Inorganic and Organic Fertilization Effects on the Growth, Nutrient Uptake, Chlorophyll Fluorescence and Fruit Quality in Solanum melongena L. Plants

Abstract

Plant growth, nutrient uptake and fruit quality may be influenced by fertilization practices. A 64-day greenhouse pot experiment, with a 6X1 factorial, i.e., Solanum melongena L. (cv. ‘Lagkadas’) plants, grown on soil substrate and submitted to six fertilization treatments (Patent Kali, Ammonium Nitrate + Patent Kali, Tree Branch Chips, Poultry Manure, Tree Branch Chips + Poultry Manure, and non-fertilization—CONTROL) was conducted. The objectives were to investigate the impact of fertilization on: (i) plant growth, (ii) nutrition, (iii) photosystem II activity and (iv) fruit quality. The main results were the following: a) the highest total plant and fruit biomass values were recorded in poultry manure, followed by those in the ammonium nitrate + patent kali treatment; (b) in most cases, total plant macronutrient content was significantly higher in the poultry manure-treated plants; (c) the optimum and most balanced plant nutrition, fruit total phenolic and flavonoid contents and antioxidant activity levels were achieved in the poultry manure, tree branch chips + poultry manure and ammonium nitrate + patent kali treatments; (d) significant decline in the values of the maximum quantum yield of photosystem II, performance index and fruit quality was found in the tree branch chips and CONTROL plants. It was concluded that the kind of fertilization significantly influenced biomass, nutrient uptake, chlorophyll content and fluorescence, as well as fruit quality of Solanum melongena L. plants. Thus, it should be thoroughly investigated, towards substituting high fertilization rates by manure applications and improving fruit quality, with human health benefits.

1. Introduction

Eggplant (Solanum melongena L.) is extensively grown on a global scale, with an annual production of 54.08 million tons and a net value of more than USD 10 billion per year, which makes it the fifth most important Solanaceous crop after potato, tomato, pepper, and tobacco [1,2,3]. It is widely grown in subtropics and tropics areas, and it is also a good source of soluble sugars, protein, anthocyanin, phenolic, and glycoalkaloid compounds [4]. Eggplant has received significant interest as a functional food among the top 10 vegetables with high antioxidant capacity because of its high content of phenolic compounds, such as delphinine derivatives and chlorogenic acid isomers [5,6]. Eggplant fruits are supplemented with sufficient amounts of starches, proteins, minerals, vitamins, dietary fibers and low-fat content, which are useful for body growth, replacement of worn-out elements, and protection of the human body [7,8]. Eggplant fruits are mainly used for cooking and pickling, which aids in digestion, and lowering high blood pressure, while it also prevents constipation; its green leaves are a good source of aphrodisiac, laxative and effective tonic for liver problems [9]. Eggplant is rich in nutrients and phytochemicals protecting against cancer, cardiovascular diseases, hypertension and fluid retention [5,10]. The National Diabetes Education Program, led by the National Institutes of Health, USA, and the American Diabetes Association, recommends eggplant as a component of the diet for people suffering from type 2 diabetes [11,12].

Eggplant is a summer vegetable crop that is grown throughout the year under the optimum temperature of 22–30 °C, while at 17 °C, its growth is inhibited [13]. Its performance is successful on a wide range of soil types, well-drained, with an optimum pH of 5.5–6.5 [14]. Eggplant is a long-duration crop, with high yields, which removes significant nutrient quantities from soil; thus, appropriate nutrient management can boost crop productivity and improve fruit post-harvest characteristics and soil properties [15]. Eggplant crop yields equivalent to 60 t ha^{− 1} remove approximately 190 kg N, 10.9 kg P and 128 kg K from one hectare, as well as significant quantities of micronutrients [16]. However, the majority of farmers apply only N-P-K fertilization for eggplant cultivation, resulting in a decline in soil fertility, low yields, poor nutritional quality and nutrient imbalances [17,18]. Considering widespread micronutrient deficiencies in soils, it is crucial to overcome them with appropriate fertilization strategies, including combined soil and foliar nutrient applications and other innovative management approaches [19]. Therefore, in order to achieve and sustain higher crop productivity and improve nutritional quality, diagnosis and correction of micronutrient deficiencies, in addition to macronutrient application, are necessary [20]. Although eggplant is one of the vegetable crops that have recently gained prominence due to the high demand for foods with nutraceutical properties, only a few published studies have evaluated the effects of organic fertilizers on crop productivity [21].

Fertilization is a crucial factor in sustaining crop productivity and yields; however, high inorganic fertilizer application rates cause environmental problems. Thus, in order to reduce and eliminate the adverse environmental effects of inorganic fertilizers, new agricultural systems have been developed, such as organic, sustainable or ecological agriculture [22,23,24,25,26]. The effects of organic manures and fertilizer application rates on growth performance and eggplant yields were studied by Vijaya and Seethalakshmi [27]. Organic manures and other biofertilizers were found to enhance eggplant productivity and constitute environmentally friendly approaches, compatible with sustainable and ecological agriculture [28]. Each kind of animal manure has a different nutrient content, depending on its origin (poultry, goat, cow manure, etc.) [29]. Manures and other by-products (e.g., biochar) play a direct positive role in soil fertility, microbial population [26] and management of disease-infested soils [30], and they also improve eggplant growth and crop productivity [31,32]. Thus, organic fertilization should be preferred to improve and provide food quality and security to people [26,33]. It was found that co-applications of 6% leaf waste biochar (LWB), together with the bio-control agent (BCA) Trichoderma harzianum, increased N, P, and K levels in eggplants by 92.74%, 76.47% and 53.73%, respectively, improved plant growth and reduced Ralstonia solanacearum-induced wilt [30].

On the other hand, inorganic fertilizers have high costs for the producers; particularly under the recent energy crisis, the prices of fertilizers have been dramatically increased, leading to high-cost production. Thus, it is fundamental to find alternative (non-conventional, mainly of organic origin) sources of nutrients to sustain crop productivity [34]; although, in some cases, these organic materials could constitute a significant cost for the producers and difficulties in easily finding them. Our study is among the first ones proving—via a combinational approach—the impact of fertilization practices and soil amendment application on eggplant nutrient uptake, physiological performance and fruit quality. In most other published studies, inorganic and organic fertilization effects in eggplants were clearly separated and compared to each other [35,36,37], while the novelty in our case is the co-application (mixed application) of conventional fertilizers with alternative sources of nutrients (e.g., ammonium nitrate + patent kali–AN + PK), as well as the application of innovative organic amendments (e.g., tree branch chips–TBC). According to our knowledge, our study is the first one investigating the effects of TBC application on eggplant biomass, nutrient uptake, physiology and fruit quality. After all the above, our research was based on the premise that non-conventional sources of nutrients (mainly of organic origin, like poultry manure and tree branch chips, as well as patent kali) could satisfy the nutritional needs of eggplants and could also influence plant growth, nutrient uptake, PSII functionality and fruit quality. This was realized via a comparative and combinational approach of application of organic amendments (poultry manure) with conventional (ammonium nitrate) and other kinds of fertilization, from natural resources (patent kali).

Thus, the aims of this study were the following: (i) to investigate the effects of different fertilization practices [non-fertilization (CONTROL), Patent Kali (PK), Ammonium Nitrate + Patent Kali (AN + PK), Tree Branch Chips (TBC), Poultry Manure (PM), Tree Branch Chips + Poultry Manure (TBC + PM)] on plant growth, fruit biomass, nutrient uptake, PSII functionality (F_v/F_m and performance index) and fruit quality, as well as (ii) to evaluate the ability of the above-mentioned organic amendments to sufficiently satisfy the nutritional requirements of eggplants; this was realized via a comparative and combinational approach of inorganic fertilization with manure application. For the needs of the research, plants of the cv. ‘Lagkadas’ were used as plant material, since it is one of the most important eggplant genotypes of Northern Greece, producing big elongated tasteful fruits of purple color.

2. Materials and Methods

2.1. Plant Material, Experimental Design and Conditions of the Experimental Greenhouse

Plant material consisted of early long purple eggplants (cv. Lagkadas), approximately 12–15 cm in height; the plants were grown in 5L pots, inside an experimental glasshouse of the faculty of Forestry and Natural Environment (Aristotle University of Thessaloniki), for 64 days (from the 17 May until 21 July 2023), on a soil substrate from parent material Gneiss. The pots were filled with soil type, collected from the upper 30 cm, from fields cultivated with vegetable crops, close to the region of Vasilika (40°28′48.0864″, 23°8′18.492″ N/E), near the city of Thessaloniki, Macedonia, northern Greece. Before the establishment of the experimentation, all the plants were fully randomized, based on their initial height, and divided into six similar groups (each one of these groups coincided with each of the six fertilization treatments described in the following paragraph). The experimental design consisted of a 6X1 factorial, i.e., Solanum melongena L. (cv. ‘Lagkadas’) plants, grown on soil substrate and submitted to six fertilization treatments. During the whole experimental period, the plants were irrigated (with high-quality tap water, 0.70 mS cm⁻¹), every second day, with an automated drip irrigation system, in order to obtain a soil moisture level of approximately 70% of its water-holding capacity. The minimum, maximum and average temperatures were 18 °C, 36 °C and 27 °C, respectively, while the relative humidity ranged from 59% to 73%. Natural (solar) light was used during the whole plant growth experimentation, for all the treatments. During the experiment, due to insect infestation, the plants were sprayed two times (on the 31 May and on the 9 June) with Karate Zeon 10 CS (Syngenta Hellas) and another two times (on the 27 June, as well as on the 5 July) with Abamectin (commercial product: Acaramik Ultra 1.8EC), combined with the biological insecticides Super VIVERE-FYT and Azadirachtin (commercial product: NeemAzal-T/S, Biogarden).

2.2. Fertilization Treatments

The following six fertilization treatments (each coincided with one of the six plant groups mentioned in the Section 2.1) were applied: (a) Patent Kali (PK) (0-0-30 + 10 Mg + 42S—suggested quantity for optimum eggplant growth under field conditions: 800 kg ha⁻¹; by performing the necessary calculations and conversions for the 5 L pots, i.e., surface pot area: πR² with mean pot diameter 18.5 cm, it was found that 2 g PK should be applied per pot, in one dose, at the beginning of the experiment). The second treatment consisted of: (b) ammonium nitrate (NH₄NO₃) and PK application (based on the suggested quantities, under field conditions, of 25 g m⁻² N, P₂O₅, K₂O and 5–8 g Mg for eggplants, and by performing the necessary calculations and conversions for the 5 L pots, i.e., surface pot area: πR² with mean pot diameter 18.5 cm, and given that NH₄NO₃ contains 14 g pure N, it was found that approximately 2 g per pot NH₄NO₃ and 2 g per pot PK should be applied, in one dose, at the beginning of the experiment). The third treatment consisted of: (c) Tree Branch Chips (TBC) application, at a suggested rate of 33% w/v (i.e., 2 parts of soil and one part of TBC); by doing the necessary calculations and conversions for the 5 L pots, i.e., surface pot area: πR² with mean pot diameter 18.5 cm, it was found that 1.38 cm³ of TBC per pot should be applied, in one dose, at the beginning of the experiment. In the 4th treatment: (d) Poultry Manure (PM) was applied, in a quantity of 108 g per pot, in one dose, at the beginning of the experiment (this quantity was determined, based on the recommended application rate of 40 tn ha⁻¹ PM, under field conditions, after performing the conversions for the 5 L pots, i.e., surface pot area: πR² with mean pot diameter 18.5 cm). In the 5th fertilization treatment: (e) both TBC and PM, in the above-mentioned application rates were applied, in one dose, at the beginning of the experiment. Finally, in the 6th treatment, no fertilizer or organic amendment was added (CONTROL soil), during the whole experimental period. In each of the six treatments described above, five plant replicates were included.

2.3. Soil Sampling and Lab Analyses

Soil sampling was performed from the upper 30 cm of an agricultural soil type (from parent material Gneiss, cultivated with vegetable crops), in order to use it as a substrate for plant growth medium. These soil samples were received from cultivated vegetable crop fields, close to the region of Vasilika, near Thessaloniki city, Central Macedonia, Northern Greece. Before filling the pots and establishing plant experimentation, all the soil samples were carefully mixed, in order to be homogenized. After mixing, a representative soil sample of approximately 1.5 kg was received, to determine the initial soil fertility (that of the CONTROL soil). The properties of the CONTROL soil (i.e., that soil without mixtures with other amendments and/or fertilizers) are presented in

Table 1. Initial and final (after organic amendments and fertilizer application) fertility of the Gneiss soil type.

Soil Type	Soil Texture	pH	Organic Matter (%)	CaCO3 (%)	NO3-N	P	K	Mg	Ca	Fe	Zn	Mn	Cu	B
					mg kg−1
GNEISS (CONTROL)	L	6.47	2.94	0	32.98	5.83	72	287	1921	7.96	1.0	14.31	2.28	0.9
PK	L	6.49	2.90	0	33.27	5.59	90	377	1960	6.36	1.0	12.20	1.98	0.9
AN + PK	L	6.62	2.85	0.1	37.78	6.02	86	358	2021	6.90	1.1	12.85	2.06	0.8
TBC	L	6.39	3.01	0.0	33.05	5.68	69	299	1897	7.71	0.9	12.45	1.95	0.9
PM	L	6.80	3.09	0.1	38.36	5.57	96	283	1886	8.04	1.1	14.46	2.09	1.0
TBC + PM	L	6.68	3.05	0.1	36.46	6.00	92	297	2045	7.62	0.9	13.35	1.99	0.8
Optimum range		5.5–7.5	>4	<10	20–40	15–25	140–200	50–100	300–750	5–25	1–2.5	12–25	0.6–1.5	0.5

. In addition, in

Table 2. (a) Nutrient composition of the organic amendments (Poultry Manure, Tree Branch Chips), applied in the Gneiss soil type. (b) Nutrient content of the fertilizers (Patent Kali, Ammonium Nitrate), applied in the Gneiss soil type.

(a)
	Macronutrient Concentration (% D.W.)					Micronutrient Concentration (mg kg−1 D.W.)
Organic Material	N	P	K	Ca	Mg	B	Mn	Zn	Fe	Cu
Poultry Manure	2.31	1.38	2.51	12.78	1.38	32.67	298.35	159.1	4200.5	51.6
Tree Branch Chips	1.66	0.14	0.08	2.23	0.27	29.66	60.38	35.55	833.05	11.98
(b)
	Nutrient Content (Nutrient Units, %)
Organic Material	N	P	K	Ca	Mg	S	Mn	Zn	Fe	B
Patent Kali	0	0	30	0	10	42.5	0	0	0	0
Ammonium Nitrate	34.5	0	0	0	0	0	0	0	0	0

, the nutrient composition of PM and TBC are presented. After achieving soil homogenization, the treatments were established, by mixing soil with fertilizers/amendments (addition of the necessary quantities/rates of fertilizers and/or amendments in the homogenized soil).

Soil samples were subjected to homogenization, they were dried at room temperature, and the stones were removed and sieved through a 10-plexus dredge, prior to chemical analysis. Particle size analysis, pH, organic matter, NO₃-N and CaCO₃ content, available P, exchangeable cations (K, Ca and Mg) and the concentrations of micronutrients (Fe, Mn, Zn, Cu and B) were defined under lab conditions. In particular, pH was estimated in a soil-distilled water paste (1:1) [38], the particle size composition (% soil content in sand, clay, and silt) according to the Bouyoucos method [39], and the organic matter and CaCO₃ content via the potassium dichromate [40] and acid neutralization methods, respectively [40]. The determination of macronutrient concentrations was achieved using the VCl₃/Griess method for nitrate nitrogen (NO₃-N) [41], the Olsen method for available P [42], and the ammonium acetate extraction method for exchangeable K, Ca and Mg [43]. The determination of micronutrient concentrations was realized with the method described by Wolf [44] for B and the DTPA method (pH 7.3) for Fe, Mn, Zn and Cu.

2.4. Plant Growth

From the beginning of the experimentation, every 10 days plant height, as well as the elongation of the main shoot (cm day⁻¹) was determined. At the end of the experiment, the plants were divided into shoots, leaves, root systems and fruits; after weighing all these plant tissues, the total plant F.W. was determined. In addition, after washing twice with high-quality tap water and once with distilled water all the plant tissues, were dried at 75 °C for 48 h, in order to determine their dry weights. By adding all the dry weights, the total plant dry weight (i.e., the total plant biomass) was determined.

2.5. Tissue Nutrient Concentrations and Total Plant Nutrient Uptake

Prior to chemical analysis, all the vegetative tissues underwent drying, processing until the formation of a fine powder, and sieving via a 30-plexus dredge. Afterward, 0.5 g of the fine powder of each sample was weighed and subjected to incineration for 5 h, in a muffle furnace, at 515 °C. Dissolution of the ash in 3 mL of 6 N HCl and dilution with double-distilled water up to 50 mL were subsequently applied. For the determination of nutrient concentrations, the ICP (OPTIMA 2100 DV optical emission spectrometer, Perkin Elmer, Waltham, MA, USA) spectrometric method was applied for P, K, Ca, Mg, Fe, Mn, Zn and Cu [45], and the Kjeldahl method for N [46]. Macronutrient concentrations (N, P, K, Ca, Mg) were expressed in % D.W. and those of micronutrients (Fe, Mn, Zn and Cu) were expressed in mg kg⁻¹ DW. At the end of the experiment, the content (absolute quantity) of each nutrient per plant part was calculated, as follows: nutrient concentration in each plant tissue x counterpart D.W. The total plant nutrient content (i.e., the total nutrient uptake) was the sum of the content of the different plant parts (leaves, shoots, roots, fruits). Fruits nutrient concentrations were determined based on the aforementioned methodology, after fruits were subjected to crushing, drying for 24 h at 75 °C and then incineration, for 5 h at 515 °C.

2.6. Chlorophyll Fluorescence and Chlorophyll Content

At the termination of the experiment, the maximum efficiency of PSII photochemistry (Fv/Fm), the performance index (PI), and the chlorophyll content were evaluated. The functionality of photosystem II is mirrored by PI, a high quantitative index representative of the prevailing performance status of plants under stress. Fv/Fm and PI were determined using a PAM-2000 fluorometer (HeinzWalz GmbH, Effeltrich, Germany). The F_v/F_m ratio was determined, where F₀ is the minimum fluorescence, F_m is the maximum fluorescence and F_v = F_m − F₀ is the variable fluorescence [47]. The measurements of Fv/Fm and PI indices via the fluorometer PAM-2000 were performed on the youngest mature, fully expanded leaves of the plant, after a previous 20-min exposure of leaves to complete darkness [48]. The total chlorophyll content index was measured by the portable chlorophyll meter CCM 200 plus Opti-Sciences.

2.7. Total Phenοlic Content (TPC), Total Flavonoid Content (TFC) and Antioxidant Activity (DPPH) in Fruits

The Folin–Ciocalteu method was adopted for evaluating the total phenolic content (TPC) [49,50], with slight amendments. The reaction solution was composed of three chemicals including 2400 µL Folin–Ciocalteu (1:10 v/v), 80% (v/v) methanolic extract (100 µL), and nanopure water (500 µL) within tubes, blended under magnetic stirring. After a 3-mim period to ensure adequate reaction of the mixture, 2 mL of Na₂CO₃ (7.5% w/v) solution was aggregated, properly integrated, and incubated at 37 °C for 5 min. The Folin–Ciocalteu reagent is associated with the oxidation of phenolic compounds in the sample. The samples within tubes were maintained for a period at room temperature (23 °C) to bedew, and then a spectrophotometer was used to measure the absorbance of samples at 760 nm. TPC was expressed as milligrams of gallic acid equivalents (GAE) per 100 g of fresh weight (mg GAE 100 g⁻¹ FW), utilizing a gallic acid standard curve.

The colorimetrical method was adopted for the determination of total flavonoid content (TFC) in the fruits of the plant. A portion of 3 g of fresh tissue was weighed and extracted in an 80% ethanol solution [51,52]. The use of a rutin (RUT) standard curve was utilized for the quantitative determination of TFC, expressed as mg RUT g⁻¹ FW.

Prior to the determination of the DPPH antioxidant activity of the eggplant fruits, 0.5 g of fresh tissue was weighed, chopped into smaller sections, placed in a mortar, extracted and blended well with 1 mL of 80% aqueous methanol solution for one second. Following that, the homogenized sample was placed in a centrifugal assembly for 20 min at 12,000 rpm and 4 °C. The hydrogen or electron donor competence non-enzymatic antioxidant activity of the fruits by remodeling the 1,1-diphenyl-2-picrylhydrazyl (DPPH) into its reduced form DPPH-H was specified. The method of Su et al. [53] was applied to assess the total antioxidant activity. Specifically, 50 µL of fruit extract was mixed with 2.95 mL of 0.1 mM DPPH solution, and after 1 h of incubation, a spectrophotometer was used to evaluate the absorbance of the reaction mixture at 517 nm threefold. The DPPH solution enriched further with 50 µL of absolute ethanol constituted the control solution.

Calibrations were enunciated as scavenging activity %. The antioxidant activity [51] was calculated using the following equation: Scavenging Activity (%) = {(Abs control − Abs sample)/Abs control} × 100, where Abs is the absorbance at 517 nm.

2.8. Statistical Analysis

The experimental design consisted of a 6 x 1 completely randomized factorial with 6 fertilization (or soil amendment) treatments and one plant species. In each of the six treatments, five plants (i.e., replicates) were included (thus, the total number of the experimental plants was 30). The data were statistically analyzed by both one-way and two-way ANOVA, using the SPSS statistical program (version 28, IBM, Armonk, NY, USA), Duncan’s multiple range test for p ≤ 0.05, and the General Linear Model to identify the effect of the main factors and their amongst interactions for p ≤ 0.05, p ≤ 0.01 and p ≤ 0.001. Mean comparison among the six treatments for plant growth parameters (plant height, main shoot elongation rate per day, total plant fresh and dry weight), leaf chlorophyll content, chlorophyll fluorescence parameters (F_v/F_m, performance index—PI), total phenolic content, total flavonoid content and DPPH antioxidant activity in the fruits was performed using one-way ANOVA. In the case of nutrient concentrations and biomass (FW, DW) measurements, one-way ANOVA was adopted for mean comparison either among the six treatments for each plant tissue type (leaves, shoots, roots, fruits) separately, or among the four plant tissue types (leaves, shoots, roots, fruits) for each treatment (PK, AN + PK, TBC, PM, TBC + PM, CONTROL), separately. The experimental design in the combined statistical analysis (two-way ANOVA/General Linear Model) was a 4 × 6 full factorial one consisting of 24 treatments in total divided into four plant tissue types (leaves, shoots, roots, fruits) × six soil substrates (PK, AN + PK, TBC, PM, TBC + PM, CONTROL). Regarding biomass and nutrient concentrations, the main effect of factors (plant tissue, soil substrate) and their interaction (plant tissue × soil substrate) was evaluated, using the General Linear Model.

3. Results

3.1. Soil Fertility

In Table 1, soil properties of the CONTROL soil type, as well as of the treatments before the beginning, are presented. As it is clear, the soil texture was LOAM (L) while pH was approximately 6.5 and the organic matter content was satisfactory (approximately 3.0%). NO₃-N concentration was within the optimum range of sufficiency, while P extracted by the Olsen method and exchangeable K concentrations were low and should be enhanced. In contrast, exchangeable Ca and Mg were high (Table 1). With regard to micronutrients, Fe, B and Cu availabilities were high, that of Mn was within the optimum levels of sufficiency, and only Zn concentration could be considered as marginal (i.e., close to the critical level of deficiency) (Table 1). Based on these data, it was decided that the Gneiss soil type mainly needed P and K applications to enhance their levels.

In Table 2a,b are presented the nutrient compositions of Poultry Manure (PM) and Tree Branch Chips (TBC), as well as the nutrient content of the fertilizers (PK and AN). As it is clear from the table, PM was better soil amendment (more convenient, as an organic fertilizer, to satisfy the nutritional needs of Solanum melongena L.), compared to TBC. More specifically, the N, P, K, Ca, Mg, Mn, Zn, Fe and Cu concentrations of PM were 1.4, 9.9, 31.4, 5.7, 5.1, 4.9, 4.5, 5.0 and 4.3 times higher, respectively, compared to those determined in TBC (Table 2). Especially with regard to P and K (which are the main nutrient deficiencies determined in the Gneiss soil type), very low P and K concentrations were found in TBC (0.14% D.W. and 0.08% D.W., respectively). Finally, the concentration of Zn in PM was 4.5 times higher compared to TBC, showing overall its superiority as an organic amendment to satisfy the nutritional needs of the Solanum melongena L. plants (Table 2).

3.2. Plant Growth

From the results of

Table 3. Effect of fertilization/Soil amendment treatments (Patent Kali, Ammonium Nitrate + Patent Kali, Tree Branch Chips, Poultry Manure, Tree Branch Chips + Poultry Manure, CONTROL) on plant height (cm), within a 2-month cultivation period of Solanum melongena L.

Fertilization/Soil Amendment Treatment	Plant Height (cm)
	17 May (Initial)	30 May	9 June	19 June	29 June	9 July	19 July (Final)
PK	20.5 ± 2.0 a	27.8 ± 2.3 a	33.0 ± 3.0 a	37.0 ± 1.4 ab	38.4 ± 1.5 a	38.4 ± 1.5 a	38.6 ± 1.8 a
AN + PK	19.3 ± 3.5 a	29.0 ± 4.24 a	34.3 ± 3.0 a	37.0 ± 2.2 ab	40.0 ± 3.4 a	40.5 ± 3.4 a	41.5 ± 3.4 a
TBC	17.7 ± 1.8 a	21.3 ± 2.8 bc	24.0 ± 1.6 b	30.0 ± 3.9 c	34.8 ± 2.5 a	36.8 ± 3.3 a	37.5 ± 3.6 a
PM	18.6 ± 2.9 a	25.2 ± 3.0 ab	33.2 ± 2.4 a	39.2 ± 2.8 a	41.0 ± 4.2 a	41.8 ± 4.4 a	42.2 ± 4.8 a
TBC + PM	17.8 ± 2.9 a	20.2 ± 3.6 c	28.4 ± 2.1 c	34.6 ± 3.6 b	36.8 ± 3.1 a	37.8 ± 3.6 a	38.2 ± 3.4 a
CONTROL	18.6 ± 2.2 a	27.5 ± 2.7 a	32.3 ± 1.5 a	37.0 ± 1.9 ab	38.3 ± 2.2 a	38.5 ± 2.1 a	38.8 ± 2.1 a
p-values	0.115 ns	0.000 ***	0.000 ***	0.001 **	0.052 ns	0.178 ns	0.180 ns

Means (n = 5) ± standard deviation (S.D.) accompanied by different letters in each column indicate significant differences at a 5% level based on Duncan’s test (p ≤ 0.05). ns: p > 0.05, ** p ≤ 0.01, *** p ≤ 0.001.

, it is concluded that, at the end of the experiment (19 July), insignificant differences in plant height among the CONTROL and the other treatments were recorded. Only in the previous stages of plant growth (i.e., between 30 May and 9 July do Solanum melongena L. plants show lower height in TBC, compared to the other treatments (mainly those of PM, AN + PK and PK) (Table 3).

Main shoot elongation rate (cm day⁻¹) was better enhanced (i) by AN + PK and CONTROL during the first 13 days of the experimentation (from the 17 May until 30 May), (ii) by PM and TBC + PM between 30 May and 9 June, and (iii) by TBC from the 9 June until the 9 July (

Table 4. Effect of fertilization/Soil amendment treatments (Patent Kali, Ammonium Nitrate + Patent Kali, Tree Branch Chips, Poultry Manure, Tree Branch Chips + Poultry Manure, CONTROL) on plant growth rate (cm day⁻¹), within a 2-month total cultivation period of Solanum melongena L.

Fertilization Treatment	Main Shoot Elongation Rate (cm) per Day
	17–30 May (1–13th day)	30 May–9 June (13–23th day)	9–19 June (23–33th day)	19–29 June (33–43th day)	29 June–9 July (43–53th day)	9–19 July (53–63th day)
PK	0.64 ± 0.09 ab	0.60 ± 0.07 b	0.30 ± 0.10 bc	0.14 ± 0.05 c	0.00 ± 0.00 d	0.02 ± 0.004 c
AN + PK	0.75 ± 0.23 a	0.60 ± 0.07 b	0.28 ± 0.16 c	0.30 ± 0.07 b	0.05 ± 0.01 bc	0.10 ± 0.03 a
TBC	0.48 ± 0.08 bc	0.17 ± 0.05 c	0.65 ± 0.11 a	0.57 ± 0.11 a	0.20 ± 0.06 a	0.08 ± 0.02 ab
PM	0.51 ± 0.09 bc	0.80 ± 0.14 a	0.60 ± 0.12 a	0.18 ± 0.06 bc	0.08 ± 0.02 b	0.04 ± 0.01 b
TBC + PM	0.37 ± 0.06 c	0.82 ± 0.19 a	0.63 ± 0.04 a	0.22 ± 0.04 b	0.10 ± 0.03 b	0.04 ± 0.01 b
CONTROL	0.68 ± 0.08 a	0.48 ± 0.13 b	0.48 ± 0.11 ab	0.13 ± 0.03 c	0.03 ± 0.005 c	0.03 ± 0.003 bc
p-values	0.000 ***	0.000 ***	0.000 ***	0.000 ***	0.000 ***	0.016 *

Means (n = 5) ± standard deviation (S.D.) accompanied by different letters in each column indicate significant differences at a 5% level based on Duncan’s test (p ≤ 0.05). * p ≤ 0.05, *** p ≤ 0.001.

); at the end of the experimental period from the 9 July until 19 July, the highest shoot elongation rate (0.10 cm day⁻¹) was recorded in the AN + PK treatment, which was significantly higher to those determined in the CONTROL, PK, PM and TBC + PM (Table 4).

The highest total plant biomass (as expressed by the total plant F.W. and D.W.) was recorded in the PM-treated plants, followed by those determined in the treatments AN + PK, TBC + PM, PK, and CONTROL and TBC (Figure 1a,b), showing the good response of eggplants to the applied organic fertilizer (poultry manure).

The combined statistical analysis of all 24 treatments (four different types of plant tissue × six different soil fertilization treatments) (two-way ANOVA, General Linear Model), revealed that PM was the most advantageous treatment for shoot, root system and fruit formation (

Table 5. Effect of fertilization/Soil amendment treatments (Patent Kali, Ammonium Nitrate + Patent Kali, Tree Branch Chips, Poultry Manure, Tree Branch Chips + Poultry Manure, CONTROL) on fresh weight and dry weight of leaves, shoots, root system and fruits of the Solanum melongena L. plants.

Treatment		Biomass
Plant Tissue Type	Fertilization Treatment	FW (g)	DW (g)
Leaves	PK	29.89 ± 6.12 ijkl B (c)	3.66 ± 0.64 def C (b)
	AN + PK	64.53 ± 16.08 fghi A (b)	9.12 ± 2.11 cd A (b)
	TBC	25.35 ± 2.92 jkl B (c)	3.56 ± 0.54 def C (b)
	PM	53.30 ± 8.71 fghijkl A (b)	6.46 ± 0.69 cdef B (b)
	TBC + PM	37.48 ± 8.25 ghijkl B (bc)	4.71 ± 0.98 cdef C (b)
	CONTROL	27.55 ± 5.00 jkl B (c)	3.37 ± 0.48 ef C (b)
Shoots	PK	23.69 ± 1.87 kl C (c)	5.51 ± 0.57 cdef BC (ab)
	AN + PK	32.40 ± 3.92 hijkl AB (b)	7.27 ± 1.50 cdef A (b)
	TBC	19.59 ± 2.66 l C (d)	4.36 ± 0.69 def C (a)
	PM	36.60 ± 3.47 ghijkl A(b)	8.08 ± 1.09 cde A (b)
	TBC + PM	30.45 ± 6.29 hijkl B (c)	6.00 ± 0.99 cdef B (b)
	CONTROL	22.86 ± 1.83 l C (c)	5.39 ± 0.60 cdef BC (ab)
Roots	PK	67.13 ± 14.80 fgh B (b)	7.34 ± 1.63 cdef B (a)
	AN + PK	60.13 ± 20.91 fghijk B (b)	7.37 ± 3.39 cdef B (b)
	TBC	37.34 ± 2.68 ghijkl C (b)	4.04 ± 0.17 def C (ab)
	PM	85.86 ± 3.83 ef A (b)	9.97 ± 1.01 c A (b)
	TBC + PM	71.01 ± 9.64 fg AB (b)	6.21 ± 1.23 cdef BC (b)
	CONTROL	64.84 ± 10.92 fghi B (b)	6.18 ± 0.77 cdef BC (ab)
Fruits	PK	129.43 ± 27.26 d B (a)	7.14 ± 2.36 cdef BC (a)
	AN + PK	248.52 ± 55.03 b A (a)	14.61 ± 2.34 b AB (a)
	TBC	61.62 ± 0.00 fghij C (a)	2.40 ± 0.00 f C (c)
	PM	291.70 ± 77.61 a A (a)	23.64 ± 11.08 a A (a)
	TBC + PM	175.53 ± 48.77 c B (a)	15.41 ± 11.48 b AB (a)
	CONTROL	115.99 ± 30.52 de BC (a)	7.72 ± 4.38 cdef BC (a)

Means (n = 5) ± standard deviation (S.D.) accompanied by different letters in each column indicate significant differences at a 5% level based on Duncan’s test (p ≤ 0.05). Small letters—differences between all the 24 treatments from the combined effect of four different plant tissue types (Leaves, shoots, roots, fruits) × six different soil fertilization treatments [Patent Kali (PK), Ammonium Nitrate + Patent Kali (AN + PK), Tree Branch Chips (TBC), Poultry Manure (PM), Tree Branch Chips + Poultry Manure (TBC + PM), CONTROL] (two-way ANOVA, General Linear Model). CAPITAL LETTERS—differences between the six different fertilization treatments but separately for each plant tissue type (one-way ANOVA). Small letters in parentheses—differences between the four different plant tissue types but separately for each fertilization treatment (one-way ANOVA).

and Table S1). AN + PK and PM were the most appropriate treatments to enhance leaf and shoot biomass, respectively. However, F.W. and D.W. of roots and fruits were better promoted in the PM treatment, compared to the CONTROL (Table 5). Thus, PM proved the most beneficial treatment for total plant and plant tissue biomass. Fruit F.W. and D.W. were significantly higher, compared to those of the other plant tissues (roots, shoots, leaves) in the AN + PK and PM treatments (Table 5).

3.3. Tissue Nutrient Concentrations and Total Plant Nutrient Uptake

Total plant N content was significantly higher in the AN + PK, compared to PK, TBC, TBC + PM and CONTROL; furthermore, PM treatment showed significantly higher N uptake, compared to PK, TBC and CONTROL (Figure 2a). Both total P (Figure 2b) and K contents (Figure 2c) were significantly higher in the PM and TBC + PM treatments. Similarly for Ca and Mg, PM showed the highest Ca and Mg contents, followed by AN + PK and then by TBC + PM (Figure 2d,e). Therefore, PM was the most beneficial fertilization treatment with regard to macronutrient uptake.

Regarding micronutrient content, PM, AN + PK, CONTROL and PK showed higher Fe and Mn levels, compared to the TBC and TBC + PM treatments (Figure 3a,b). In contrast, total Zn did not significantly differ among the treatments (Figure 3c), while significantly higher Cu content was determined in the CONTROL, compared to TBC, PM and TBC + PM treatments (Figure 3d).

The combined statistical analysis of all 24 treatments (four plant tissues x six fertilization treatments) (Two-way ANOVA, General Linear Model) revealed that the highest N concentration in leaves was determined in the AN + PK fertilization treatment, while the highest leaf P concentration was found in the TBC + PM treatment; the highest K concentration in leaves was recorded in PM (

Table 6. Effect of fertilization/Soil amendment treatments (Patent Kali, Ammonium Nitrate + Patent Kali, Tree Branch Chips, Poultry Manure, Tree Branch Chips + Poultry Manure, CONTROL) on foliar, shoot, root and fruit nitrogen, phosphorous and potassium concentrations of the Solanum melongena L. plants.

Treatment		Macronutrient Concentrations
Plant Tissue Type	Fertilization Treatment	N	P (% D.W.)	K
Leaves	PK	1.53 ± 0.11 c C (a)	0.09 ± 0.01 jk C (b)	1.77 ± 0.27 jkl BC (c)
	AN + PK	2.33 ± 0.32 a A (a)	0.06 ± 0.01 lm D (c)	1.87 ± 0.15 ijk ABC (b)
	TBC	1.55 ± 0.12 c C (a)	0.29 ± 0.01 c B (a)	1.57 ± 0.23 l C (c)
	PM	1.68 ± 0.22 c C (a)	0.29 ± 0.02 c B (a)	2.15 ± 0.28 fghi A (b)
	TBC + PM	2.03 ± 0.22 b B (a)	0.39 ± 0.01 a A (a)	2.09 ± 0.36 ghi AB (c)
	CONTROL	1.63 ± 0.10 c C (a)	0.10 ± 0.01 ij C (b)	1.52 ± 0.26 l C (c)
Shoots	PK	0.52 ± 0.09 g B (c)	0.05 ± 0.01 mn D (c)	2.06 ± 0.16 ghi BC (bc)
	AN + PK	1.00 ± 0.16 f A (c)	0.04 ± 0.01 n D (d)	1.52 ± 0.17 l D (c)
	TBC	0.54 ± 0.05 g B (c)	0.10 ± 0.01 ij C (d)	1.87 ± 0.22 ijk C (b)
	PM	0.59 ± 0.09 g B (c)	0.12 ± 0.02 h B (c)	2.26 ±0.12 efg AB (b)
	TBC + PM	0.50 ± 0.07 g B (d)	0.19 ± 0.03 f A (c)	2.38 ± 0.19 ef A (c)
	CONTROL	0.57 ± 0.11 g B (c)	0.05 ± 0.00 mn D (c)	1.93 ± 0.06 hij C (b)
Roots	PK	1.13 ± 0.15 def B (b)	0.10 ± 0.00 ij D (b)	2.20 ± 0.14 efgh B (b)
	AN + PK	1.70 ± 0.11 c A (b)	0.08 ± 0.01 kl E (b)	0.97 ± 0.13 m E (d)
	TBC	1.23 ± 0.11 d B (b)	0.19 ± 0.01 f C (c)	1.65 ± 0.14 jkl D (bc)
	PM	1.14 ± 0.09 def B (b)	0.25 ± 0.02 de B (b)	1.93 ± 0.32 hij C (b)
	TBC + PM	1.25 ± 0.07 def B (b)	0.37 ± 0.01 b A (a)	2.89 ± 0.09 c A (b)
	CONTROL	1.19 ± 0.07 def B (b)	0.11 ± 0.01 hi D (b)	1.59 ± 0.27 kl D (c)
Fruits	PK	1.02 ± 0.14 ef C (b)	0.17 ± 0.02 g D (a)	2.70 ± 0.30 cd B (a)
	AN + PK	1.72 ± 0.12 c A (b)	0.10 ± 0.01 ij E (a)	2.48 ± 0.18 de BC (a)
	TBC	1.18 ± 0.00 def BC (b)	0.25 ± 0.00 de AB (b)	2.61 ± 0.00 a B (a)
	PM	1.21 ± 0.17 de B (b)	0.25 ± 0.02 e B (b)	2.75 ± 0.21 c B (a)
	TBC + PM	1.02 ± 0.11 ef C (c)	0.27 ± 0.01 d A (b)	3.25 ± 0.16 b A (a)
	CONTROL	1.09 ± 0.13 def BC (b)	0.19 ± 0.01 f C (a)	2.45 ± 0.06 de C (a)

Means (n = 5) ± standard deviation (S.D.) accompanied by different letters in each column indicate significant differences at a 5% level based on Duncan’s test (p ≤ 0.05). Small letters—differences between all the 24 treatments from the combined effect of four different plant tissue types (Leaves, shoots, roots, fruits) x six different soil fertilization treatments [Patent Kali (PK), Ammonium Nitrate + Patent Kali (AN + PK), Tree Branch Chips (TBC), Poultry Manure (PM), Tree Branch Chips + Poultry Manure (TBC + PM), CONTROL] (two-way ANOVA, General Linear Model). CAPITAL LETTERS—differences between the six different fertilization treatments per plant tissue type (one-way ANOVA). Small letters in parentheses—differences between the four different plant tissue types per fertilization treatment (one-way ANOVA).

and Table S2). With regard to fruit nutrient concentrations, the maximum values for K, P and N were recorded in the TBC, TBC + PM and AN + PK treatments, respectively, likely showing the potential ability of TBC and PM to promote fruit K and P concentrations. Similarly to fruit nutrients, the optimum root K, P and N concentrations (which were significantly higher than all the other treatments) were determined in TBC + PM (both for K and P) and AN + PK (for N) (Table 6). Regarding the plant tissue effect (one-way ANOVA), the highest N concentrations were detected in leaves, followed by root systems and fruits (Table S2). In contrast, for P only in TBC, PM and TBC + PM were detected its highest concentrations in leaves; in the other three treatments (CONTROL, PK and AN + PK), the highest P concentrations were found in fruits, followed by root system and leaves. Finally, the maximum K concentrations were recorded in fruits (Table 6).

The combined statistical analysis of all 24 treatments (derived from the combined effect of four different types of plant tissue x six different soil fertilization treatments) (two-way ANOVA, General Linear Model), revealed that foliar Ca concentration was significantly higher in TBC + PM, compared to all the other treatments (

Table 7. Effect of fertilization/Soil amendment treatments (Patent Kali, Ammonium Nitrate + Patent Kali, Tree Branch Chips, Poultry Manure, Tree Branch Chips + Poultry Manure, CONTROL) on foliar, shoot, root and fruit calcium, magnesium and sodium concentrations of the Solanum melongena L. plants.

Treatment		Ca, Mg and Na Concentrations
Plant Tissue Type	Fertilization Treatment	Ca (% D.W.)	Mg (% D.W.)	Na (% D.W.)
Leaves	PK	1.86 ± 0.11 c C (a)	0.59 ± 0.05 c C (a)	0.15 ± 0.02 j ABC (c)
	AN + PK	1.50 ± 0.25 d D (a)	0.76 ± 0.09 a A (a)	0.18 ± 0.02 hij A (b)
	TBC	2.09 ± 0.07 b BC (a)	0.67 ± 0.03 b B (a)	0.13 ± 0.01 j BCD (d)
	PM	2.25 ± 0.25 a AB (a)	0.79 ± 0.05 a A (a)	0.11 ± 0.05 j D (c)
	TBC + PM	2.32 ± 0.11 a A (a)	0.80 ± 0.02 a A (a)	0.13 ± 0.01 j CD (c)
	CONTROL	1.92 ± 0.08 c C (a)	0.64 ± 0.04 bc BC (a)	0.16 ± 0.02 j AB (c)
Shoots	PK	0.70 ± 0.07 e B (b)	0.26 ± 0.04 ghi BC (c)	0.25 ± 0.01 fghi B (b)
	AN + PK	0.69 ± 0.06 e B (b)	0.29 ± 0.01 gh AB (c)	0.29 ± 0.02 f A (b)
	TBC	0.70 ± 0.08 e B (b)	0.21 ± 0.02 i D (c)	0.27 ± 0.02 fg AB (b)
	PM	0.80 ± 0.04 e A (b)	0.22 ± 0.01 i CD (c)	0.24 ± 0.02 fghi B (b)
	TBC + PM	0.78 ± 0.05 e AB (b)	0.28 ± 0.05 gh AB (c)	0.25 ± 0.03 fghi B (b)
	CONTROL	0.76 ± 0.06 e AB (b)	0.31 ± 0.03 g A (c)	0.26 ± 0.02 fgh B (b)
Roots	PK	0.48 ± 0.03 fg B (b)	0.44 ± 0.07 cd AB (b)	0.56 ± 0.10 d D (a)
	AN + PK	0.57 ± 0.04 f A (b)	0.39 ± 0.02 ef BC (b)	0.96 ± 0.17 b B (a)
	TBC	0.47 ± 0.03 fg B (c)	0.43 ± 0.03 def ABC (b)	0.72 ± 0.08 c C (a)
	PM	0.48 ± 0.01 fg B (c)	0.40 ± 0.04 ef BC (b)	0.43 ± 0.11 e D (a)
	TBC + PM	0.49 ± 0.06 fg B (c)	0.47 ± 0.03 d A (b)	0.44 ± 0.08 e D (a)
	CONTROL	0.51 ± 0.01 fg B (c)	0.38 ± 0.01 f C (b)	1.12 ± 0.09 a A (a)
Fruits	PK	0.29 ± 0.04 hi B (b)	0.27 ± 0.05 ghi BC (c)	0.15 ± 0.03 j BC (c)
	AN + PK	0.26 ± 0.07 i B (c)	0.24 ± 0.02 hi C (c)	0.17 ± 0.02 ij B (b)
	TBC	0.46 ± 0.00 fg B (c)	0.43 ± 0.00 def A (b)	0.19 ± 0.00 ghij A (c)
	PM	0.23 ± 0.05 i B (d)	0.25 ± 0.01 hi BC (c)	0.15 ± 0.01 j B (c)
	TBC + PM	0.23 ± 0.05 i B (d)	0.26 ± 0.01 ghi BC (c)	0.17 ± 0.01 ij B (c)
	CONTROL	0.40 ± 0.05 gh A (d)	0.28 ± 0.03 gh B (c)	0.13 ± 0.01 j C (c)

Means (n = 5) ± standard deviation (S.D.) accompanied by different letters in each column indicate significant differences at a 5% level based on Duncan’s test (p ≤ 0.05). Small letters—differences between all the 24 treatments from the combined effect of four plant tissue types [Leaves (L), shoots (S), roots (R), fruits (F)] × six soil fertilization treatments [Patent Kali (PK), Ammonium Nitrate + Patent Kali (AN + PK), Tree Branch Chips (TBC), Poultry Manure (PM), Tree Branch Chips + Poultry Manure (TBC + PM), CONTROL] (two-way ANOVA, General Linear Model). CAPITAL LETTERS—differences between the six fertilization treatments per plant tissue type (one-way ANOVA). Small letters in parentheses—differences between the four plant tissue types per fertilization treatment (one-way ANOVA).

and Table S3). The highest leaf Mg concentration was recorded in AN + PK, PM, and TBC + PM, compared to CONTROL, PK and TBC. Regardless of fertilization treatment, Ca and Mg concentrations were higher in leaves, compared to other plant tissues (shoots, root system and fruits) (Table 7).

Regarding micronutrient concentrations, significantly higher foliar Fe was recorded in the PM treatment, compared to PK, TBC and TBC + PM; in fruits, the highest Fe concentration was found in TBC, which was significantly higher than all the other treatments (

Table 8. Effect of fertilization/Soil amendment treatments (Patent Kali, Ammonium Nitrate + Patent Kali, Tree Branch Chips, Poultry Manure, Tree Branch Chips + Poultry Manure, CONTROL) on foliar, shoot, root and fruit iron, manganese, copper and zinc concentrations of the Solanum melongena L. plants.

Treatment		Micronutrient Concentrations (μg g−1 D.W.)
Plant Tissue	Fertilization	Fe	Mn	Cu	Zn
Leaves	PK	247.4 ± 11.1 fgh C (b)	97.4 ± 6.2 bc B (a)	10.2 ± 1.3 e A (b)	48.9 ± 1.9 ghi A (c)
	AN + PK	300.6 ± 34.7 def AB (b)	132.3 ± 52.6 a A (a)	8.4 ± 1.0 e B (b)	51.5 ± 11.4 ghi A (c)
	TBC	266.7 ± 25.1 efg BC (a)	80.7 ± 5.0 cd B (a)	7.6 ± 0.5 e B (c)	44.7 ± 4.3 hi A (c)
	PM	346.4 ± 39.3 cd A (b)	107.2 ± 15.8 b AB (a)	8.1 ± 0.9 e B (b)	47.9 ± 9.6 ghi A (a)
	TBC + PM	224.2 ± 7.9 ghi C (b)	78.2 ± 6.5 d B (a)	8.4 ± 1.1 e B (b)	44.5 ± 7.9 hi A (c)
	CONTROL	306.5 ± 60.9 de AB (b)	91.4 ± 4.2 bcd B (a)	9.9 ± 1.1 e A (b)	52.3 ± 4.8 ghi A (c)
Shoots	PK	205.5 ± 22.9 hij BC (c)	26.3 ± 4.4 f B (c)	7.5 ± 1.0 e BC (b)	83.0 ± 10.5 e B (b)
	AN + PK	174.2 ± 9.6 ij D (c)	34.7 ± 5.6 ef A (bc)	6.3 ± 0.5 e CD (b)	128.2 ± 16.8 a A (a)
	TBC	208.2 ± 18.9 hij BC (b)	22.8 ± 1.4 f BC (c)	9.1 ± 1.4 e A (bc)	96.7 ± 6.1 bcd B (a)
	PM	216.5 ± 18.0 ghij AB (c)	21.8 ± 1.7 f BC (c)	6.0 ± 0.9 e D (b)	58.7 ± 5.7 fg C (a)
	TBC + PM	167.7 ± 32.0 ij D (b)	19.3 ± 1.6 f C (c)	9.0 ± 0.7 e A (b)	91.2 ±11.5 cde B (b)
	CONTROL	248.2 ± 44.8 fgh A (c)	31.3 ± 5.0 ef A (b)	8.3 ± 1.1 e AB (b)	89.6 ± 9.1 de B (b)
Roots	PK	394.1 ± 34.1 bc A (a)	47.4 ± 20.8 e B (b)	58.6 ± 12.0 b AB (a)	102.3 ± 11.9 bc A (a)
	AN + PK	409.0 ± 113.5 b A (a)	73.0 ± 24.9 d A (b)	62.3 ± 3.2 ab A (a)	68.0 ± 6.5 f B (b)
	TBC	202.9 ± 53.7 hij B (bc)	37.0 ± 2.9 ef B (b)	48.7 ± 6.2 c B (a)	98.5 ± 7.3 bcd A (a)
	PM	443.6 ± 60.4 ab A (a)	33.8 ± 4.6 ef B (b)	22.6 ± 3.6 d C (a)	58.4 ± 5.1 fg B (a)
	TBC + PM	478.1 ± 81.5 a A (a)	33.2 ± 5.3 ef B (b)	57.4 ± 12.5 b AB (a)	105.3 ± 10.5 b A (a)
	CONTROL	415.3 ± 27.7 b A (a)	36.1 ± 2.5 ef B (b)	66.6 ± 5.6 a A (a)	102.4 ± 10.9 bc A (a)
Fruits	PK	40.2 ± 13.0 k D (d)	24.1 ± 5.8 f B (c)	8.1 ± 0.7 e C (b)	42.7 ± 2.9 i B (c)
	AN + PK	75.3 ± 14.8 k B (d)	22.5 ± 2.3 f BC (c)	7.6 ± 1.0 e C (b)	40.5 ± 5.3 i B (c)
	TBC	164.4 ± 0.0 j A (c)	35.6 ± 0.0 ef A (b)	12.3 ± 0.0 e A (b)	55.9 ± 0.0 gh A (b)
	PM	44.7 ± 8.2 k CD (d)	19.1 ± 2.1 f CD (c)	7.7 ± 1.2 e C (b)	41.4 ± 4.7 i B (b)
	TBC + PM	30.8 ± 3.5 k D (c)	17.2 ± 0.7 f D (c)	7.3 ± 0.6 e C (b)	44.6 ± 5.8 hi B (c)
	CONTROL	58.2 ± 19.6 k C (d)	24.5 ± 1.8 f B (c)	10.2 ± 1.8 e B (b)	42.5 ± 5.1 i B (c)

Means (n = 5) ± standard deviation (S.D.) accompanied by different letters in each column indicate significant differences at a 5% level based on Duncan’s test (p ≤ 0.05). Small letters—differences between all the 24 treatments from the combined effect of four plant tissue types [Leaves, shoots, roots, fruits] x six soil fertilization treatments [Patent Kali (PK), Ammonium Nitrate + Patent Kali (AN + PK), Tree Branch Chips (TBC), Poultry Manure (PM), TBC + PM, CONTROL] (two-way ANOVA, General Linear Model). CAPITAL LETTERS—differences between the six fertilization treatments per plant tissue type (one-way ANOVA). Small letters in parentheses—differences between the four plant tissue types per fertilization treatment (one-way ANOVA).

). The highest leaf Mn concentration (approximately 132 μg g⁻¹) was determined in the AN + PK treatment, while in fruits, the highest Mn concentration was recorded in TBC (approximately 36 μg g⁻¹), which was significantly higher than all the other treatments (Table 8). Similarly for Zn and Cu, the highest concentration in fruits was found in TBC (approximately 56 and 12 μg g⁻¹, respectively), while insignificant differences were recorded among the treatments in foliar Zn (Table 8). Zinc concentration in shoots was significantly higher in AN + PK, compared to all the other treatments, while the highest shoot Cu concentrations were determined in TBC and TBC + PM; finally, leaf Cu was significantly higher in CONTROL and PK, compared to the other treatments (Table 8). With regard to the plant tissue effect on micronutrient concentrations, significantly higher Fe, Zn and Cu concentrations were recorded in the root system, compared to the other plant tissues, while the highest Mn concentrations were determined in leaves (approximately 80–130 μg g⁻¹) (Table 8 and Table S4).

3.4. Chlorophyll Fluorescence Parameters and Chlorophyll Content

The maximum efficiency of photosystem II (as indicated by the ratio F_v/F_m and performance index—PI) was significantly decreased in TBC and CONTROL, compared to the other treatments (Figure 4a,b). The highest chlorophyll content was recorded under AN + PK, which was also significantly higher than all the other treatments (CONTROL, PK, TBC, TBC + PM) (Figure 4c).

3.5. Total Phenolic Content (TPC), Total Flavonoid Content (TFC) and Antioxidant Activity—DPPH Method in Fruits of Solanum melongena L. Plants Under Different Fertilization Treatments

TPC (Figure 5a), TFC (Figure 5b), and DPPH (Figure 5c) were significantly increased by AN + PK, PM, and TBC + PM fertilization, compared to the other treatments. The comparison among the five fertilization treatments, considering simultaneously all the parameters of the phytochemical profile, showed the higher efficiency of TBC + PM, in terms of TPC (16.0 mg GAE g⁻¹ FW), TFC (60.6 mg RUT g⁻¹ FW), and DPPH (88.8%) (Figure 5a–c).

4. Discussion

CONTROL soil was of relatively low fertility, with limited P and K availability for optimum eggplant growth (Table 1); thus, K application was necessary in order to enhance their availability for plants. This is why PK was chosen as fertilizer (to improve K availability, which was the main nutritional problem); between PM and TBC, the first one showed approximately 1.4, 10, 31, 5.7, 5.1, 5, 4.5 and 5 times higher N, P, K, Ca, Mg, Mn, Zn and Fe concentrations, compared to TBC, respectively, while both had similar B content (Table 2). Especially for K, TBC had very low K content (only 0.08% D.W.) (Table 2), showing its low ability to act as a potential soil amendment to improve K availability for eggplants. The higher impact of the elevated nutrient content of PM was clear on plant growth and nutrition; indeed, the highest plant growth was recorded in PM, followed by AN + PK (Table 3 and Table 4, Figure 1a,b). In addition, the highest total P, K, Ca and Mg contents and foliar K concentration were recorded in the PM-treated plants, compared to the other treatments (Figure 2b–e, Table 6 and Table 7). In other studies, it was also found that organic fertilizers, like poultry manure, improved plant growth and yields, by providing macro- and micronutrients in available forms [32,33,54]. Since the highest foliar P, Ca and Mg concentrations were not found in PM (Table 6 and Table 7), their highest total contents recorded in this treatment were owed to higher plant biomass (Figure 1a,b), rather than to higher nutrient uptake. Similar were the conclusions in the study of Chatzistathis et al. [29] for olive plants, fertilized with different organic (cow and goat manures) and inorganic (N-controlled release fertilizer) sources; more specifically, they found that despite the higher macronutrient content recorded in the plants treated with N-controlled release fertilizer, their lowest foliar P, K and Mg concentrations revealed that the higher nutrient accumulation, compared to the organic treatments, should be ascribed to higher plant biomass and not to higher nutrient uptake [29].

Despite the fact that the effects of fertilization treatments on plant height at the end of the experiment were non-significant (Table 3), the same effects on total plant biomass were significant; more specifically, the lowest total biomass was determined in TBC (Figure 1a,b). This finding could be ascribed to the insignificant foliar Zn concentrations found among the treatments (Table 8). Zinc influences plant height via its crucial role in tryptophan biosynthesis, which is the previous stage of IAA (auxin) synthesis; IAA concentration is significantly reduced in vegetative tissues suffering from Zn deficiency [55,56]. In our study, leaf Zn concentrations were not deficient (varied from approximately 44 to 52 mg kg⁻¹ D.W., which are considered sufficient for normal plant growth) (Table 8) and were also insignificant among the treatments; this could probably explain the insignificant differences in plant height determined at the end of the experiment. In contrast, the significant differences found among the treatments in total plant biomass, and especially the lowest plant biomass determined in TBC, could be possibly ascribed to the lowest foliar N, K concentrations (Table 6), and/or to the significant decline in leaf Mg (Table 7), Fe and/or Mn (Table 8) concentrations found in this treatment; both leaf N and K concentrations were approximately 1.50% D.W. in TBC (Table 6), which were clearly below the optimum range of nutrient sufficiency (for N the optimum range of nutrient sufficiency under field conditions is 4.0–5.0% D.W., while for K the optimum range of nutrient sufficiency varies from 2.50% D.W. to 4.0% D.W.). Similarly to N and K, foliar Mn in the TBC and TBC + PM treatments was also in the range of insufficiency since its concentrations were approximately 80 mg kg⁻¹ D.W. (Table 8); the optimum range for Mn sufficiency in eggplants is 100–150 mg kg⁻¹ D.W. In contrast, Mg and Fe concentrations were not deficient, since their concentrations were higher or within the range of sufficiency (Table 7 and Table 8), which is 0.40–0.60% D.W. and 100–200 mg kg⁻¹ D.W., respectively.

Based on the above-mentioned classification for optimum nutrient sufficiency, it seems that N, K and/or Mn deficiency (or insufficiency) could be probably responsible for the lowest total plant biomass determined in the TBC treatment (Figure 1a,b). Indeed, N is a primary component of nucleic acids, purines, pyrimidines and chlorophyll [56]; in addition, it exerts a significant effect on plant growth, since it reduces perennial bearing and increases the percentage of perfect flowers [55,57]. This negative impact of N starvation on flowering and fruiting could be also a reality in our study, since in TBC the lowest fruit production and biomass were recorded (Table 5). With regard to the impact of N on chlorophyll content, this tendency of insufficient N on chlorophyll limitation was also verified by the data of our study; indeed, the significant decline in foliar N detected in the TBC, PK, PM treatments, as well as in the CONTROL plants (Table 6), was accompanied by a significant decrease in chlorophyll content in these treatments, compared to AN + PK (Figure 4c). In the last treatment, the highest leaf N concentration was found (2.33% D.W.) (Table 6), and the optimum chlorophyll levels (Figure 4c) were determined. Apart from N, Fe also plays an important role in chlorophyll synthesis, while Mg is part of its molecule [56]. However, Mg and/or Fe insufficiency was not a case in our study (Table 7 and Table 8), since both nutrients were within the sufficiency range. Thus, the differences in chlorophyll content levels among the treatments (Figure 4c) could be ascribed to the differences in plant N availability (Table 6) and not to the differences found in plant Mg and/or Fe availability (Table 7 and Table 8).

Fertilizer applications (PK, AN + PK, PM, TBC + PM) significantly improved total plant biomass (Figure 1a,b), which is in accordance with the data of Al Ali et al. [58], who found that organic and inorganic fertilizer applications had a significant effect on plant vegetative growth. Similarly to the previous results, Al-Bayati and Hamdoon [59] found that soil mulching and organic fertilizer spraying increased plant height, number of leaves, number of branches per plant, leaf area, fruit number and eggplant yields, while Antonious et al. [60] concluded that eggplants grown on a chicken manure-amended soil were 62% and 67% higher in size, compared to those treated with sewage sludge–biochar or vermicompost–biochar amended soil. Although these data are not fully comparable to our results due to different growth conditions (plants grown in soil, under unheated plastic house conditions, in the studies of Al-Bayati and Hamdoon [59] and Antonious et al. [60] vs. pot experiment, under glasshouse conditions, in our case), it is clear from the data of Table 5 that fruit biomass and production was significantly enhanced under organic fertilization (PM), followed by the treatments AN + PK and TBC + PM, compared to the CONTROL (Table 5); this partially confirms the results of Al-Bayati and Hamdoon [59], which means that both fertilization regimes (organic and inorganic) were beneficial on fruiting. In alignment with our data, several published studies in eggplant (Solanum melongena L.) demonstrate the beneficial effect of animal manures and other organic fertilizers on the growth, plant height, total plant FW and DW, number of fruits per plant, fruit length and weight and crop yields [14,21,32,61,62,63,64,65,66,67,68,69]. In contrast, Antonious et al. [60], in their study with eggplants, found that the number and weight of fruits obtained from inorganic treatments were not significantly different from those obtained from organic fertilization.

Apart from the role of N on plant biomass and fruiting, K also plays a crucial role in carbohydrate and N metabolism, as well as in protein synthesis, enzyme activities and the opening and closing of stomata (thus, to the regulation of photosynthetic and transpiration rates) [56,70,71,72]. These effects of K availability on plant growth regulation could be also responsible for the differences detected in the total biomass of the experimental eggplants (Figure 1a,b), since the lowest foliar K concentrations found in the TBC and CONTROL plants (Table 6) were accompanied by the lowest values of the ratio F_v/F_m (Figure 4a) and performance index—PI (Figure 4b), as well as by significant decline in fruit quality, as expressed by TPC (Figure 5a), TFC (Figure 5b) and DPPH levels (Figure 5c). This shows the influence of soil amendments on K availability for plants (Table 6), and therefore on photosystem II functionality (F_v/F_m and PI) and consequently on fruit quality, since it was found to be significantly influenced by K availability [34]. Indeed, the impact of K, along with the application of P and some micronutrients (Fe, Mo, B, Zn, etc.), could be strong enough to improve the biosynthesis of phenolics and flavonoids [73] and to ameliorate fruit quality, since, in our case, PM was a rich source of nutrients (mainly of K and P, along with that of some micronutrients), contributing to optimum fruit quality in the PM, TBC + PM and AN + PK treatments (Figure 5a–c).

Fruit quality, as expressed by TPC, TFC and DPPH, significantly declined in CONTROL, TBC and PK treatments (Figure 5a–c), which means that PM (including that with TBC addition) and AN + PK applications were significantly beneficial on fruit qualitative characteristics (Figure 5a–c). This agrees with the results of Aminifard et al. [74], who found that compost application had a strong impact on fruit quality and antioxidant compounds of pepper plants. Similarly, Antonious et al. [60] found that animal manure-amended soils significantly influenced fruit quality (vitamin C, total phenols and soluble sugars) of eggplants; especially with regard to total phenols, the highest concentration was determined in fruits of eggplants grown on vermicompost, mixed with biochar, compared to sewage sludge and horse manure-amended plants [60]. In addition, and similarly to our data, Jagessar et al. [75] concluded that total carotenoid, flavonoid and phenolic contents in sweet peppers were higher in the plants subjected to organic fertilizer treatments, compared to those subjected to inorganic ones [75]. In our study, the increase in DPPH activity under PM, TBC + PM and AN + PK treatments (Figure 5c) could be probably attributed to the high content of fruits in TPC (Figure 5a) and TFC (Figure 5b), as previously reported by Fedeli et al. [76] and Ye et al. [77]. The fact that PK did not positively influence fruit quality could be possibly explained by the lack of N and P availability in this fertilizer, since the combinational application of PK with AN (i.e., the treatment AN + PK) was clearly positive, and within the optimum three treatments, based on fruit TPC, TFC and DPPH contents (Figure 5a–c), compared to the CONTROL plants. This means that PK should be co-applied either with AN, or with other N-P fertilizers, in order to increase N and P availability for plants and to significantly ameliorate fruit qualitative characteristics. Indeed, the co-application of PK with AN (i.e., the treatment AN + PK) significantly improved the availability of some nutrients (mainly those of N, Fe and Mn) for plants, compared to the simple PK application (Table 6 and Table 8), which could be subsequently beneficial for the amelioration of TPC, TFC and TPPH levels in fruits (Figure 5a–c). Of course, this hypothesis needs further, long-term, experimentation, under field conditions, in order to be fully verified. Gonzalez-Coria et al. [78], in their study, concluded that organic fertilization with reduced N application did not negatively influence tomato yield, but positively affected phenolic compound levels in tomato fruits. Perhaps a similar approach (i.e., co-application of limited N, together with different organic amendments and manures) would be also beneficial for experimentation with eggplants in the near future, aiming at further improving fruit quality.

The optimum nutrition effects in the PM, TBC + PM and AN + PK treatments, as expressed by enhanced most macronutrients and micronutrient uptake (Table 6, Table 7 and Table 8), could be due to the increased secondary metabolism (higher TPC, TFC and DPPH levels of eggplant fruits) (Figure 5a–c), as well as due to enhanced enzyme activities for physiological growth [79]. It could be speculated that the improved bioactive composition of the PM, TBC + PM and AN + PK treated plants could either (i) promote sink strength for continuous mineral element flow and accumulation [80], and/or (ii) stimulated genes encoding nutrient transporters in cell membranes, and/or (iii) stimulated root system growth to facilitate higher nutrient uptake and translocation [81]. Root growth stimulation was a case only in the PM-treated plants (Table 5) and not in the other two treatments (TBC + PM and AN + PK), where one of (or both) the first two hypotheses could be responsible for the improved nutrient uptake and not the third one, based on the data of Table 5. Of course, all these hypotheses need further thorough experimentation in the near future, also including molecular approaches to investigate gene stimulation and transporters under different fertilization regimes and application rates.

5. Conclusions and Future Perspectives

Poultry manure, ammonium nitrate + patent kali and tree branch chips + poultry manure treatments provided the optimum effects on total and fruit biomass, plant nutrition, photosystem II functionality and fruit quality of Solanum melongena L. plants; in contrast, a significant decline in all the above parameters was found in the CONTROL and tree branch chip-treated plants. These data clearly show the importance and significant influence of optimum, balanced fertilization on the improvement of nutrient uptake, photosystem II functionality and fruit quality of eggplants. Based on these data, it is suggested to include additional, multi-year field experimentation on the combined (mixed) inorganic and organic fertilization effects on the improvement of eggplant productivity, nutrition, physiological performance and fruit quality, with maximum benefits for human health. Finally, the—even partial—substitution of high inorganic fertilization rates with manure and other soil amendment applications (with organic residues and by-products of agricultural production) could constitute a promising strategy for decreasing the high fertilization cost for eggplant growers, especially under the light of the recent energy crisis. However, additional (including cost) consideration on how to choose the suitable fertilizers/amendments for nutrient inputs in eggplants should be included by the local farmers, based also on their local experience, cost of these selected materials, expected prices from improved fruit quality after organic material application, etc.