Influence of Arbuscular Mycorrhizal Fungi on Physiology and Yield of Eggplant in Organic Soilless Production System

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  • 1 Department of Plant and Soil Science, Davis College of Agricultural Sciences and Natural Resources, Texas Tech University, Lubbock, TX 79409

Recent studies suggest that arbuscular mycorrhizal fungi (AMF) have the potential to improve the growth and yield of eggplant (Solanum melongena L.) under soil-based organic production systems. However, the application of AMF in organic soilless vegetable production in a greenhouse has not been well studied, creating an important knowledge gap. Therefore, two greenhouse experiments [Experiment 1 (E1) and Experiment 2 (E2)] were conducted to investigate the effect of AMF (Glomus spp.) on the growth, gaseous exchange, and yield of eggplant fertilized with various liquid organic fertilizers (OFs) and inorganic fertilizers (IFs) in a soilless greenhouse production system. The experiment was conducted in a split-plot design with four replications in which liquid OFs [OF1 (5N–1P–1K), OF2 (0N–5P–5K and 3N–3P–3K), OF3 (3N–1P–1K), OF4 (5N–1P–2K), OF5 (3.7N–2.7P–3.7K), and OF6 (3N–3P–5K)], and IFs [IF1 (6N–4P–4K) and IF2 (4N–0P–1K and 1N–3P–5K)] were randomized as main plot factor, and AMF [inoculated and uninoculated (control)] as a subplot factor. Results indicate that AMF inoculation had no significant effect on the growth, gaseous exchange, and yield parameters of eggplant. Among different OFs, the eggplant fertilized with OF6 resulted in a 4.3% and 3% reduction of leaf area compared with top-performing IF1 treatment in E1 and E2, respectively. Further, the OF6 treatment resulted in a 12% and 15% reduction in total yield per plant compared with IF1 in E1 and E2, respectively. The differences in plant gaseous exchange parameters were also nonsignificant for eggplants fertilized with different OF and IF treatments in both E1 and E2. These results conclude that Glomus spp. were not associated with a significant increase in the yield of eggplant in the soilless production system. However, OFs were performing similar to IFs in terms of growth and yield, which could be due to a higher nutrient availability of these OFs, which are highly useful for the production of eggplant in greenhouse soilless production systems.

Abstract

Recent studies suggest that arbuscular mycorrhizal fungi (AMF) have the potential to improve the growth and yield of eggplant (Solanum melongena L.) under soil-based organic production systems. However, the application of AMF in organic soilless vegetable production in a greenhouse has not been well studied, creating an important knowledge gap. Therefore, two greenhouse experiments [Experiment 1 (E1) and Experiment 2 (E2)] were conducted to investigate the effect of AMF (Glomus spp.) on the growth, gaseous exchange, and yield of eggplant fertilized with various liquid organic fertilizers (OFs) and inorganic fertilizers (IFs) in a soilless greenhouse production system. The experiment was conducted in a split-plot design with four replications in which liquid OFs [OF1 (5N–1P–1K), OF2 (0N–5P–5K and 3N–3P–3K), OF3 (3N–1P–1K), OF4 (5N–1P–2K), OF5 (3.7N–2.7P–3.7K), and OF6 (3N–3P–5K)], and IFs [IF1 (6N–4P–4K) and IF2 (4N–0P–1K and 1N–3P–5K)] were randomized as main plot factor, and AMF [inoculated and uninoculated (control)] as a subplot factor. Results indicate that AMF inoculation had no significant effect on the growth, gaseous exchange, and yield parameters of eggplant. Among different OFs, the eggplant fertilized with OF6 resulted in a 4.3% and 3% reduction of leaf area compared with top-performing IF1 treatment in E1 and E2, respectively. Further, the OF6 treatment resulted in a 12% and 15% reduction in total yield per plant compared with IF1 in E1 and E2, respectively. The differences in plant gaseous exchange parameters were also nonsignificant for eggplants fertilized with different OF and IF treatments in both E1 and E2. These results conclude that Glomus spp. were not associated with a significant increase in the yield of eggplant in the soilless production system. However, OFs were performing similar to IFs in terms of growth and yield, which could be due to a higher nutrient availability of these OFs, which are highly useful for the production of eggplant in greenhouse soilless production systems.

Consumer demand for organic products has been growing in double digits since the early 1990s, providing market opportunities for U.S. farmers to produce a wide range of organic products (Greene et al., 2017). According to the Organic Trade Association, the U.S. total organic product sales for 2015 were estimated at $43.3 billion, in which organic food sales and organic nonfood sales accounted for $39.7 billion and $3.6 billion, respectively (McNeil, 2016). Moreover, nearly 6% of all the food sold in the United States in 2015 was certified organic. Among all the major organic categories, organic fruits and vegetables were the top-selling organic category with a sale value of $14.4 billion (McNeil, 2016). Thereby, the consumer continuous demand for organic food leads to further opportunities for the U.S. organic growers to participate in high-value markets in the United States and other countries (Greene et al., 2017). In 2015, the area under open-field and covered (greenhouse and high tunnels) organic farms in the United States was 74,998 ha and 137 ha, respectively, whereas the gross sales value of covered organic vegetable systems was $535k·ha−1 compared with $18.1k·ha−1 of open organic systems (USDA NASS, 2015). This data statistic further concludes that organic vegetable production under covered systems in 2015 was more profitable with a gross sales value 29.4 times higher than open organic vegetable systems. Therefore, the greenhouse and high tunnel growers have more opportunities to obtain benefit through adopting the production and sales of certified organic vegetables. However, the adoption of organic production practices is highly challenged by nutrient management (Burnett and Stack, 2009; Moncada et al., 2021), which is an important factor to influence the yield and quality of organic crops (Dordas et al., 2008; Pokhrel et al., 2019).

Organic nutrient management for vegetable production often includes OFs, which may be applied as substrate-incorporated OFs (SOFs) (added before transplanting), top-dressing, or sidedressing (added after transplanting), and water-soluble or liquid organic fertilizers (LOFs) (added through drip irrigation). The SOFs are further categorized as conventional mineral, organic, or noncertified organo-minerals (Burnett et al., 2016). The SOFs may be derived from single or blended sources of plant and animal by-products or permissible mined organic sources (Burnett et al., 2016; Treadwell et al., 2011). Plant-based SOFs include alfalfa (Medicago sativa L.) meal, kelp (included under Order Laminariales and family Phaeophyceae), and soybean (Glycine max L.) meal (Gaskell and Smith, 2007), whereas fish meal/powder, bone meal and blood meal, pelleted chicken manure, seabird and bat guano, and feather meal are commonly used animal-based SOFs (Gaskell and Smith, 2007). In plant and animal-based residue fertilizers, the organic nutrients are usually not immediately available to plants, unlike IFs, and must require a microbial mineralization process in the substrate to convert into plant-available forms (Bi et al., 2010; Burnett et al., 2016). The microbial mineralization process highly depends on the substrate temperature, air porosity, moisture content, and particle size and composition of the OF source (Gaskell and Smith, 2007; Guajardo-Ríos et al., 2018). Because of aforementioned challenges, OFs are usually less soluble than conventional IF sources (Banados et al., 2012; Guajardo-Ríos et al., 2018; Hirzel et al., 2012; Tamada, 2004), and it is difficult with OFs to adjust the rate of nutrient release to the plant's nutrient requirements (Treadwell et al., 2011). With high substrate pH and electrical conductivity (EC) (Bi et al., 2010), the release of high ammonia (NH3) with the application of blood meal and feather meal (Zandvakili et al., 2019), and release of allelopathic compounds from plant-based alfalfa residues (Nair et al., 2011) are further challenges while using plant- and animal-based SOF (Burnett et al., 2016), whereas LOFs are highly refined, concentrated, and water-soluble with higher plant-available nutrient forms compared with SOFs (Burnett et al., 2016; Hartz et al., 2010; Pichyangkura and Chadchawan, 2015; Pokhrel et al., 2019). Application of these commercially available LOFs could be a suitable alternative to limit the challenges in using SOFs by imparting adequate fertility to high-value greenhouse-grown crops (Pokhrel et al., 2019), especially for vegetable cultivation (Zhai et al., 2009).

The LOFs are derived from plant and animal sources, including fish and seaweed emulsions, fish hydrolysates, and oilseed extract (Burnett et al., 2016) by the microbial digestion process. The LOFs consist of readily available nitrogen (N) (Burnett et al., 2016; Hartz et al., 2010) that could potentially improve the growth and yield of vegetables in organic soilless production systems. Compared with SOFs, the LOFs consist of higher organic matter and soluble nutrients that could maintain soil sustainability and the overall health of plants (Dordas et al., 2008; Hou et al., 2017; Ji et al., 2017). Moreover, specific compounds like chitin, humic and fulvic acids, and other biopolymers in LOFs can act as biostimulants to plants (Canellas et al., 2015; Du Jardin, 2015; Ji et al., 2017; Sharp, 2013; Tang et al., 2014). However, lesser nutrient availability from OFs results in 5% to 34% lower yields in the organic system than the conventional inorganic production system (Pokhrel et al., 2019; Seufert et al., 2012; Shaik et al., 2022). Further, lack of beneficial microorganisms in soilless media may reduce rate of microbe-mediated mineralization, which eventually reduces the nutrient availability to plant growth. Thereby, a sustainable production system must include efficient OFs with a combination of plant beneficial microorganisms to optimize yield in organic vegetable production systems (Burnett et al., 2016).

Plant beneficial microorganisms such as biofertilizers play a crucial role in sustainable agriculture (Zandi and Basu, 2016) because they can be used as mineral solubilizing agents, biocontrol agents, and biological fungicide to improve productivity (Burnett et al., 2016). The AMFs can be used as biofertilizers to increase the yield and quality in the vegetable production (Baum et al., 2015). AMFs are obligate symbiotic fungi that help in an extension of the plant root system to enhance immobile nutrient uptake, such as phosphorus, zinc, and copper (Neumann and George, 2010), through a mutualistic symbiosis with more than 80% of vascular plants (Douds et al., 2017; Smith and Read, 2010). The addition of AMF to soilless production systems could improve plant efficiency to uptake organic nutrients through mycorrhizal colonization. Thereby, substrate inoculation with AMF can be a promising strategy for mobilization and uptake of unavailable nutrients from the LOFs in the soilless production system.

Vegetables are complex foods that consist of numerous vitamins, minerals, fiber, carotenoids, flavonoids, and other bioactive substances (sterols, indoles, and phenols) that may help in cancer prevention (Marmot et al., 2007). Eggplants (Solanum melongena L.) are a fruit vegetable crop that belongs to the Solanaceae family. The eggplant fruits are good sources of fiber, vitamin B1, and copper. Nasunin, an anthocyanin present in eggplant skin, acts as an antioxidant that can scavenge reactive oxygen species (Douds et al., 2017; Ichiyanagi et al., 2006; Noda et al., 2000; Sakamura et al., 1963). Moreover, eggplant also consists of phenols, especially chlorogenic acid, that work as antioxidants in the human diet (Douds et al., 2017; Stommel and Whitaker, 2003; Whitaker and Stommel, 2003).

Various studies have shown that several vegetables, including eggplant, provide high yields and good quality of produce when cultivated in a greenhouse soilless system using inorganic fertilization (Miceli et al., 2003; Moncada et al., 2021; Settanni et al., 2012). However, there is little research aiming at fruit vegetable production with a combination of fertilization (Shaik et al., 2022; Treadwell et al., 2011), especially LOF and AMF inoculation in the soilless production system. For this reason, the objective of the present work was to assess the potential of different LOFs and AMF in improving the growth, gaseous exchange, and yield attributes of eggplant in a soilless production system under greenhouse conditions. We hypothesized that the higher nutrient availability through LOFs and the higher nutrient uptake through AMF, would work synergistically to improve eggplant growth and yield in the soilless production system.

Materials and Methods

Experimental site and growth conditions.

Two experiments were conducted in a glasshouse at the horticulture gardens and greenhouse complex, Department of Plant and Soil Science, Texas Tech University, Lubbock, TX (lat. 33°35′03.3″N, long. 101°53′13.2″W, 979 m mean sea level). The first experiment (E1) was conducted in the glasshouse from 3 Mar. to 9 June 2020, and the second experiment (E2) was conducted in the same glasshouse from 18 July to 21 Oct. 2020 (Supplemental Fig. 1A). The soilless substrate used in these two experiments was Resilience Silicon Enriched Metro-Mix MM 902 RSi (Sun Gro Horticulture, Agawam, MA), which consisted of 45% to 55% processed softwood bark, 25% sphagnum peatmoss, 20% vermiculite, and 5% to 10% perlite.

Seed sowing and transplanting.

In both the experiments, seeds of eggplant cultivar Jylo (Johnny’s selected seeds, Winslow, ME) were sown into 72-cell plug trays (Hydrofarm, Petaluma, CA) on 12 Feb. 2020 and 30 June 2020 in E1 and E2, respectively. The seedlings were transplanted at the four leaves stage (18 d after sowing) into 30-cm-diameter plastic pots (10 L) filled with the soilless substrate on 3 Mar. 2020 (E1) and 18 July 2020 (E2). The transplanted plants in pots were spaced at a distance of 72 cm between the rows and 70 cm within rows, maintaining a plant density of two plants/m2.

Treatments and their nutrient composition.

The LOFs of various sources such as OF1 (5N–1P–1K), OF2 (0N–5P–5K and 3N–3P–3K), OF3 (3N–1P–1K), OF4 (5N–1P–2K), OF5 (3.7N–2.7P–3.7K), and OF6 (3N–3P–5K), and IF sources as IF1 (6N–4P–4K) and IF2 (4N–0P–1K and 1N–3P–5K) were used as treatments. These LOF products are listed under the Organic Material Review Institute for organic crop production. The different sources and nutrient composition of these OFs and IFs are given in Table 1. Each treatment was prepared in a 49-L capacity plastic container by using reverse osmosis (RO) water with a final EC of 2.0 dS·m−1 measured by an Orion Star A329 pH/ISE/Conductivity/Dissolved Oxygen Portable Multiparameter Meter (Thermo Fisher Scientific, Waltham, MA). The pH, EC, and total dissolved solids of prepared nutrient solution were measured before application. The respective fertilizer treatments were applied at 1 L per plant every 3-d interval with RO water applied between fertilizations. The commercial product AMF, called Micronized Endomycorrhizal Inoculant (BioOrganics LLC, New Hope, PA), which contains a blend of nine endomycorrhizal spores (Glomus aggregatum, Glomus etunicatum, Glomus clarum, Glomus deserticola, Glomus intraradices, Glomus monosporus, Glomus mosseae, Gigaspora margarita, and Paraglomus brasilianum), was mixed with water at 30 mL·L−1 and used for drenching of seedling trays for 24 h before transplanting.

Table 1.

Organic and inorganic fertilizers used in the present study at Lubbock, TX.

Table 1.

Experimental design and data collection.

The experiment was conducted in a split-plot design with OFs (OF1–OF6) and IFs (IF1 and IF2) treatments as the main plot factor, and with and without inoculation of AMF as subplot factors. Plant growth parameters such as plant height, stem diameter, and total leaf area were recorded at the end of the experiment [i.e., 90 d after transplanting (DAT)] by taking four plant replicates in each treatment. The plant height was recorded as the distance from soilless substrate surface to the tip of eggplant. The stem diameter of each plant was measured with Digital Caliper (Carrera Precision; Max Tool, Ontario, CA). The leaf area was measured with LI-3100C Area Meter (LI-3100C; LI-COR, Lincoln, NE). The yield parameters like total marketable yield, average single fruit weight, and the number of fruits per plant were recorded from four plants during the harvest period. The fruits were harvested at baby size (226 g) for every 5-d interval and fruit weight was recorded using a weighing balance (Ohaus R31P15, Ranger 3000 Compact Bench Scale; Ohaus, Parsippany, NJ).

The physiological responses of eggplant to fertilizer and AMF treatments were assessed at 45 DAT using four plants from each treatment by measuring assimilation rate [net photosynthesis (Pn)], stomatal conductance (gS), and transpiration rate (E) using a portable photosynthesis system (Model LI-6800, LI-COR Biosciences). These gas exchange measurements were recorded at 45 DAT in both E1 and E2. In addition, chlorophyll accumulation (Chl) was measured from four plants using the Apogee chlorophyll concentration meter (Apogee Instruments, Inc., Logan, UT) at every 14-d interval throughout the growing season. All physiological measurements were recorded using young fully expanded leaves from each plant. The portable photosynthesis system was used at a steady state by keeping 1500 µmol⋅m−2⋅s−1 photosynthetically active radiation, 400 µmol·mol−1 reference CO2 concentration, 700 μmol·s−1 air flow rate, 65% of relative humidity, and switching off the temperature control (Parkash et al., 2021). All these physiological observations were recorded between 12:00 am and 2:00 pm.

Statistical analysis.

All data were statistically analyzed using analysis of variance with a split-plot design in R version 3.5.2 with Agricolae package version 1.2.8. Data were analyzed separately for each experiment. The least significant difference test at a 5% significance level was used to compare treatment means. SigmaPlot software version 14.5 (Systat Software, San Jose, CA) was used to make figures.

Results and Discussion

Greenhouse weather conditions.

The greenhouse weather conditions during both eggplant experiments, E1 and E2, are described in Fig. 1. The average temperature and humidity from March to May (E1) were 25.6 °C and 44.4%, respectively (Fig. 1A). In E2, the average temperature and humidity from mid-July to mid-October were 25.3 °C and 44.8%, respectively (Fig. 1B). The daily average solar radiation in E1 and E2 was 750 µmol·m−2⋅s−1 and 591 µmol·m−2⋅s−1, respectively (Fig. 1C and D).

Fig. 1.
Fig. 1.

Daily maximum, minimum, and average air temperatures, and average relative humidity in E1 (A) and E2 (B). Photosynthetically active radiation inside the greenhouse during crop season in E1 (C) and E2 (D) at Lubbock, TX.

Citation: HortScience 57, 7; 10.21273/HORTSCI16612-22

Effect of fertilizer treatment and mycorrhizae on plant growth parameters.

The effect of various fertilizers and AMF treatments on growth parameters (plant height, stem diameter, and leaf area) of eggplant is presented in Table 2. The interaction among various fertilizer types and AMF treatments for the measured plant growth parameters were nonsignificant during E1 and E2. The eggplant fertilized with OF and IF treatments showed no significant difference in plant height and stem diameter of eggplant in E1 and E2. The inoculated AMF treatment resulted in 1.6% and 6% higher plant height than the uninoculated treatment in E1 and E2, respectively. In both the experiments, there was no significant difference in stem diameter of eggplant inoculated with AMF compared with uninoculated AMF treatment.

Table 2.

Effect of fertilizers and arbuscular mycorrhizal fungi (AMF) on plant height, stem diameter, and leaf area of eggplant (Solanum melongena L. cv. Jylo) at 90 d after transplanting in Experiment 1 and Experiment 2 at Lubbock, TX.

Table 2.

Similar to our study, An et al. (2017) also reported no significant differences in plant height of cucumber (Cucumis sativus L.) with the application of fish meal LOF and chemical fertilizer. Hasnelly et al. (2021) found that the LOF from landfill leachate did not affect plant height and stem diameter in soybean (Glycine max L. Merrill) compared with no fertilizer treatment. The LOFs used in the aforementioned studies might be functionally similar to selected LOFs in our study in terms of providing sufficient nutrients to plant growth that eventually resulted in similar plant height and stem diameter in comparison with IFs. In contrast to our study, Wang et al. (2017) reported that OF treatments like vermicompost and chicken manure compost effectively promoted greenhouse tomato (Solanum lycopersicum L.) plant growth, including higher plant height and thicker stem diameter compared with other fertilizer treatments in a soil-based production system, which could be due to the higher available N in the soil through vermicompost application. In our study, nonsignificant differences in plant height and stem diameter of eggplants among OF and IF treatments could be because the OFs and IFs used in the study have sufficient amount of available N for eggplant growth. In contrast to our study concerning AMF results, El Maaloum et al. (2020) reported that the phosphor-compost inoculated with AMF and phosphate-solubilizing bacteria resulted in significantly higher plant height of tomato seedlings because of better absorption of nutrients through fungal hyphae in the soil. It seems like AMF could not establish a strong association with eggplant roots in our study, and similar results were found in Dasgan et al. (2008), who reported that AMF species Glomus fasciculatum did not significantly influence plant height and stem diameter of tomato compared with uninoculated tomato plants grown in recycling and open systems of hydroponics. Therefore, AMF used in our study did not improve the plant height and stem diameter of eggplant in an organic soilless production system.

The leaf area of eggplant was significantly influenced by the application of OF and IF treatments in E1 and E2 (Table 2). In E1, the eggplant treated with IF1 resulted in a higher leaf area per plant followed by the OF6 treatment. The leaf area of IF1 treatment was 33.6% and 4.4% higher than treatment of IF2 and OF6, respectively. The OF6 treatment resulted in increased leaf area per plant compared with OF3 and OF4 treatments where differences between the treatments were nonsignificant. In the case of E2, the leaf area per plant was also the highest in the IF1 treatment followed by the OF6 treatment. Among different OF treatments, the OF6 resulted in the highest leaf area per plant among OFs, which was 3% lower than the IF1 treatment. In both the experiments, the application of AMF did not significantly affect the leaf area of the eggplant compared with the uninoculated control treatment.

Similar to our results, Shaik et al. (2022) reported that lettuce (Lactuca sativa L.) plants produced the highest leaf area when fertilized with IF, which was followed by plants fertilized with different OFs. Researchers found that fish-based and fish- and plant-based OFs enhanced the lettuce growth compared with other OFs from various sources. In our study also, eggplants fertilized with fish- and plant-based OF6 treatment resulted in increased leaf area compared with other OFs, which could be because the plant essential nutrients are sufficiently supplied through both fish- and plant-based resources. Further, Ekinci et al. (2019) also recorded the highest leaf area of spinach (Spinacia oleracea L.) plants when fertilized with a combination of fish manure and IF. Irshad et al. (2006) reported that fish fertilizers act as a good rooting media conditioner that promotes root growth, which increases nutrient uptake and leads to higher leaf area of the plant. The OF6 in our study is also a fish-based fertilizer, which might have enhanced eggplant root and plant growth, and ultimately leaf area. We could conclude that fish- and plant-based OFs are more efficient in terms of nutrient availability than any other sources of OFs for organic soilless greenhouse vegetable production.

Fertilizer treatments and mycorrhizae effects on chlorophyll content and gaseous exchange of eggplant.

The interactions among fertilizers and AMF treatments for Chl and the gaseous exchange parameters such as Pn, gS, and E were nonsignificant in both experiments. The Chl varied with the application OF and IF treatments in both experiments (Fig. 2A). In E1, the total Chl was the highest in IF1, which was 4% higher Chl than IF2. Among the different OF treatments, the OF5 resulted in the highest Chl, which was 0.1% lower than the IF1 treatment. In E2, there was no significant difference in Chl of eggplant fertilized with different OFs and IFs. Moreover, the Chl did not differ between eggplants inoculated with AMF and uninoculated control plants in both experiments. Leaf color is predominantly determined by Chl, which can be affected by fertigation, particularly in terms of N availability (Moncada et al., 2021). Sage et al. (1987) and Cabrera (1998) reported a strong correlation between leaf color and N content. Lower N supply to the plants resulted in lower Chl; however, high Chl in leaves is essential for photosynthesis, and leaves with more Chl gain more light energy and produce photosynthates more efficiently. According to Lesing and Aungoolprasert (2016), leaves of kale (Brassica oleracea var. sabellica L.) showed higher Chl with organic fertilization, which was due to a high content of N in the organic fertilizer. A study conducted by Phibunwatthanawong and Riddech (2019) reported that the highest chlorophyll a, chlorophyll b, and total Chl of Green Cos Lettuce were found in LOF-treated plants, which was similar to IF. In a greenhouse study, Aboutalebi et al. (2013) also noticed that there was no significant difference in Chl of organically grown basil plants compared with IF treatments in a soilless production system. Further, El-Shinawy and Gawish (2006) showed that there was no significant difference in Chl of lettuce fertilized with different OFs and IFs in a hydroponic system. Shaik et al. (2022) reported that lettuce plants fertilized with different OFs derived from different plant and animal sources resulted in no significant difference in Chl of lettuce compared with IF. In our study, most of the cases, the eggplant fertilized with different OFs and IFs resulted in similar Chl, which could be because these OFs were composed of different plant and animal sources with an adequate N content. Díaz Franco et al. (2013) observed that a chlorophyll index was increased with AMF-inoculated pepper plants compared with uninoculated plants. However, in our study, eggplant inoculated with AMF did not show any significant difference in Chl compared with noninoculated eggplant.

Fig. 2.
Fig. 2.

Chlorophyll content (A), photosynthetic assimilation rate (B), stomatal conductance (C), and transpiration rate (D) of eggplant (Solanum melongena L. cv. Jylo) influenced by fertilizers and arbuscular mycorrhizal fungi (AMF) during Experiment 1 (E1) and Experiment 2 (E2) at Lubbock, TX. Vertical bars are standard errors of the mean. Line graphs following the same letter in each experiment are not significantly different at P ≤ 0.05. IF = inorganic fertilizer; OF = organic fertilizer; In = AMF inoculated; Unin = Uninoculated.

Citation: HortScience 57, 7; 10.21273/HORTSCI16612-22

Similarly, the effect of different OF and IF treatments on Pn, gS, and E was not significant. In E1, the rate of leaf Pn ranged between 28.4 and 31.4 µmol⋅m−2⋅s−1 with a maximum leaf Pn recorded in IF1 followed by OF4 (Fig. 2B), whereas, 28.3 to 31 µmol⋅m−2⋅s−1 range of Pn was recorded in E2 (Fig. 2B). In E1 and E2, gS of eggplant fertilized with different OFs and IFs ranged between 0.7 and 1.1 mol·m−2⋅s−1 (Fig. 2C). In E1, E of different fertilizer treatments was similar, that is, 0.02 mol·m−2⋅s−1, which was 0.01 mol·m−2⋅s−1 in E2 (Fig. 2D). There were no significant differences in gaseous exchange parameters of eggplant with the application of AMF inoculant compared with control (Fig. 2B–D).

The plant photosynthetic process depends on leaf chlorophyll molecules for harvesting the light energy and to drive the electron transport reaction. In this process, the gained photosynthates are mostly translated into plant growth benefits (Kirschbaum, 2011). Moreover, carbon is made available to plants with increased photosynthetic rate, which may lead to an increase in plant growth depending on the nature of the limiting factors, such as nutrient availability (Kirschbaum, 2011). Several studies consistently demonstrated a relationship between leaf Chl and the amount of leaf Pn in different plant species (Croft et al., 2017; Evans, 1996; Evans and Poorter, 2001). The leaf Pn is directly proportional to leaf Chl, but at a certain point the increase in Chl has no more to contribute to increase in leaf Pn (Emerson, 1929). Dordas and Sioulas (2008) reported that safflower (Carthamus tinctorius L.) plants with increased leaf Chl resulted in a higher photosynthetic assimilation rate. Similarly, many studies found that there is a liner relationship between leaf Pn and gS, which represents the contribution of gS to photosynthetic CO2 assimilation (Cechin and de Fátima Fumis, 2004; Del Pozo et al., 2007; Dordas and Sioulas, 2008; Zhao et al., 2005). Lawson and Blatt (2014) reported that the plants with higher gS had greater Pn which resulted in faster plant growth under optimal conditions, whereas low gS restricted Pn by limiting CO2 diffusion into the leaf, which substantially affected plant growth. A study conducted by Zlatev and Popov (2013) reported young tomato plants treated with two certified OFs (Emosan and Lumbrical) resulted in increased leaf gas exchange parameters. They also concluded that the photosynthetic apparatus was positively affected by OFs. In our study, the results of gaseous exchange (Pn, gS, and E) indicated that eggplant fertilized with different OFs (OF1 to OF6) performed physiologically similar to eggplant fertilized with IFs (IF1 and IF2). This could be attributed to a nonsignificant increase in leaf Chl among OF and IF treatments, resulting in a similar Pn and gS. Further, nonsignificant differences for Pn, gS, and E among OF and IF treatments could be due to similar nutrient availability from OFs and IFs.

Effect of fertilizer treatment and mycorrhizae on yield parameters.

The interactions among different fertilizer and AMF treatments were nonsignificant for all the measured yield parameters of eggplant in E1 and E2. Moreover, the AMF inoculant had no significant effect on yield parameters of eggplant, while significant differences resulted because of the application of different OF and IF treatments to the plants in E1 and E2 (Supplemental Fig. 1B). The number of fruits per plant differed significantly with fertilization of different OFs and IFs in E1 (Fig. 3A). The IF1 resulted in the highest number of fruits per plant, and which was 14% higher than IF2. Among OF treatments, OF6 resulted the highest number of fruits per plant, which was 13% lower than IF1. Whereas, in E2, IF2 treatment resulted in 10 fruits/plant, which was 17% higher than IF1 treatment (Fig. 3B). The OF6 resulted in a 6% lower fruit number per plant than IF2 but 10% higher fruits per plant than IF1 treatment. In both E1 and E2, there was no significant influence of inoculated treatment over control treatment. The average single fruit weight of eggplant was significantly influenced by different OFs and IFs in E1 and E2. In E1, the IF2 resulted in the highest average single fruit weight compared with other fertilizer treatments and that was 5% higher than the IF1 treatment (Fig. 4A). Among the OF treatments, OF2 and OF3 resulted in higher average single fruit weights, which were each 9% lower fruit weight than the IF2 treatment, whereas in the E2, the IF2 resulted in a higher average single fruit weight compared with other fertilizer treatments (Fig. 4B). Among the different OFs, OF2 treatment resulted in a higher average single fruit weight compared with other OF treatments and that was 9% lower than the IF2 treatment. The average single fruit weight was not significantly influenced with AMF inoculation in both E1 and E2. In E1, the highest total yield per plant was recorded in the IF1 followed by IF2 (Fig. 5A). Among different OFs, OF6 treatment recorded the highest fruit yield per plant, which is 12% lower than IF1. In E2, the IF2 resulted in a higher yield per plant, which was 18% and 67% higher than OF6 and OF1 treatments, respectively (Fig. 5B). Among OF treatments, OF6 resulted in a higher yield per plant, which was 15% lower than IF2, but 5% higher than IF1 treatment. There was no significant difference observed in yield per plant of eggplant with inoculated and uninoculated treatments of AMF in both experiments. However, the per plant eggplant yield was higher with inoculated AMF treatment, which was 4% and 6% higher than uninoculated treatment in E1 and E2, respectively. The regression analysis between the fruit yield and fruit number (R2 = 0.74; P = 0.05) (Fig. 6) showed an increase in fruit yield with per unit increment in fruit number in various treatments.

Fig. 3.
Fig. 3.

Effect of different fertilizers and arbuscular mycorrhizal fungi (AMF) on the number of fruits per plant in eggplant (Solanum melongena L. cv. Jylo) in (A) Experiment 1 (E1) and (B) Experiment 2 (E2) at Lubbock, TX. Vertical bars are standard errors of the mean. Bar graphs following the same letter in each experiment are not significantly different at P ≤ 0.05. IF = inorganic fertilizer; OF = organic fertilizer.

Citation: HortScience 57, 7; 10.21273/HORTSCI16612-22

Fig. 4.
Fig. 4.

Effect of different fertilizers and arbuscular mycorrhizal fungi (AMF) on average single fruit weight of eggplant (Solanum melongena L. cv. Jylo) in (A) Experiment 1 (E1) and (B) Experiment 2 (E2) at Lubbock, TX. Vertical bars are standard errors of the mean. Bar graphs following the same letter in each experiment are not significantly different at P ≤ 0.05.

Citation: HortScience 57, 7; 10.21273/HORTSCI16612-22

Fig. 5.
Fig. 5.

Effect of different fertilizers and arbuscular mycorrhizal fungi (AMF) on total yield per plant of eggplant (Solanum melongena L. cv. Jylo) in (A) Experiment 1 (E1) and (B) Experiment 2 (E2) at Lubbock, TX. Vertical bars are standard errors of the mean. Bar graphs following the same letter in each experiment are not significantly different at P ≤ 0.05. IF = inorganic fertilizer; OF = organic fertilizer.

Citation: HortScience 57, 7; 10.21273/HORTSCI16612-22

Fig. 6.
Fig. 6.

Relationship between the total yield per plant and fruit number per plant of eggplant (Solanum melongena L. cv. Jylo) subjected to various fertilizer treatments and arbuscular mycorrhizae inoculation at Lubbock, TX.

Citation: HortScience 57, 7; 10.21273/HORTSCI16612-22

Our results were similar to Xu et al. (2001), who reported a 13% and 24% decrease in tomato fruits per plant and total yield in OF treatment, respectively, compared with IF. Researchers explained that nutrient mineralization of OF in soil takes time, and thereby organically fertilized plants grew more slowly at early stages, which resulted in lower yield compared with those fertilized with IF. In another study, Pokhrel et al. (2019) reported that the application of plant-based OFs without amending with N-enriched water resulted in a 21% to 26% reduction in the yield of parsley (Petroselinum crispum Mill.) compared with IF. Results from the greenhouse experiment conducted by Shaik et al. (2022) reported 18% to 30% reduction in lettuce yield while using various OFs compared with IFs. The authors concluded that the yield reduction in OF treatments was due to lower nutrient availability in OFs compared with IF. These findings are in accordance with our study in which eggplant yield reduction was noticed with OFs compared with IFs. The yield reduction due to OFs (OF1–OF6) ranged between 12% and 39% compared with the best performing IF1 in E1. Similarly, a 15% to 40% eggplant yield reduction was noticed with organic fertilization (OF1–OF6) compared with IF2 in E2. This yield reduction range could be due to the lower mineralization and availability of nutrients in OFs compared with IFs at the early plant growth stage. The mineralization and availability of nutrients in OFs are highly dependent on multiple factors, such as timing of application and rate, microbial activity, soil temperature and moisture, substrate component, and the nature of the fertilizer (Bi et al., 2010; Rosen and Allan, 2007).

Our results are in contrast to a study conducted by Pradana et al. (2021) who reported that a combination of LOF and AMF significantly increased red chili yield by 33% compared with control. The authors explained that LOF improved the physical, chemical, and biological properties of soil through increased nutrient availability and increased area of root absorption through mycorrhizal inoculation. Further, Xu et al. (2001) also confirmed a 15% increase in tomato yield in treatments where plants were treated with OFs and microbial inoculation compared with plants treated with OFs without inoculation. They explained that microbial inoculum produced some plant growth regulators during reproduction that eventually changed the properties of the organic material leading to more nutrient availability in inoculated treatments. These two studies were conducted in a soil production system in which the AMF was positively influenced by soil growing conditions, which might have improved plant-root-mycorrhizal colonization and eventually the nutrient uptake process. However, Maboko et al. (2013) reported that application of AMF neither enhanced plant growth, nor yield of hydroponically grown tomato plants. They explained that higher air temperature (>28 °C) negatively affected AMF colonization with host root. Moreover, Croll et al. (2008) reported that a specific AMF species could show a significant preference to a single host plant or multiple host plants, and the application of several AMF species is not always beneficial. In our studies, AMF inoculant did not show a significant effect on growth and yield parameters of eggplant, which could be due to the high ambient temperature range of 28.7 to 34.7 °C in E1 (Fig. 1A) and 29.4 to 33.5 °C in E2 (Fig. 1B) during the growing season of eggplant, which negatively affected eggplant root colonization. Further, AMF species used in this study might have a poor association with eggplant in a soilless production system.

Conclusion

The commercial inoculation product Glomus spp. had no significant effect on eggplant growth, plant physiological responses, and yield attributes. This shows that Glomus spp. were not able to make symbiotic association with eggplants in the soilless production system. The use of different LOFs showed no significant differences in growth parameters of eggplant compared with IFs. Among different OFs, the OF6 performed similarly to IFs in terms of total yield and could be used for soilless organic fruit vegetable production under greenhouse conditions. We believe that future studies must be carried out to understand more combinations of AMF species and OFs.

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Supplemental Fig. 1.
Supplemental Fig. 1.

The greenhouse experimental site (A) and growth of eggplant (Solanum melongena L. cv. Jylo) (B) at Lubbock, TX.

Citation: HortScience 57, 7; 10.21273/HORTSCI16612-22

Contributor Notes

S.S. is the corresponding author. E-mail: s.singh@ttu.edu.

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    Daily maximum, minimum, and average air temperatures, and average relative humidity in E1 (A) and E2 (B). Photosynthetically active radiation inside the greenhouse during crop season in E1 (C) and E2 (D) at Lubbock, TX.

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    Chlorophyll content (A), photosynthetic assimilation rate (B), stomatal conductance (C), and transpiration rate (D) of eggplant (Solanum melongena L. cv. Jylo) influenced by fertilizers and arbuscular mycorrhizal fungi (AMF) during Experiment 1 (E1) and Experiment 2 (E2) at Lubbock, TX. Vertical bars are standard errors of the mean. Line graphs following the same letter in each experiment are not significantly different at P ≤ 0.05. IF = inorganic fertilizer; OF = organic fertilizer; In = AMF inoculated; Unin = Uninoculated.

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    Effect of different fertilizers and arbuscular mycorrhizal fungi (AMF) on the number of fruits per plant in eggplant (Solanum melongena L. cv. Jylo) in (A) Experiment 1 (E1) and (B) Experiment 2 (E2) at Lubbock, TX. Vertical bars are standard errors of the mean. Bar graphs following the same letter in each experiment are not significantly different at P ≤ 0.05. IF = inorganic fertilizer; OF = organic fertilizer.

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    Effect of different fertilizers and arbuscular mycorrhizal fungi (AMF) on average single fruit weight of eggplant (Solanum melongena L. cv. Jylo) in (A) Experiment 1 (E1) and (B) Experiment 2 (E2) at Lubbock, TX. Vertical bars are standard errors of the mean. Bar graphs following the same letter in each experiment are not significantly different at P ≤ 0.05.

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    Effect of different fertilizers and arbuscular mycorrhizal fungi (AMF) on total yield per plant of eggplant (Solanum melongena L. cv. Jylo) in (A) Experiment 1 (E1) and (B) Experiment 2 (E2) at Lubbock, TX. Vertical bars are standard errors of the mean. Bar graphs following the same letter in each experiment are not significantly different at P ≤ 0.05. IF = inorganic fertilizer; OF = organic fertilizer.

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    Relationship between the total yield per plant and fruit number per plant of eggplant (Solanum melongena L. cv. Jylo) subjected to various fertilizer treatments and arbuscular mycorrhizae inoculation at Lubbock, TX.

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    The greenhouse experimental site (A) and growth of eggplant (Solanum melongena L. cv. Jylo) (B) at Lubbock, TX.

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