Abstract
Water deficit in young fruit trees can reduce growth and future orchard productivity. Exogenous silicon (Si) applications have been associated with induced resistance to biotic and abiotic stresses such as water deficit, but the role of Si in fruit trees is still largely unexplored. The aim of the study was to evaluate the effect of Si applications on water status and gas exchange of young peach trees. This study comprises two experiments arranged in a factorial design with two water regimens (well-irrigated or water-stressed) and three Si concentrations (0, 10, or 20 mg⋅L−1 in the first experiment; 0, 20, or 40 mg⋅L−1 in the second experiment). Si applications via foliar spray were performed weekly after the water regimens were clearly established. Tree water status (midday stem water potential), and gas exchange parameters (CO2 assimilation, leaf transpiration, stomatal conductance, leaf water use efficiency) were measured. Si application at 10 or 20 mg⋅L−1 improved water status of water-stressed trees without affecting gas exchange, but 40 mg⋅L−1 reduced CO2 assimilation. Thus, foliar applications of Si could be a promising strategy for nonirrigated, nonbearing orchards to maintain their water status during dry periods and/or improve their recovery from water deficit.
The peach tree [Prunus persica (L.) Batsch] is one of the most important stone fruits in temperate zones and contributes millions to regional, state, and local economies in the southeastern United States, producing ≈30% of the fresh peaches in the United States (CAED 2019; Kramer et al. 2021; White 2019). This region has a subtropical humid climate and many growers do not typically irrigate young peach trees until the third year after planting, which is when they enter the adult phase and start producing fruit (Okie 2011). Growers certainly save water and irrigation costs during rainy years, but severe and extreme droughts are becoming more common in the region (Konrad and Knox 2022), and water stress in young peach trees impairs the growth and future production of the orchard (Konrad and Knox 2022; Layne et al. 2002).
Previous studies have shown that Si plays an important role in enhancing growth and mitigating the adverse effects of various abiotic and biotic stresses in plants (Ma 2004; Tubana et al. 2016; Wang et al. 2021a). Two types of mechanisms have been proposed to explain the role of Si in improving water stress tolerance: 1) when foliarly applied, Si may act as a physical barrier that reduces water loss and maximizes water use efficiency as it is accumulated as amorphous silica on the epidermis and in the sheath cells of vascular bundles, thus forming a cuticle-Si double layer (Ma et al. 2001); and 2) Si activates genomic and biochemical pathways that improve plant responses to stresses such as inducing expression of aquaporin genes and increasing root water uptake (Chen et al. 2018), increasing osmolyte accumulation (Wang et al. 2021b), adjustment of phytohormone levels (Kim et al. 2014; Yin et al. 2016) and polyamines (Liu and Xu 2007), and modulating plant antioxidant defense systems (Kim et al. 2017). The role of Si in the tolerance to abiotic and biotic stresses is still poorly understood for most fruit trees, as most studies have been carried out in row crops (Fernández-Escobar 2019). Among the few published studies concerning peach, Si has been reported to potentially control pre- and postharvest peach brown rot, to increase the total polyphenol synthesis, and to maintain a higher flesh firmness (Pavanello et al. 2016). Al-Hamadani and Joody (2021) observed that Si applications in peach trees significantly increased growth, chlorophyll, and carbohydrate content. However, to date, there is no published literature on the potential effects of Si in improving tolerance to water deficit in peach trees. Peach trees are often not irrigated during the first 2 years of growth in subtropical humid climates, such as that of the Southeastern United States. Lack of irrigation during dry periods could eventually lead to water deficit and negatively impact tree growth and future productivity. The objective of this study was to assess the role of foliarly sprayed Si on young peach trees under water deficit. Our hypothesis was that Si may improve water status and gas exchange of water-stressed trees.
Material and Methods
Two experiments were carried out at the Clemson University’s Musser Fruit Research Farm (Seneca, SC, USA; lat. 34.61°N, long. 82.87°W). Sixty 2-year-old peach trees [Prunus persica (L.) Batsch cv. Contender] were established in 19-L plastic pots filled with a mixture of 2:1 potting soil Fafard 3B (Sun Gro, Agawam, MA, USA): sand, 3.5 g⋅L−1 of lime and 3.5 g⋅L−1 Osmocote® 14–14–14 fertilizer (Scotts, Marysville, OH, USA). All trees were well-watered before the beginning of both experiments and gravimetric water content was estimated as in Romero-Conde et al. (2014), with pots having a gravimetric water content of 42.1% ± 1.2%.
In the first experiment, the 2-year-old peach trees were subjected to six treatments (10 trees per treatment, each tree being a replicate) resulting as the combination of 1) two water regimens (well-watered or water-stressed), and 2) three Si concentrations applied foliarly [0%, 0.2%, or 0.4% (v/v), equivalent to 0, 10, or 20 mg⋅L−1 Si, respectively]. Well-watered trees were maintained at the optimal irrigation level by receiving 100% of their evapotranspirative needs (Romero-Conde et al. 2014), which corresponded to 1 L per day, whereas water-stressed trees were subjected to 50% of the amount applied to well-watered trees. Irrigation was applied by an automated drip system. Tree water status was assessed through weekly measurements of midday stem water potential in a minimum of six trees per treatment (McCutchan and Shackel 1992). YaraVita Actisil® (Bio Minerals N.V., Destelbergen, Belgium), whose active compound is choline-stabilized orthosilicic acid (containing 0.5% Si, w/v) was applied as the Si source. Because this product also contains 1% calcium, a similar set of trees received the same Si concentrations from a pure (+99%, not containing choline or calcium), powdered formulation (Sigma Aldrich, St. Louis, MO, USA). Nevertheless, because no differences among products were found, this experiment only reports the results from the trees receiving YaraVita Actisil®. Si spray solutions had an average pH of 4.4 ± 0.3, did not cause phytotoxicity, and no pH adjustments were made. Si applications began when first differences in tree water status were observed (i.e., 3 weeks after water deficit treatments began) and was sprayed once per week for 5 weeks until dripping point (0.42 L per tree) with the use of battery-powered backpack sprayers.
The first experiment was carried out between September and October of 2019. After the first experiment ended, trees were well-irrigated, and went through senescence and dormancy. Eighteen months later (i.e., after trees grew new shoots and went through leaf abscission, dormancy, and vegetative growth twice), trees were used for a second experiment. The second experiment was carried out between April and May of 2021 using a similar experimental design (factorial with two water regimens × three Si concentrations); however, the level of water stress and the concentrations of Si were increased in the second experiment, with water-stressed trees receiving 30% evapotranspirative needs and Si concentrations of 0%, 0.4%, or 0.8% (v/v), equivalent to 0, 20, or 40 mg·L−1 Si, respectively. The same Si compound as in the first experiment was used.
Gas exchange parameters [CO2 assimilation, leaf transpiration, stomatal conductance, and leaf water use efficiency (CO2 assimilation/transpiration)] were measured with a gas exchange analyzer (LI-6400XTR; LI-COR, Lincoln, NE, USA). In both experiments, the last water status and gas exchange measurements were taken after all trees (including water-stressed trees) had received water from rainfall.
The experimental design was a factorial design with two factors (water stress and Si) and six treatments. Data were subjected to two-way analysis of variance (Statistix 13.0, Analytical Software, Tallahassee, FL), using Tukey’s test for mean separation (P ≤ 0.05) when a significant F-test was observed.
Results
In the first experiment, Si applications (10 or 20 mg⋅L−1) improved tree water status of water-stressed peach trees, which had less negative midday stem water potential than water-stressed trees that did not receive Si applications (Fig. 1). However, Si applications did not have any effect on the water status of well-watered trees. Si did not improve any of the gas exchange parameters measured in water-stressed trees, but a decrease in leaf CO2 assimilation was observed with increasing applications of Si in well-watered trees (Fig. 2).
Stem water potential of peach trees cv. Contender receiving three rates of foliar Si (0, 10, and 20 mg⋅L−1) under two levels of irrigation (water-stressed and well-watered). Different letters indicate significant differences and n.s. indicates nonsignificant differences at P ≤ 0.05.
Citation: HortScience 58, 4; 10.21273/HORTSCI16881-22
Stem water potential of peach trees cv. Contender receiving three rates of foliar Si (0, 10, and 20 mg⋅L−1) under two levels of irrigation (water-stressed and well-watered). Different letters indicate significant differences and n.s. indicates nonsignificant differences at P ≤ 0.05.
Citation: HortScience 58, 4; 10.21273/HORTSCI16881-22
Stem water potential of peach trees cv. Contender receiving three rates of foliar Si (0, 10, and 20 mg⋅L−1) under two levels of irrigation (water-stressed and well-watered). Different letters indicate significant differences and n.s. indicates nonsignificant differences at P ≤ 0.05.
Citation: HortScience 58, 4; 10.21273/HORTSCI16881-22
CO2 assimilation of peach trees cv. Contender receiving three rates of foliar Si (0, 10, and 20 mg⋅L−1) under two levels of irrigation (water-stressed and well-watered). Different letters indicate significant differences and n.s. indicates nonsignificant differences at P ≤ 0.05.
Citation: HortScience 58, 4; 10.21273/HORTSCI16881-22
CO2 assimilation of peach trees cv. Contender receiving three rates of foliar Si (0, 10, and 20 mg⋅L−1) under two levels of irrigation (water-stressed and well-watered). Different letters indicate significant differences and n.s. indicates nonsignificant differences at P ≤ 0.05.
Citation: HortScience 58, 4; 10.21273/HORTSCI16881-22
CO2 assimilation of peach trees cv. Contender receiving three rates of foliar Si (0, 10, and 20 mg⋅L−1) under two levels of irrigation (water-stressed and well-watered). Different letters indicate significant differences and n.s. indicates nonsignificant differences at P ≤ 0.05.
Citation: HortScience 58, 4; 10.21273/HORTSCI16881-22
In the second experiment, water status of water-stressed trees receiving Si at 20 mg⋅L−1 was comparable to that of water-stressed trees that had not received Si applications throughout the first 4 weeks of the experiment (Fig. 3). However, on the fifth week, after a rain event, water-stressed trees that had received 20 mg⋅L−1 had an improved tree water status compared with those that had not received Si or those sprayed with 40 mg⋅L−1 (Fig. 3). Si did not have a consistent effect on well-watered trees and, only by the end of the experiment, well-watered trees receiving Si at 40 mg⋅L−1 had a more negative stem water potential than those sprayed with 20 mg⋅L−1, although comparable to control trees that did not receive Si. Regarding gas exchange, foliar application of Si at 20 mg⋅L−1 did not affect CO2 assimilation of water-stressed trees throughout the experiment but water-stressed trees receiving 40 mg⋅L−1 had reduced CO2 assimilation compared with control trees and trees receiving 20 mg⋅L−1 (Fig. 4). Other gas exchange parameters were unaffected in water-stressed trees treated with Si (data not shown). Si did not have any effect on CO2 assimilation (Fig. 4), or any other gas exchange parameter of well-watered trees (data not shown).
Stem water potential of peach trees cv. Contender receiving three rates of foliar Si (0, 20, and 40 mg⋅L−1) under two levels of irrigation (water-stressed and well-watered). Different letters indicate significant differences and n.s. indicates nonsignificant differences at P ≤ 0.05.
Citation: HortScience 58, 4; 10.21273/HORTSCI16881-22
Stem water potential of peach trees cv. Contender receiving three rates of foliar Si (0, 20, and 40 mg⋅L−1) under two levels of irrigation (water-stressed and well-watered). Different letters indicate significant differences and n.s. indicates nonsignificant differences at P ≤ 0.05.
Citation: HortScience 58, 4; 10.21273/HORTSCI16881-22
Stem water potential of peach trees cv. Contender receiving three rates of foliar Si (0, 20, and 40 mg⋅L−1) under two levels of irrigation (water-stressed and well-watered). Different letters indicate significant differences and n.s. indicates nonsignificant differences at P ≤ 0.05.
Citation: HortScience 58, 4; 10.21273/HORTSCI16881-22
CO2 assimilation (CO2 µmol⋅m−2⋅s−1) of peach trees cv. Contender receiving three rates of foliar Si (0, 20, and 40 mg⋅L−1) under two levels of irrigation (water-stressed and well-watered). Different letters indicate significant differences and n.s. indicates nonsignificant differences at P ≤ 0.05.
Citation: HortScience 58, 4; 10.21273/HORTSCI16881-22
CO2 assimilation (CO2 µmol⋅m−2⋅s−1) of peach trees cv. Contender receiving three rates of foliar Si (0, 20, and 40 mg⋅L−1) under two levels of irrigation (water-stressed and well-watered). Different letters indicate significant differences and n.s. indicates nonsignificant differences at P ≤ 0.05.
Citation: HortScience 58, 4; 10.21273/HORTSCI16881-22
CO2 assimilation (CO2 µmol⋅m−2⋅s−1) of peach trees cv. Contender receiving three rates of foliar Si (0, 20, and 40 mg⋅L−1) under two levels of irrigation (water-stressed and well-watered). Different letters indicate significant differences and n.s. indicates nonsignificant differences at P ≤ 0.05.
Citation: HortScience 58, 4; 10.21273/HORTSCI16881-22
Discussion
Our experiments demonstrated that foliar applications of Si may improve tree water status on water-stressed peach trees; however, Si effectiveness to improve tree water status in our experiments might have been influenced by leaf age. To our knowledge, this is the first report on the influence of foliarly applied Si on water status of fruit trees. In our first experiment, carried out between early summer and late fall, when there was no new leaf formation, foliar Si improved water status of water-stressed trees from the first week; however, when similar and higher Si applications were made in spring, at a time when shoots and leaves were actively growing, Si did not seem to improve tree water status of water-stressed trees. Nevertheless, when trees were rewatered, water-stressed trees receiving 20 mg⋅L−1 in spring improved their water status faster than trees that did not receive Si or that received 40 mg⋅L−1. On the other hand, Si applications did not have a consistent effect on tree water status of well-watered trees in any of the experiments.
Most Si studies on fruit or nut tree water relations applied Si to the soil. Soil-applied Si increased relative water content in leaves in pistachio (Habibi and Hajiboland 2013) and grapevine (Kamangar and Haddad 2016) plants under water-stressed conditions. Although there have been some studies on the use of foliar applications of Si to fruit trees, their focus was on yield and fruit quality (Artyszak 2018), and there is very limited research on the role of foliarly applied Si on water relations and gas exchange. A study on water-stressed chestnut trees fertilized with Si (Carneiro-Carvalho et al. 2020) suggested that foliarly applied Si can reduce cuticular transpiration and permeability to water vapor, limiting the loss of water and, thus, promoting better tree water status, although they used a combination of soil-applied and foliarly applied Si.
In our experiment, Si did not influence CO2 assimilation of water-stressed trees when sprayed at 10 or 20 mg⋅L−1 but did cause a reduction when sprayed at 40 mg⋅L−1. Most research on the effect of Si on gas exchange has been done in annual crops but, in the past decade, gas exchange parameters improvement by Si application under water stress conditions has been documented in a few fruit crops, including chestnut (Zhang et al. 2013), pistachio (Habibi and Hajiboland 2013), and strawberry (Dehghanipoodeh et al. 2018). Nevertheless, Si accumulation has also been documented to reduce the flexibility of stomata walls, which tend to remain closed, thus, reducing gas exchange parameters (Botelho 2006; Zanetti et al. 2016). This could have been the reason for the reduction in photosynthesis of water-stressed trees receiving the highest Si concentration (40 mg⋅L−1). However, there are no studies on the effect of Si on cuticular transpiration, which can represent between 5% and 10% of the total leaf transpiration in higher plants (Taiz and Zeiger 2002). Thus, our study agrees with Carneiro-Carvalho et al. (2020), and we suggest that the improvement of tree water status without affecting stomatal gas exchange could have been induced by a reduced cuticular conductance as a consequence of the foliar Si applications, which were probably more effective in creating a physical barrier in mature leaves (first experiment) than in young leaves (second experiment).
Conclusion
This research shows that Si may improve water status of young, water-stressed peach trees and may improve their recovery from water deficit. Foliar applications of Si could be a promising and affordable strategy for nonirrigated, nonbearing orchards to maintain their water status during dry periods. There is a need to understand the mechanism behind Si improving tolerance to water deficit, specifically the role of Si as a physical barrier on leaf permeability to water vapor and cuticular transpiration. This knowledge would be especially relevant concerning foliar applications to species that are considered nonaccumulators like most fruit trees, such as peach.
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