Chlorophyll a, chlorophyll b and carotenoids were determined in leaves (Vernon and Seely, 1966). The results were calculated as mg.g^-1 fresh weight.

Ascorbic acid was measured as described by (Jagota and Dani, 1982). Fresh weight (0.2 g) sample was homogenized in 1.5 mL 10% (w/v) trichloroacetic acid (TCA) at 4 °C. After centrifugation at 3000 × g for 5 min, 0.3 mL of the supernatant was made up to 2 mL volume with distilled water. A 0.2 mL 10% (v/v) Folin phenol reagent (Sigma Chemical Co.) was then added to the mixture, and vigorously shaken. After 10 min reaction time, maximum absorbance was measured at 760 nm. A result of the reaction between ascorbic acid and Folin phenol reagent, L (+) – ascorbic acid (Hamburg chemistry, Germany) was used as a standard.

Sample (0.5 g) from each treatment were homogenized in 100 mM pre-chilled sodium phosphate buffer (pH 7.0) containing 0.1 mM EDTA and 1% polyvinyl pyrrolidone (PVP) (w/v) at 4 ºC. The extraction ratio was 4.0 mL buffer for each one gram of sample. The homogenate was centrifuged at 15.000 x g for 15 min at 4 ºC. Supernatant was used to estimate the activities of peroxidase and polyphenol oxidase. Proteins content was determined in the enzymes extract (Bradford, 1976).

Peroxidase was assayed following method the method of Hammerschmidt et al. (1982). The reaction mixture (2.9 mL) consisted of 0.25% (v/v) guaiacol in 10 mM sodium phosphate buffer (pH 6 containing 10 mM hydrogen peroxide). Volume of 100 μL of the enzyme extract was added to launch the reaction which was determined spectrophotometrically (Jasco V-530, Jappan) at 470 nm per min. One unit of PPO activity was defined as the amount of enzyme that causes an increase in absorbance of 0.001 min^-1 mL^-1. The POD activity expressed as unit min^-1 mg^-1 protein.

Polyphenol oxidase was assayed following method the method of Oktay et al. (1995). The reaction mixture consisted of 600 μL catechol (0.1 M) and 100 μL enzyme extract was completed to 3.0 mL with 0.1 M phosphate buffer pH 7. The absorbance was registered at 420 nm by spectrophotometer. One unit of PPO activity was defined as the amount of enzyme that causes an increase in absorbance of 0.001 min^-1 mL^-1. The enzyme activity was expressed as unit min^-1 mg^-1 protein.

The fruits of sweet pepper were freeze dried (Virtis model 10-324) and ground. The ground samples were extracted using ethanol 80% and used in chemical analyses. Phenolic content was determined (Shahidi and Naczk, 1995) using the Folin-Denis reagent. The results were expressed as mg.g^-1 of gallic (GAE) equivalent of the dry weight.

Power reduction was determined by Oyaizu (1986) technique. Each 0.5 mL sample was mixed with 0.2 M sodium phosphate buffer 2.5 mL and 1% potassium ferricyanide 2.5 mL and incubated at 50 °C for 20 min. Following the addition of 2.5 mL of 10% trichloroacetic acid, the mixture was centrifuged for 10 min at 200 g. The upper layer (5.0 mL) was blended with 5.0 mL of deionized water and the test sample was read against the blank at 700 nm.

Metal chelating activity was consequently evaluated by 0.1 mM FeSO₄ (0.2 mL) and 0.25 mM ferrozine (0.4 mL) in 0.2 mL of samples (Chew et al., 2009). After 10 min of incubating at room temperature, absorbance of the blend was recorded against a void reagent was registered at 562 nm. The lower absorption of the test sample suggested greater chelating capacity of ferrous ion. The control contained all the reagents except test sample.

Organic acids were separated by high performance liquid chromatography (HPLC) Knauer, Germany, flow rate was set at 0.6 mL.min^-1, UV detector set at λ = 214 nm, column oven temperature kept constant at 65 °C. The column used was Rezex@ column for organic acids analysis, mobile phase was 0.005 M H₂SO₄, data integration by clarity-chrom software.

All the statistical analyses were performed using an ANOVA, and Duncan’s multiple range tests (Duncan, 1955) were applied to compare the results of the experiments (p ≤0.05). A three replicates was used.

Fulvic acids and algae extracts applications affected fruit chlorophyll content during both two seasons (Figure 1).

Results indicate that fruits chlorophyll content increased with most of treatments. The highest content of fruit chlorophyll was obtained from algae extract (1 g.L^-1), followed by fulvic acid (6%).

Seaweed extracts contain cytokinins that stimulate physiological activities (such as activation of some enzymes that involved in photosynthesis), increased total chlorophyll in the plant and photosynthesis activity that will be positively reflected on shoot characteristics (Thomas, 1996).

The effect of spray treatments with fulvic acids and algae extracts on ascorbic acid content, depends on the concentration used (Figure 2). All extracts stimulated ascorbic acid over control. Spraying with the algae extract (1 g.L^-1) gave the highest content of ascorbic acid (5.38 and 4.75 mg.g^-1 FW in the first and second seasons, respectively) followed by spraying fulvic acid 6% (4.48 and 3.86 in both seasons, respectively). Humic substance application at different concentrations improved tomato vitamin C (Dorais et al., 2008; Mauromicale, Longo and Lo Monaco, 2011).

The results in Figure 3 showed that spraying of algae extract at 1 g.L^-1 resulted in peroxidase and polyphenol oxidase activities 240.08 and 353.64 unit min^-1 mg^-1 protein, respectively while fulvic acid (6%) gave 236.00 and 346.44 unit min^-1 mg^-1 protein, respectively compared with control (195.22 and 261.70 unit min^-1 mg^-1 protein, respectively) in the first season. The same trend was observed in the second season. Treatment with algae extract (1 g.L^-1) gave the highest value for the activity of each enzyme. In general, the best results were obtained from algae extract (1 g.L^-1) followed by fulvic acid extract (6%).

It is clear from Figure 4 that all sprayed concentrations of fulvic acid and algae extracts increased the phenolic content of sweet pepper fruits compared to the control. The highest values in this respect were recorded when plants were sprayed with algae extract at 1 g.L^-1 (15.21 and 14.24 mg.g^-1 DW in the first and second seasons, respectively) followed by fulvic acid at 6% (14.62 and 13.89 mg.g^-1 DW in the first and second seasons, respectively) compared to control (11.06 and 10.93 mg.g^-1 DW in the first and second seasons, respectively).

The organic fertilizer applications significantly affected total phenolic content. Plants cannot allocate resources for growth and defence at the same time, and there is competition between proteins and phenols in plants for common precursors involved in their biosynthesis (Aminifard et al., 2012; Riipi et al., 2002). The use of humic acid and calcium nitrate individually improves the quality of fruits, total phenols and antioxidant activity in pepper fruits under normal and saline conditions (Akladious and Mohamed, 2018). These results led us assume that pepper plants may benefit from fulvic acid fertilizer for protein synthesis and growth development. On the other hand, organic acids and amino acids act as precursors or stimulants of plant hormones and growth substances, as well as secondary compounds in plants (Vernieri et al., 2006). Also, when the light is sufficient for regular photosynthetic rates, additional carbon (C) may be allocated for the synthesis of C-based secondary compounds such as phenols in plants treated with organic fertilizers (Toor et al., 2006).

Figures 5 and 6 show the effect of spray treatments with the extracts of fulvic acid and algae on the antioxidant activity. This indicates a significant difference between all treatments and control in metal chelating activity. The best results were obtained from algae extract at 1 g.L^-1 followed by 6% fulvic acid in both seasons. However, there were no significant differences between all treatments and the control in reducing power in both seasons.

Organic fertilizers increased the antioxidant activity (Aminifard et al., 2012). There are a number of factors that can affect the total antioxidant capacity in plant tissues such as light intensity, temperature and cultivar as well as soil type and soil content of humic compounds (humic acid and fulvic acid) where the high content of humic compounds in the soil led to increase antioxidant activity (Rimmer, 2006). Another view that explains the increase of antioxidant compounds in organic foods is that as the use of insecticide, fungicide and herbicide is limited in organic agriculture, the plants allocate more resources to fight pathogenic attacks, which include the generation of antioxidant compounds (Winter and Davis, 2006). Thus, these findings reveal that fulvic acid had a positive influence on the antioxidant activity of sweet pepper fruits.

Organic acids have sour and fresh taste that impart unique flavour to the food (Williams, 2001). Citric, malic, and succinic acids are the major organic acids in peppers (Luning et al., 1995). The current study reports the determination of oxalic, citric, malic, succinic, glutaric and propionic acids in sweet pepper by HPLC. The results which summarized in Table 4, showed that the organic acids in the pepper fruits decreased in the order of malic >citric > succinic >oxalic >glutaric >propionic. Malic and citric acids are the principal organic acids found in peppers (Jensen, 2007). While, the concentrations of glutaric and propionic acids in the sweet pepper were low. The concentrations of malic, succinic and glutaric acids were higher in 1 g.L^-1 algae extract treatment, but the concentration of citric acid was higher in 6% fulvic acid treatment (Luning et al., 1995).