In this study, intercropping, biochar and vermicompost increased photosynthesis pigments. The increase in leaf chlorophyll and carotenoids content is basically done through improving nitrogen absorption and increasing leaf nitrogen. Intercropping increases the absorption of photo synthetically active rays and the possibility of using the canopy space, also increases the efficiency of nutrients usage, and the relative productivity of each plant from water and nutrients increases (Hussain et al. 2020; Madembo et al. 2020; Maitra 2020). Vermicompost with biochar make suitable conditions for basil by providing nutrients and water and also prevent to nutrient leaching and increased soil water capacity, so they improved basil photosynthesis pigments and by increasing photosynthesis, promote soluble sugars and also yield and yield compounds of basil (Paczka et al. 2021; Tavallali et al. 2022). The reduction of soluble sugars reduces the nutrients needed for plant growth, and intercropping has increased the amount of soluble sugars due to the better use of organic matter in the soil by plants (Dai et al. 2018).
Increasing the membrane stability index (ion leakage) causes a lot of damage to the plant. This damage leads to preventing the production of photosynthetic substances and ultimately causes a decrease in plant growth. By increasing the leaf RWC, intra-cellular pressure is provided for cell growth, and as a result, the expansion of the cell wall becomes possible, and finally, it increases the flexibility of the cell membrane to provide the basis for cell growth (Amer et al. 2021). It can be said that by using biochar and improving the physical and chemical conditions of the soil, including the water holding capacity of the soil, the plant is less prone to drought stress and has shown less tendency to increase the stability of the membrane and provides most of the conditions for the expansion and growth of the wall. In this study, biochar and vermicompost increased RWC. Biochar causes water retention around the roots, in the control treatment (without biochar), as water is removed from the soil and not replaced, the water potential in the root area is reduced and if the resistances remain constant in the plant, in order to maintain the speed Sweating, the water potential in the plant decreases significantly. The decrease in the relative water content of the leaves in response to the decrease in the supply of moisture needs of the plant indicates that in this case, the supply of water from the roots does not match the amount of loss from the leaves. Vermicompost by improving the physical, chemical and biological characteristics of the soil, leads to an increase in the plant's access to water, thus, prompts the RWC and reduces the stability index of the membrane in the plant (Raza et al. 2013; Voko et al. 2022; Yan et al. 2020). Probably, vermicompost can actually act as a growth regulator. Because vermicompost containing plant hormones such as gibberellins and containing large amounts of mineral substances such as calcium, it can reduce the leakage of electrolytes and improve the stability of leaf cell membranes (Lahbouki et al. 2021; Zhao et al. 2020). Our results are similar to the work of Yan et al. (2020), these researchers stated that the use of biochar increased the quantitative and qualitative yield of five mint species.
Catalase is one of the superlative antioxidant enzymes in plants. It supports cells from the effects of hydrogen peroxide and also preventing cell wall degradation and helps plants by removing active oxygen species. Catalase is evaluated as one of the iron including proteins and acts in plants when the hydrogen peroxide in the environment is high, so when the catalase activity is low; it showed that environment conditions are suitable for plants (Tang et al. 2019; Farooq et al. 2021). Therefore, in this study, intercropping 70% forage turnip + 30% basil decreased catalase activity compare to another levels. Usage of vermicompost and biochar mostly motivate plants to improve the antioxidant system activity by stimulant specific antioxidant genes (Lahbouki et al. 2021). Similarly, Tikoria et al. (2021), reported that vermicompost increased catalase activity of tomato (Solanum lycopersicum L.).
Biochar and vermicompost usage improved total anthocyanin of basil. Vermicompost is full of elements and increased the availability of nutrients for plants, which enhanced the higher photosynthetic activity and in turn, corresponded to the higher amounts of anthocyanin. On the other hand, light intensity and temperature are two important environmental factors for red pigments development such as anthocyanin (Lalay et al. 2021; Naser et al. 2016), and in present study intercropping in level of 70% forage turnip + 30% basil made suitable conditions for basil. Other studies reported that application of biochar increased Rapeseed (Brassica napus L.) anthocyanin (Danish and Zafar-ul-Hye 2019).
Plant height increased by usage of biochar and vermicompost. It seems that, increasing the content of chlorophyll, as an important factor in the plant's photosynthetic system, make increased the growth and height of the plant. In this study, biochar improved plant height by increasing the plant's access to nitrogen. On the other hand, vermicompost can increase plant height due to the availability and increase of soil nitrogen. Also, biochar and vermicompost increased the plant height by improving the qualitative characteristics of the soil (Ouertatani 2021; Greco et al. 2021).
The use of biochar and increasing the amount of vermicompost (18.5 ton ha− 1) raised leaf/stem ratio, which can be said that the availability of nitrogen in vermicompost and increasing plant access to this element by biochar; in addition, increased the leaf area and surface durability. It became delayed the aging of leaves and their fall, thereby improving the leaf/stem ratio. Considering that leaves and flowers have the highest amount of essential oil in basil, therefore, increasing the ratio of leaves to stems can play an effective role in increasing the quality and quantity of essential oil produced (Shushupti et al. 2022; Mumivand et al. 2023).
The essential oil yield increased by increasing the use of vermicompost and also application of biochar, due to the increase in the leaf/stem ratio. Because vermicompost and biochar prepare nitrogen for basil and increased leaf area (Mumivand et al. 2023). Also, light is one of the most important factors for photosynthesis, which has a great effect on increasing the percentage of essential oil. In this study the height of intercropping plants (basil and forage turnip) were the same and forage turnip was not exposed to shading stress and also increased absorption of photosynthetically active radiation. Thus, basil can use light as well as forage turnip and improved essential oil (Husain et al. 2020).
Probably, the reason for the larger stem diameter of the basil in 70% forage turnip + 30% basil level of intercropping can be due to the absorption of more sunlight by basil, which causes photosynthesis and the production of more photosynthetic materials, and as a result, the presence of more vascular elements for the rapid transfer of cultivated materials to other plant organs (Maitra et al. 2021; Glaze-Corcoran et al. 2020).
Vermicompost contains nitrate, phosphorus, potassium, calcium and magnesium, which are essential minerals for plants, and vermicompost made easily nutrient available to plants and provide suitable conditions for plant growth. On the other hand, biochar prevent nutrient leaching and increased availability of nutrients for plants. Therefore it makes improved dry matter yield and yield compounds (Voko et al. 2022; Greco et al. 2021). Our outcomes are also similar to the work of Mumivand et al. (2023), they reported that application of biochar increased peppermint (Mentha × piperita L.) yield and yield compounds and also photosynthesis pigments (chlorophylls and carotenoid).