Silicon mitigates the adverse effects of drought on Lolium perenne physiological, morphometric and anatomical characters

Introduction

Periodic droughts recurring in recent years in Central Europe (including Poland) have caused large losses in plant production (Łabędzki & Bąk, 2017; Trnka et al., 2020). The increase in the frequency of droughts is primarily the result of an increase in air temperature during the growing season and the occurrence of periods without precipitation or periods with repeated rainfall lower than average. Water deficiency causes deterioration of growth and development conditions and decreases the production of plant biomass (Górski, Kozyra & Doroszewski, 2008). Under projected water deficits, a severe reduction in grassland productivity (Smith, 2011) and ecosystem functions (Jentsch et al., 2011) is expected. The health benefits of turf areas in parks and other urban green spaces have been highlighted in recent years (Neale et al., 2020). During extended droughts, there may be less water available for lawn irrigation due to water shortages and growing human demands for water (Milesi, Elvidge & Nemani, 2009; Gelernter et al., 2015). Studies on the ability of turfgrasses to recover from drought without irrigation are therefore of growing importance (Braun et al., 2022).

Lolium perenne is the most important forage grass cultivated in the temperate climate zone. This species provides high yields with a good forage value for ruminants (Chapman et al., 2022). It is also appreciated for its turf properties. L. perenne germinates quickly and tolerates intensive management. However, it requires a relatively large amount of water to maintain growth (Robins & Bushman, 2023). It belongs to shallow-rooting grasses with a limited drought tolerance (Sampoux et al., 2011).

The way that plants react to drought is a complicated process that differs greatly based on the species or cultivar, the stage of plant development, the severity and duration of stress, and other factors (Lisar et al., 2012). Under conditions of water deficit, plants lose turgor and reduce transpiration by closing their stomata to prevent excessive water loss. At the same time, the uptake of carbon dioxide is hindered, which results in a decrease in the intensity of photosynthesis (Farooq et al., 2009). One technique for examining how the light-dependent phase of photosynthesis responds to abiotic stress is to measure the physiological state of the photosynthetic process by measuring the chlorophyll fluorescence (λ_max = 685 nm), which is present in 95% of photosystem II (PSII). Additionally, it provides an indirect estimate of the tissue’s chlorophyll content. Stress causes alterations in the levels of chlorophyll a fluorescence and a decrease in the photochemical efficiency of PSII (Kalaji et al., 2014). The shortage of water hinders the effective use of the absorbed light energy, which is manifested by its increased dissipation by heat and increased fluorescence (Kalaji et al., 2017).

The roots of many plants experience drought stress first; however, they continue to grow while the growth of the shoots is slowed down (Spollen & Sharp, 1991). Lateral root growth is greatly inhibited during drought, primarily due to the inhibition of lateral root meristem activation (Deak & Malamy, 2005). Adaptive characteristics for enduring drought stress include the existence of specialized tissues in plant roots, such as rhizodermis, with a thicker outer cell wall or suberized exodermis, as well as a decrease in the number of cortical layers (Basu et al., 2016). An important trait of perennial grasses is the ability to regrow after a period of drought. As water is lost in plant organs, metabolic processes are gradually inhibited, and when water is reabsorbed, the organs are quickly hydrated and metabolic processes are activated. The level of dehydration tolerance shows variations in the ability of grass species and cultivars to withstand stress, but it can also reveal the potential for plant adaptation (Staniak & Kocoń, 2015). Leaf buds are an essential organ that determines the survival of periods of drought in grasses. They can withstand lower osmotic potential values than a fully grown leaf blade, and they start plant regrowth. The dying of leaves during a prolonged drought allows the movement of proteins, fats and other macromolecules just to buds of young leaves, flowers or seeds, and thus, the plant can regenerate after the stress is over (Volaire, 2003). Recently, there has been a growing interest in the use of elements in plant production that positively affect plant growth, but are not included in either macro- or micronutrients. One of them is silicon (Si). It is the second most abundant element in the lithosphere, after oxygen (Elsokkary, 2018). The silicon is taken up by plants from the soil solution in the form of monosilicic (Si[OH₄]) and orthosilicic (H₄SiO₄) acids, commonly found at concentrations ranging from 0.1 to 0.6 mM at the pH levels found in most agricultural soils (Knight & Kinrade, 2001). Apart from the pH of the soil, Si bioavailability is mostly determined by parameters such as soil texture, temperature, organic matter, and accompanying ions (Tripathi et al., 2021). Application of silicon to the soil can rapidly increase the concentration of orthosilicic acid in the soil solution and can, therefore, be a major treatment in intensive agriculture, especially in soils that are characterized by a low content of soluble silicon (Schaller et al., 2021). Some authors report that this element is mostly or even completely taken up by the roots (Ma & Yamaji, 2006; Guével, Menzies & Bélanger, 2007), while others indicate a high effectiveness of foliar Si application (Sattar et al., 2020; Othman et al., 2021). Plants can be categorized based on silicon (Si) content in tissues as accumulators (e.g., rice, wheat, maize, and sorghum), intermediates (e.g., cucumber, bitter gourd, and melon), or excluders (e.g., tomato, potato, canola, and lentil). The differences are attributed to the different modes of Si uptake (active, passive, and rejective) (Sacała, 2009; Luyckx et al., 2017). Furthermore, these differences primarily due to the ability of various plant species’ roots to absorb Si, which is linked to the expression and function of Si transporters (Souri et al., 2021). In recent decades, there has been a notable increase in interest in the foliar application of silicon as a fertilizer. The effectivity of foliar fertilization is influenced by various factors like plant physiology, species-specific properties (e.g., leaf structure and form), abiotic as well as biotic stressors, spray application, and physico-chemistry of the sprayed solution (Puppe & Sommer, 2018). Silicon may be regarded as an almost necessary element for plants since its deficiencies can result in many problems in plant growth, development, and reproduction (Epstein & Bloom, 2005; Mir et al., 2022). Many studies have shown that the Si application has a positive effect on yielding and also reduces the negative effects of abiotic and biotic stresses of various plant species (Ma, 2004; Epstein & Bloom, 2005; Artyszak, 2018; Siddiqui et al., 2020; Krzymińska, 2021; Mastalerczuk, Borawska-Jarmułowicz & Darkalt, 2023). The influence of Si on physiological parameters greatly depends on plant species, genotype, growth stage, and stress severity (Thorne, Hartley & Maathuis, 2020). Under drought conditions, Si positive effects photosynthesis and chlorophyll levels (Maghsoudi, Emam & Pessarakli, 2016). Additionally, Si consistently lowers oxidative damage across a range of crops and environmental conditions, which is correlated with a notable rise in antioxidant enzyme activity (Sattar et al., 2019). Numerous article provide evidence that Si fertilizers change the plant’s water status during drought, which is partly a consequence of Si increasing water use efficiency (WUE) (Hajiboland, Cheraghvareh & Poschenrieder, 2017). However, the mechanism(s) of this process are still under investigation.

The majority of research focuses on a variety of drought-related characteristics of plants, but they often are related to the sections of the plants that are only above or below ground. This complicates comparisons between studies on grass’s drought tolerance. In this study, the simultaneous response of plant shoots and roots to drought and Si application was assessed. The objectives of the research were: (i) to determine the possibility of reducing the negative impact of drought stress on the photosynthetic efficiency, the morphometric features of plant shoots and roots, and the distribution of biomass of L. perenne lawn cultivars in the initial period of growth, by applying biostimulant with silicon; (ii) to learn how drought and Si application affect leaf and root anatomical structure of L. perenne plants.

Materials and methods

Results

Influence of drought and Si application on physiological and biometric parameters

In the studies, the influence of water conditions and silicon application on the values of the analyzed features differed for the tested cultivars (Table 1). Our studies showed that L. perenne cvs. differed significantly in terms of F_m′, ETR, NBI and Chl. The Stadion cv was characterized by higher F_m′ and ETR values (by 7.2% and 6.8%, respectively), while the Bokser had a higher relative content of Chl and NBI (by 3.2% and 7.3%, respectively) (Table 1). Regardless of other factors, drought significantly reduced the values of almost all the tested features. The highest difference was found for ETR, F_m′ and Φ_PSII compared to the control conditions (114%, 85.5% and 70.1%, respectively). The application of Si significantly increased the values of Φ_PSII, ETR, NBI and RWC (by 2.3%, 3.8%, 6.3% and 4.9%, respectively) and also decreased by 10.9% the Flv values. The physiological condition of the plants also changed depending on the measurement term. Both cvs were characterized by lower values of most of the tested parameters in the following days of drought (except Flv) (Figs. 1 and 2; Tables S1 and S2). Simultaneously, providing plants with optimal soil moisture after the end of the drought resulted in a significant increase in the values of F_m′, Φ_PSII, ETR and RWC (35 DAT).

Table 1:

Significance of the effect of main factors on the analyzed variables based on the analysis of variance.

Mean values of the studied features.

	Fs	Fm′	ΦPSII	ETR	NBI	Chl	Flv	RWC	SN	SDM	RDM	RD	RL	RA	SRL	R:S ratio
Cultivars (A)	ns	**	ns	**	**	*	ns	ns	**	**	ns	**	**	**	ns	*
Bokser	649	1726a	0.526	58.9a	43.9b	28.9b	0.706	71.1	22.5b	0.24b	0.26	0.41b	25.5b	299b	79.2	1.22a
Stadion	668	1850b	0.533	62.9b	40.9a	28.0a	0.707	72.0	16.6a	0.22a	0.25	0.39a	21.1a	259a	77.0	1.28b
Water conditions (B)	**	**	**	**	**	**	**	**	**	**	**	ns	**	**	**	**
Control	787b	2323b	0.667b	83.0b	51.7b	32.9b	0.692a	87.4b	28.0b	0.39b	0.43b	0.4	40.6b	482b	82.8b	0.99a
Drought	530a	1252a	0.392a	38.8a	33.2a	24.1a	0.721b	55.7a	11.0a	0.06a	0.08a	0.4	6.0a	76a	73.4a	1.51b
Si application (C)	ns	ns	**	**	**	ns	*	**	ns	**	**	**	**	*	ns	**
Si–	668	1782	0.524a	59.7a	41.1a	28.2	0.743b	69.8a	19.9	0.22a	0.24a	0.41b	22.8a	272a	77.5	1.17a
Si+	650	1793	0.536b	62.0b	43.7b	28.7	0.670a	73.2b	19.2	0.23b	0.27b	0.39a	23.9b	286b	78.6	1.33b
DAT (D)	**	**	**	**	**	**	**	**	**	**	**	**	**	**	**	**
7	649b	2131b	0.686d	79.0d	62.4d	40.7d	0.660a	88.0c	12.3a	0.06a	0.06a	0.47c	3.7a	55a	63.9a	0.94a
14	613ab	1440a	0.437b	52.7b	52.5c	34.3c	0.734bc	69.5b	16.6b	0.12b	0.11b	0.36a	8.2b	94b	73.9b	0.94a
21	597a	1423a	0.341a	41.7a	30.8b	22.4b	0.746c	58.5a	21.6c	0.32c	0.20c	0.37a	16.0c	176c	80.6c	1.46b
35	775c	2157b	0.655c	70.1c	24.0a	16.5a	0.686ab	70.1b	27.7d	0.40d	0.65d	0.40b	65.3d	792d	93.9d	1.66c

DOI: 10.7717/peerj.18944/table-1

Notes:

The mean values of features in columns marked with the same lower-case letters did not differ significantly at *p ≤ 0.05, **p ≤ 0.01; ns–not significant.

F_s, steady-state chlorophyll fluorescence yields (relative units); F_m′, maximal fluorescence signal (relative units); Φ_PSII, quantum efficiency of photosystem II (relative units); ETR, photosynthetic electron transport rate (μmol m^–2 s^–1); NBI, nitrogen balance index (Dualex units); Chl, content of chlorophyll (Dualex units); Flv, content of flavonols (Dualex units); RWC, relative water content (%); SN, number of shoots (pcs.); SDM, shoot dry mass (g plant^–1); RDM, root dry mass (g plant^–1); RD, average diameter (mm); RL, root length (m); RA, root area (cm²); SRL, specific root length (m g^–1); R:S, ratio of the root mass to the shoot mass.

The application of Si under optimal soil moisture conditions did not significantly affect the values of chlorophyll fluorescence and RWC. In the case of the relative content of Chl and Flv, it was shown, that their values decreased under the influence of Si (Figs. 2C–2F). Under drought conditions, Si slightly increased the values of the tested parameters, especially at 14 DAT (Table S2). Only in the case of Flv content both L. perenne cultivars were characterized by lower values of this parameter.

It was found that, regardless of other factors, L. perenne cvs. were characterized by similar values of RDM and SRL (Table 1). In the case of other features, significant differences between cvs. were demonstrated. Drought reduced the values of all morphometric parameters, except RD. The use of Si, regardless of other factors, significantly increased SDM, RDM, RL, RA and R:S ratio (by 4.5, 12.5, 4.8, 5.1 and 13.8, respectively). There was no effect of Si on SN and SRL, while a 5.1% reduction in RD values after Si application was demonstrated. The application of Si in drought did not significantly differentiate most of the morphometric features (Figs. 3 and 4). However, a considerable increase in the value of R:S ratio was found (Figs. 4G and 4H; Table S2). In the case of the Bokser cv, the values increased by 30.2% and in the Stadion cultivar by 23.1%.

The studies showed that the values of most of the tested parameters (except RD, R:S and Flv) under drought conditions were lower compared to the control (Fig. 5). After a regeneration period under optimal soil moisture conditions (35 DAT), high values (close to the control) were recorded for Φ_PSII, NBI, R:S and SRL in both cvs., especially after Si application.

According to PCA, the first component explained 77.90% of the examined variability under control conditions, whereas the second component explained 8.98% (Figs. 6A and 6B). At optimal soil moisture (70% CWC), silicon application and measurement term influenced both morphometric (except RD) and physiological features. A strong positive correlation was found between the morphometric parameters of roots (RDM, RA and RL) and above-ground parts of plants (SN and SDM) and between physiological parameters (ETR, RWC and NBI). However, a strong negative relationship under control conditions was demonstrated between F_s and Φ_PSII and Chl and between NBI and roots features: RDM, RA and RL.

The PCA analysis carried out for drought conditions showed that the first component was responsible for 50.54% and the second for 33.79% of the analyzed variability (Figs. 6C and 6D). Drought had the strongest impact on RWC. Positive relationships were found between RL, RDM and RA and between Φ_PSII, F_m′ and ETR. Moreover, a negative correlation was found between R:S and Chl and NBI. The NBI and Chl parameters reached the highest values at 7 DAT, while after the regeneration period (35 DAT) the highest values were found in the case of RL, RDM and RA.

Influence of drought and Si application on leaf and root anatomy

There were differences in the anatomical characteristics of plant leaves according to water conditions and Si application. Generally, in the case of both cvs, Si application during drought, compared to drought without Si, increased the leaves thickness, the height of the ridges and reduced the width and distance between them (Table 2). This reduced distance between the ridges in Si-treated leaves allows for greater bulliform cell depth in the upper epidermis compared to drought. These cells are contribute to water loss reduction via the curling leaf blade. The same relationships were observed after the regeneration period.

Table 2:

Main anatomical characters observed in the leaf cross-sections of L. perenne cultivars under various water and Si conditions after the 7 DAT (drought) and 35 DAT (recovery).

Cultivar	Water and Si conditions	Characters
		high of the ridges in the leaf (µm)	width of the ridges (µm)	distance between the ridges (µm)
		7 DAT (drought)
Bokser	Control Si−	263.8 ± 33.3	336.5 ± 22.2	58.9 ± 7.5
	Drought Si−	237.1 ± 34.8	245.8 ± 46.2	77.7 ± 16.6
	Control Si+	283.8 ± 17.0	220.4 ± 18.2	32.1 ± 5.3
	Drought Si+	278.8 ± 53.8	216.6 ± 14.2	42.9 ± 2.9
Stadion	Control Si−	305.5 ± 35.0	250.3 ± 35.6	73.5 ± 13.0
	Drought Si−	232.8 ± 48.5	262.4 ± 29.2	56.0 ± 8.1
	Control Si+	357.3 ± 18.9	236.3 ± 40.0	67.3 ± 8.0
	Drought Si+	277.0 ± 10.4	204.9 ± 36.4	55.1 ± 8.4
		35 DAT (recovery)
Bokser	Control Si−	266.3 ± 34.1	230.5 ± 62.7	51.5 ± 5.1
	Drought Si−	236.7 ± 10.9	233.7 ± 2.0	43.4 ± 1.1
	Control Si+	295.3 ± 52.7	256.4 ± 75.0	39.1 ± 5.8
	Drought Si+	260.0 ± 21.2	226.9 ± 2.5	33.3 ± 2.0
Stadion	Control Si−	283.4 ± 29.6	233.7 ± 18.1	52.1 ± 2.6
	Drought Si−	231.4 ± 4.9	241.3 ± 9.5	41.2 ± 3.7
	Control Si+	349.6 ± 18.9	196.6 ± 59.9	18.9 ± 5.6
	Drought Si+	270.0 ± 5.9	204.9 ± 36.4	19.2 ± 3.5

DOI: 10.7717/peerj.18944/table-2

Note:

Cultivars, Bokser and Stadion; water conditions, control and drought; silicon application, lack of silicon–Si− and silicon application–Si+.

The adaxial surface of the leaves was corrugated (Fig. 7). The upper (adaxial) epidermis contains stomata and bulliform (motor) cells. Independently on water and Si conditions, mesophyll cells were usually spherical in shape. Mesophyll cells, adjacent to the lower epidermis developed as a tightly packed cell layer. External cell walls of the upper and lower epidermis were better developed under drought conditions than in the control. Moreover, Si application stimulated the development of the external cell walls of both epidermis.

In the roots, we observed exoderm development regardless of the water conditions and Si application. The second subepidermal cell layer developed as an exoderm independent of water conditions, Si application, and plant cvs. (Stadion or Bokser). In the case of plants in control without Si application (control Si‒), exodermal cells exhibited full turgor. In contrast to the lack of Si (Si‒), its application (Si+) resulted in the maintenance of turgor in exodermal cells of both cultivars under drought conditions. Furthermore, Si presence often resulted in the development of 2-layered exodermis under drought conditions in the case of Stadion cv (Fig. 7). In the root cortex, characteristic lacunas were formed by the disintegration of superordinately arranged cells in the radial direction. Lacunas developed in the root cortex regardless of the treatment and cultivar. One or two cortical cell layers adjacent to endodermis developed as sclerenchymatic cells, which, independently of treatment, did not participate in lacunas formation. After the recovery period (35 DAT), in the case of both cultivars, Si application resulted in better development of root lacunas both in control and drought conditions (Fig. 8).

Discussion

In the case of turf grasses, the main goal of cultivation is to provide an aesthetic aspect and good soil cover with plants. Therefore, the ability of plants to maintain healthy and good condition of both leaves and roots is crucial, especially under stress. We investigated the effects of drought stress under silicon application conditions on the physiological and morphometric features of L. perenne cultivars as well as the anatomy of the leaves and roots of the plants.

Plants tolerate drought by suspending the growth of their shoots (Fry & Huang, 2004). They often become dormant. The stem apex and leaf primordia endure under such circumstances, but the leaves may desiccate and die (Seleiman et al., 2021). When sufficient rainfall and favorable growing conditions are restored, the plants recover. In our studies, drought lasting 21 days inhibited the growth of both grass shoots and roots. This was confirmed by low values of biometric parameters on subsequent days of drought (SN, SDM, RDM, RL and RA) and decreasing values of physiological parameters (especially F_m′, Φ_PSII and ETR). Plants under drought stress produce less dry matter of roots due to adaptation mechanisms meant to prevent excessive water loss (Mastalerczuk & Borawska-Jarmułowicz, 2021). It also allows the reduction of the adverse effect of drought on the physiological metabolism of plants, especially their photosynthetic activity (Fariaszewska et al., 2020). Closing the stomata due to drought reduces the efficiency of the CO₂ assimilation reaction and also reduces the consumption of ATP and NADPH (Dias & Brüggemann, 2010). As a result, in our studies, electron transport (ETR) in the light phase slowed down and caused a decrease in the Φ_PSII value.

Harvesting light energy to drive the electron transport reactions in the photosynthesis process is closely related to the chlorophyll content in the leaves (Croft et al., 2017). According to the literature, the degree of chlorophyll degradation caused by a moisture deficit can be linked to stress sensitivity (Chauhan et al., 2023). In the case of drought-sensitive genotypes, components of the photosynthetic apparatus may be damaged, whereas drought-tolerant genotypes may show good adaptability to reduce/avoid drought stress disturbances (RongHua et al., 2006; Staniak et al., 2020; Mastalerczuk & Borawska-Jarmułowicz, 2021). Our research showed that the Chl content in L. perenne leaves decreased on subsequent days of drought, which could indicate chlorophyll metabolism disorders. One of the crucial processes of drought adaptation is the buildup of substances that boost antioxidant capacity and support the detoxification of reactive oxygen species (Hasanuzzaman et al., 2020). An important role in the protection of the photosynthetic apparatus is played by non-enzymatic flavonoids, which are preferentially located in the epidermal cells of leaves (Agati et al., 2012; Tan & Gören, 2024). In our study, increased flavonol accumulation was evident throughout the stress period (up to 21 DAT). After providing the plants with optimal soil moisture after a period of drought (35 DAT), a significant increase in the values of chlorophyll fluorescence parameters (F_m′, Φ_PSII, ETR) and RWC and also a decrease in relative Flv content was demonstrated, which may indicate the tolerance of L. perenne to drought conditions. Drought independently on treatment and variety resulted in thicker outer cell wall development of the upper and lower leaf epidermis, which may also give better resistance to water evaporation.

In many studies, the positive effects of silicon were observed almost exclusively under stressful conditions (Coskun et al., 2019). Similarly in our research, Si supplementation under control conditions did not differentiate the values of chlorophyll fluorescence parameters and RWC. However, it limited the relative content of Chl and Flv in L. perenne plants. Due to their antioxidative properties, flavones protect the plant against reactive oxygen species (ROS) (Mierziak, Kostyn & Kulma, 2014). According to our earlier research, plants with a very dry and moderately moist year had a higher relative flavonoid content than those with a rainy year. In that moisture condition, the application of Si caused a decrease in flavone content, especially in L. perenne plants (Mastalerczuk et al., 2020).

Several studies have demonstrated that when Si was applied to drought-stressed plant species, the concentration of photosynthetic pigments increased. In the studies of Shen et al. (2010), Si application under drought increased total chlorophyll contents in soybean leaves, while Yin et al. (2014) reported a similar reaction in the case of sorghum. The high levels of Si in plant leaves promote the biosynthesis of chlorophyll pigments and slow down their breakdown, which is caused by water stress (Verma et al., 2022). According to Rizwan et al. (2015), Si application in drought causes changes in values of gas exchange parameters, water potential, and reduction in oxidative stress in plants. Additionally, it can improve gas exchange, which has a positive effect on photosynthesis, nutrient uptake, and eventually, plant biomass and growth. The beneficial effect of silicon fertilization on plants under drought conditions may be due to an improved water balance, more effective osmoregulation, and reduced water loss through transpiration (Sacała, 2009; Wang et al., 2021).

In our study, the application of Si in drought increased values of chlorophyll a fluorescence parameters (Φ_PSII, ETR) and RWC. Furthermore, RWC values in plants treated with Si were higher after the recovery period (35 DAT). The higher RWC demonstrates that Si enhanced water uptake and retention in L. perenne plants under drought stress. As suggested by Gong et al. (2003) the increase in leaf water content and water potential in wheat plants under drought and Si presence may be caused by the leaves’ thickness in comparison to those without Si treatment. Additionally, since water molecules may not be able to escape from the leaf surface, the deposition of Si in leaves may decrease transpiration (Ahmed, Asif & Hassan, 2014; Keller et al., 2015).

Most plant water loss occurs through leaf transpiration through stomata. Water loss through transpiration significantly decreases as leaf area decreases (Trlica & Biondini, 1990). In the present study, we observed that Si-treated leaves were thicker, and the upper blade surface had greater protuberances compared to drought. The relative humidity of the leaf surface can be maintained by a wavy surface, thicker mesophyll and leaf layers, and structures that aid in plant water storage. Additionally, the greater depth of the bulliform cells of Si-treated leaves facilitates the rolling of the leaf blade to avoid water loss during water stress (Dickison, 2000).

Based on data from the literature, silicon is primarily found in the apoplast, where it can aid in the formation of mechanical strength and physical barriers, such as in the root endodermis or leaf epidermal cell wall (Hodson & Sangster, 1989; Dietrich et al., 2003; Luyckx et al., 2017; Coskun et al., 2019). Our studies showed that, root cortex parenchyma cells arranged in radial rows disintegrate by the disruption of the tangential walls. Because radial walls remain untouched, so-called lacunas are formed. In the literature, lacunas are described as evident features of aerenchyma formation. In both cultivars of L. perenne, drought without Si application resulted in at least partially collapsed exodermal cells, which is a sign of turgor loss (Cuneo et al., 2016). Supplementation of Si resulted in fully rounded exodermal cells, which indicates their full turgor. Microscopic observations suggest that exodermis may be crucial for efficient resistance to drought conditions, especially together with Si application, which leads to the development of two-cell-layer exodermis (cv. Stadion, Fig. 5).

According to Farooq et al. (2009), applying Si during drought stress is crucial for maintaining root development and water transport. In our earlier studies (Mastalerczuk, Borawska-Jarmułowicz & Darkalt, 2023), we found that, under the drought and Si applications, both the water use efficiency and also the length and mass of roots of the plants increased significantly compared to the conditions without silicon. Additionally, plants transferred carbon from their leaves to form fine roots (with small diameters) in response to drought. In this study, we found that, under similar conditions, not only the RWC values but also the root-to-shoot ratio increased significantly compared to the treatments without silicon, both in drought and after the recovery period. These findings show that when Si supplementation was applied, the tested plants developed roots in response to drought stress, reducing the amount of photosynthetic products allocated to the aboveground organs (leaves and stems). One explanation for this could be that plants use newly generated biomass to strengthen their roots, which can enhance their ability to access water in the soil. This is a survival strategy for plants experiencing water scarcity.

An indicator that describes the plant’s responses to modifications in the soil environment is specific root length (Ostonen et al., 2007; Liu et al., 2024). Plants demonstrating a higher share of fine roots of smaller diameter (higher value of SRL) have better root branching, allowing them to survive periods of water or nutrient deficiencies (Hill et al., 2006; Fort & Freschet, 2020). In our study, L. perenne plants subjected to drought conditions were characterized by low SRL values. Only after recovery in the presence of Si they were distinguished by the better branching of the roots than plants without Si. This indicates that Si mitigates the harmful impacts of drought conditions, accelerates root regeneration, and promotes plant development after a drought by increasing most of the studied physiological parameters.

Conclusion

The present study demonstrated that 21-day drought stress in the tillering phase of young L. perenne seedlings markedly decreased RWC, chlorophyll content and photosynthesis activity while increasing the flavonol content. These adverse impacts were reflected in the reduction of biomass-related traits: shoot number, mass of shoots and root, as well as area and length of roots. Foliar Si supplementation under drought stress conditions influenced plant growth through better electron transport efficiency, more efficient quantum yield of photochemical reactions in PSII and increased RWC and chlorophyll content, promoting photosynthesis and stimulating biomass allocation to the roots (higher R:S values). Under drought stress conditions, Si application leads to the development of two-cell-layer exodermis, which reduces the water losses in L. perenne roots and shoots and, as a result, improves the drought tolerance of plants. To maintain green areas in cities under changing climate conditions, it is essential to comprehend how drought affects turfgrass species and to develop strategies that improve plant adaptation to drought. Given the complex relationships between Si and different grass species, genotypes and environments, detailed studies are needed to understand the interactions between Si and plant responses under drought conditions.

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