To determine the physiochemical properties of soil, the detailed method of [22] was followed. In brief, 15–20 g of soil samples were collected for the field, and impurities were removed. The obtained soil sample was subject to air-dried and sieved through a 4-mm mesh to obtain clean and fine soil for further investigation. 5 g of the obtained soil sample was diluted with 25 ml of deionized water, followed by vortex for 5 min, and incubated at room temperature at continuous shaking for 1 h. After that, the pH and electrical conductivity were measured using pH and EC meters, respectively.

The endogenous ABA was extracted and quantified through gas chromatography-mass spectrophotometry (GC-MS) following the detailed methodology as previously described by [23] with slight modifications. Briefly, 0.5 g of sample was diluted with 10 mL of extraction buffer and was then subjected to shaking (130–140 rpm) at room temperature for 2 h. After shaking, the sample was then subjected to vacuum filtration and was diluted four times with a 5 mL extraction buffer. The obtained supernatant was treated with 50 ng of ABA standard before being evaporated in a rotary evaporator to a minimum of 2 mL. A 2 mL of 1 N (sodium hydroxide) NaOH was added to RBF and sonicated before being transferred to the long test tube. The pH was adjusted to 12.5, and 4 mL of dichloromethane was added to an empty RBF and sonicated before being transferred to respective test tubes, followed by vertex, and allowing to form layers.

The obtained upper layers were acidified by lowering the pH to 2.5–3.5, followed by adding 4 mL acetyl acetate three times for solvent-solvent partition through vertexing. The upper layer was then subjected to vacuum dry, and a 1 g of PVPP was added to the new flask while the RBF was washed three times with 4 mL of phosphate buffer pH 8.0, sonicated, and then transferred to the respective flask, followed by shaking for 1,30 Hr and they were the cleaned twice with 5 mL of P buffer. The filtrate pH was adjusted to a range of 2.5 to 3.5, and added ethyl acetate was three times, and vortexed again three times; the top layer was removed and added DW pH with 2.5 to the ethyl acetate vortex, and thrower away the bottom part. The solution was dried to 1 mL before transferring it to RBF. One ml was transferred to small vials and dried under an N-dryer. After that, 1 mL of 100% ethyl acetate was added twice and sonicated. Sixty uL of diazomethane was added, vortexed, and subjected to incubation in the dark at room temperature for 30 min. After incubation for 30 min, the vials were dried using N-dry gas; this incubation with diazomethane was repeated two times. Finally, the vials were diluted with 20 µL of dichloromethane and injected into GC-MS (6890N network gas chromatograph, Agilent Technologies), software to monitor signal ions (m/z 1162 and 190 for Me-ABA and m/z 166 and 194 for Me-[²H₆]-ABA).

The antioxidant activity was estimated using the method described by Adhikari et al. 2020 [24]. Briefly, to quantify SOD, a 0.2 g of the plant samples were added to 5 mL of extraction buffer (5 mM Tris HCl (60 mL) + 40 mL EDTA (10 mL) and were then centrifuged at 10,000 xg for 10 min. A 50 uL of the obtained supernatant was diluted by the addition of 150 µL of extraction buffer + 50 µL of pyrogallol to record A values, and 50 uL of supernatant was added 200 µL of extraction buffer to record B values, while a 50 uL of supernatant was added 150 uL of extraction buffer + 100 µL of pyrogallol to record C values at an absorbance wavelength of 420 nm.

SOD activity = [1−(A−B/C)]

To measure flavonoid content, 0.2 g of plant samples were extracted with NaNO₂, followed by centrifuge for 10 mint at 10000 rpm. The obtained supernatant was then diluted with 60 µL of 10% AlCl_3, and 1 M sodium hydroxide was added. The absorbance was measured at 510 nm, as described by Adhikari et al. [25].

The 1,1-diphenyl-2-picrylhydrazyl (DPPH) scavenging activity was determined as described by Blois [26] with modifications. Briefly, 0.2 g of sample was extracted using 90% ethanol followed by centrifuge for 10 mint at 10000 rpm. The obtained supernatant was then diluted with 0.5 mM DPPH and 100 mM acetate buffer pH 5.5. The absorption was measured using a spectrophotometer at 517 nm and deliberated using the following equation:

 Scavenging effect (%) = 1 − (Abssample−Abscontrol) ∗ 100

To measure LPO lipid peroxidation levels, a 0.2 g of freeze-dried samples were diluted with 10 mM phosphate buffer pH 7.8% and 5% TCA and followed by centrifuge at 15,000 rpm for 12 min at 4°C. After centrifugation, 1 ml of supernatant and 2 ml of 0.5% TBA was added and incubated at room temperature for one hour, and then absorbance was measured at 532 nm.

For MDA quantification, a 0.2 g of grounded fresh leaves were combined with 5 mL of 5% TCA solution, followed by centrifuging at 10,000 xg for 10 min; the obtained 100 uL of supernatant was added to 1 mL TBA reagent and heated in a water bath at 45°C or 10 mins, and cooled immediately in a refrigerator, later centrifuged again at 10,000 xg for 10 mins and 250 uL of supernatant was taken, the absorbance was read as follows.

R₁ at 530 nm, R₂ at 600 nm, and the final value were obtained using MDA equation R₃ (R₁–R₂) as the absolute value.

For Chlorophyll a, 0.2 g of freeze-dried grounded shoot sample was extracted by adding 150 uL of 80% methanol and centrifuged at 10,000 xg for 10 min, 200 uL of supernatant was removed and read at 470 nm.

The ion uptake was determined using a previously described method [27]. In a microwave, freeze-dried powdered plant samples of 0.1 g were mixed with 3 ml of nitric acid and 2 ml of H₂O₂. The samples were injected using inductively coupled plasma mass spectrometry (ICP-MS).

The experiment was repeated thrice, and each replicate contained 20 plants. GraphPad Prism was used for graphical representations. One-way ANOVA in MS Excel determined the statistical difference between treatment means. Duncan’s multiple range test (DMRT) was used to determine critical values for comparison between the mean of treatments at a significance level of p < 0.05. Statistical Analysis System (SAS 9.1) was used for the DMRT Analysis. For the graphical representation, we used GraphPad Prism software (version 6.0, San Diego, CA, USA).

The results of the current study indicate that the application of food waste significantly improved soil electrolytic conductivity. As shown in Fig. 1a significant increase in EC was found in FW4 (1.4 mS/m) and decreased in FW2 (0.8 mS/m: No, FW2 was lower than the control) treated plants when compared to control plants. Moreover, further addition of Chlorella slightly increases the EC level by (0.7 mS/m) when compared to the control. However, the combined application of FW and CR significantly increases the EC level by 133.3%, compared to the control plant followed by the sole application of FW and chlorella (Fig. 1A). Similarly, the application of FW lowers the soil pH to 3.0–3.5, when compared to control plants, while the combined application of FW + CR significantly balances soil pH to optimum ranges between 3.8–4.0, which is the optimum level for the availability of most nutrients. These parameters further improve the soil’s physiochemical properties and nutrient availability (Fig. 1B).

The current results showed that food waste application (FW2 and FW4) reduced pepper plants’ growth characteristics (Table 1). As plants treated with FW4 significantly decreased the shoot length (15.9 ± 2.50 cm), root length (21.9 ± 2.65 cm) shoot fresh weight (11.6 ± 3.13 g), dry weight (1.8 ± 0.68 g), and stem thickness (4.4 ± 0.43 mm), when compared to control plants (Fig. 2). While, combine inoculation of Chlorella along with food waste enhances the morphological features in red pepper plant such as increases in shoot length (28 ± 0.91 cm), root length (30 ± 3.37 cm), fresh shoot weight (15.6 ± 1.60 g), shoot dry weight (2.7 ± 0.44 g) and stem thickness (5.7 ± 0.31 mm).

Red pepper plants treated with food waste alone and in combination with Chlorella were further investigated to quantify endogenous ABA. The current results show that sole FW2 and FW4 cause a significant increase in the ABA content by 41.8% and 67.5%, when compared to control plants. However, the sole CR and FW2 + CR application shows a significant reduction in ABA content by 39.7% compared FW4 treated plant, followed by the combined application of FW4 + CR (Fig. 3).

Major ROS-scavenging enzymes, such as SOD, PPO, MDA, and DPPH, and phytochemicals, such as flavonoids and phenolics, were investigated to assess their influence on food waste and on the regulation of the endogenous antioxidant defense system of red pepper. The current results indicate that a slight increase in the SOD, MDA, El, and DPPH content was found in the plant treated with sole CR application, whereas a significant decrease was found in the total flavonoids and phenolic content by sole CR application when compared to control plants (Figs. 4–7). On the other hand, FW2 and FW4 application cause significant increases in the SOD (21.5%), MDA (8.6% and 17.3%), EL (9.8% and 14.1%), DPPH (24.1%), total flavonoids (15.5%), and phenolic content (19.6%) compared to control plants. Were the combined application FW2 + CR and FW4 + CR significantly reduced this increase in the SOD (9.1% and 13%), MDA (15.4% and 7.3%), EL (13.2% and 7.1%), total flavonoids (11.2% and 16.1%), and phenolic content (8.6% and 12.4%) when compared to sole FW application. However, in the case of DPPH, the FW2 + CR significantly increased its content by 12.1% and 22.4% compared to FW-treated and control plants, while the FW4 + CR shows a significant reduction in the DPPH content by 7.5% and 16.7% when compared to control and FW treated plants (Fig. 6B).

The accumulation of different elements such as Ca, Na, and K was investigated through ICP analysis (Fig. 8). The current results show a significant increase in the Na content by FW2 (21.5%) and FW4 (48.6%) compared to control plants, where the sole CR and combine FW2 + CR application shows a slight decrease in Na content compared to control plant (Fig. 8A). in the case of K content, its level was significantly reduced in the FW4 (19.3%), CR (11.2%), and FW4 + CR (11.6%) treated plant compared to control plants, where the FW2 + CR application cause a significant increase in the K content by 13.8% compared to control plants (Fig. 8B). Moreover, the Ca content shows a significant increase by the FW2 (10%) and FW4 (8.6%) treated plants compared to control plants, where the sole CR and combine application of FW and CR cause significant reduction in the Ca content by 8% and 12% when compared to a sole FW treated plants (Fig. 8C).