**3. Enhancement of yield and antioxidant contents in strawberry by various natural products including chitosan**

To overcome the challenge of increasing strawberry production with a significant reduction of agrochemical use and environmental pollution (especially from synthetic chemicals), a great deal of interest and research has been devoted to natural products and beneficial microbes in recent days. Many growers and researchers are actively looking for ways to create a more sustainable production system through use of natural inputs while simultaneously improving yield and antioxidant properties. A large body of literature suggested that integration of these products with conventional management tools could significantly reduce chemical use and make strawberry production more sustainable. Various natural products have been tested in strawberry production to improve yield and quality by preventing disease and stimulating growth and development. Among the natural products, chitosan is the most tested that has shown growth and yield stimulating effect together with efficacy against diseases in strawberries and other crops [51]. Chitosan is a polysaccharide derived from chitin outer skeletons of shell fish and crustaceans such as crab, crayfish, lobster, and shrimp. As chitin is deacetylated by sodium hydroxide to obtain chitosan, it is slightly basic and is soluble in dilute aqueous acidic solution (pH < 6.5). Once dissolved, it can be further diluted with water to apply on plants at all different growth stages. In general, it is nontoxic to humans and considered safe for agricultural uses due to its quick degradation in the environment. Once chitosan or its derivatives come in contact with plants, they bind with the cell plasma membrane and elicit defense responses through expression of pathogenesis-related (PR) genes, accumulation of phytoalexins, callose, oxidative burst, and formation of reactive oxygen species. Expression of these PR genes and accumulation of antimicrobial phytoalexins are believed to play a major role in controlling pre- and postharvest pathogenic diseases. A large body of published reports supports antimicrobial activities of chitosan against a wide range of phytopathogens [52]. Similar studies also found that the biostimulant chitosan promoted plant growth and development and provided enhanced disease suppression capability to plants through multiple mechanisms including induced systemic resistance [51, 53]. Chitosan has been widely used as fruit coatings to enhance storability and preserve anthocyanin and other antioxidants in strawberry [51], and various other fruit mainly for protection

*Strawberry - Pre- and Post-Harvest Management Techniques for Higher Fruit Quality*

prevent oxidative stress.

consumers of strawberry.

carotenoids, flavonols, and polyphenols, with the diet being the main source

[36–39]. Exogenous antioxidants play a key role in this delicate equilibrium between oxidation and antioxidation in living systems [36, 37, 40, 41]. Under physiological conditions, the human antioxidative defense system allows the elimination of excess ROS. However, our endogenous antioxidant defense systems are incomplete without exogenous reducing compounds such as vitamin C, vitamin E, carotenoids, and polyphenols. Therefore, there is a continuous demand for exogenous antioxidants to

Strawberry polyphenolic phytochemicals perform nonessential functions in plants but have large impacts on humans. Of the polyphenolic compounds, anthocyanins in strawberries are the best-known and quantitatively the most important. Studies have determined total anthocyanin content as 150–600 mg/kg of fresh weight. [17]. Strawberries also contain small amounts of other phenolic compounds as shown in **Table 2**. Evidence from in vitro studies shows that strawberry phenolics may have anti-inflammatory effects and suppress mutagenesis through antioxidative and genoprotective properties. Additionally, the content and composition of flavonols have been studied [42], and these compounds are identified as derivatives of quercetin and kaempferol, with quercetin derivatives being the most abundant [43]. The contents of the flavonoid groups, flavonols, and anthocyanins in strawberry extracts have been associated indirectly and directly, respectively, with the total antioxidant capacity for low-density lipoproteins [21]. Flavonoids in strawberries exhibit antioxidant [44, 45] and anticancer properties as well [46]. Elevated levels of these secondary metabolites should provide better health benefits to the

Among numerous studies conducted on antioxidant contents in fruits and vegetables, results have shown that strawberry possessed a high level of antioxidant activity compared with others in the same group, and the activity was directly linked to the levels of phenolic compounds in the fruit [1, 21, 22]. A comparative study on the antioxidant activity of strawberry extract with other fruits based on the oxygen radical absorbance capacity assay indicated that its antioxidant capacity was higher than extracts from plum, orange, red grape, kiwifruit, pink grapefruit, white grape, banana, apple, tomato, pear, and honeydew melon [47]. However, Sun et al. [22]

> Cyanidin-3-rutinoside Pelargonidin-3-glucoside Pelargonidin-3-rutinoside Pelargonidin-3-malylglucoside

Quercetin-glucoside Kaempferol-3-glucoside

Proanthocyanidin B1 (EC-4,8-C) Proanthocyanidin B3 (C-4,8-C)

Flavonols Quercetin-3-glucuronide

Flavanols (+)-catechin

**Class Group Compound**

Flavonoids Anthocyanins Cyanidin-3-glucoside

Phenolic acids Hydroxycinnamic acids p-coumaroyl hexose

**84**

**Table 2.**

*Adapted from [17].*

*Polyphenol composition reported in strawberries.*

from postharvest losses due to microbial infections [51, 54]. In addition, many investigators reported that chitosan use as a foliar spray increased vegetative growth, yield, and biochemical contents in plants [55–58]. Improvement of yield and functional properties of strawberry fruit through application of chitosan should be considered a sustainable option. A recent study by Rahman et al. [58] showed that multiple application of low concentrations (ppm level) of chitosan on the canopy of field grown strawberry plants at the prebloom stage significantly improved growth and yield. Authors also reported concurrent increase in various antioxidant contents and total antioxidant activities in treated fruit compared to nontreated control. This is an interesting and significant finding as total antioxidants and pigments such as anthocyanins are determinants of health benefits of strawberry fruit. Rahman et al. [58] also determined the effect of different doses of chitosan biopolymer on growth, fruit yield, and human health benefiting antioxidant properties of strawberry and found that both yield and contents of antioxidants are increased in a dose-dependent manner to some extent compared to untreated control. These findings indicate that the biostimulant chitosan can be an attractive agent for production of high quality and human health benefiting strawberry [58]. Results also indicated that foliar application of varying doses of chitosan on strawberry canopy stimulated all aspects of vegetative growth (plant height and root length) that may have influenced fruit yield and fruit quality compared with untreated control (**Table 3**). These findings were also interesting as all doses of chitosan improved growth of strawberry plants to some extent and may be experimented in similar crops being grown in soils with varying physical, chemical, and biological characteristics. This study was one of the few of its kind that determined the effects of natural products such as chitosan application on field-grown strawberry plants influencing yield and contents of multiple antioxidants in fruit. Experimental protocol for this study can be found in Rahman et al. [58].

Among a few different chitosan concentrations tested in the study, 500 ppm provided the highest fruit yield (42% higher than untreated control) in "Strawberry Festival" compared with untreated control (**Table 3**). Similar to yield response and a few other antioxidants, chitosan spray application on the canopy of strawberry also significantly increased fruit anthocyanin contents in a dose-dependent manner that plateaued at 500 ppm with 184.3 mg cyanidin-3-*O*-glucoside/100 g fruit. This increase of anthocyanin contents was equivalent to 2.3-fold higher compared


*Five different concentrations, 0, 125, 250, 500, and 1000 ppm, of chitosan solution were prepared by dissolving the required amount in 0.1 N HCl and diluting with distilled water with pH adjusted at 6.5 by NaOH. Freshly prepared chitosan solutions were applied onto strawberry plants in each experimental unit prior to flowering and at 10% flowering stage by spraying up to run off at five different times with 10-d intervals. Cumulative fruit harvest from each plot was recorded. The required amounts of fruit tissues from first harvest were subjected to analyses for phenolics and other antioxidants mentioned in the table. Values are means ± standard errors of three independent replications (n = 3). Different superscripted letters within the column indicate statistically significant differences among the treatments according to Fisher's protected LSD (least significance difference) test at p* ≤ *0.05, adapted from [58].*

**87**

*Improving Yield and Antioxidant Properties of Strawberries by Utilizing Microbes and Natural…*

with untreated control. The fruit produced by the plants treated with 1000 ppm chitosan solution in the study by Rahman et al. [58] had the highest total phenolic content (370.9 μg gallic acid/g fruit) indicating that chitosan concentration should be adjusted depending on the intended quality improvement in strawberry fruit. Total antioxidant activities of strawberry fruit obtained from both varied rates of chitosan treated and untreated control plants were assayed by utilizing the DPPH method, and the results were expressed as butylated hydroxytoluene (BHT) equivalents per gram of strawberry fruit. The highest total antioxidant activity was quantified in strawberry fruit obtained from 1000 ppm chitosan (415.6 μg BHT/g fruit) treated plants. These results reveal that application of chitosan on the canopy of strawberry could increase antioxidant activity in fruit up to 1.7-fold compared to

A few other examples of natural products that have been investigated on strawberry with varying results are derived either from seaweed or compost. Seaweed products are used as nutrient supplements, biostimulants, and biofertilizers to augment plant growth and yield in agriculture. A study by Masny [59] found no effect on disease suppression of *Botrytis cinerea* on strawberry by applying seaweed products. However, application of these products had a significant influence on yield with an increase in the range of 17–42%. Compost or tea extracts were also used for plant disease control and for plant nutrition and growth promotion. Welke [60] assessed the effect of compost extract application on strawberry. Aerobically prepared extracts were effective in both disease suppression (*B. cinerea*) and

**4. Strawberry growth, yield, and quality improvement by probiotic** 

Beneficial microorganisms especially bacteria that are associated with host plants either as rhizoplane, phylloplane, or endophyte and enhance growth of the host plants including yield are popularly known as plant probiotic bacteria (PPB). These PPB can also suppress plant diseases by various modes of action when applied proactively in adequate amounts [61]. PPB that are used as biofertilizers or biostimulants possess the ability to colonize the rhizosphere, plant roots, or both when applied to seeds or crops. Some of these microbes have shown potential to promote strawberry plant growth by the release of metabolites into the rhizosphere that may inhibit various pathogens as biocontrol agents [5–8]. However, Tomic et al. [9] found that the response to bacterial inoculation is cultivar-related in strawberries, which indicates that a specific microbial strain should be tested for efficacy against a specific strawberry variety before large scale use. Microbes belonging to this group are also known as plant growth promoting rhizobacteria (PGPR) and were reported to improve availability of plant nutrient and support plant development under natural or stressed conditions as well as increase yield and quality. Although beneficial microbes have not been widely researched or used for improving yield and quality of strawberry, a large body of evidence indicates that many available beneficial microbes were found to provide growth and yield enhancement to diverse crop commodities [10–12], which can be tested for similar efficacy on strawberry and thus may play a crucial role in sustainable strawberry production in the future. A significant body of literature suggests that these microbes can increase strawberry fruit quality in terms of taste and nutritional value and thereby have a positive impact on human health with associated reduction of healthcare costs [13, 14]. A few relevant examples of positive effects of antagonistic microbes on multiple crops include protection against *Verticillium dahliae* [62] and protection of tomato

*DOI: http://dx.doi.org/10.5772/intechopen.84803*

untreated control (**Table 3**) [58].

increasing yield compared to the control.

**bacteria**

#### **Table 3.**

*Effect of chitosan application on yield and content of antioxidants in strawberry fruit.*

*Improving Yield and Antioxidant Properties of Strawberries by Utilizing Microbes and Natural… DOI: http://dx.doi.org/10.5772/intechopen.84803*

with untreated control. The fruit produced by the plants treated with 1000 ppm chitosan solution in the study by Rahman et al. [58] had the highest total phenolic content (370.9 μg gallic acid/g fruit) indicating that chitosan concentration should be adjusted depending on the intended quality improvement in strawberry fruit. Total antioxidant activities of strawberry fruit obtained from both varied rates of chitosan treated and untreated control plants were assayed by utilizing the DPPH method, and the results were expressed as butylated hydroxytoluene (BHT) equivalents per gram of strawberry fruit. The highest total antioxidant activity was quantified in strawberry fruit obtained from 1000 ppm chitosan (415.6 μg BHT/g fruit) treated plants. These results reveal that application of chitosan on the canopy of strawberry could increase antioxidant activity in fruit up to 1.7-fold compared to untreated control (**Table 3**) [58].

A few other examples of natural products that have been investigated on strawberry with varying results are derived either from seaweed or compost. Seaweed products are used as nutrient supplements, biostimulants, and biofertilizers to augment plant growth and yield in agriculture. A study by Masny [59] found no effect on disease suppression of *Botrytis cinerea* on strawberry by applying seaweed products. However, application of these products had a significant influence on yield with an increase in the range of 17–42%. Compost or tea extracts were also used for plant disease control and for plant nutrition and growth promotion. Welke [60] assessed the effect of compost extract application on strawberry. Aerobically prepared extracts were effective in both disease suppression (*B. cinerea*) and increasing yield compared to the control.
