Growth regulators for VSL-2 rootstock at the propagation stage
As many as 60% of viable sterile explants during the
in vitro introduction phase were obtained. To stimulate their regenerative activity and shoot growth, media MS variants with Epin (0.2-2.0 ml/L), as well as its combination with 6-BAP at 0.5 mg/L, were used. After 8 weeks of growth, Epin and 6-BAP influence on the studied morphometric parameters and the multiplication factor in microshoots of the VSL-2 rootstock was determined (Graph 1). The correlation analysis showed dependence for the node number (r= 0.57) and the multiplication factor (r= 0.68) on the shoot length. The statistically significant differences between the shoot length in the control and the experimental variants were found (Graph 1). No significant differences between the experimental variants with Epin were found.
No notable effect on the shoot number was recorded except the variant with Epin (0.2 ml/L). The shoot number value (1.9) was almost two times higher than the control value (1.0). The correlation analysis (r = 0.13) did not show a positive dependence of the multiplication factor on the shoot number. 6-BAP at 0.5 mg/L was ineffective; the multiplication factor was only 2.7. The combination of Epin (0.1-2.0 ml/L) and 6-BAP (0.5 mg/L) had a negative effect: it led to the shoot’s conglomerate formation and their vitrification (Fig 1b).
Epin used in the nutrient media on apple rootstocks
in vitro increased the reproduction rate to 112% and stimulated adventitious bud formation in cauliflower and coconut
in vitro (Ali, 2019). Epin (0.5 mg/L) also contributed to an increase in the raspberry microshoots average length but had no significant effect on their number. The combined use of brassinosteroid with 6-BAP 0.5 mg/L contributed to an increased shoot number
(Makarov et al., 2022). 24-epibrassinolide is one of the regulators that specifically modulates the response of plants to abiotic stress and can improve some indicators that affect growth, for example, in potatoes
(Khalid and Aftab, 2016).
The positive effect of 6-BAP (0.5-1.0 mg/L) is mentioned by
Jafarlou et al., (2022) during micropropagation of plants of the genus Prunus to lengthen the shoots of dwarf cherry rootstocks. In our study, such a modification of the medium gave a negative result, causing the shoot vitrification. The multiplication factor is greater than the control value by 0.4. Elongation, leaves twisting and shoots hyperhydration significantly reduced the regenerative explants capacity at the propagation stage. Using 6-BAP (0.5 mg/L) for VSL-2 didn’t stimulate shoot formation (Graph 1). When combined with Epin, caused vitrification (Fig 1b), a similar response in cherry rootstocks was observed with 6-BAP (0.5 mg/L) in MS medium
(Pronina and Matushkina, 2020).
Kinetin has been successfully used
In vitro and has a stimulatory effect on the regenerants’ growth and development
(Prameswari et al., 2019). The kinetin’s introduction (0.05 to 0.5 mg/L) had a positive effect on the VSL-2 regenerants at the reproduction stage. After 8 weeks of growth, the shoot length increased from 2.55 to 4.95 cm, while the node number was limited to the range of 3.16 to 6.50 per shoot. The maximum shoot length (9.12-10.31 cm) with 11.75-13.00 nodes was recorded on media with 0.45 and 0.50 mg/L kinetin (2.3 times higher than control; Graph 2). The shoot number per explant, the internode length and the leaves number were evaluated (Graph 2). The first parameter amounted to 1.0-1.5. The distance between nodes varied within 0.53-1.38 cm; at low concentrations, uneven and significant stretching of internodes and a large accumulation of nodes in the apical and basal parts of regenerated plants were noted.
An increase in the shoot length and the node number at high kinetin concentrations also contributed to a significant increase in the leaves number, which amounted to 18.75 and 22.00 on media with kinetin (0.45 and 0.50 mg/L), respectively; 2 times more than the control value (10.80 pcs). Experimental data showed a high percentage (90%) of rooted explants in the control. With an increase in the kinetin concentration, the number of rooted regenerants decreased proportionally, the minimum value being 35% with 0.50 mg/L. A positive kinetin’s effect on root length was observed with 0.30-0.50 mg/L of hormone with an increase to 5.2 cm (Graph 2).
A linear relationship on the effect of low kinetin concentrations on the VSL-2 rootstock cells proliferation
in vitro was revealed: with its increasing in the medium, the regenerative explant’s ability increased, which contributed to the shoot growth and the appearance of additional nodes and leaves (Fig 1c and Graph 2). Probably, the combined kinetin and auxin use have a positive effect.
Effect of auxin IBA on In vitro root induction
For the rooting step, one of the most abundant auxins in numerous studies in this area
(Tsafouros and Roussos, 2022;
Kaviani et al., 2023) is indolyl-3-butyric acid, used alone or in combination with other compounds that stimulate rhizogenesis. Its addition (0.1 to 1.0 mg/L) in full- and half-strength MS media had a positive effect on the VSL-2 rooting, but statistically significant differences were not shown (Graph 3). Microshoots were rooted in 90-100% of cases on all experimental media. However, a dependence of quantity of regenerants with second-order roots on the auxin concentration was revealed. The best result was achieved on full-strength MS and IBA (0.5 mg/L); second-order root formation was 83.3%. On the half-strength MS medium and IBA (0.5 mg/L), this indicator was two times lower; there were no second-order roots on the control medium. Significant differences in the root length in comparison with the control and a high average root number (7.5 pcs) were also observed on media with IBA (0.5 mg/L). Using the half-strength MS had no effect on the rooting and differences were observed only between the variants with IBA (0.5 mg/L); no positive effect was noted. During the increase of IBA to 1.0 mg/L, leaf curl and vitrification were fixed. The variant (IBA 0.5 mg/L) on MS medium (Graph 3) turned out to be optimal for the successful VSL-2 rooting; the maximum values of regenerants with second-order roots, the root number and length were obtained. Similar results were mentioned by
Kumar et al., (2020).
Evaluation of the stomatal apparatus in the abiotic stress conditions In vitro
Under osmotic stress influence, there is a decrease in the leaves number, leaf plates twisting and partial drying after 6 weeks of growth of VSL-2 regenerants on media containing PEG 6000 (Fig 2). The root system development of the experimental groups’ explants was inhibited. All studied regenerants were viable.
Osmotic stress adversely affected leaf weight (Table 1). The leaf area in the experimental groups decreased, but there were no statistically significant differences between the medium MS and the PEG content media. When assessing changes in the stomatal apparatus, leaves formed under osmotic stress were used as material for analysis. There were no significant differences in the leaf area of regenerated plants in the control medium and in media with PEG content (Table 1). Leaf curl in explants cultured on osmotic media is a morphological change to reduce water loss and light absorption area.
During prolonged drought, mature leaves are able to generate signals for developing leaf plates in order to form the optimal stomatal apparatus under specific conditions
(Li et al., 2021). At the same time, a balance must be maintained between transpiration and CO2 uptake for the normal operation of the photosynthetic apparatus
(Hasanuzzaman et al., 2023).
Changes in the VSL-2 leaf plate stomatal apparatus occurred in the absence of available water (Table 1). The lowest value of stomatal density was recorded in the control group (21.79) and the highest value was in the plant leaves on PEG 20 (33.93).
The stomatal cells’ area in the experimental groups decreased by 2-2.75 times. The length and width ratio of stomatal cells L1/D1 in the experimental groups was statistically significantly higher than on PEG 0 by 15%. When assessing changes in the stomatal apparatus caused by osmotic stress, the smallest changes in this parameter were recorded.
The stomatal gap area under osmotic stress decreased by 5-10 times. The ratio L2/D2 (Table 1) change indicates a stomatal cell’s shape modification. Statistically significant differences in L2/D2 to control (1.59) were observed on PEG 20 (1.9) and PEG 60 (2.4). When there is a scarcity of water, increasing stomatal density in VSL-2 explants (Table 1) results in higher conductivity, which increases carbon uptake but decreases water use efficiency
(Hasanuzzaman et al., 2023). The observed disproportionate decrease of the stomatal fissure area size (Table 1) from the total stomatal cell area in the control from 15.5% to 3.6-6.1% on PEG-content media occurs in accordance with the hydropassive and hydroactive mechanisms of stomatal gap narrowing.
(Wankmüller and Carminati, 2022). These changes characterize the rootstock VSL-2 as a xerophytic plant
(Babaei et al., 2021).
No significant difference was observed between the control and PEG 20 examples when analyzing the total leaf plate chlorophyll content (Graph 4). When comparing the mean values of the control group, PEG 40 and PEG 60, the obtained data were in the uncertainty zone. The hypothesis put forward about the significance of the differences was not confirmed. We noted a statistically significant increase in the carotenoids in the experimental groups (2.77-3.15) compared with the control (1.89). When comparing the chlorophyll
a to chlorophyll
b ratio no statistically significant differences between experimental and control groups were noticed. At the same time, the total chlorophyll to carotenoids ratio in the experimental groups was statistically significantly reduced in relation to the control. The carotenoid concentration increases in the experimental groups (Graph 4), it confirms the plant photosynthetic apparatus stability under a lack of available water
(Salsinha et al., 2021).