Effect of gamma rays on germination.
Irradiated seeds germinated on day 12 after sowing at the same time as non-irradiated seeds. This can be observed in the graph (Fig. 3). As the days passed, the doses of 0.5, 1.5, and 3.0 Gy presented a higher percentage of germinated seeds. In all the doses applied (0.5, 1.5, 3.0, 5.0, and 7.5 Gy), a steep curve was maintained and stabilized until 21, 20, 19, 21, and 20 days, respectively, resulting in the final germination percentage. The greatest increase in the percentage of germinated seeds was obtained with the 1.5 Gy dose from day 16 to 17, increasing from 19.23% to 34.05%.
Germination capacity and germination energy are the variables that define seed quality for plant production (Bonner et al. 1994; Trujillo, 1996). Kolotelo et al. 2001). The faster and more uniform the germination rate (Kolotelo et al. 2001); the higher the germination capacity and the greater the uniformity of plant production (Bonner et al. 1994; Trujillo, 1996). Therefore, the effect of different doses of the linear accelerator on germination capacity (GC), germination energy (GE), peak value (PV), and germination value (GV) of seeds can be observed in Table 1. The 0.5 Gy dose presented the highest percentage of germination capacity (50%), and as the irradiation dose increased, the germination capacity decreased. However, no dose presented a lower percentage than non-irradiated seeds. The germination energy (50%) was lowest at the 3.0 Gy dose (12.8 days), followed by the 0.5 Gy dose (13.2 days), and the highest peak value was 2.38 % on day 1. The highest germination values were observed with the 0.5 and 3.0 Gy doses (4.76 and 4.75, respectively).
Table 1 Average values of the means of the effect of different doses of irradiation on seed germination parameters of P. pseudostrobus
Dose
(Gy)
|
Germination Capacity (%)
|
Germination Energy (EG) (50%)
|
Maximum value (% day1)
|
Germination value (% day1)
|
0
|
38.46a
|
15.00e
|
2.13ab
|
3.28a
|
0.5
|
50.03e
|
13.25b
|
2.38cd
|
4.76e
|
1.5
|
46.46f
|
16.5d
|
2.32c
|
4.31d
|
3
|
47.50d
|
12.8a
|
2.5d
|
4.75e
|
5
|
45.15c
|
14.5c
|
2.15b
|
3.88c
|
7.5
|
42.31b
|
13.2b
|
2.11a
|
3.57b
|
Standard Error ±
|
1.8
|
0.5
|
0.32
|
0.65
|
Values with different letters in the column are statistically different. Tukey's (P ≤ 0.05).
When these results were compared to those obtained by Rangel et al. (2017), lower data was obtained in terms of germination capacity for P. pseudostrobus, but this could be attributed to the germination method, which is carried out in a germination chamber with controlled environmental conditions. On the other hand, this same author mentions that it is common in the propagation practices of different Pinus species to resort to direct sowing in soil with the objective of ensuring higher survival percentages, since not having to manipulate the seedlings to establish them in substrate once they have already germinated avoids damaging the radicle during this procedure (Rangel et al. 2017).
Seedling growth
Regarding growth, in the height variable, it was observed that during the first 3 months of evaluation there were no significant differences between treatments (Tukey; P ≤ 0.05). However, in the fourth month, a greater increase was noted in seedlings of doses of 0.5 and 3.0 Gy for both diameter and height. As the dose increases, this effect on growth decreases (Fig. 4 and Fig. 5).
The percentage of seedling survival was evaluated for each dose used, and it was found that seedlings of 0.5, 1.5, 3.0, 5.0, and 7.0 Gy had no more than 10% mortality, while seedlings from non-irradiated seeds had a 15% mortality rate (Table 2).
According to Mexal and Landis (1990), plant height is a good predictor of future height in the field, although not for survival; on the other hand, Romero-Arenas et al. (2019) mention that the diameter of a forest seedling is perhaps the most important variable to evaluate since it is directly related to the survival capacity of the plant and defines the robustness of the stem and is therefore associated with the vigor and survival of the plantation. In the last month of evaluation, values with statistically significant differences were obtained with respect to the non-irradiated treatment. The 0.5 Gy dose showed greater height and diameter (26.4 cm and 4.63 mm) (Table 2). The results obtained for seedling height and diameter are higher than those referred to by Rangel et al. (2017), who at six months reported mean height values of 14.90 cm and a diameter of 2.10 mm for P. pseudostrobus. It has been indicated (Fonseca et al. 2012) that low-dose radiation applications lead to the formation of free radicals, ions, and molecules that contribute to greater efficiency in biochemical-metabolic pathways, which is reflected in improved plant growth and development.
For root length, it was found that the 1.5 Gy dose presented a greater length (16.20 cm) than the 0 Gy dose. Córdoba-Rodríguez et al. (2011) mentioned that an important characteristic for the successful establishment and survival of plants is the growth and development of the root since the absorption of water and essential nutrients for various physiological processes depends to a great extent on it.
Table 2 Survival and growth variables of P. pseudostrobus seedlings irradiated with linear accelerator
Dose
|
Total average
height (cm)
|
Stem
diameter (mm)
|
Root length (cm)
|
Survival
(%)
|
0 Gy
|
22.515 ± 1.2a
|
3.97 ± 0.63a
|
13.00± 0.71a
|
85
|
0.5 Gy
|
26.442 ± 1.2b
|
4.63 ± 0.63c
|
15.66 ± 0.71c
|
95
|
1.5 Gy
|
26.262 ± 1.2b
|
4.53± 0.63bc
|
16.20± 0.71c
|
90
|
3.0 Gy
|
26.073 ± 1.2b
|
4.38± 0.63abc
|
14.26± 0.71abc
|
90
|
5.0 Gy
|
24.27 ± 1.2ab
|
4.37 ± 0.63abc
|
13.56 ± 0.71ab
|
95
|
7.5 Gy
|
24.13 ± 1.2ab
|
4.00 ± 0.63ab
|
15.33 ± 0.71bc
|
90
|
Values with different letter in the column are statistically different Tukey (P≤ 0.05).
Effect of linear accelerator on plant quality.
In the present work, Tukey's test showed significant differences (P ≤ 0.05) for root height/length ratio (RAR) and aerial biomass/root biomass ratio (RAS), obtaining the highest average for 3.0 Gy plants of 1.82 and 5.65, respectively. According to Rodriguez-Ortiz et al. (2020), values < 2 in the RAR refer to high seedling quality. This ratio will help to improve seedling survival.
The Slenderness Index (SI) is an indicator of plant resistance to wind desiccation, survival, and growth in dry sites (Rodríguez-Ortiz et al. 2020). For example, values equal to or less than 6 were found, which indicates that the seedlings are of high quality; however, higher values indicate that the plant has a thin stem in relation to its height (Prieto et al. 2009). The dose that presents the thinnest stem in relation to height is 7.0 Gy. These results are like those found by Rodriguez-Ortiz et al. (2020) values in a range of 5.5 to 6.1 for P. pseudostrobus.
The Dickson quality index (DI) and RAR are indicators that predict the success of the plantation (Ortiz et al. 2021). The doses that presented a higher ID were those of 0.5 and 1.5 Gy (0.24 and 0.25). These values refer to the medium quality of the seedlings. However, the doses of 0, 3.0, 5.0, and 7.5 Gy presented values lower than 2. Therefore, it is considered of poor quality according to the classification for Pinus made by Rodriguez-Ortiz et al. (2020). Prieto et al. (2009) have indicated that there must be a certain balance between the aerial part and the root system of the seedlings for them to survive. The lignification index showed lower values (18.54%) in doses of 5.0 Gy than in the rest of the seedlings, and the dose of 1.5 Gy presented the highest value (28.66%) (Table 3).
Table 3 Quality indices of P. pseudostrobus seedlings from seeds irradiated with different doses in a linear accelerator
Doses
|
RAS
|
RAR
|
Slenderness Index (SI)
|
Dickson Index (DI)
|
Index of Lignification (IL%)
|
0 Gy
|
5.45c
|
1.73bcd
|
5.67ab
|
0.14a
|
28.3e
|
0.5 Gy
|
4.3bc
|
1.68abc
|
5.71ab
|
0.24c
|
25.86c
|
1.5 Gy
|
3.47a
|
1.62ab
|
5.8bc
|
0.25c
|
28.66e
|
3.0 Gy
|
5.65 cd
|
1.82d
|
5.95bc
|
0.15ab
|
27.77de
|
5.0 Gy
|
4.7bc
|
1.78cd
|
5.55a
|
0.16ab
|
18.54a
|
7.5 Gy
|
3.88ab
|
1.57a
|
6.03c
|
0.15ab
|
22.09b
|
Standard
Error ±
|
0.67
|
0.06
|
0.36
|
0.01
|
1.3
|
RAS = aboveground biomass to belowground biomass ratio; RAR = height to root ratio; values in the column with different letters are statistically different. Tukey's (P ≤ 0.05).
Photosynthetic pigment content
Statistically significant differences (P≤0.05) were found between the different linear accelerator doses. At the 3.0 and 5.0 Gy doses, the lowest values were obtained for the variables evaluated (chlorophyll a, chlorophyll b, chlorophyll a + b and carotens). On the other hand, the 1.5 Gy dose did not present significant differences (P≤0.05) with respect to seedlings from unirradiated seeds in all the variables evaluated (chlorophyll a, chlorophyll b, chlorophyll a + b, and carotens) (Fig. 6).