Published online 25 April 2006
Published in Crop Sci 46:1336-1340 (2006)
© 2006 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
SEED PHYSIOLOGY, PRODUCTION & TECHNOLOGY
Aerosol Smoke and Smoke-Water Stimulation of Seedling Vigor of a Commercial Maize Cultivar
S. G. Sparg,
M. G. Kulkarni and
J. van Staden*
Research Centre for Plant Growth and Development, School of Biological and Conservation Sciences, University of KwaZulu-Natal Pietermaritzburg, Private Bag X01, Scottsville 3209, South Africa
* Corresponding author (rcpgd{at}ukzn.ac.za)
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ABSTRACT
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The effect of smoke as a germination cue is well documented. Recent developments in smoke technology have suggested that smoke also improves vigor. It has been reported that indigenous storage methods of South African maize landraces using smoke enhances seedling vigor. This present study highlights the effects of aerosol smoke and smoke–water on the germination and seedling vigor of a commercial maize cultivar, Zea mays L. var. PAN 6479. Various presoaking and smoking combinations were investigated. Treating seeds with aerosol smoke showed significant (p < 0.05) stimulatory effects on vigor. However, prolonged exposure to aerosol smoke resulted in reduced germination which was alleviated by rinsing the seeds. Furthermore, presoaking seeds before exposure to aerosol smoke also significantly (p < 0.05) reduced the inhibitory effect of prolonged smoking. The combination of presoaking and smoking had a significant (p < 0.05) improvement on the percentage germination. This study also showed that smoke has the potential to improve not only the percentage germination but also seedling vigor of commercially bred maize seeds.
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INTRODUCTION
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ALTHOUGH THE CHEMICAL IDENTITY of a highly active germination promoter from smoke, 3-methyl-2H-furo[2,3-c]pyran-2-one, has only recently been discovered (Flematti et al., 2004; van Staden et al., 2004), the effects of smoke on seed germination is widely known and utilized in various ways (Roche et al., 1997; Brown and van Staden, 1997; van Staden et al., 2000). Smoke has the potential to be used for a variety of applications related to seed technology (Light and van Staden, 2004). Furthermore, smoke has the potential to be used in the horticultural and agricultural industry for the production of healthier and more vigorous crops (Light and van Staden, 2004). Much of the research in this field, however, has focused on smoke as a germination cue for the release of seed dormancy. To date, there is very little documented work on postgermination effects of smoke (Sparg et al., 2005). Baxter and van Staden (1994) reported that the seedlings of the fire-climax grass Themeda triandra Forssk. from smoke-treated seeds did not have any morphological abnormalities and grew more vigorously. A similar effect was observed for Erica species and species of Asteraceae (Brown, 1993). More recently, Sparg et al. (2005) stated that although smoke treatment may not necessarily have an effect at the germination stage, it may play a role at the postgermination stage and suggested that in previous studies where many species have not responded to smoke treatments, these species may show some response at their postgermination stages, i.e., improved seedling vigor. Therefore, it may be necessary to extend germination studies to include seedling vigor when smoke is used as a germination cue. All these previous studies have been conducted on wild species, with no evidence to suggest whether this postgermination effect of smoke can be seen in commercial crop plants. The use of fire and smoke in agriculture is not a new practice. Farmers have traditionally used fire and smoke in grain drying practices. It is thought that these methods improve germination and seedling vigor (Paasonen et al., 2003). In South Africa, some rural farmers store maize cobs over a fireplace, thus subjecting the seeds to smoke and heat. Modi (2002, 2004) investigated the effect of this storage method on the seed quality of two traditional maize landraces. This storage method improved the germination rate and final germination in comparison with untreated seeds. Furthermore, seeds stored above the fireplace produced significantly more vigorous seedlings, which were heavier and taller, in comparison with untreated seeds.
In this investigation, the effect of different smoke applications (aerosol smoke and smoke–water) on the germination and seedling vigor of a common commercial maize variety (PAN 6479) was examined.
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MATERIALS AND METHODS
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Seed Source
All experimental studies were conducted on the commercial maize cultivar PAN 6479, obtained from Pannar Seed (Pty) Ltd, Pietermaritzburg, South Africa.
Smoke–Water Treatment
An aqueous smoke extract was prepared by continuously bubbling smoke from smoldering Themeda triandra Forssk. (Poaceae) leaf material through 500 mL water for 45 min (Baxter et al., 1994). Solutions of this smoke extract were prepared by diluting 1 mL of the concentrated solution in 250 mL of distilled water and diluting this further, as required. For the germination experiments, seeds were placed in 9-cm Petri dishes on two layers of filter paper (Whatman No. 1) moistened with 5 mL distilled water or smoke–water (1:250, 1:500, 1:1000 and 1:2000; pH 7.5, 7.8, 7.9, and 8.2 respectively). Each treatment consisted of five replicates of 20 seeds. Experiments were repeated twice. Seeds were incubated at 25 ± 0.5°C under a 16:8 h light/dark period with a photosynthetic photon flux density of 101 (±5) µmol m–2 s–1 provided by cool-white fluorescent lamps. The filter paper was remoistened when required with a few drops of distilled water during the course of the trial. Germination, and shoot/root length were recorded after 7 d.
Exposure to Aerosol Smoke
Seeds were placed in sieves and exposed to cooled (
28°C) aerosol smoke for 30, 60, and 90 min. This was achieved by placing the sieves atop a 1.5-m chimney, above smoldering semidry grass Heteropogon contortus (L.) Roem. & Schult. (Poaceae). Following the aerosol smoke treatment, seeds were divided into two batches, rinsed and nonrinsed. The first batch was washed twice with 1000 mL water, after which the seeds were transferred to Petri dishes moistened with 5 mL distilled water. The second batch was not rinsed and transferred directly to Petri dishes moistened with 5 mL distilled water. The filter paper was remoistened when required with a few drops of distilled water during the course of the trial. Seeds were incubated as described above and germination and shoot/root length were recorded after 7 d.
Presoaking and Exposure to Aerosol Smoke
Seeds were presoaked in distilled water for 60, 120, and 180 min, before being exposed to aerosol smoke for various exposure times (30, 60, and 90 min). Following the aerosol smoke treatment, seeds were divided into two batches, rinsed and nonrinsed. The first batch was washed twice with 1000 mL water, after which the seeds were transferred to Petri dishes moistened with 5 mL distilled water. The second batch were not rinsed and transferred directly to Petri dishes moistened with 5 mL distilled water. The filter paper was remoistened when required with a few drops of distilled water during the course of the trial. Seeds were incubated as described above and germination and shoot/root length were recorded after 7 d.
Seed Soaking in Smoke–Water
To compare presoaking in distilled water with soaking in smoke–water, seeds were soaked in various concentrations of smoke–water (1:250, 1:500, 1:1000, and 1:2000) for 60, 120, and 180 min, blotted dry, and transferred to Petri dishes moistened with 5 mL distilled water. The filter paper was remoistened when required with a few drops of distilled water during the course of the trial. Seeds were incubated as described above and germination and shoot/root length were recorded after 7 d.
Vigor Index
The vigor index of 1-wk-old seedlings was calculated as VI = [shoot length (mm) + root length (mm)] x percentage germination (Dhindwal et al., 1991).
Statistical Analysis
The germination data in each treatment were arcsine transformed and chi-square test (p < 0.05) was used. The arcsine percentage transformation was performed before statistical analysis and significant difference (p < 0.05) is indicated on untransformed data in the figures. The vigor index was calculated on the basis of actual mean percentage germination. Root length and shoot length data were analyzed by one-way analysis of variance (ANOVA) with MINITAB (1998) and Fisher's pair-wise comparison was evaluated at 5% level of significance.
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RESULTS
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Smoke–water and aerosol smoke improved the percentage germination of maize (Fig. 1
). Compared with the control, smoke–water at concentrations of 1:500, 1:1000, and 1:2000 resulted in significantly higher germination. Although the pH of these solutions differed by a range of 0.7, preliminary experiments showed that this did not affect the germination of these seeds. Smoke–water at a concentration of 1:500 resulted in significantly higher germination with a mean of 92.5% and showed the highest vigor index (3361) of the smoke–water concentrations tested (Fig. 1A). Exposure to aerosol smoke for 30 min, followed by rinsing, resulted in significantly higher germination (90.0%) compared with the control (72.5%). This treatment gave the highest seedling vigor (3860), which was more than three times that of the control (1165) (Fig. 1B). Increased exposure time to aerosol smoke had a negative effect on the percentage germination, with seeds exposed for 90 min and not rinsed resulting in 61.2% germination and having the lowest vigor index of 1028 (Fig. 1B). Similar results were observed with the increase in smoke–water concentration. At a concentration of 1:250 the vigor index was significantly lower in comparison with the other treatments (Fig. 1A).
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Fig. 1. Effects of (A) smoke–water and (B) aerosol smoke on the percentage germination of maize after 7 d under 16:8h light/dark period at 25 ± 0.5°C. Asterisks indicates values significantly different from the control (p < 0.05). Values above standard error bars represent vigor indices.
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Presoaking maize seeds in water and smoke–water (1:1000) improved germination. Presoaking of seeds in water for 180 min before aerosol smoking for 90 min, resulted in 97.5% germination (Fig. 2
). Presoaking the seeds in water before aerosol smoke, significantly improved the vigor indices when seeds were exposed to aerosol smoke for both 60 and 90 min (Fig. 3
). Seedlings from seeds presoaked in water for 180 min and exposed to aerosol smoke for 60 min had the highest vigor index of all treatments, with a value of 5987 (Fig. 3). Both smoke–water and aerosol smoke improved the growth of young maize seedlings (Fig. 4
). All four smoke–water concentrations significantly improved shoot length. Root length increased with an increase in smoke–water concentration up to 1:500, decreasing significantly at a concentration of 1:250 (Fig. 4A). The longest roots and shoots were recorded in 30 min aerosol smoke exposure followed by rinsing (Fig. 4B). As the exposure time to aerosol smoke increased, both the root and shoot lengths decreased. Nonrinsed aerosol-smoked (60 and 90 min) seedlings showed no significant improvements in root and shoot growth in comparison to the control. Figure 5A
and B clearly shows the promotive effect of different smoke–water concentrations and different exposure periods of aerosol smoke on root and shoot elongation of maize seedlings.
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Fig. 2. Effect of various soaking treatments with smoke–water (SW, 1:1000 dilution) and aerosol smoke (90 min exposure) on the percentage germination of maize after 7 d under 16:8h light/dark period at 25 ± 0.5°C. Control seeds were not soaked. Symbols are with standard error bars.
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Fig. 3. Effect of presoaking seeds following aerosol smoke treatment for different periods on the vigor index of maize seedlings after 7 d under 16:8h light/dark period at 25 ± 0.5°C. Seeds were presoaked in water for different durations before exposure to aerosol smoke. Vigor index for the control was 1165. Average coefficient of variation (%) for 30 min smoke were 13.2 (rinsed) and 18.2 (nonrinsed), 60 min smoke were 26.4 (rinsed) and 40 (nonrinsed), 90 min smoke were 26 (rinsed) and 50 (nonrinsed).
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Fig. 4. Effects of (A) smoke–water and (B) aerosol smoke on the growth of maize seedlings after 7 d under 16:8h light/dark period at 25 ± 0.5°C. Bars with the same letters are not significantly different at 5% level. Error bars denote standard error.
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Fig. 5. Seven-day-old maize seedlings germinated from seeds treated with (A) smoke–water at different concentrations and (B) aerosol smoke for different exposure periods (germinated after rinsing in water). Bar represents 10 mm.
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DISCUSSION
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This study highlights the effects of aerosol smoke and smoke–water on the germination and seedling vigor of a commercial maize cultivar, PAN 6479. Smoke–water and aerosol smoke improved the percentage germination of maize (Fig. 1). However, increasing the exposure time of the seeds to aerosol smoke resulted in a negative effect on the percentage germination. For example, seeds exposed to 90 min of aerosol smoke had only 61.2% germination compared with 72.5% in the control (Fig. 1B). At high concentrations, smoke extracts are known to inhibit seed germination (Brown and van Staden, 1997). According to Brown and van Staden (1997) more dilute solutions improved the germination in dormant seeds of the everlasting wildflower Syncarpha vestita (L.) B. Nord. (Asteraceae). Similarly, results from the study on smoke–water showed that at a concentration of 1:500, germination is significantly improved, but at a concentration of 1:250 the germination decreases significantly. This study suggests that, by rinsing the seeds after exposure to aerosol smoke, the percentage germination can be increased. Rinsing aerosol smoked seeds also improved germination in medicinal bulbous plants (Sparg et al., 2005). As reported by Sparg et al. (2005), the apparent inhibition of germination by aerosol smoke appears to be reversible and indicates the possible presence of water-soluble inhibitory compound(s) as suggested by Light et al. (2002). In the natural environment, these inhibitory compounds would be leached from the seeds by rainfall, but with in vitro germination studies using Petri dishes, they are retained at a high concentration.
Presoaking maize seeds in water has been reported recently by Murungu et al. (2005) to improve germination. Results from this study concur with their findings. However, presoaking for 180 min in water followed by aerosol smoking for 90 min (Fig. 2) resulted in even significantly higher germination (97.5%). By presoaking the seeds, seeds can be exposed to 90 min aerosol smoke without any inhibitory effects. Aerosol smoked seeds do not necessarily need rinsing if presoaked, since germination and vigor still improved significantly compared with the untreated control. Similarly, soaking maize seeds in smoke–water also showed significant improvements in germination.
The application of aerosol smoke showed significant stimulatory effects on vigor (Fig. 3
–5). Seedlings grown from seeds exposed to aerosol smoke produced longer roots and shoots. However, with aerosol smoking, rinsing is required for seedlings to achieve significant improvement in vigor. The prolonged exposure to aerosol smoke not only affected percentage germination but also vigor. Seedlings formed from seeds exposed to 90 min aerosol smoke and rinsed showed slight improvements in vigor, but these were not significant. However, seeds exposed for 90 min that are not rinsed developed shorter shoots than those of the untreated control. These results suggest that the inhibitory action observed with germination is due to either a high smoke concentration or to some inhibitory compound(s) as reported by Light et al. (2002), which may be responsible for a decrease in vigor. It would therefore appear that the overall growth of maize, from germination to seedling establishment, is affected by the presence of such compounds.
Various presoaking and smoking combinations were investigated. Exposure of maize seeds to aerosol smoke for 60 min resulted in the highest vigor. Results showed that smoke-treated seed requires rinsing with water to eliminate the inhibitory effect of smoke compound(s). Soaking seeds before exposure of aerosol smoke significantly reduced the inhibitory effect generated by prolonged smoking. The combination of presoaking in water and smoking had a significant improvement on the percentage germination.
In field conditions, germination of maize seeds takes place after imbibition of sufficient water to activate growth. This imbibition may be interrupted by drying soil, by low soil water potential, decreased hydraulic conductivity, insufficient seed-soil contact, and other environmental factors (Hadas, 1977; Brar et al., 1992). Under such situations, seed water uptake rate and total germination were shown to be greatly decreased (Collis-George and Hector, 1966; Hadas, 1977). To overcome these stresses, rapid germination and seedling emergence are very important (Murungu et al., 2005). This study has shown that smoke treatments (e.g., using smoke solutions) have the potential to improve not only the percentage germination but also the seedling vigor of commercially bred maize seeds. This can assist with seedlings coping under unavoidable stresses generated under field conditions. The findings from the present investigation suggest that although commercial varieties of maize seeds are genetically bred, the application of smoke technology (seed priming with smoke) can further improve percentage germination and more vigorous seedlings can be obtained.
The recent isolation and identification of a butenolide compound (3-methyl-2H-furo[2,3-c]pyran-2-one) from smoke (Flematti et al., 2004; van Staden et al., 2004) which stimulates seed germination in a number of species will now allow for research into the physiological mode of action to carefully unravel the mechanisms and responses of seeds to smoke.
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ACKNOWLEDGMENTS
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The financial support of the National Research Foundation (NRF), Pretoria, and the University of KwaZulu-Natal Research Fund is gratefully acknowledged.
Received for publication September 27, 2005.
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REFERENCES
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- Roche, S., J.M. Koch, and K.W. Dixon. 1997. Smoke enhanced seed germination for mine rehabilitation in the southwest of Western Australia. Restor. Ecol. 5:191–203.
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ABSTRACT
INTRODUCTION
