3.2. Evaluation of the Growth-Stimulating and Growth-Inhibitory Activities of the SI Derivatives
A study of the growth-promoting and growth-inhibitory effects of the SI derivatives
1–
9 on the germination of wheat seeds was carried out using five concentrations of the test compounds (10
−4 to 10
−8 mol/L). No significant changes in the germination of wheat seeds were noted in the experiments. However, the notable effects of some compounds on the growth of axial organs of wheat seedlings (cv. Mironovskaya 808) were found (
Table 1). Compounds
1,
3, and
5 showed growth-inhibitory activity, whereas compounds
4,
6, and
8 exhibited growth-promoting effects.
A more detailed analysis of the concentration dependence of the growth-inhibitory activity of SI derivatives showed the following regularities. Compound 1 at all concentrations decreased root growth by 17.0–30.8%. However, this compound had a weaker effect on the shoot growth, with notable changes in growth (13.8–25.1%) being found only at concentrations of 10−8, 10−7, and 10−4 mol/L. Compound 3 inhibited the root and shoot growth at all concentrations, with these effects being very similar and more pronounced for roots (about 30%) than for shoots (about 20%) at all concentrations, excepting the lowest one, where the effects were found to be reversed quantitively. Compound 5 was found to be similar to compound 1 in growth-inhibiting activity; however, the root growth was inhibited by compound 5 at all concentrations by 17.8–32.6%, while the shoot growth was inhibited only at concentrations of 10−4 and 10−6 mol/L by 19.6% and 18.9%, respectively.
An evaluation of the dependence of the growth-promoting effect of SIs on their concentrations showed that compound 4 at any concentration stimulated the growth of roots and shoots by 23.7% to 38.4% and 14.0 to 38.0%, respectively. Compound 6 had a stimulating effect (13.6%) on root growth only at a concentration of 10−6 mol/L, whereas it promoted shoot growth at all concentrations by 10.8–54.3%. Compound 8 stimulated root growth (10.1–26.6%) at all concentrations, while promoted shoot growth (up to 16.7%) only at 10−6 to 10−8 mol/L concentrations.
Three other SI derivatives showed ambiguous effects on the growth of the axial organs of wheat. Compound 2 stimulated root growth at concentrations ranging from 10−8 to 10−5 mol/L, with a maximum (20.5% above the control) at 10−6 mol/L. However, this compound slightly inhibited (14.4%) shoot growth at a concentration of 10−5 mol/L; at lower concentrations, compound 2 showed only a growth-inhibition trend. Compound 7 behaved similarly, promoting root growth (up to 24.9%) at concentrations of 10−5 to 10−8 mol/L; however, it inhibited shoot growth at concentrations of 10−6 and 10−7 mol/L (11.7–13.4%). Finally, compound 9 showed a tendency to stimulate root growth (the differences with the control were not significant at p = 0.05) and to inhibit shoot growth (9.9% at the concentration of 10−6 mol/L).
It was interesting to evaluate the effect of the SI derivatives on the shoot-to-root length ratio (SRR). In most cases, the wheat seedlings (cv. Mironovskaya 808) showed an increase in SRR value under treatment with compounds 1 (except for a concentration of 10−8 mol/L), 3 (except for a concentration of 10−8 mol/L), 5 (except for a concentration of 10−4 mol/L), and 6. On the contrary, a noticeable decrease in the SRR index was noted for seedlings grown from seeds treated with compounds 2, 4, 7, 8, and 9. Thus, the SI derivatives inhibiting the growth of the axial organs of wheat (cv. Mironovskaya 808) caused a notable increase in the SRR values. On the contrary, the growth-promoting compounds decreased this index.
Since the response of plants to the action of PGRs can differ not only among species but also among varieties of crop plants, we conducted similar experiments with wheat seeds cv. Moskovskaya 39. In addition, a different concentration range of the SI solutions (10
−9–10
−6 mol/L) was chosen for the treatment of the seeds. Compared with the results obtained with cv. Mironovskaya 808, both similarities and differences were revealed in the growth of the axial organs of wheat (
Table 2). Specifically, compounds
4 and
8 were found to promote both the root and shoot growth of wheat cv. Moskovskaya 39. A promotion of the root but not shoot growth was noted for compounds
2,
7, and
9. On the contrary, compound
6 promoted shoot but not root growth. Lastly, compounds
1,
3, and
5 were disclosed to inhibit both root and shoot growth of cv. Moskovskaya 39.
To summarize, a unidirectional effect on both cultivars of wheat was found for compounds 4 and 8 (positive) and 1, 3, and 5 (negative). For the rest of the SI derivatives, the effect was found to be mixed (both positive and negative). As for the SRR index, a decrease in the SRR values for cv. Moskovskaya 39 was discovered in most of the studied concentrations of SI derivatives, excepting compounds 5 and 6. Some of the studied compounds showed an ambiguous effect on the SRR index, including an increase in the SRR values in some variants of the experiment.
The concentration range of the SI solutions of 10
−9–10
−6 mol/L was chosen for the treatment of maize seeds. Again, the diverse effects of SI derivatives on the growth of axial organs of maize seedlings (hybrid ROSS 199 MV) were discovered (
Table 3). Growth inhibition was found for compounds
3,
6, and
9. Compound
3 most strongly inhibited root (by 25.6–38%) and shoot growth (by 17.9–32.1%, except for a concentration of 10
−8 mol/L). Compound
6 inhibited root growth by 18.8–23.0% (except for a concentration of 10
−8 mol/L) and shoot growth by 14.1–26.0% (except for a concentration of 10
−9 mol/L).
SI derivatives 2, 4, 7, and 8 exhibited growth-promoting activity. All compounds showed a pronounced activation of root growth (2: 10.0–29.0%; 3: 15.0–37.1%; 7: 37.6–67.0%; 8: 21.2–74.6% (except for a concentration of 10−6 mol/L)). However, their effects on shoot-growth activation were weaker and did not manifest at every concentration. Thus, compounds 2, 4, and 8 only significantly increased the shoot length at two concentrations, with the maximum growth enhancement being as high as 20.2, 32.7, and 31.0% of the control, respectively. Only compound 7 exhibited growth-stimulating activity at all concentrations, revealing no pronounced maximum. The remaining two compounds, 1 and 5, either showed no growth stimulation of the axial organs or this activity was ambiguous with respect to the root and shoot growth.
As a rule, the treatment of maize seeds (ROSS 199 MB) with the SI derivatives resulted in a decrease in the SRR values. Interestingly, in the cases of an increase in the RSS index, this phenomenon took place mainly at the lowest (compounds 1–5) or, conversely, the highest concentrations of the corresponding SIs (compounds 8 and 9).
The experiments with maize hybrid Voronezhskij 158 SV showed only a poor effect of SI derivatives on plant growth (
Table 4). In the majority of variants of the experiment, the difference with the water control was unreliable. A stimulating effect on the growth of axial organs was noted for the SI derivative
2 and, to a lesser extent, for compounds
5 (roots) and
4,
7, and
8 (shoots). The inhibitory effect on the growth of both roots and shoots was shown for compound
3. Compounds
1,
6, and
9 inhibited root growth, but the similar effect on shoots was poorly resolved.
With maize (hybrid Voronezhskij 158 SV), an influence of the SI derivatives on the SRR index was found to be ambiguous. A notable decrease in the index value was triggered by compounds 1 (excepting a concentration of 10−9 mol/L), 3, 5, and 9. The treatment of the seeds with compound 2 (excepting a concentration of 10−9 mol/L) resulted in an increase in the SRR value. Finally, compounds 4, 6, 7, and 8 showed positive or negative effects on the SRR index depending on the concentration.
Taking into account the results described above, we selected some SI derivatives (compounds 4, 5, 7, and 8) to test their effect on plant growth for a longer time period. The seeds treated with the selected compounds were planted in pots filled with soil and the germinants were cultivated for 3 weeks.
The SI derivatives
4,
5,
7, and
8 were found to differently effect the growth of wheat cv. Mironovskaya 808 (
Table 5). The length of the aerial part of the plants significantly increased when treated with any of these compounds, whereas only compounds
7 and
8 promoted the growth of roots. Compound
7 appeared to be the only SI derivative to increase the leaf area compared with the control; the remaining compounds only exhibited a tendency to behave similarly. The raw weight of the plants (the sum of the roots and aerial parts) differed significantly from the control after pre-sowing treatment with all compounds, excepting for a high concentration of compound
8. Among them, compounds
6 and
7 promoted the growth of roots and compounds
4,
6, and
8 promoted the same with the shoots. There were less significant differences in the dry weights compared with the control. The maximum dry weight of the roots was found for compounds
6 and
7. The maximum dry weight of the shoots, as well as the total dry weight of the plants, was obtained after treatment with compounds
4,
6, and
7. There were no significant differences in the dry weights between the variants of the experiment.
Slightly different results were obtained from the pre-sowing treatment of maize seeds with the same SI derivatives. With the ROSS 199 MB hybrid (
Table 6), most of the SIs, excepting the lowest concentration of compound
8, stimulated an elongation of the roots. However, the growth of the aerial part of the plants was significantly promoted only on the treatment of the seeds with compounds
4,
6, and
8 (at low concentration). Compounds
4,
6, and
7 enlarged the leaf area. All SI derivatives contributed to the increase in the root fresh weight, but only some compounds contributed to that of the shoots. The maximum increase in the fresh weight of plants was found for compound
7. All SIs increased the dry weight of the roots and some performed the same with the aerial part of the plants. As a result, the dry weight of plants increased relative to the water control of the treatment with any of the studied SIs.
The treatment of maize seeds hybrid Voronezhskij 158 SV with the SI derivatives caused diverse effects on different growth parameters (
Table 7). All compounds exhibited a tendency to increase the growth of both the roots and the aerial part of the plants, however the differences with the water control were not always significant. The maximal root length was obtained after treatment of the seeds with compound 8 at a concentration of 10
−7 mol/L, while the maximum value of the length of an aerial part was achieved after treatment with compound
2. Both the leaf area and the root fresh weight were the highest in the case of treatment with compound
8 (10
−8 mol/L), while the fresh weights of the aerial part of the plants and the whole plant were the highest after treatment with compound
4. The maximum values of the dry weights of roots, shoots, and the whole plant were found for every SI derivative. These values significantly exceeded the water control, thus showing the high growth-stimulating effect of the SI derivatives on maize plants.