Photosynthetic Pigments and Biochemical Response of Zucchini (Cucurbita pepo L.) to Plant-Derived Extracts, Microbial, and Potassium Silicate as Biostimulants Under Greenhouse Conditions

Abstract

There are many technological innovations in the field of agriculture to improve the sustainability of farmed products by reducing the chemicals used. Uses of biostimulants such as plant extracts or microorganisms are a promising process that increases plant growth and the efficient use of available soil resources. To determine the effects of some biostimulants' treatments on the photosynthetic pigments and biochemicals composition of zucchini plants, two experiments were conducted in 2019 and 2020 under greenhouse conditions. In this work, the effects of beneficial microbes (Trichoderma viride and Pseudomonas fluorescens), as well as three extracts from Eucalyptus camaldulensis leaf extract (LE), Citrus sinensis LE, and Ficus benghalensis fruit extract (FE) with potassium silicate (K₂SiO₃) on productivity and biochemical composition of zucchini fruits, were assessed as biostimulants. The results showed that E. camaldulensis LE (4,000 mg/L) + K₂SiO₃ (500 mg/L) and T. viride (10⁶ spore/ml) + K₂SiO₃ (500 mg/L) gave the highest significance yield of zucchini fruits. Furthermore, the total reading response of chlorophylls and carotenoids was significantly affected by biostimulants' treatments. The combination of K₂SiO₃ with E. camaldulensis LE increased the DPPH scavenging activity and the total phenolic content of zucchini fruits, in both experiments. However, the spraying with K₂SiO₃ did not observe any effects on the total flavonoid content of zucchini fruits. Several phenolic compounds were identified via high-performance liquid chromatography (HPLC) from the methanol extracts of zucchini fruits such as syringic acid, eugenol, caffeic acid, pyrogallol, gallic acid, ascorbic acid, ferulic acid, α-tocopherol, and ellagic acid. The main elemental content (C and O) analyzed via energy-dispersive X-ray spectroscopy (EDX) of leaves was affected by the application of biostimulants. The success of this work could lead to the development of cheap and easily available safe biostimulants for enhancing the productivity and biochemical of zucchini plants.

Keywords: beneficial microbes; phenolic compounds; plant-derived biostimulants; potassium silicate; zucchini plants.

Figure 1

The measured climatic conditions in the greenhouse during 2019 and 2020. (A) Minimum and maximum temperature (°C); (B) RH (%).

Figure 2

Fruits number and total yield of zucchini as affected by biostimulant treatments. (A) Total number of fruits/ha and (B) The total yield (ton/ha) (means ± S.E) of zucchini as affected by the extracts, microbial inoculations, and K₂SiO₃. Letters in the figure indicated that the means ± S.E of treatments with the same letter/s were not significantly different according to Tukey's HSD level of probability.

Figure 3

Elemental compositions of zucchini leaves with EDX analysis as affected by plant extracts, microbial inoculations, and potassium silicates biostimulants. Analysis was taken from three points for each treatment. (A1–A3) Control; (B1–B3) T. viride + K₂SiO₃; (C1–C3) P. fluorescens + K₂SiO₃; (D1–D3) T. viride + P. fluorescens + K₂SiO₃; (E1–E3) E. camaldulensis LE + K₂SiO₃; (F1–F3) C. sinensis LE + K₂SiO₃; (G1–G3) F. benghalensis FE + K₂SiO₃.

Figure 4

DPPH (2,2-diphenyl-1-picrylhydrazyl) inhibition percentages (means ± S.E) of methanol extracts from zucchini fruits as affected by the plant extracts, microbial inoculations, and potassium silicate biostimulants. Letters in the figure indicated that means ± S.E of treatments with the same letter/s were not significantly different according to Tukey's HSD at a 0.05 level of probability.

Figure 5

Phenolic and flavonoid contents (means ± S.E) of zucchini fruits as affected by the plant extracts, microbial inoculations, and potassium silicate biostimulants. (A) Total phenolic content and (B) total flavonoid content. Letters in the figure indicated that the means ± S.E of treatments with the same letter/s were not significantly different according to Tukey's HSD at a 0.05 level of probability.