms of size and weight. Seeds from old trees are often smaller than from young trees which are entering the phase of seed production . Due to the need to protect and preserve genetic diversity, large and heavy seeds should be as important as small and light ones. Gradual elimination of small seeds may remove from the crop the seeds of old trees, which are the most likely to be of native character, and thus best suited to that particular habitat .
What follows, the weight of seeds is usually their basic physical characteristic, indicating their robustness, depending largely on moisture. An increased content of stored substances in the seed provides better development of the embryo, resulting in greater resistance to negative environmental influences [8, 33]. However, the weight of 1000 seeds, commonly used as an indicator of seed quality assessment, is inadequate and should be extended by analysis of the distribution of individual seeds in the whole resource, all the more that the analysis of the relationship between vitality and weight of 1000 seeds does not usually indicate the existence of a correlation between the above-mentioned traits . Moreover, mass is not a distribution characteristic of seeds from the point of view of the technical capabilities of their separation on a commercial scale because there are no separators using mass as a separative feature that is fundamental, rather than only secondary. For the purposes of purification, what is important is the density of seeds (specific weight), which constitutes the mass of a totally filled volume unit. Seed density is used in practice as the primary separative feature (during flooding) and the secondary one (during separation on aero-vibratory tables, pneumatic separators, vibro-pneumatic concentrators). The weight of seeds can be measured easily but, due to their small dimensions, accurate measurement of the volume of seeds, which is the basis for the calculation of density, requires the use of special equipment. In general, determination of volume (and thus of density) cannot be performed for individual seeds, but for larger samples .
Obtaining certified seed material often requires the implementation of a range of physical activities that require the knowledge of seed agrophysics and the laws governing selection processes. Knowledge is also required of the construction components (tanks, trays, filters, pneumatic channels, etc.) of machinery and devices for seed material purification as well as efficient use of cleaners and mechanical sorting machines and other machines for seed treatment. The machines that operate nowadays do not clearly differ from the ones that functioned even in the nineteenth century, and they do not meet the requirements of current technology. Both modern and old structures of separators are based on the experience of practitioners rather than theoretical concepts motivated by a thorough knowledge of seed agrophysics. Far-reaching simplifications at the stage of machinery design often lead to producing malfunctioning components or entire engineering systems . It must be remembered that seeds are organisms that continue to exhibit vital functions in response to external influences. Sorting and cleaning cannot cause their mechanical damage or changes in their physiology or biochemistry .
In the case of oak seed preparation, what is cumbersome is the multi-stage character of the processes. After the initial cleaning from heavy contamination, it is necessary to separate the seeds infested by insects and the empty ones, where the percentage may be as high as several percent or several dozen percent . Then acorns are subjected to phytosanitary treatments: thermotherapy and fungicide seed treatment, aimed at reducing their mummification, which is the result of the attack of Ciboria batschiana fungus [16, 23]. After drying and cooling, seeds are stored in a cold room at a temperature of -3°C. Before sowing in a nursery, in order to accelerate and align the seeding process, acorns are subjected to mechanical scarification by cutting their ends with the cotyledons at 1/4 to 1/3 of their length on the side of the stigma. It should be noted that scarification which is too intense results in worse smorphological parameters of seedlings, as in the case of damage to seedling cotyledons [10, 11]. Limiting the number of seeds which undergo the above treatments unnecessarily and reducing the weight of stored seeds of poor seeding quality or completely devoid of vitality would provide tangible economic benefits. Usually the vitality of the seeds of forest trees is correlated with a variety of physical features [6, 14, 29]. In connection with the above, it was hypothesized that the degree of development of the embryo and the mummification of acorns affects their physical characteristics. Thus, the aim of this study is to determine the properties of acorns, such as mass, volume and density, depending on their vitality, which may lead to the knowledge of the potential distribution characteristics which are the basis for the design of mechanical seed separation processes and the creation of new technical solutions in sorters. The study also aims to find out the range in which the isolated seed fractions store water. Another goal is to find possible relations between the physical characteristics of oak seeds, which might allow for simplification of agrophysical metrology procedures by elimination of those measurements which are particularly troublesome in terms of methodology.
MATERIALS AND METHODS
The object of the present research are the acorns of pedunculate oak (Quercus robur L.), whose harvest was carried out in 2014 in a managed stand in Oleszyce Forest District. Until the time of the analyses, the acorns had been kept in a cooler. The research method involved the measurement of particular parameters on individual acorns placed in a segregator. Subsequent acorns (400 items) were collected from the segregator and after the measurement of each of the parameters they returned to corresponding, labeled cells. This allowed for their later identification: each was described in terms of anatomical and physical characteristics. This way of conducting research is laborious but allows for precise assessment of the influence of the anatomy of acorns on their distribution characteristics.
Mass measurement and density determination were done using an analytical balance (with accuracy of 0.1 mg) with a set for density determination of solids and liquids (Fig. 1). Seed weight was measured in two media of different density: in the air (when placed on the dry pan basis) and in a liquid (when placed on a wet pan).
Fig. 1. Diagram of the stand for measurement of seeds mass density: 1 – stand locate on the analytical balance platter, 2 – wet platter, 3 – weighting seed, 4 – perforating plate, 5 – dry platter, 6 – thermometer, 7 – beaker with alcohol, 8 – beaker base, 9 – plunger for evaluate of liquid mass density
The density of the seeds was determined indirectly from the relation:
ρ – density of seeds,
m – mass of seeds in the air,
ma – mass of seeds in liquid (alcohol),
ρa – density of the liquid.
The liquid used was ethanol (difficult to wet the seeds), whose density at 18°C was 0.812 g∙cm-3. The alcohol density was determined using a glass dipper of calibrated volume. Due to the fact that some of the seeds did not sink in the alcohol, the method was modified by maintaining the seeds below the surface of the liquid by keeping them down with a perforated plate. The weight of the plate was taken into account during wet pan taring. Thus instead of seed weight in liquids, their buoyancy was measured and the measurement result was interpreted with the negative symbol .
Vitality was determined by cutting the acorns. It is a destructive method; therefore, it was performed after the mass and density measurements. Cutting is one of approximate seed evaluation methods, which assesses the vitality of seeds on the basis of macroscopic examination: embryonic development, cotyledon tissue condition, the root ovaries and the internal damage caused by fungi and insects. The assessment is made only on the basis of the external appearance of the section through the tissue; hence the cutting method is the least unreliable in relation to seeds freshly harvested and stored properly and briefly. Distinguished are the following seeds: healthy, rotting, spoiled, damaged by larvae, empty (without an embryo). Adopting the development of the embryo and the topography of mummification changes as the basis, one can estimate the expected seed germination ability, assuming that for healthy seeds it is 100%, for rotting seeds: 50% whereas the others are considered to be incapable of germination .
In view of the fact that, following the evaluation of the vitality, it was found that fractions of the acorns damaged by larvae or empty were represented only by single individuals, it was decided to exclude them from further analysis. The other acorns were divided into 3 classes of vitality: healthy (fraction I), rotting (fraction II) and rotten (fraction III). Irrespective of the hypothetical destination of seeds, fraction III should be rejected in the selection process while fraction I should constitute the proper material. The problem of fraction II (seeds partially capable of germination) is different. The batch of seeds for sowing in the ground nursery should include this fraction while the seeds for long-term storage or for precision sowing (especially in containers) should be devoid of this fraction. Therefore, when determining the theoretical criterion for divisibility, fractions I and II were joined when preparing a batch of seeds for sowing; in the other variant fraction III was isolated from the remaining ones when preparing a batch of seeds of the highest quality.
Separability index λ was determined from the relationship :
R – range of coverage of value of the physico-mechanical characteristic of the two mixture components,
R0 – the range of values of the physico-mechanical characteristic of the mixture (Fig. 2).
Fig. 2. Outline of the determination of separability of seeds into fractions: R – scope cover of physico-mechanical feature of both mixture ingredients, R0 – value range of physico-mechanical feature of mixture
This criterion distinguishes between mixtures in the following way: indivisible, i.e. λ = 0, when R = R0; difficult to divide, i.e. 0 < λ < 1, when R < R0; easily divided, i.e. λ = 1, when R = 0; and absolutely divisible, when R < 0.
The difficulty of developing a clear characterization of the organic material is associated with the lack of formal methods for their determination and description. The input mixture may vary in relation to many indicators, including the concentration of seeds with an expected feature, the number and content of the fractions with negative features and the size of the differences in characteristics. In view of the above, on the basis of empirical work, the values of the different physical characteristics of acorns were juxtaposed for defined measurement fractions. Describing the structure of the data, in addition to the mean values and their variability, the maximum and minimum values were given, providing the information on the dissipation rate that is significant from the point of view of the separation process. Due to the deviation of the actual distribution of feature variability from the normal distribution, the Shapiro-Wilk test was used to investigate hypotheses about the lack of normality of the considered probability distributions. Therefore the results which are statistically significant mean the lack of normal distribution while the remaining ones point to the lack of possibility to reject that hypothesis.
Due to the methodological difficulty of measuring the volume of seeds, they are sometimes compared to geometric solids with a simple mathematical description, and the replacement volume is calculated [28, 29]. Due to their oval-cylindrical shape, acorns may be compared to the ellipsoid (a solid whose all flat sections are ellipses), calculating the replacement volume from the relationship:
VZ – substitute volume,
a, b, c – linear dimensions of a seed: thickness, width, length.
Given that the difference between the thickness and width of the acorns is negligible , their shape can be likened to a special case of an ellipsoid: a spheroid; hence equation (3) takes the form:
RESULTS AND DISCUSSION
The number of individual fractions with properly assigned moisture of acorns is presented in Table 1. As follows from the juxtaposition, the moisture of stored seeds is heterogeneous: spoiled seeds store water worse than viable and rotting seeds. This can cause difficulties in determining the optimum moisture content for the storage; moreover, due to the efficiency of mechanical separation, it will have adverse consequences for the use. The mass characteristics of seeds depend strictly on their moisture: seed density decreases with the increase of moisture [2, 9, 12, 17].
Table 1. Numbers of seeds in the defined measurement fractions
Based on the measurements, Table 2 summarizes the mass, volume and density of the seeds. The shape of the distributions was further characterized by means of kurtosis (a measure of the flattening) and the coefficient of skewness (a measure of the asymmetry) (Tab. 3). These measures have an impact on the process of distribution, and should be taken into account in the process of designing the cleaning and sorting equipment because the separating element may be charged more than it would result from analysis of the average values. Analyses have shown compliance with the normal distribution of characteristics such as weight and volume; density is characterized by distribution with the leptokurtic (slim) course.
Table 2. Characteristics of seeds mass and density
Table 3. Characteristics of the distributions of the mass features of seeds
„+” conforming with normal distribution, „–” lack of conformity with normal distribution
The average mass of rotten oak seeds (fraction III) is significantly lower than that of healthy seeds (fraction I); the difference is 20%. A slightly higher value (23%) was obtained by comparing the average weight of rotten seeds to empty and rotting ones (fraction II). These are, however, differences which are significantly or even several times smaller than in the case of seeds of other forest-forming species [26, 27]. The weight of the heaviest acorns is almost 10 times higher than that of the lightest acorns. Such a large difference within the examined characteristic is unparalleled in the case of seeds of other forest-forming species. In turn, the variability of the characteristic (characterized by the coefficient of variation) does not differ from the variability observed in other species [14, 28]. Related trends were found when considering the volume of the seeds, wherein the differences in mean values for individual fractions of vitality amount to only a few percent. Therefore, it should be noted that the process of mummification affects the acorns of different sizes to an almost identical degree, which is confirmed in the observations by Tylek , and which shows that the shape of the acorns does not affect the threat of mummification, either.
Comparing the differences between the mean values of seed density in individual fractions, it was found that they are almost twice smaller than the corresponding differences in seed weight. From the perspective of the separation process, this may cause difficulties in the precision of the division, all the more that the distribution of density differs from the normal distribution the most, particularly in relation to fraction II: a very flat course, kurtosis on the level of 2.36 (Tab. 3).
In turn, from the utility point of view, the coefficient of variation is very positive, assuming much lower values as compared to the characteristics of seed weight. The level of less than 7% for healthy seeds is the lowest of all analyzed distribution traits. High stability of a trait is usually related to the efficiency of divisibility of a granular mixture. In the case of separation of seeds for long-term storage or for sowing in containers (rejecting fractions II and III), the separation index is only 0.17; but when rejecting only fraction III (separation in terms of seed preparation for sowing in the ground), the separation index is relatively high (0.53), which is the highest recorded value (Tab. 4). A certain flaw of the construction of the separation index becomes exposed here: that index takes into account the extent of occurrence of a trait in individual fractions without a comprehensive analysis of the feature distribution. This is particularly evident for very inclined and flat distributions, which is the case of the distribution of density of rotting seeds. It includes a considerable range of occurrence of the feature, where the extreme values are adopted by scarce, sometimes single seeds.
Table 4. Evaluation of differences in seeds mass features between measurement classes
* – statistically significant differences at α < 0.05 level
** – statistically significant differences at α < 0.01 level
n – statistically insignificant differences
Evaluation of the utility of the examined physical characteristics in separation processes was done by determining the intergroup differences for each measurement fraction of the seeds, using analysis of variance, while maintaining control over the uniformity of distribution. Statistically significant results were obtained by testing, with the use of the Fisher test, the differences between particular pairs of fractions (Tab. 4). There were no statistically significant differences in the seed size characterized by their volume. However, there is variation within mass and density, although in both cases it is not possible to separate just the healthy seeds; it can be done only together with the rotting seeds, separating them all from the remaining (rotten) ones.
In addition, Figure 3 shows the results of measurements (grouped in distribution rows) presented graphically using a frequency polygon, which makes it easier to assess the effectiveness of the separation of seeds [13, 14]. To allow comparison of results for different physical characteristics of seeds, a distribution row always consists of 10 class divisions.
Fig. 3. Graphical analysis of mass features distribution of seeds broken by measurement fractions: a – mass, b – volume, c – density
Using the geometric features of acorns, characterized by Tylek , and the actual seed volume, a high correlation was shown between the volumes defined above (Fig. 4). This may significantly simplify complex measurement procedures of the parameter of volume, that are useful even in analyses in the field of seed aerodynamics. It should be noted that the replacement volume is in any case greater than the actual volume, since the acorn stigma is flat (an acorn can be placed vertically) while its top is slightly pointed .
Fig. 4. Actual volume of seeds expressed as function of substitute volume
The density of oak acorns, and to a limited extent also their weight, depends on vitality, determined by the cutting method. They may therefore be regarded as separation characteristics, used in the construction and operation of machines and other equipment for sorting seeds. There is, however, no possibility to separate only healthy seeds; they may only be separated together with rotting ones from the remaining, rotten acorns.
Acorn volume is not a separating feature; the process of mummification affects acorns of different sizes to an almost identical degree.
The weight and volume of seeds are continuous random variables, and can be characterized by a normal distribution of probability (excluding the volume of spoilt seeds). Density is characterized by left-skewed distribution of probability.
The traditional way of drying and storing acorns does not allow obtaining homogeneous final moisture: spoiled and rotting seeds store water worse than viable seeds.
The relationship between the actual volume of acorns and the replacement volume make it possible to simplify the agrophysical metrology procedures, eliminating the measurement that is difficult methodologically and that requires specialized equipment. The relationships can be described by the equation of regression:
V = 0.69VZ + 0.41 with the coefficient of determination of 0.93.
The study was done within the project no. PBS3/A8/34/2015 financed by the National Centre for Research and Development (NCBR), Poland.
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Accepted for print: 15.10.2015
Department of Forest Work Mechanisation, University of Agriculture in Krakow, Poland
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Department of Heavy Duty Machines and Research Methodology, University of Warmia and Mazury, Poland
Institute of Machinery Management, Ergonomics and Production Processes, University of Agriculture in Krakow, Poland
Department of Forest Work Mechanisation, University of Agriculture in Krakow, Poland
29 Listopada 46
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