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Introduction
The physic nut (Jatropha curcas L.) is a tropical species of the family Euphorbiaceae, which is native to the Americas. It is appreciated for its multiple uses and for its oil, which can be used as an alternative source of energy (Kumar & Sharma, 2006). This plant has thickly-coated seeds that, shortly after harvesting, present high levels of viability in contrast to their low germination levels, indicative of innate vegetative rest (Echeverría, Valles, & Rengifo, 2013). To induce sprouting, it is recommended to apply pregerminative treatments such as seed soaking and alternate soaking and drying times (Salazar & Soihet, 2001), which disinhibit latency, reinvigorate the seed and accelerate and increase germination (Sánchez, Orta, & Muñoz, 2001).
On the other hand, the selected substrate is essential to obtain quality plants, being the first mean of contact with the plant (Echeverría et al., 2013). Although there are several papers on pregerminative treatments of J. curcas seeds (Gairola, Nautiyal, & Dwivedi, 2011; Valdés-Rodríguez, Pérez-Vázquez, & Martínez, 2014), they do not consider the efficacy of using rodent manure or ash. However, it has been found that the manure of some animals improves the germination of certain seeds (Traveset, Bermejo, & Willson, 2001), while rubbing with ash could function like sandpaper to scarify the testa and facilitate moisture penetration.
In view of this information, the hypothesis that such treatments could be beneficial for J. curcas seeds was considered. Therefore, the aim of this research was to evaluate 13 pregerminative methods (with water, Manilkara bidentata ash and cuy [Cavia porcellus] manure) on germination and uniformity in the emergence of physic nut seeds.
Materials and methods
The experiment was carried out from August to October 2009. Physic nut seeds harvested during August 2009 from mature fruits and vigorous, healthy plants of the Totorillayco ecotype of the San Martín region, Peru were used. The average seed length and width were 19 and 8 mm, respectively.
Study site
The study was carried out in a 6.0 x 4.0 x 3.5 m brick greenhouse open at the sides, with a transparent acrylic gable roof, located in the district of San Martín, Peru (6° 29’ 27’’ south latitude and 76° 22’ 14’’ west longitude, at 293 masl). The environmental variables recorded were temperature and relative humidity, with averages of 26 °C and 78.5 %, respectively.
Treatments and sowing
The experiment consisted of 13 different pregerminative treatments with two substrates (Table 1). The substrates used were disinfected 1 h before sowing with hot water at 100 °C. The 1 m2 germinators were constructed with wooden slats. The boxes were filled with substrate (up to 5 cm in height). The pre-treated seeds were sown horizontally to the depth of the seed thickness (approx. 1 cm) and with the caruncle down. They were watered daily by spraying to maintain moisture in the substrates.
Table 1: Description of treatments evaluated in the germination and emergence of physic nut seeds (Jatropha curcas L.).
| Substrate | Treatment | Key | Description |
|---|---|---|---|
| Sawdust or sand | Control | T1 | Direct sowing. |
| Seed immersion in water for 12 h | T2 | Water at room temperature. | |
| Seed immersion in water for 24 h | T3 | Water at room temperature. | |
| Seed immersion in water for 24 h with water change at 12 h | T4 | Water change at room temperature every 12 hours. | |
| Seed immersion in decomposed cuy (Cavia porcellus) manure tea for 12 h | T5 | Three-month-old cuy manure diluted in water (0.5 kg in 0.5 L of water), left to stand for seven days and then diluted again in water (15 mL∙L). | |
| Seed immersion in fresh cuy manure extract for 12 h | T6 | The fresh cuy manure diluted in water (0.5 kg in 0.5 L of water) was sieved to obtain the solution in which the seeds were immersed. | |
| Rubbing seeds with ash for 15 min + immersion in water for 12 h | T7 | Manilkara bidentata ash with particles smaller than 2.5 mm and water at room temperature. | |
| Rubbing seeds with ash for 15 min + immersion in fresh cuy manure tea for 12 h | T8 | Manilkara bidentata ash with particles smaller than 2.5 mm and fresh manure tea. | |
| Seed immersion with fresh cuy manure tea for 12 h | T9 | Seeds submerged for 12 hours at a dilution of 15 mL∙L of fresh manure tea prior to sowing. | |
| Seed immersion in fresh cuy manure for 24 h | T10 | Seeds submerged for 24 hours in 500 g of fresh manure tea prior to sowing. | |
| Seed immersion in decomposed cuy manure for 24 h | T11 | Seeds submerged for 24 hours in 500 g of three-month-old manure prior to sowing. | |
| Seed immersion in water at 62 °C for 10 min | T12 | 1 L of water at 100 °C was left to stand 20 min (tfinal = 62 °C). | |
| Seed immersion in water at 47 °C for 10 min | T13 | 1 L of water at 100 °C was left to stand 30 min (tfinal = 47 °C). |
A completely randomized split-plot design with the substrates as large plots and the pregerminative treatments as small plots was used. Three replicates of 50 seeds were made for each treatment.
The evaluated variables were germination onset, counting the days from sowing to the appearance of the radicle (approximately 0.4 cm), germination percentage every 24 hours, as the number of germinated seeds over the total planted, and emergence percentage, as the number of seedlings already emerged by the eighth day after sowing.
The analysis of variance was performed using the Statistical Analysis System program (SAS, 2004) and the means comparison was done with the Tukey test (P ≤ 0.05).
Results and discussion
Germination onset
Highly significant statistical differences (P ≤ 0.01) were obtained due to the effect of substrates, pregerminative treatments and their interaction (Table 2). In sawdust, germination started on average 1.3 days earlier than in sand. In both substrates, seed immersion in water for 24 h, cuy manure and ash rubbing allowed germination to start on average 1.41 and 11.95 days earlier than with direct sowing in sawdust and sand, respectively (Table 3). This indicates that the imbibition phase (the rapid absorption of water) was favored by soaking (Suárez & Melgarejo, 2010), while ash rubbing and cuy manure can be considered to affect the permeability of the testa and allow for better hydration of the embryo (Finch-Savage & Leubner-Metzger, 2006).
Table 2: Significances obtained in the analysis of variance for the response of Jatropha curcas L. seeds to different treatments.
| Source of variation | Degrees of freedom | Germination onset (das) | Germination percentage | Emergence percentage |
|---|---|---|---|---|
| S1 | 1 | 32.05** | 576.21** | 1063.38** |
| Error a | 4 | 0.24 | 1.28 | 5.64 |
| PT | 12 | 51.51** | 5446.06** | 5240.43** |
| PT x S | 12 | 18.47** | 968.09** | 992.94** |
| Error b | 48 | 0.17 | 17.48 | 24.03 |
| Total | 77 | |||
| CV (%) | 13.45 | 6.12 | 7.21 |
1S: substrates, PT: pregerminative treatments and CV: coefficient of variation.
** Significant with P < 0.01
Table 3: Comparison of means of substrates and treatments for the germination of Jatropha curcas L. seeds
| Substrate | Treatment | Germination onset (days) | Germination (%, 5 das) | Emergence (%, 8 das) |
|---|---|---|---|---|
| Sawdust | T11 | 3.3 dz | 62.7 de | 74.0 cd |
| T2 | 6.7 b | 43.3 f | 43.3 f | |
| T3 | 1.3 f | 80.7 bc | 80.7 bc | |
| T4 | 1.7 ef | 82.7 bc | 82.7 bc | |
| T5 | 2.0 ef | 80.0 bc | 80.0 bc | |
| T6 | 2.0 ef | 80.0 bc | 80.0 bc | |
| T7 | 2.0 ef | 80.0 bc | 80.0 bc | |
| T8 | 2.0 ef | 90.7 ab | 90.7 ab | |
| T9 | 2.0 ef | 100.0 a | 100.0 a | |
| T10 | 2.0 ef | 100.0 a | 100.0 a | |
| T11 | 2.0 ef | 60.0 de | 60.0 de | |
| T12 | 0.0 h | 0.0 g | ||
| T13 | 5.0 c | 70.7 cd | 60.0 de | |
| Sand | T1 | 14.0 a | 0.0 h | 0.0 g |
| T2 | 13.7 a | 12.7 g | 12.7 g | |
| T3 | 2.0 ef | 81.3 bc | 81.3 bc | |
| T4 | 1.7 ef | 66.7 d | 60.7 de | |
| T5 | 2.0 ef | 100.0 a | 100.0 a | |
| T6 | 2.0 ef | 91.3 ab | 100.0 a | |
| T7 | 2.0 ef | 91.3 ab | 82.0 bc | |
| T8 | 2.0 ef | 91.3 ab | 91.3 ab | |
| T9 | 2.7 de | 82.0 bc | 82.0 bc | |
| T10 | 2.0 ef | 91.3 ab | 91.3 ab | |
| T11 | 2.0 ef | 100.0 a | 82.0 bc | |
| T12 | 0.0 h | 0.0 g | ||
| T13 | 2.7 de | 52.0 ef | 52.0 ef | |
| HSD | 1.33 | 13.43 | 15.61 |
zMeans with the same letter within each column do not differ statistically (Tukey, P ≤ 0.01).
HSD: honest significant difference, das: days after sowing.
1T1: control, T2: seed immersion in water for 12 h, T3: seed immersion in water for 24 h, T4:
seed immersion in water for 24 h with water change at 12 h, T5: seed immersion in decomposed cuy manure tea for 12 h, T6: seed immersion in fresh cuy manure extract for 12 h, T7: rubbing of seeds with ash for 15 min + immersion in water for 12 h, T8: rubbing of seeds with ash for 15 min + immersion in fresh cuy manure tea for 12 h, T9: seed immersion with fresh cuy manure tea for 12 h, T10: seed immersion in fresh cuy manure for 24 h, T11: seed immersion in decomposed cuy manure for 24 h, T12: seed immersion in water at 62 °C for 10 min and T13: seed immersion in water at 47 °C for 10 min.
Direct sowing in sand and immersion in water for 12 h in combination with sand showed the latest germination onsets with 14 and 13.7 days, respectively. This could be due to the poor water retention capacity of the sand, which prevented seeds from reaching adequate hydration to initiate germination (40-60% moisture, according to Montes [1998]); on the other hand, under the same conditions, germination in sawdust began on average 8.8 days earlier. The sawdust, by storing moisture inside its fibers and having porosity greater than 80 % (Maher, Prasad, & Raviv, 2008), provided a better balance between moisture and aeration for the seeds, accelerating their germination.
On the other hand, immersion in water at 62 °C was lethal to the embryo. Similar damage is reported with Centrosema rotundifolium seeds (Sanabria, Silva, Oliveros, & Barrios, 2001). However, Sánchez-Paz and Ramírez-Villalobos (2006) obtained improvements in germination by immersing Leucaena leucocephala seeds for 10 min in water at 80 °C, so this factor is thought to depend on the species (Probert, 2010).
Germination percentage
Highly significant statistical differences (P ≤ 0.01) were obtained due to the effect of substrates, pregerminative treatments and their interaction (Table 2). The substrate with the highest germination percentage was the sawdust, which surpassed the sand with 5.4 %. In sawdust, soaking the seeds for 24 h or in warm water improved germination by 15.3 % compared to the control, while in sand it was better by 66.67 % under the same conditions. On the other hand, the combinations that used manure, ash or manure plus ash improved the germination percentage by 21.69 and 92.46% in sawdust and sand, respectively. In this respect, ash is considered to have worked as an abrasive to thin the testa, to allow the action of microorganisms that degrade it and to facilitate imbibition, while cuy manure, thanks to its humic acids, provided nutrients and phytohormones that promoted physiological activities and stimulated seed activation (Hartwigsen & Evans, 2000; Restrepo-Rivera, 2007).
With seed soaking for 12 h and sowing in sand, germination was 60 % lower than that reported in other studies with Mexican physic nut seeds (Valdés-Rodríguez, Sánchez-Sánchez, & Pérez-Vázquez, 2013). This may be due to the type of sand used, which in this case was river sand, since Valdés-Rodríguez et al. (2013) used dune sand, which could be finer and retain more water, favoring the hydration of the seed.
Emergence percentage
Highly significant statistical differences (P ≤ 0.01) were obtained due to the effect of treatments, substrates and their interaction (Table 2). On average, emergence was 7.38 % higher in sawdust than in sand, since it was higher in T1 and T2 (Table 3). However, both substrates recorded similar emergence percentages in T3, T7 and T8 (Table 3). In the treatments where the lowest percentages were obtained, seed rot and radicle damage problems were observed.
According to these results, the sand, as compared to the sawdust, considerably limited germination and emergence in T1 and T2 (Table 3). This could be due to the fact that sand has a lower capacity to store moisture, so the hydration of the seeds was less efficient. However, by using ash or manure the testa was successfully penetrated and emergence in sand improved by 90 % in relation to direct sowing. On average, these treatments with both substrates increased emergence by 50 % in relation to T1 and 27 % in relation to soaking in water for 12 or 24 hours. Therefore, the use of organic materials such as manure or inorganic substances such as ash provided protection to the seeds due to the presence of phytohormones, minerals and beneficial microorganisms (Restrepo-Rivera, 2007).
Conclusions
Pregerminative treatments that involve soaking the seed in water for 24 hours and especially the use of cuy manure or rubbing with ash improve the germination of Jatropha curcas L. seeds and the emergence of the seedlings, by being able to soften their thick testa and facilitate the hydration of the seed and the embryo's respiration.
