3.1 Si absorption and translocation patterns among strawberry leaf and root samples
SEM-EDS results from the foliar-applied potassium silicate in strawberry plant samples showed silica distribution and intensity variations. In the elemental maps, Figure 1 panel-A of SEM image in black and white shows the "knobs" of silica in white precipitation; however, the high intensity of white precipitate in 2 mL treated plant leaf samples suggests the higher accumulation of silica. In panel B, the high-intensity orange color for the element Si indicates a higher presence of Si than potassium, represented in blue. The high intensity of orange color in 2 mL treated leaf samples suggests a higher accumulation of Si than other samples, both panel-B and panel-C. A similar trend of having a high intensity of orange color in the 2 mL treated leaf sample image in panel-D was observed, suggesting the higher accumulation of Si over all the elements overlayed in the same picture (Fig. 1A-D). EDS spectra from the strawberry leaf samples treated with 2 mL of potassium silicate show a high peak area compared to other treatments (0 mL, 3 mL, and 5 mL). The samples of 2 mL treatment showed higher Si peak intensity and area and well-structured phytoliths. Other elements, such as O and Au (internal coating material for the sample fixation), were also observed in higher intensities (Fig.1E). On the other hand, the root sample showed less intensity of orange color in 2 mL treated samples compared to other treatments, and higher intensity in the root samples treated with the higher Si treatment concentrations suggests the higher translocation of silica into the root samples from the treated leaves (Fig. 2 A-D). The EDS spectra in the root samples for 2 mL of potassium silicate treatment showed less peak area of Si than other treatments (Fig. 2E), increasing the Si peak area observed on 3- and 5-mL treatment root samples.
There are no comprehensive studies on the absorption and translocation of Si in strawberry plants and their ratios in shoots and roots [25]. Therefore, the present study is focused on assessing the localization of Si in the leaves and roots of the strawberry plants upon different concentrations of Si foliar spray treatments. Various methods are available for qualitative and quantitative analysis of Si presence in plant samples [30]. However, among these, visualization methods with quantifications provide a simple and reliable approach to assessing the localization of Si in various plant parts [41, 42]. Similar visualization observations resulting from SEM-EDS image analysis in the present study assist in understanding the mechanisms of absorption and translocation of Si to various plant parts. Overall, the SEM-EDS image analysis suggests that the 2 mL treatment is effective in absorbing and retaining the Si in leaf samples compared to other treatments, and the dumbbell-shaped phytoliths of silica structures were observed in the strawberry leaves (Fig. 3) possibly strengthening the plant growth and health of the aerial parts through biosilification. Phytolith denotes phyto (plant) and lithos (stone); both originated from Greek. Si application-induced phytolith formation in plants is reported to strengthen the plant tissues and act as a mechanical barrier against pests and regulations of nutrient uptake and abiotic and biotic stress tolerance [43, 44].
3.2 Si accumulation in strawberry plants
Spectrophotometer analysis on leaf samples revealed a higher accumulation of silicon (48 mol/L) upon 2 mL treatments than other treatments (0 mL-23 mol/L, 3 mL-33 mol/L, and 4 mL-42 mol/L) (Fig. 4A). On the other hand, root samples accumulated a less amount of silica in 2 mL treatment (119 mol/L) compared to other treatments (0 mL-151 mol/L, 3 mL-141mol/L, and 5 mL-226 mol/L, Fig. 4B), suggesting the 2 mL treatment shows lower translocation of silica to the roots and well partition in the leaf samples; the same pattern was visible in the 3 mL and 4 mL samples. Higher accumulation of Si upon foliar spray application in plants was reported at the whole plant level, and an enhanced accumulation of Si concentration was observed in leaves [25, 45]. The quantification of Si using both SEM-EDS and spectrophotometer analysis in our study provided the knowledge to understand the efficient Si treatment combination and Si partition in the different plant tissues, similar to previous studies [42, 46, 47]. Upon Si treatment, the higher accumulation of Si in the leaf tissues in 2 mL treatment strengthened the morpho-physiological and yield traits [47].
Table 1 Efficacy of potassium silica foliar spray for improving plant growth and yield traits in strawberry plants under high tunnel conditions. Plants were treated every two weeks with a potassium-silicate solution containing concentrations of 2 mL, 3 mL, and 4 mL mixed with one gallon of water in five applications during the fall of 2018 and spring of 2019.
Treatment
|
Marketable yield (g)
|
Non-marketable yield (g)
|
Plant height (cm)
|
Plant width (cm)
|
|
Flavorfest
|
Rutgers
ScarletTM
|
Flavorfest
|
Rutgers
ScarletTM
|
Flavorfest
|
Rutgers
ScarletTM
|
Flavorfest
|
Rutgers
ScarletTM
|
0 mL
|
33.00b
|
31.57b
|
13.20bc
|
9.35c
|
20.22
|
19.55
|
150.02b
|
143.21b
|
2 mL
|
80.20 a
|
82.80 a
|
28.40a
|
24.00ab
|
22.83
|
19.41
|
183.16a
|
152.81b
|
3 mL
|
88.33 a
|
40.00 ab
|
13.60abc
|
13.60abc
|
19.91
|
17.75
|
156.73b
|
111.99c
|
4 mL
|
57.80 ab
|
47.20 ab
|
19.80abc
|
16.60abc
|
19.83
|
22.91
|
152.55b
|
158.83b
|
Treatment (T)
|
**
|
*
|
NS
|
***
|
Genotype (G)
|
NS
|
NS
|
NS
|
**
|
G*T
|
NS
|
NS
|
NS
|
**
|
|
|
|
|
|
3.3 Effect of potassium silicate on improved physiological and agronomic traits in strawberry plants
The strawberries grown under high tunnels were subjected to four different potassium silicate treatments at two-week intervals in the Fall of 2018 and Spring of 2019. Plant height, width, marketable (MKY), and non-marketable (NMY) fruit yields were recorded. The potassium silicate treatment significantly affected yield (marketable and non-marketable) and canopy size (width). However, no interaction between the genotype and treatment was observed for any measured morpho-physiological traits except for the plant width (Table 1). There was no significant difference between the treatments for plant height among the two genotypes; however, plant width was significantly higher in the 2 mL treatment (183 cm) in 'Flavorfest' compared to other treatments (0 mL-150 cm, 3 mL-156 cm, and 4 mL-152 cm). We observed improved plant size and yield on strawberry plants with 2 mL potassium silica foliar spray. The following studies also reported that Si treatments significantly improved vegetative growth in strawberry plants [46, 48, 49] and other plants [50, 51]. The differential response of the potassium silicate treatment in strawberry plants improved the 'plant's physiological traits, such as leaf area, root and shoot growth, and biomass [20, 22, 24, 28, 46].
The marketable fruit data (yield/plant) revealed that 'Flavorfest' produced higher yields with Si treatments (2 mL [80 g], 3 mL [88 g], and 4 mL [57 g]) than the control (33 g). 'Rutgers ScarletTM' marketable fruit yield for 2 mL (82 g) was more than doubled compared to control (32 g), 3 mL (40 g), and 4 mL (47 g) treatments. Overall, the non-marketable yield revealed that 2 mL treatments for 'Flavorfest' and 'Rutgers ScarletTM' produced more yield than other treatment levels (Table 1). Similarly, different Si treatments in strawberries and other fruit crops were reported to increase the fruit yield [23, 28, 52]. However, most studies were conducted under controlled environmental conditions [24, 47, 48, 52] or open fields [23]. Our study evaluated the effective concentration of Si foliar spray application for strawberry production and overall plant health under high tunnel conditions. Nevertheless, not much information is available on studies related to foliar spray application under high tunnel conditions [25]. Previous studies in strawberry plants reported the improvement of both quantitative and qualitative traits upon Si treatment [46]. Similarly, in the present study, the Si application helped strawberry plants to achieve improved morphophysiological characteristics (Suppl Fig. 1). Higher accumulation of Si upon 2 mL foliar treatment might help strawberry plants improve their agronomic traits and yield parameters.
3.4 Reduced mycelial growth of the B. cinerea upon potassium silicate treatments.
The efficacy of potassium silicate against B. cinerea growth was measured daily for five days. The 2 mL treatment significantly affected the growth of the fungal mycelial. The substantial differences in radial growth of the fungus were observed from the 2 DAI to 5 DAI upon 2 mL treatment compared to other treatments. On day three, again, 2 mL concentration was effective and provided more inhibition of mycelium growth (437mm) compared to other treatments (0 mL-667, 1 mL-666, 3 mL-6810, 4 mL-716, and 5 mL-907 mm) (Fig. 5). On day four, 2 mL concentration continued to provide more inhibition in mycelium growth (1667 mm) while the control (2721 mm), 1 mL (2580 mm), 3 mL (2650 mm), 4 mL (2850 mm), and 5 mL (3165 mm) mycelium growth increased (Fig. 5). On the last day of observation, 2 mL (4263 mm) concentration showed a higher inhibition in mycelium growth than all other treatments (5165-5440 mm), suggesting the possible effective dose against B. cinerea to improve the resistance (Fig. 5).
Earlier in vitro studies reported the potassium silicate antifungal activity against several phytopathogenic fungi grown on PDA media [53, 54]. Similarly, the antifungal activity of potassium silicate and significant reduction of fungal mycelial growth was reported in fungal isolates collected from apples. Also, silicon dioxide nanoparticles and other silica forms showed reduced mycelial growth of Botrytis under in-vitro conditions [55–57]. Likewise, previous studies reported the antifungal efficacy of Si against Botrytis in several crop plants using fruit assays [58, 59]. In the present study, the reduced mycelial growth of the Botrytis upon 2 mL potassium silicate treatment corroborates with the prior reports of mycelial growth reduction upon different forms of Si treatments in various fungal species under in-vitro conditions [53, 55, 58]. However, further studies are needed to unravel the mechanism of the antifungal functions of potassium Si against Botrytis.