Evolution of leaf Si concentration in response to foliar application of Si at different doses in ‘Arbequina’ and ‘Picual’. Measurements were made in fully expanded leaves monthly. Bars represent the standard error of the mean (Expt. 2).
Effect of soil and foliar application of Si at different doses on leaf Si concentration in ‘Arbequina’. Leaves of sampling 1 (A) were collected 16 weeks after the initiation of the Si treatments. Leaves of sampling 2 (B) were collected from plants subjected to 28 weeks of Si treatments 2 weeks after the last Si application. Bars represent the standard error of the mean (Expt. 3).
Effect of soil and foliar Si application at different doses on leaf Si concentration in ‘Picual’. Leaves of sampling 1 (A) were collected 16 weeks after the initiation of the Si treatments. Leaves of sampling 2 (B) were collected from plants subjected to 28 weeks of Si treatments, 2 weeks after the last Si application. Bars represent the standard error of the mean (Expt. 3).
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Silicon (Si) is a nonessential element for plant growth, but it influences the tolerance to biotic and abiotic stresses in many plant species. Most research about the uptake and beneficial effects of Si in plants has been carried out on monocotyledonous species, Si-accumulating plants. Little attention has been paid to woody crops, characterized as low-Si-accumulating plants. In this sense, available information about Si nutrition in olive trees is scarce. Therefore, this work aimed to study the effect of Si application on the uptake, accumulation, and organ distribution of Si in young olive plants by analyzing the influence of the dose, the method of application, and the cultivar. Three experiments were conducted under shade-house conditions with mist-rooted ‘Arbequina’ and ‘Picual’. The treatments consisted of different Si doses, ranging from 0 to 20 mg·L−1, depending on the experiment, applied by foliar sprays or through the irrigation water. Choline-stabilized orthosilicic acid (H4O4Si) was used as the source of Si. Results indicated that after 120 days of Si treatments, this element was accumulated in major proportion in the roots, followed by the leaves and the shoot of the plants. Si organ concentration increased according to the doses applied, independently of the olive cultivar and the method of Si application. Differences in leaf Si accumulation between treated and control plants were evident 60 days after its application. The dose of 20 mg·L−1 was the most effective to increase Si level in leaves under the trial conditions. Si is recommended to be applied periodically to ensure its accumulation in growing leaves.
Si is a nonessential element for plant growth, but it positively influences the tolerance to biotic and abiotic stresses in many plant species, promoting plant growth and productivity (Debona et al., 2017; Ma, 2004). After oxygen, Si is the second most abundant element in the earth’s crust. Silicon in soils can be found in a solid phase composed mainly of silica (SiO2) and silicates adsorbed to soil particles and Fe and Al oxides and hydroxides, or in a liquid phase mainly in the form of monosilicic acid (H4SiO4) (Tubana et al., 2016). Plants take up Si from the soil solution as monosilicic acid, which is the prevailing form of Si when the soil solution is at pH below 9 (Epstein, 1994; Ma and Takahashi, 2002). All plants growing in soils contain Si in their tissues. Si concentration in the shoot may vary from 1 to 100 mg·g−1 on the dry weight basis depending on the plant species (Epstein, 1994; Hodson et al., 2005). In this sense, most monocotyledons are characterized as Si-accumulating plant species, whereas dicotyledons show low Si accumulation (Guntzer et al., 2010; Ma et al., 2001). These differences among plant species have been attributed to the different ability of the roots to uptake Si (Ma and Takahashi, 2002). Related to this, grasses extract Si from the soil more intensively than other species. Plant Si accumulation also depends on the soil type (Debona et al., 2017; Tubana et al., 2016), sandy soils or soils with important levels of Fe and Al oxides usually have lower silicon available contents.
Silicon is taken up by roots in three different modes: rejective, passive, or active, and then translocated via the xylem to the aerial organs through the transpiration stream (Pontigo et al., 2015). Dicotyledonous plants tend to uptake Si passively, and Si-accumulating species, such as grasses, do it actively (Barber and Shone, 1966; Casey et al., 2003; Epstein, 1999). In the latter, different Si transporters involved in root uptake, translocation, and distribution of Si to aboveground organs have been identified (Ma and Yamaji, 2006; Mitani et al., 2009, 2011; Yamaji and Ma, 2009). Plants with a rejective mode of uptake tend to strongly discriminate against Si during the uptake process (Epstein, 1999).
The distribution and accumulation of Si in the aerial organs is related to their transpiration rate (Jones and Handreck, 1969). Once in the leaves, most of the Si remains in the apoplast and is deposited at the endpoints of the transpiration stream, after water evaporation, as amorphous silica on the walls of epidermal cells forming a silica gel layer between the cuticle and the epidermis (Debona et al., 2017; Wang et al., 2017). In this way, the translocation of Si to new, growing leaves is prevented. This silica layer constitutes a physical barrier that has been observed to reduce the incidence of pests and diseases, as well as to improve photosynthetic rates and tolerance to salinity, drought conditions, and other abiotic stresses (Luyckx et al., 2017; Marschner, 2012). Because the Si deposited in the leaves is immobile, a continuous supply of Si is required in newly formed leaves to ensure optimal tolerance to these stresses (Huber et al., 2012). Also, it is suggested that the more Si accumulates in the shoots, the larger its effect (Ma, 2004).
Foliar application of Si solutions has been considered a viable method to increase Si accumulation in plant tissues, especially for low-Si-accumulating species (Pilon et al., 2014; Wang et al., 2015). So far, the mechanisms of Si absorption by the leaves have not been well characterized. Because silicic acid has certain similarities with boric acid (Broadley et al., 2012), it has been speculated that the mechanisms of foliar absorption are similar. In any case, Savvas and Ntatsi (2015) have indicated that soil Si application is more effective than foliar application.
Si accumulation also forms a chemical barrier, inducing the production of phenolic compounds, phytoalexins, and other products that activate plant defense mechanisms (Debona et al., 2017; Wang et al., 2017). Consequently, although Si is present in the soil and available for plants, Si application has been observed to improve plant resistance or tolerance to several biotic and abiotic stresses (Luyckx et al., 2017). This could be particularly important in low Si-accumulating plant species, such as the olive (Hodson et al., 2005).
Although many studies investigating the effects of Si fertilization on plants have been recently undertaken (Broadley et al., 2012; Savvas and Ntatsi, 2015), limited information is available related to fruit tree crops, especially to olive. Therefore, the present work aimed to study the effect of Si application on the uptake, distribution, and accumulation of Si in different organs of young olive plants, paying attention to the influence of the dose, the method of application (foliar or soil), and the cultivar.
Three experiments were conducted using mist-rooted olive plants of ‘Arbequina’ and ‘Picual’ obtained from a certified nursery. Plants were transferred to 1.5-L plastic pots and placed in a sheltered shade-house located at the Experimental Farm of Rabanales, University of Córdoba, Spain (37°55′N, 4°43′W). The experiments were carried out between July and December in the range of 14 to 30 °C. In all experiments, Si treatments were initiated 30 d after the acclimation of the plants to the shade-house climate conditions. During this period, the plants were regularly watered with tap water. YaraVita Actisil [Bio Minerals N.V., Belgium (Actisil)], whose active compound is choline-stabilized orthosilicic acid, was applied as the Si source. Actisil contains a minimum of 0.5% (w/v) Si.
A preliminary experiment (Expt. 1) was conducted with olive plants of both cultivars growing in pots containing a mixture of washed sand and peat (2:1 by volume). Two methods of Si application were used, foliar or soil. An aqueous solution of Actisil at concentrations of 0%, 0.05%, 0.1%, or 0.2% (v/v) (equivalent to 0, 2.5, 5, or 10 mg·L−1 of Si, respectively), was uniformly sprayed onto leaves until the dripping point (foliar treatment) or applied to the soil through the irrigation water (soil treatment) at concentrations of 0%, 0.025%, 0.5%, or 0.1% (v/v) (equivalent to 0, 1.25, 2.5, or 5 mg·L−1 of Si, respectively). Control plants (0% of Actisil) were sprayed or irrigated with deionized water without the addition of Si in all three experiments. Silicon treatments were applied for 16 weeks, and 1 week later plants were harvested. For each cultivar, the experiment was arranged in a randomized complete block design with the two methods of Si application, foliar or soil, with three single plant replications per treatment.
In the second experiment (Expt. 2), plants were grown in pots with a mixture of washed river sand and perlite (2:1 by volume). In this case, foliar Si treatments were applied, increasing the doses and replications used in Expt. 1. Following the same procedure described before, an aqueous solution of Actisil at concentrations of 0%, 0.1%, 0.2%, or 0.4% (v/v) (equivalent to 0, 5, 10, or 20 mg·L−1 of Si, respectively) was applied by foliar spray for 16 weeks. One week later the plants were harvested. The experiment was arranged in a randomized complete block design with 10 single plant replications per Si treatment and cultivar.
In the third experiment (Expt. 3), the same substrate as in Expt. 1 was used. For each cultivar, plants were arranged in a factorial experiment design with six replications and two factors: method of Actisil application (foliar or soil), and dose used: 0%, 0.1%, 0.2%, or 0.4% (v/v), equivalent to 0, 5, 10, or 20 mg·L−1 of Si, respectively. The Si treatments were applied for 28 weeks. Two weeks later the plants were harvested.
In all experiments, Actisil was applied foliar once per week (∼50 mL per pot), and three times per week when it was applied to the soil through the irrigation water (∼100 mL per pot each time). To prevent nutritional deficiencies, plants were watered monthly with 100 mL per pot of 2 g·L−1 of Hakaphos Verde fertilizer 15–10–15 (Compo, Germany), containing 15% N, 4.4% P, 12.4% K, 1.2% Mg, 12% S, 0.01% B, 0.05% Fe, 0.05% Mn, 0.02% Zn, 0.02% Cu, and 0.001% Mo.
In Expt. 1 and Expt. 2, total shoot length was measured every 15 days to determine new shoot growth. The evolution of Si concentration in leaves was analyzed along Expt. 2 and Expt. 3. For this purpose, fully expanded current-year leaves from each treatment were collected, every month in Expt. 2, and at two different moments in the Expt. 3: 16 weeks after the initiation of Si treatments (sampling 1) and 2 weeks after the last Si application, this is, from plants subjected to 28 weeks of Si treatments (sampling 2).
At the end of all experiments, plants were harvested and individually separated into different organs: leaves, stems, and roots, to determine dry matter and Si concentration of each organ. All plant material was rinsed with deionized water, dried at 70 °C until constant weight (72 h), ground, and stored in an oven at 60 °C until Si analysis.
Si concentration in the samples was determined as described by Kleiber et al. (2015). Briefly, the plant material was digested in nitric acid (33%) under high pressure in a microwave. Si concentration was measured by electrothermal atomic absorption spectrometry with inverse longitudinal Zeeman background correction (Perkin Elmer Analyst 800; Perkin Elmer, Waltham, MA) and pyrocoated graphite tubes with an L’vov platform (Perkin Elmer).
An analysis of variance (ANOVA) was performed on the data using the Statistix 10.0 software package (Analytical Software, Tallahassee, FL). Where a significant F-value was observed in the ANOVA, mean separation among treatments was obtained by polynomial contrasts or using Tukey’s test and a 5% rejection level.
No significant differences in Si concentration on leaves, stems, and roots of both cultivars were found in Expt. 1 when Si was applied by foliar spray (Si foliar) or through the irrigation water (Si soil) (Tables 1 and 2).
Si was mainly accumulated in the roots, followed by the leaves and the stems of the plants. No effect of the dose or the method of Si application was observed.
Although Si application did not affect shoot growth in ‘Arbequina’, it had a stimulating effect in ‘Picual’ for both methods of Si application (Table 3).
In Expt. 2, Si concentration significantly increased in all plant organs with the amount of Si applied by foliar spray in both cultivars when compared with control plants (0 mg·L−1 Si). As occurred in the first experiment, the root was the organ with the higher Si content followed by the leaves and the stems of the plants (Table 4).
Leaf Si concentration increased according to the doses of Si applied (Fig. 1). Differences in Si accumulation in leaves between treated and control plants (0 mg·L−1 Si) were observed 60 d after the initiation of Si treatments. The dose of 20 mg·L−1 was the most effective to increase Si level in leaves.
Citation: HortScience 57, 12; 10.21273/HORTSCI16750-22
Nonsignificant effects of Si foliar applications were observed in vegetative growth in both cultivars (Table 5).
In Expt. 3, the combined effect of the dose and the method of Si application, foliar and soil, on leaf Si concentration was studied at two different moments in both cultivars (Figs. 2 and 3). In ‘Arbequina’, 16 weeks after the onset of Si treatments a significant interaction between both factors was observed: at the highest doses of Si used, foliar application significantly increased leaf Si concentration (Fig. 2A; Sampling 1; P < 0.01). After 28 weeks of Si treatments, plants did not receive Si for 15 d, in this case, foliar Si application was also more effective than soil in increasing Si levels in leaves (Fig. 2B; Sampling 2; P < 0.001). For both methods of Si application, a decrease in leaf Si concentration was observed at the higher doses applied (Fig. 2B). In ‘Picual’, after 16 weeks of Si applications, leaf Si concentration increased significantly with the amount of Si applied, but no differences were found between the two methods of Si application (Fig. 3A; Sampling 1; P > 0.001). In sampling 2, significant interaction between the dose and method of application of Si was found: Si accumulation in leaves was significantly reduced at the highest dose of the soil application method (Fig. 3B; P < 0.05).
Citation: HortScience 57, 12; 10.21273/HORTSCI16750-22
Citation: HortScience 57, 12; 10.21273/HORTSCI16750-22
Few published data are available on Si nutrition in dicotyledonous plants and nothing in olive trees. To have some information on this respect in olive, we have studied the effect of Si application on the distribution and accumulation of this element in young olive plants, to identify the best doses at which Si could be applied in the field in future studies. This knowledge may be critical for the potential use of Si as an environmentally friendly alternative to reduce the impact of abiotic and biotic stresses on olive trees.
In a preliminary experiment (Expt. 1), we used doses ranging from 0 to 10 mg·L−1 of Si, applied by foliar spray or to the soil through the irrigation water. In the literature, it has been reported that both methods of Si application can be effective in increasing Si concentration in plant tissues (Ashour et al., 2017; Nascimento et al., 2017), although there is some controversy in this respect. Some authors have reported that soil applications are more effective (Abd El Gayed, 2019; Abed-Ashtiani et al., 2012; Agostinho et al., 2017; Savvas and Ntatsi, 2015; Zajaczkowska et al., 2020), and others highlight the effectiveness of foliar Si applications (Laane, 2017, 2018; Pilon et al., 2014). However, in our first experiment, none of the doses of Si used or application methods were effective in increasing Si levels in plant tissues. Consequently, no influence of Si was found on the vegetative growth except in ‘Picual’, although this effect was no longer observed in subsequent experiments. Because the essentiality of Si in higher plants has not been demonstrated yet (Marschner, 2012), the stimulation of plant growth after its application could not be expected. Despite that, some authors have observed that Si promotes vegetative growth in different crops (Korndörfer and Lepsch, 2001; Krupa-Małkiewicz and Calomme, 2021; Liang et al., 2015; Pati et al., 2016; Sá et al., 2015), particularly when plants are under biotic and abiotic stress conditions (Ma, 2004). Furthermore, the highest Si concentration in plants was observed in the roots. This result agrees with the results reported in other dicotyledons, such as cowpea (Pereira et al., 2018), citrus (Mvondo-She and Marais, 2019), passion fruit (Linhares, 2019), and petunia (Boldt and Atland, 2021), and can be explained by differences in the ability of the roots to absorb Si among plant species (Ma and Yamaji, 2006). Monocotyledons, in general, take up Si actively by specific proteins called Low Silicon (Lsi): it enters from the external solution to the root cortical cells through a specific influx channel (Lsi1) and reaches the xylem vessels by a specific efflux transporter (Lsi2). Then, the unloading of Si from the xylem vessels to the leaf is mediated by another influx transporter (Lsi6) (Mitani and Ma, 2005). Therefore, the uptake and transport of Si from the roots to the aerial organs is efficient, which could explain the lower Si concentration in the roots than in the aerial organs commonly observed (Hodson et al., 2005). But in dicotyledons, Si uptake occurs passively, that is, it moves across the root cortex following decreasing concentration gradients, and once in the xylem vessels it is transported to the aerial organs. Along the radial transport in roots, Si can be retained in cortical cell walls, preventing the translocation to the aerial organs (Heine et al., 2005; Raven, 2001).
Increasing the doses of Si applied up to 20 mg·L−1 and the number of plant replicates (Expt. 2) promoted leaf Si accumulation as the amounts of Si applied raised. Continuous application of Si for a longer time did not result in a higher leaf Si concentration. In the literature, no critical toxic levels of Si for plants have been defined yet, and even it has been stated that an excess of Si is not harmful for plants (Guntzer et al., 2010; Ma et al., 2001). However, some studies have reported toxicity effects. Mantovani et al. (2018) observed a reduction in dry matter accumulation of 10% in two genera of orchids, Phalaenopsis and Dendrobium, after 18 months of foliar applications of Si at concentrations higher than 39 and 18 mmol·L−1, respectively. In different potato genotypes growing hydroponically, the addition of NaSiO3 to the growth medium at concentrations greater than 2.5 mM reduced different shoot growth parameters, mainly the leaf area (Dorneles et al., 2018). Similar phytotoxic effects have also been found in pineapple because of excessive applications of Si (Santos, 2016). In this study, vegetative growth was reduced and simultaneously a lower accumulation of N, P, Ca, Mg, and S was observed. The author attributes these effects to the fact that the excess of Si is deposited as amorphous silica, which polymerizes in the cuticle and the cell walls of the leaves, forming a thick and inflexible layer that reduces the opening and closing of stomata. Consequently, gas exchange could have been reduced, influencing the reduction of vegetative growth, biomass accumulation, and mineral nutrients uptake. It should be noted that an excess of Si caused toxic symptoms in other preliminary works carried out in our laboratory (unpublished data).
The method of Si application does not seem to influence the absorption of Si, although in ‘Arbequina’ at the highest amounts of Si used, foliar application was more effective than in ‘Picual’. These results could have an important interest in olive culture practices because most of the olive orchards are cultivated under rainfed conditions, and foliar sprays are the common application form of phytosanitary products (Nascimento-Silva et al., 2019).
In the leaves, once Si is deposited in epidermal cells it becomes immobile and cannot be translocated to newly formed leaves (Huber et al., 2012; Ma and Takahashi, 2002; Tubana et al., 2016). Continuous application of Si seems to be critical to protect growing organs against biotic and abiotic stresses. In this respect, we have observed that after 15 d without Si applications, there was a certain decrease in Si accumulation in leaves at the highest application doses.
In young olive plants, Si accumulates in leaves as the concentration of Si applied increases. Differences in leaf Si accumulation between treated and control plans were evident 60 d after its application. The dose of 20 mg·L−1 was the most appropriate to increase Si level in leaves under the trial conditions. It seems that Si should be applied periodically to avoid a reduction in Si accumulation in leaves. Si is accumulated highly in the root, followed by the leaves and stems. The method of Si application, sprayed onto leaves or through the irrigation water, seems to be equally efficient to increase Si organ concentration independently of the olive cultivar. Therefore, the foliar method could be recommended to be used in rainfed olive orchards.
Evolution of leaf Si concentration in response to foliar application of Si at different doses in ‘Arbequina’ and ‘Picual’. Measurements were made in fully expanded leaves monthly. Bars represent the standard error of the mean (Expt. 2).
Effect of soil and foliar application of Si at different doses on leaf Si concentration in ‘Arbequina’. Leaves of sampling 1 (A) were collected 16 weeks after the initiation of the Si treatments. Leaves of sampling 2 (B) were collected from plants subjected to 28 weeks of Si treatments 2 weeks after the last Si application. Bars represent the standard error of the mean (Expt. 3).
Effect of soil and foliar Si application at different doses on leaf Si concentration in ‘Picual’. Leaves of sampling 1 (A) were collected 16 weeks after the initiation of the Si treatments. Leaves of sampling 2 (B) were collected from plants subjected to 28 weeks of Si treatments, 2 weeks after the last Si application. Bars represent the standard error of the mean (Expt. 3).
Contributor Notes
This work was supported by project AGL2017-85246-R financed by the Agencia Estatal de Investigación and European Regional Development Funds (AEI/FEDER, UE). We also acknowledge Dr. Mario Calomme for his help in the analysis of Si.
M.B.-G. is the corresponding author. E-mail: g72begom@uco.es.
Evolution of leaf Si concentration in response to foliar application of Si at different doses in ‘Arbequina’ and ‘Picual’. Measurements were made in fully expanded leaves monthly. Bars represent the standard error of the mean (Expt. 2).
Effect of soil and foliar application of Si at different doses on leaf Si concentration in ‘Arbequina’. Leaves of sampling 1 (A) were collected 16 weeks after the initiation of the Si treatments. Leaves of sampling 2 (B) were collected from plants subjected to 28 weeks of Si treatments 2 weeks after the last Si application. Bars represent the standard error of the mean (Expt. 3).
Effect of soil and foliar Si application at different doses on leaf Si concentration in ‘Picual’. Leaves of sampling 1 (A) were collected 16 weeks after the initiation of the Si treatments. Leaves of sampling 2 (B) were collected from plants subjected to 28 weeks of Si treatments, 2 weeks after the last Si application. Bars represent the standard error of the mean (Expt. 3).
