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Highlights:
Moringa peregrina cultivation is limited by variable seed quality.
Seeds from the middle positions were the heaviest (0.73 g).
Germination was highest (>80 %) in seeds from the middle positions of the pod.
Seeds leached for 48 h reached 88 % germination.
Growth was higher in seedlings from seeds of the middle position, leached for 48 h.
Introduction
Moringa peregrina (Forssk.) Fiori belong to Moringaceae. It is medium sized (5 m to 15 m) and it is deciduous, perennial tree and its main root is thick. The leaves of M. peregrina are feathery, pale green, compound tripinnate, and 30 cm to 60 cm long with many small leaflets. Flowers are fragrant, creamy white to pale pinkish color, 2.5 cm in diameter, born in sprays, with five yellow stamens at the top of the flower. The fruits or pods of M. peregrina are pendulous ridged, brown and triangular. The pod splitting lengthwise into three parts containing about 20 to 25 trigonous seeds embedded in the pith.
M. peregrina is at risk of extinction and loss of its genetic resources in some regions (Hajebi, 2014). Overall, the International Union for Conservation of Nature's (IUCN) identifies the population patterns of the declining species (Oldfield, 2020). A similar trend is noted for Moringa borziana Mattei (Luke et al., 2018), Moringa hildebrandtii Engl. (Letsara et al., 2019), Moringa drouhardii Jum (Andriamanohera, 2019) and Moringa rivae Chiov (Musili et al., 2018).
To succeed in the development and breeding of moringa in harsh, arid and semi-arid habitats, it is necessary to develop appropriate propagation techniques. To accelerate germination and create high quality seedlings, chemical and mechanical pretreatments might be needed. Seed pretreatments might also be designed to increase seed tolerance to adverse environmental conditions, through germination to seedling growth (Nouman et al., 2012).
The main challenge in propagating moringa by seed is hardseededness. Moringa seeds germinate in 11 to 15 days after treatment in water for 24 h, i.e. a soaking treatment, with germination in control seeds being just as high (94 %) but taking longer (Padilla et al., 2012). Materechera (2017) applied several treatments to seeds of moringa and found that abrasion with sandpaper was the most effective, more so than sulfuric acid treatment for 2 min and soaking seeds in cold or warm water. These results suggest that moringa seeds have some form of physical dormancy that slows down the germination process. Even when the seed is committed to germinate, the efficiency of the process will depend on environmental conditions, particularly temperature. Hassanein and Al-Soqeer (2017) compared M. oleifera and M. peregrinanje and observed that high temperature led to an increase in germination speed but a decrease in germination percentage; and low temperature delayed germination. Moreover, the authors note that the response varies among species with M. oleifera and M. peregrina showing high germination totals under optimum temperatures, but M. oleifera showed the highest germination index (Hassanein & Al-Soqeer, 2017).
The outcome of treatments can be noticed after the initial germination phase per seed. For example, Njehoya et al. (2014) found that seedlings from M. oleifera seeds immersed in water for 36 h had higher stem length than that achieved with water immersion or scarification for 76 h. Similarly, when the water immersion pretreatment is extended to 48 h, germination may decrease (Padilla et al., 2012; Yerima et al., 2016). Therefore, excessive pretreatment of seeds can be detrimental to germination and seedling growth.
The experimental environment for evaluating germination and seedling growth is also important. Hassanein and Al-Soqeer (2017) indicate that the germination efficiency in greenhouse was different from that in the laboratory. On the other hand, not only the physiological condition of the seed can influence seedling performance, but also its physical condition that can be affected by the position in the pod. In M. oleifera, it has been observed that seeds in the middle part are heavier than those near the end of the pod, have the highest germination percentages (79.58 %) and the highest number of leaves (~18.44) (Bayé-Niwah & Mapongmetsem, 2014). Seed size also affects relative leaching rate, germination and vegetative characteristics (Padilla et al., 2012).
The lack of information on regenerative techniques, especially through seeds, is the main obstacle to the domestication of a species and its use as part of local agroforestry systems or plantations (Moupela et al., 2013). There is a clear need to better understand how seed physiology, physical characteristics and pretreatments affect seed quality in moringa species (Hajebi, 2014). In addition to water immersion, leaching can have effects on seed germination. Thus, in the present study on M. peregrina, the objective was to research the effect of seed position in the pod and leaching on germination and seedling growth.
Materials and methods
Study area
The habitat of M. peregrina is located in the Sahara-Sindi climate zone in the northwestern heights of Hormozgan province, near Bashagard (Sorheh heights) in Iran, at 26° 20' 42'' LN and 57° 42' 48'' LE. According to the Domarten climate classification (Keneshloo et al., 2014), the region has dry climate with hot summers and mild winters without glacial period, mean annual temperature of 24 °C to 27 °C, mean annual rainfall of 150 mm to 200 mm mainly in winter. The soils are alkaline, shallow to medium with light to medium texture. The main plant species are Periploca aphylla Decne., Artemisia aucheri Boiss, Ficus glomerata Roxb., M. peregrina, Amygdalus scoparia Spach. and Pistacia atlantica Desf. on the slopes and Vitex agnus-castus L. and Nannorrhops ritchiana within the waterways (Keneshloo et al., 2014).
Seedling production through seeds
After forest monitoring in Bashagard area (Moringa distribution point), 30 elite trees were identified for seed collection from three habitats of Sagan, Gafr and Parmon. Figure 1 shows the seeds of Moringa. Seeds pods were collected in mid-August 2018, packed in polyethylene bags (Bayé-Niwah & Mapongmetsem, 2014) and transferred to the laboratory. All pods of similar size were also grouped together.
The most frequent number of seeds per pods was six. These pods were used to determine the effect of seed position on germination and vegetative characteristics. The seeds were separated from 48 pods, the seeds numbered 1 to 6 (the position near the attached part to stem was No. 1) and weighed to ±0.01 mg (Bayé-Niwah & Mapongmetsem, 2014). Thus, 48 seeds from each pod position were used in the experiments.
To investigate the effect of leaching on germination and the vegetative characteristics, seeds were placed in a leaching instrument (Moslehi et al., 2020). After placing in cotton bags, 16 seeds from each position were exposed to running water for 0, 24 and 48 h (Padilla et al., 2012). The seeds were dried on a cotton cloth in the shade, a posteriori, sown for germination. The experiment was performed in a completely randomized design as a factorial arrangement.
In the standard germination test, 100 seeds of each treatment were placed in Petri dishes (9 cm diameter) with three replicates. Each plate included Whatman No. 1 filter paper, where 15 mL of sterile distilled water was added. The plates were transferred to an incubator at 25 °C. The test lasted 12 days and germination was recorded daily at the same time. This was necessary until day 8; thereafter, germination was slower (Agrawal, 2003). To determine germination quality, the following characteristics were evaluated: 1) germination percentage (GP) at the end of the trial; 2) mean daily germination (MDG) according to the equation of Hunter et al. (1984): MDG = GP / number of days to reach maximum germination; and 3) germination rate (GR), using Maguire's method (Maguire, 1962) which is based on the relationship ΣTG = Ni / Ti in which Ni is the number of seeds germinated per day and Ti is the number of days after sowing (1 to 16 days).
To evaluate seedling quality, seeds from each position were sown in pots (18 × 12 cm) at a depth of 1 to 2 cm with a mixture of light soil (crop soil with perlite) (3/4) and sand (1/4). Seeds were watered daily for the first three weeks and then once every other day. For each seed position, four replicates (4 group of 4 seeds) (n = 4) were considered. For each leaching treatment, 96 seeds were sown (six seed positions × n). In total, 288 sowings were made for seed position and leaching (six seed positions × three leaching treatments × n). Pots were maintained in a greenhouse and seedling emergence above the mixed soil was recorded every day for two weeks (Figure 2). After 45 days, vegetative characteristics all seedlings from each group were measured: collar diameter (mm), seedling length (cm), number of leaves, root length (cm), fresh and dry weight of leaves, roots and stems (g).
Statistical analysis
The experimental design was a completely randomized design (CRD) with three replications. The experimental design had a 2×2 factorial arrangement. Data were stored in Excel and analyzed with SPSS version 24. The effects of treatments on vegetative characteristics were examined with two-way analysis of variance. Multiple comparisons were performed using Duncan's multiple range tests (P ≤ 0.05). Normality of data was tested by Kolmogorov-Smirnov and Shapiro-Wilk tests.
Results
Statistical analysis showed a normal distribution pattern of the data (P ≥ 0.05) by Kolmogorov-Smirnov test for seed germination parameters (germination percentage, mean daily germination and germination rate). The results of the experiment with factorial design (treatment type and seed position) revealed significant differences in seed germination, mean daily germination and germination rate (P ≤ 0.05) (Tables 2, 3 and 4).
Seed position in the pod
Seed weight was significantly (P < 0.05) affected by its position in the pod. Seed weight ranged from 0.60 g in the first position to 0.73 g in the middle position. The seeds from position 1 had the lowest weight (Table 1). Seed position in the pod also had significant (P ≤ 0.05) effect on germination and overall germination rate (Tables 2 and 4), and mean daily germination (P ≥ 0.05; Table 3 and Figure 3B). The results of mean comparisons, using Duncan's test, showed that the highest germination rate occurred in seedlings derived from seeds with positions 4 and 5 (Figure 3A); while the highest germination rate was observed in seeds of position 3 (Figure 3C).
Figure 3 A) Germination percentage, average daily germination and germination rate according to seed position (position close to the part attached to the stem was number 1) and water treatments evaluated. Error bars denote standard deviation of the mean (n = 72); different letters indicate significant differences according to Duncan's test (P ≤ 0.05).
Table 1 Moringa peregrina seed weight according to the position in the pod. Seeds were numbered from 1 to 6; the position close to the part attached to the stem was number 1.
| Seed position | Seed weight (g) |
|---|---|
| P1 | 0.60 ± 0.13 b |
| P2 | 0.65 ± 0.14 ab |
| P3 | 0.73 ± 0.14 a |
| P4 | 0.72 ± 0.114 a |
| P5 | 0.72 ± 0.13 a |
| P6 | 0.68 ± 0.10 a |
| Total | 0.68 ± 0.14 |
± standard deviation of the mean (n = 48).
Table 2 Results of the experiment with factorial arrangement to determine the effect of seed position in the pod and leaching treatments on Moringa peregrina germination.
| Source | Type III Sum of Squares | Degrees of freedom | Mean Square | F | P-value |
|---|---|---|---|---|---|
| Leaching treatment | 546.71 | 2 | 273.35 | 88.41 | 0.000 |
| Pod | 682.99 | 5 | 136.59 | 44.18 | 0.000 |
| Leaching × pod | 2 197.02 | 10 | 219.7 | 71.05 | 0.000 |
| Error | 166.95 | 54 | 3.09 | ||
| Total | 523 227.18 | 72 |
Table 3 Results of a factorial design experiment to determine the effect of seed position in the pod and leaching treatments on the mean of daily germination of Moringa peregrina.
| Source | Type III Sum of Squares | Degrees of freedom | Mean Square | F | P-value |
|---|---|---|---|---|---|
| Leaching treatment | 84.25 | 2 | 42.12 | 136.84 | 0.000 |
| Pod | 6.33 | 5 | 1.26 | 4.11 | 0.003 |
| Leaching × pod | 29.78 | 10 | 2.97 | 9.67 | 0.000 |
| Error | 16.62 | 54 | 0.31 | ||
| Total | 7 107.91 | 72 |
Table 4 Results of the experiment with factorial arrangement to determine the effect of seed position in the pod and leaching treatments on the germination rate of Moringa peregrina.
| Source | Type III Sum of Squares | Degrees of freedom | Mean Square | F | P-value |
|---|---|---|---|---|---|
| Leaching treatment | 0.32 | 2 | 0.16 | 9.73 | 0.000 |
| Pod | 1.23 | 5 | 0.24 | 14.97 | 0.000 |
| Leaching × pod | 0.75 | 5 | 0.07 | 4.55 | 0.000 |
| Error | 0.88 | 54 | 0.016 | ||
| Total | 95.06 | 72 |
Seed position in the pod also had a significant effect on vegetative characteristics, except for leaf number and root fresh weight. For most of the parameters measured, seeds positioned in the center of the pod produced the best seedlings (Table 5). Seeds from position 3, 4, and 5 had seedlings with significantly higher fresh (0.46 to 0.49 g) and dry (0.08 g) leaf weights. Seedlings from position 4 seeds had better neck diameter (3.51 mm) and stem length (19.48 cm) growth, while seeds from position 5 had greater length (10.55 cm) and root dry weight (0.46 g). As with other features, the highest stem weights corresponded to seedlings grown from seeds of middle positions 3 and 5; i.e., 0.62 and 0.56 g (dry weights) and 0.99 and 0.88 g (fresh weights), respectively.
Table 5 The effect of seed position in pod and leaching treatment on all of vegetative characteristics of Moringa peregrina.
| Treatment | Leaf | Stem | Root | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Leaf number | Leaf fresh weight (g) | Leaf dry weight (g) | Collar diameter (mm) | Stem length (cm) | Stem dry weight (g) | Stem fresh weight (g) | Root length (cm) | Root fresh weight (g) | Root dry weight (g) | ||
| Seed position | 1 | 13.47 ± 0.83 a | 0.39 ± 0.03 b | 0.07 ± 0.00 ab | 3.13 ± 0.07 b | 13.05 ± 0.37 d | 0.06 ± 0.00 b | 0.51 ± 0.03 b | 7.96 ± 0.27 c | 2.77 ± 0.16 a | 0.38 ± 0.23 bc |
| 2 | 12.50 ± 0.79 a | 0.40 ± 0.03 b | 0.07 ± 0.00 ab | 3.14 ± 0.11 b | 13.03 ± 0.54 d | 0.07 ± 0.00 b | 0.51 ± 0.03 b | 9.31 ± 0.24 b | 2.88 ± 0.16 a | 0.43 ± 0.24 ab | |
| 3 | 11.87 ± 0.67 a | 0.46 ± 0.23 ab | 0.08 ± 0.00 a | 3.25 ± 0.10 ab | 14.57 ± 0.51 c | 0.09 ± 0.00 a | 0.62 ± 0.02 a | 9.35 ± 0.4 b | 2.98 ± 0.19 a | 0.44 ± 0.24 ab | |
| 4 | 14.03 ± 1.00 a | 0.42 ± 0.03 ab | 0.08 ± 0.00 ab | 3.51± 0.08 a | 19.48 ± 0.47 a | 0.07 ± 0.00 b | 0.55 ± 0.03 b | 7.86 ± 0.29 c | 2.55 ± 0.13 a | 0.34 ± 0.23 c | |
| 5 | 13.60 ± 1.50 a | 0.49 ± 0.05 a | 0.08 ± 0.01 a | 3.34 ± 0.12 ab | 17.01 ± 0.42 b | 0.08 ± 0.00 a | 0.56 ± 0.02 ab | 10.55 ± 0.64 a | 3.02 ± 0.27 a | 0.46 ± 0.04 a | |
| 6 | 11.70 ± 0.89 a | 0.40 ± 0.04 b | 0.06 ± 0.00 b | 3.14 ± 0.08 b | 13.73 ± 0.28 cd | 0.06 ± 0.00 b | 0.54 ± 0.03 b | 7.76 ± 0.26 c | 2.78 ± 0.20 a | 0.40 ± 0.02 ab | |
| Leaching treatment | Control | 10.95 ± 0.49 b | 0.34 ± 0.01 b | 0.06 ± 0.00 b | 3.13 ± 0.06 b | 14.85 ± 0.43 a | 0.06 ± 0.00 b | 0.49 ± 0.01 b | 7.69 ± 0.24 c | 2.36 ± 0.08 b | 0.35 ± 0.01 b |
| 24 h | 10.81± 0.54 b | 0.37 ± 0.01 b | 0.06 ± 0.00 b | 3.13± 0.07 b | 14.84 ± 0.47 a | 0.07 ± 0.00 b | 0.57 ± 0.01 a | 8.61 ± 0.22 b | 2.53 ± 0.12 b | 0.37 ± 0.01 b | |
| 48 h | 16.82 ± 0.73 a | 0.57 ± 0.02 a | 0.10 ± 0.00 a | 3.50 ± 0.06 a | 15.74 ± 0.38 a | 0.08± 0.00 a | 0.58 ± 0.02 a | 10.10 ± 0.32 a | 3.60 ± 0.13 a | 0.50 ± 0.02 a | |
Seeds were numbered from 1 to 6; the position close to the part attached to the stem was number 1. In each treatment factor, values in the same column with different letters are significantly different according to Duncan's multiple range tests (P ≤ 0.05). ± standard error of the mean (n = 72)
Effects of seed leaching
For all the measured characteristics, seed leaching with running water provided better conditions for seed germination. In particular, the highest germination level (i. e., 88.1 %) was observed when seeds had been subjected to 48 h of leaching in water (Figure 3D). The average daily germination was highest (10.8) for seeds given 48 h leaching and was similar to that found in 24 h leached seeds (10.4), but significantly greater that the control (Figure 3E). The highest germination rate (1.22) was observed in 48 h leached seeds (Figure 3F). Also, the number of leaves per seedling (16.82) was significantly higher than that observed with the 24-hour treated seeds and the control (10.8 and 11.0, respectively). Collar diameter (3.50 mm) and stem dry weight (0.08 g), fresh (0.57 g) and dry weight of leaves (0.11 g), and fresh (3.60 g), dry (0.5 g) and length (10.10 cm) of roots of seedlings from seeds subjected to leaching for 48 h were significantly higher than those of the other treatments (Table 5). Finally, stem fresh weight (0.58 g) in the 48 h treatment was in the same group with the 24 h treatment (0.57 g) and had statistically significant difference compared to the control treatment (0.49 g).
In general, the analysis of variance showed that all characteristics measured after leaching treatments (0, 24 h and 48 h) with tap water were significantly different from the control treatments, with the exception of stem length (between 14.9 and 15.5 cm).
The main effects of pod position and leaching treatments (P ≤ 0.05) are tabulated in Table 5 and their interactions are shown in Table 6, where it is observed that these were significant. The interaction was not significant for collar diameter and stem length. The results showed that the 48 h treatment on mid-position seeds caused positive effects on parameters.
Table 6 Effects of seed position × leaching treatment interaction on vegetative characteristics of Moringa peregrina.
| Treatment | Variable | |||||||
|---|---|---|---|---|---|---|---|---|
| Leaching treatment | Seed position | Root length (cm) | Leaf fresh weight (g) | Leaf dry weight (g) | Stem fresh weight (g) | Stem dry weight (g) | Root fresh weight (g) | Root dry weight (g) |
| Control | 1 | 7.41 ± 0.20 b | 0.29 ± 0.03 a | 0.057 ± 0.01 a | 0.47 ± 0.02 ab | 0.05 ± 0.01 cd | 2.24 ± 0.25 a | 0.29 ± 0.03 bc |
| 2 | 9.48 ± 0.42 a | 0.35 ± 0.03 a | 0.06 ± 0.01 a | 0.48 ± 0.03 ab | 0.06 ± 0.01 bcd | 2.67 ± 0.24 a | 0.41 ± 0.03 a | |
| 3 | 7.34 ± 0.68 b | 0.40 ± 0.04 a | 0.08 ± 0.01 a | 0.54 ± 0.02 a | 0.08 ± 0.01 a | 2.34 ± 0.17 a | 0.38 ± 0.03 ab | |
| 4 | 6.96 ± 0.64 b | 0.34 ± 0.04 a | 0.07 ± 0.01 a | 0.57 ± 0.05 a | 0.07 ± 0.01 ab | 2.36 ± 0.21 a | 0.27 ± 0.03 c | |
| 5 | 7.92 ± 0.81 ab | 0.32 ± 0.05 a | 0.052 ± 0.01 a | 0.51 ± 0.03 a | 0.07 ± 0.01 abc | 2.29 ± 0.20 a | 0.38 ± 0.03 ab | |
| 6 | 7.02 ± 0.39 b | 0.31 ± 0.03 a | 0.05 ± 0.01 a | 0.40 ± 0.03 b | 0.05± 0.01 d | 2.24 ± 0.18 a | 0.36 ± 0.03 ab | |
| 24 h | 1 | 7.18 ± 0.42 c | 0.42 ± 0.03 ab | 0.08 ± 0.01 a | 0.56 ± 0.06 b | 0.06 ± 0.01 b | 2.96 ± 0.24 a | 0.42 ± 0.03 a |
| 2 | 8.80 ± 0.31 ab | 0.35 ± 0.03 bc | 0.07 ± 0.01 ab | 0.49 ± 0.06 b | 0.06 ± 0.01 b | 2.70 ± 0.31 a | 0.38 ± 0.05 a | |
| 3 | 10.09 ± 0.59 a | 0.50 ± 0.04 a | 0.09 ± 0.01 a | 0.73 ± 0.02 a | 0.09± 0.00 a | 2.89 ± 0.3 a | 0.43 ± 0.04 a | |
| 4 | 8.50 ± 0.46 bc | 0.38 ± 0.05 ab | 0.07 ± 0.01 ab | 0.52 ± 0.03 b | 0.05 ± 0.00 b | 2.48 ± 0.31 a | 0.36 ± 0.05 a | |
| 5 | 9.14 ± 0.65 ab | 0.33 ± 0.05 bc | 0.05 ± 0.01 bc | 0.62 ± 0.04 ab | 0.10 ± 0.00 a | 2.05 ± 0.39 a | 0.29 ± 0.05 a | |
| 6 | 7.95 ± 0.44 bc | 0.24 ± 0.04 c | 0.04 ± 0.01 c | 0.54 ± 0.03 b | 0.06 ± 0.00 b | 2.10 ± 0.23 a | 0.34 ± 0.04 a | |
| 48 h | 1 | 9.29 ± 0.48 cd | 0.47 ± 0.05 b | 0.09 ± 0.01 b | 0.50 ± 0.04 a | 0.06 ± 0.01 b | 3.12 ± 0.32 cd | 0.41 ± 0.05 b |
| 2 | 9.63 ± 0.51 bc | 0.49 ± 0.05 b | 0.10 ± 0.01 b | 0.58 ± 0.04 a | 0.08 ± 0.01 b | 3.26 ± 0.28 bcd | 0.49 ± 0.04 b | |
| 3 | 10.61 ± 0.29 b | 0.48 ± 0.04 b | 0.09 ± 0.01 b | 0.60 ± 0.04 a | 0.10 ± 0.01 a | 3.70 ± 0.36 bc | 0.50 ± 0.05 b | |
| 4 | 8.13 ± 0.28 d | 0.54± 0.06 b | 0.11 ± 0.01 b | 0.55 ± 0.07 a | 0.08 ± 0.01 b | 2.80 ± 0.17 d | 0.39 ± 0.04 b | |
| 5 | 14.60 ± 0.34 a | 0.81 ± 0.08 a | 0.15 ± 0.01 a | 0.56 ± 0.05 a | 0.08 ± 0.01 b | 4.73 ± 0.19 a | 0.71 ± 0.06 a | |
| 6 | 8.32 ± 0.48 d | 0.64 ± 0.04 b | 0.10 ± 0.01 b | 0.67 ± 0.04 a | 0.08 ± 0.01 b | 4.00 ± 0.29 ab | 0.51 ± 0.04 b | |
Seeds were numbered from 1 to 6; the position close to the part attached to the stem was number 1. Values in the same column with different letters are significantly different according to Duncan's multiple range tests (P ≤ 0.05). ± standard error of the mean (n = 72)
Discussion
Moringa is a drought-resistant tree. The drought resistance characteristics of this tree, seed and seedlings enable the species to grow in arid and semi-arid areas where water shortage is one of the main problems (Mirdha, 2015). Consequently, moringa has attracted international interest in its economic potential for drylands (Mirdha, 2015). To realize the species’ potential, it is important to be able to select trees that produce high quality seeds (Bayé-Niwah & Mapongmetsem, 2014) and to germinate the seeds efficiently and effectively (Eghobor et al., 2015; Materechera, 2017; Padilla et al., 2012).
The effect of leaching on germination and vegetative growth
Seed germination begins with water uptake that activates metabolic processes, leading to the emergence of radicles as the first visual signs. Water flow in a seed plays a key role in the efficiency of germination and, therefore, in the rate of seedling emergence. The present work demonstrates that immersion of moringa seeds in water is one of the factors that improves germination (Table 2-4; Figure 3), presumably, by initiating physiological changes in the embryo; furthermore, leaching for 48 h not only increased germination, but also improved mean daily germination and germination rate. Seed leaching also resulted in increased vegetative growth, which contradicts the results of other studies (Padilla, 2012); for example, Yerima et al. (2016) found that germination, number of leaves, height and diameter of the seedling collar of M. oleifera in control seeds (without immersion) were 68.7 %, 3.9, 6.5 cm and 0.5 cm, respectively, being higher than in seeds immersed in water for 4, 8 and 12 days.
Increasing the hydration level promotes enzyme activation of carbohydrate metabolism (Gupta & Kaur, 2002) as well as liberates ATP (adenosine triphosphate) for root protrusion and consequently seed germination (Lobato et al., 2009). Pre-soaking seeds might also increase the vigor of the seeds (priming-like response) enable the seeds to germinate better under stress conditions (Kaur et al., 2002). The need to soak the seed before sowing depends on the level of hard seededness in the seed lot and this can vary between species, as with M. oleifera and Leucaena leucocephala, which means that the need for intervention may not always be necessary before planting (Medina et al., 2007). In the case of species in the genus Moringa, the findings are equivocal with some studies noting the importance of soaking seeds before sowing (Eghobor et al., 2015) and others concluding that this is not needed to obtain good, high-quality germination (Hajebi, 2014). In some species, soaking the seeds in water is also needed for washing out inhibitors (Pardos, 2004), but we do not think this was a feature of the soaking response in M. peregrina. Despite of positive effect of leaching on germination, some of the differences in findings regarding the effects of pre-sowing soaking of seeds could relate to the growing conditions used in the experiments. In this regard, Padilla et al. (2012) and Medina et al. (2007) observed few evident benefits of seed soaking, which could be due to the fact that germination occurred under high irrigation conditions. In fact, for seeds of some tree species, excess soil moisture can decrease germination and root growth (Pardos, 2004)
Effect of seed position in the pod on germination and vegetative growth
Seed position of M. peregrina has a profound effect on weight, with the middle seeds being significantly superior to seeds 1 and 6 at the pod ends. These results are consistent with those of Bayé-Niwah and Mapongmetsem (2014).
There are several hypotheses to explain the variation in seed mass among seeds in the same pod. The trophic hypothesis states that there is competition among seeds for nutrients during fruiting and that those in the middle zone of the pod are favored. As an alternative, the structural hypothesis proposes that seeds in the middle zone have more room to grow than those in the distal and proximal poles (Mapongmetsem et al., 2004). The present study did not set out to resolve which of the two hypotheses is more likely; in fact, both could operate in parallel. Nevertheless, it can be concluded that seeds in the middle part of the pod are heavier than those at the ends (Mapongmetsem et al., 2004); moreover, larger seeds germinate quickly and produce stronger seedlings with higher biomass. In the case of average stem length of middle seeds, the observed response is close to that reported in this species by Bayé-Niwah and Mapongmetsem (2014). The efficiency of germination and seedling growth is important for the species to grow fast enough to outgrow competing plants in the same ecological niche in grassland ecosystems (Padilla et al., 2012).
Conclusions
Moringa peregrina seeds from the middle part of the pod are heavier and show better results in germination (percentage and total) and vegetative characteristics of seedlings (number of leaves and most biomass parameters). Overall germination performance and seedling growth can be improved by immersing seeds from the middle position of the pod in leaching treatment for 48 h. Therefore, to increase the production of good quality seedlings in nurseries or forest plots, it is recommended to select seeds by size and pre-treat them in water before planting them in pots or directly into the landscape.
