ABSTRACT
Post harvest management i.e. drying seed crops at different drying times and temperatures studies showed that pea seeds (Pisum sativum L.) were very sensitive to either high drying temperatures or longer drying times. Drying temperatures of 40, 60, 80 and 100oC were tested. It was found that only 40oC was suitable for pea seeds.
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DOI: 10.3923/ajps.2003.978.982
URL: https://scialert.net/abstract/?doi=ajps.2003.978.982
INTRODUCTION
Every year large volumes of high quality seeds are lost for planting purposes because of excess moisture. Seed moisture content is one of the factors which determines whether or not seed can be stored safely without loss of germination and vigour. When moisture content is too high the seed may heat and various moulds can grow. Therefore it is absolutely vital to ensure that harvested seed is at a safe moisture content before putting it into store. A seed crop is often harvested when the seed moisture content is higher than desirable for safe storage. Safe seed moisture contents vary with crop species, but generally 14% or less is considered satisfactory for short term storage (Kelly, 1988). Most high moisture seed lots could be saved for seed purposes if properly dried. Seeds are also susceptible to drying injury in several ways. First, they are sensitive to high temperatures, depending on the species (Kernick, 1961). Seeds may also be injured by drying too rapidly or by over drying. To obtain a good dryer performance seed moisture content, depth of seed in bin, air temperature and air volume must be controlled (Copeland, 1976). A farmer wants to dry seed quickly, but high temperature may cause damage seed viability and vigour. The effect of high temperature is most damaging when the moisture content of the seed is high (Almekinders and Louwaars, 1999).
The objectives of drying are to reduce grain respiration by removal of excess moisture and to prevent the qualitative deterioration of seeds in storage which may arise from the growth of microorganisms and the activities of insects and mites (McLean, 1989). Seed drying depends on the climate at harvest. In a dry climate, seed can go direct from the combine harvester to store. In wet regions, particularly in the tropics, large scale drying plants are necessary. It is a characteristic of agricultural seeds that they can withstand the removal of water and remain viable even if the moisture content is reduced to as low as 5% (Thomson, 1979c). He also commented that artificial drying can depress the germinability of agricultural seed, giving rise to abnormal seedlings, affecting the permeability of the seedcoat, destroying enzymes, or causing the outer layers to become hard so that when the embryo subsequently imbibes water and swells, fractures and cracks develop. In some cases it seems to depend on the temperature of the hot air and the time during which the seed is exposed to it. In other cases it seems to be caused by rapid drying, even if the temperature is low, e.g. soya seed dried by air with a relative humidity of less than 40% loses viability even at low temperatures. In general, temperatures up to 45oC are safe, but higher temperatures may be used in continuous flow dryers than in batch dryers, because the time of exposure is shorter. Final moisture content for safe storage depends upon the initial moisture content of the seed crop species. Final moisture contents for safe storage of seed of peas is 14% (McLean, 1989). After harvest moist seeds can be dried down either by using a short time at a high temperature or a longer time at a lower temperature. To find out a suitable time and temperature combination set for peas a series of time and temperature combination sets were tested. The objectives of this study were to determine the effects of different seed drying times and temperatures on moisture percentage and seed quality (viability and vigour) of pea seeds.
MATERIALS AND METHODS
These experiments were conducted utilising seed crops grown in pots out side the glasshouse of Henfaes Research Centre, University of Wales, Bangor, United Kingdom. Seed crops were sown on 04.06.1998 and were managed according to standard good husbandry practices. They did not experience any drought or water logging conditions and there were no attacks by pest and diseases.
Treatments and experimental designs: Four different temperatures i.e. 40, 60, 80 and 100oC were used as temperature treatments and 0, 2, 4, 8, 24, 48, 72, 120 and 168 h were used as time treatments. Different lengths of time were tested for different temperatures. At the lowest temperature i.e. at 40oC, up to 168 h and at 60, 80 and 100oC drying times up to 120, 72 and 48 h, respectively were tested.
Experimental details and sampling procedures: Seeds were obtained from plants grown in pots in the open air, but protected from wind, at the Henfaes Research Centre of the University of Wales, Bangor, UK. The plants were grown in pots each having a surface area of 0.1521 m2. After maturity all the plants were harvested and all the pods were collected and threshed gently by hand. Initial seed moisture content was determined taking four samples of seed (which were randomly collected from the bulk harvest) straight away after harvest and by drying at 105oC in an oven for 48 h. Moisture percentage was calculated following the procedures described by ISTA (1976a,b). The remaining seeds were then immediately dried at different temperatures for different amounts of time according to the design. The initial seed moisture content was around 25% (fresh wet basis). Four separate ovens were used for four different temperatures. Three small bench incubators (Gallenkaup) were used for temperatures of 40, 60 and 100oC and a larger oven (Unitherm Drying Oven) was used for 80oC. Seeds were dried in trays and in each oven three separate trays were used as three separate replicates. At the end of each drying period three samples of seed were collected from each temperature and kept separate as separate replicates. Seeds were mixed four times a day within the trays to help maintain uniform heat and moisture content. Seed moisture content was measured at the end of each drying period. Seeds were then stored in paper bags in a laboratory at room temperature to allow for dormancy breaking.
Testing seed quality: Seed quality testing started on 11.12.98, approximately two and half months after storage. Before starting seed quality measurements, seed dormancy was tested by taking a sample of seed and testing germination. Seed samples from the individual replicates were kept separate and treated as the replicates of the seed quality tests. A germination test was performed on the seeds and subjected to different drying time x temperature combinations.
Statistical analysis: Two different statistical analyses were performed on the data. As the experiment did not include all possible combinations (drying times x temperatures) both one way and two way analysis of variance were performed on the data. A two way Factorial ANOVA (balanced design) was performed on the data for germination percentage including values for drying times of 0, 2, 4, 8, 24, 48 and 72 h at 40, 60 and 80oC. Data for 100oC was excluded as no seeds germinated and allowing times above 72 h were also excluded as the design became unbalanced. For seed moisture content after drying, data for 100oC was not excluded (because this does not have zero values) but drying times above 48 h were again excluded for the same reason as above. A one way ANOVA was also performed to look at the effect of time at each temperature. These analysis excluded times when germination reached 0%. However, for moisture content, all the data at all temperatures were included as there were no zero values. The data, which were recorded as counts and expressed as percentages (germination percentage) were transformed, following the procedures described by Gomez and Gomez (1984).
RESULTS
Figure 1 shows the effects of drying time and temperatures on the moisture content of pea seeds. The initial moisture content of seeds after harvest was 25% (fresh weight basis). In the two way ANOVA the effects of time, temperature and the temperature x time interaction were all significant (p<0.001). As drying temperatures increased moisture percentage decreased and as drying time increased moisture content decreased as well. Moisture percentage decreased faster at higher temperature than at low temperature. At 40oC the decrease in moisture percentage was relatively slow. In the case of the time x temperature interaction all the differences were significantly different except between 24 and 48 h at 80oC and 8 to 24 h at 100oC. Fig. 1 also shows that at all temperatures there was a short period of rapid moisture loss, followed by a longer period of slower loss. At 100oC a moisture content of 1.4% was reached after only 8 h. At 40oC the initial loss was also rapid and a moisture content of 13.7% was reached after 8 h. Moisture content continued to decrease slowly up to 168 h. The trend of moisture content decrease was very rapid at higher temperatures. At 80oC almost all seed moisture was lost after drying for 24 h. At 100oC almost all seed moisture was lost after drying for 8 h.
Fig. 1: | Changes of moisture percentage of pea seeds with different drying times and temperatures |
Fig. 2: | Effect of different drying times and temperatures on germination percentage of pea seeds |
Fig. 3: | Relationship between moisture percentage and germination percentage of pea seeds dried at 40, 60, 80 and 100°C |
Figure 2 and Table 1 show the effects of drying time and temperature on the germination percentage of pea seeds. Fig. 2 shows the original data and Table 1 shows the transformed data values. The effects of time, temperature and the temperature x time interaction were all statistically significant (p<0.001). As drying temperature increased germination percentage decreased. There was a small decrease in germination percentage between 40 and 60oC and a much larger decrease between 60 and 80oC. At 100oC no seeds germinated. The effects of drying time were different at the different temperatures.
Table 1: | Effects of time of exposure of seeds to different drying temperatures on germination percentage of peas (arc sine transformed data) |
At 40oC drying time had no significant effect on germination percentage. At 60oC drying seeds for up to 8 h did not result in a significant decrease in germination percentage. As drying time increased above 8 h, germination percentage decreased. This decrease was not significant at 24 h, but was significant at 48 and 72 h. Drying seed at 80oC resulted in a rapid and significant decrease in germination percentage. Germination percentage was significantly decreased after drying at 80oC for 2 h and no seed germinated after 8 h. At 100oC drying time was not at all important as no seeds germinated.
DISCUSSION
In a developing country like Bangladesh, where drying is necessary, traditionally farmers have, on the basis of experience, evolved methods which utilize the sun and wind and which are suitable for small quantities. Threshed seed may be spread out in a thin layer on a smooth earthen floor or on straw matting (Thomson, 1979). Kelly (1988) also reported that the rate of drying is important; if it is either too rapid or too slow, the seed is liable to be damaged. Rapid drying can harm the seed, either because water is withdrawn too quickly or because of the high temperature. Slow drying may have the effect of maintaining the seed at a high moisture content and a relatively high temperature and so accelerate the deterioration which drying is intended to prevent (Thomson, 1979). The experiments reported in this paper, efforts were made to look at the effects of different drying times and temperatures on moisture percentage and seed germination. After threshing seeds were allowed to dry at a range of times and temperatures. After different time interval samples were taken and their seed quality (germination) tested.
Relationship between seed moisture content and germination percentage of pea seeds: Figure 3 shows the effects of drying at 40, 60, 80 and 100oC on moisture percentage and germination percentage of pea seeds. At 40oC there was no large effect of moisture percentage on germination percentage. Even after 7 days drying, when moisture percentage had decreased to 6%, germination percentage was still high. At 60oC after 48 h moisture content decreased to around 3%, but germination percentage was still over 80%. At 80oC after 4 h moisture content decreased to 11% but germination was only 40%. At 100oC after 2 h moisture content decreased to only 11% but no seed germinated. As moisture content decreased over drying time germination percentage was decreased at all temperatures. This decrease was large and significant at 80 and 60oC but very small and non significant at 40oC and no seed germinated at 100oC. Drying seeds at 100oC for two h decreased seed moisture percentage to around 11% but killed all the seeds and no seed germinated. Drying seeds at 40oC for 48 h dried the seeds to a lower moisture percentage (8%), but still 96% of seed germinated. This indicates that there is no strict relationship between moisture percentage and germination percentage of pea seeds. It depends on the temperature used to dry the seeds. The results are in agreement with the results of Norden (1975) who worked on peanut and found similar results.
Relationship between seed drying time at different temperatures and germination percentage of pea seeds. It was found that 40oC was completely safe for peas. Even after 168 h drying germination was still very high and close to 100%. To decrease moisture content to 14% (required for safe storage) seeds would need to dry for 8 h (Fig. 1). At 60oC initially with a short drying time up to 48 h germination decreased, but after that especially between 48 and 120 h germination stayed almost constant. This suggests that some seeds, possibly immature and low vigour seeds, were killed at 60oC during the first 24 h, but the majority of seeds with comparatively high vigour were able to survive. Hence drying seed at 60oC will give more reliable germination results and be a better indicator of performance in the field, especially in adverse field conditions, as low vigour seeds have been killed. Seeds dried at 40oC would germinate in the laboratory germination test but might not similarly perform in the field. 80oC was not a very suitable temperature for peas, as after 4 h drying germination dropped to 40%. 100oC was not at all suitable for peas. Even after two h germination was zero when the seed moisture content was around 11%. Loss of germination at 100oC was not only due to moisture content, because after 2 h drying at 100oC moisture content had decreased from 25 to 11% only. At other temperatures, especially 40 and 60oC, seeds with 11% moisture content still germinated.
Effects of high temperature on seed germination: It has been concluded from the above study that high temperatures damage the seed. The type of damage associated with the use of excessive heat during the drying of seed may be summarised as the loss of viability. The cause of this loss of viability may be associated with internal cracks, split seed coats and discolouration. Nellist and Hughes (1973) demonstrated that the extent of damage depends on the interaction of temperature with time of exposure. Cellular examination (Hutchinson et al., 1945) of damaged grains from heating did not reveal any chromosome aberrations but did show blister like protrusions of the nucleolus. These were thought to be associated with a loss of the power of selective permeability which also caused heat damaged grains to be sensitive to excess water. Although severe heat damage did involve enzyme inactivation, which is also a cause of rapid loss of viability (Abdalla and Roberts, 1968; Nellist and Hughes, 1973). In the present study discolouration was observed in high temperature dried seed and it was found that these seeds were highly sensitive to excess water as the seed coats split during the germination test and in this way may have caused imbibition damage, which is associated with loss of seed viability. In these studies at 100oC seeds lost moisture very fast and no seed germinated. The results are in agreement with the results of Nautiyal and Zala (1991) who reported that in Spanish groundnut seed higher temperature and faster rate of moisture loss during the drying process may lead to greater damage to seed membranes, which is associated with loss of viability. Seyedin et al. (1984) reported that in corn seed high drying temperatures result in hydrolysis of starch in the embryonic axis. This process, which is likely enzymatic, would be supported by rapid loss of moisture. The high drying temperature may also result in increased membrane permeability. However, in this study pea seed showed high sensitivity to high temperature drying. This may be the type of the seed coat of the species. Thomson (1979) reported that in artificial drying, the testa of the pulse seeds cracks and separates from the cotyledons and the seedlings that emerge show reduced vigour and poor establishment.
ACKNOWLEDGMENTS
We thank Mrs. Ll G. Hughes of Henfaes Research Centre of the University of Wales for her expert technical assistance and the World Bank for providing a studentship to A.B. Siddique.
REFERENCES
- Abdalla, F.H. and E.H. Roberts, 1968. Effects of temperature, moisture and oxygen on the induction of chromosome damage in seeds of barley, broad beans and peas during storage. Ann. Bot., 32: 119-136.
Direct Link - Seyedin, N., J.S. Burris and T.E. Flynn, 1984. Physiological studies on the effects of drying temperatures on corn seed quality. Can. J. Plant Sci., 64: 497-504.
CrossRefDirect Link - Gomez, K.A. and A.A. Gomez, 1984. Statistical Procedures for Agricultural Research. 2nd Edn., John Wiley and Sons Inc., Hoboken, New Jersey, ISBN: 978-0-471-87092-0, Pages: 704.
Direct Link