Exogenous Application of Humic Acid Mitigates Salinity Stress on Pittosporum (Pittosporum tobira) Plant by Adjusting the Osmolytes and Nutrient Homeostasis

Pittosporum is an ornamental greenhouse plant widely used in landscaping and along roadsides, as well as in parks and gardens (Abou Kubaa et al. 2020). Pittosporum genus belongs to the family Pittosporaceae which includes about 200 species, the Pittosporum tobira plants typified by about 2–3 m high, the leaves are dark green (dark green with white zones in Pittosporum tobira var. variegate type) (Tian-Tain et al. 2011). The genus Pittosporum is considered a source of essential oils such as monoterpenes, aliphatic hydrocarbons, sesquiterpenes. Also, α‑pinene as major compound was obtained from the flowers of pittosporum plants (Nickavar et al. 2004). Furthermore, El-Dib et al. (2015) reported that the n-butanol fractions from pittosporum leaves possess antimicrobial activity and cytoprotective effects against breast carcinoma, hepatocellular carcinoma colon carcinoma cancer cell lines. Besides pittosporum is regarded as one of popular ornamental decorative shrubs in landscape purposes because of its beautiful shape and leave colors and white odor flowers.

Distinctive adverse effects on crop yield and quality are realized owing to eco-stresses occurrence (Saudy et al. 2020a; Mubarak et al. 2021; El-Metwally and Saudy 2021; El-Bially et al. 2022a; El-Metwally et al. 2022a; Saudy et al. 2023a). Under various environmental stresses, several changes in plant physiology expressed in nutrient uptake and osmolytes accumulation occur (Hadid et al. 2023), affecting crop growth and productivity (Saudy and El-Metwally 2019, 2023). Salinity is one of the most widespread abiotic stresses which results in significant losses in agricultural crop production, especially in arid and semiarid areas (Salem et al. 2021; Hernández 2019; Alsamadany et al. 2022; El-Bially et al. 2022b). Saline conditions (consists of Cl and or Na ions) results in retarding growth, chlorophyll damage with low crop quantity and quality (Abd El-Mageed et al. 2022; Salem et al. 2022; Shaaban et al. 2023a).

Humic acid is a natural soil organic matter compound resulted from either plant, animal or microbial residues decomposition as well as from metabolic activity of microbes (Du jardin 2015). Several studies showed that humic acid not only enhances root, leaf and shoot growth, but also, stimulates the germination of many plant species (Trevisan et al. 2010). Humic acids are the main fractions of humic substances and the most active components of soil and compost organic matter. Humic acids have been shown to stimulate plant growth and consequently yield by acting on mechanisms involved in cell respiration, photosynthesis, protein synthesis, water and nutrient uptake, enzyme activities (Chen et al. 2004a; Dinçsoy and Sönmez 2019; Ramadan et al. 2023). Because of their potential to stimulate nutrient content, phenols, flavonoids and antioxidant activity in plant leaves, humates compounds considered natural antioxidants (Bayat et al. 2021). It is well known that the increase in sodium and chloride ions accumulation in plant tissues associates salt stress, causing disruption ion homeostasis, with physiological disturbance, and reduction in potassium uptake (Ketehouli et al. 2019). Also, the accumulation of osmolytes such as proline and soluble proteins increased in salt-stressed plants (Rahman et al. 2016). Contrary, humic acid reduced the accumulation of sodium and salinity-induced increase in proline while increased the antioxidant enzymes, protecting the plant from hazards of salinity (Abbas et al. 2022; Abu-Ria et al. 2023). The optimal concentrations able to affect and stimulate plant growth have been generally found in the range of 50–300 mg L^–1, but positive effects have been also exerted by lower concentrations Chen et al. (2004b). A distinction on the effects of humic acid should be made between indirect and direct effects on plants growth. Indirect effects are mainly exerted through properties such as enrichment in soil nutrients, increase of microbial population, higher cation exchange capacity, improvement of soil structure (Roudgarnejad et al. 2021). Whereas direct effects are various biochemical actions exerted at the cell wall, membrane or cytoplasm and mainly of hormonal nature (Varanini and Pinton 2001). The hormone like activities of humic acids is well documented in various papers, in particular auxin-, cytokinin- and gibberellins like effects (Nardi et al. 2016).

Based on the distinctive properties and mode of action of humic acid, the current study hypotheses that the negative impacts of salinity on pittosporum could be mitigated by exogenous supply of humic acid via adjusting the osmolytes and nutrient homeostasis. Therefore, the main objective of the research was to assess the change in physiology and nutrient contents f pittosporum plants owing to application of humic acid under different salt stress levels.

Huge efforts have been exerted to overcome the issues and sustain the crop productivity and quality under harsh growth conditions (El-Bially et al. 2018; El-Metwally et al. 2021; Saudy et al. 2021; El-Metwally et al. 2022b; Abd–Elrahman et al. 2022; Shaaban et al. 2023b). Plants exposed to salinity stress exhibited a reduction in growth and yield potential (Liu et al. 2020; Shahid et al. 2020). However, applications of humic acid mitigated the negative effects of salinity, as evidenced by the increases in plant height, lateral branches number, stem diameter, leaves number as well as root and shoot weights of pittosporum (Table 1 and 2) and decrease in fallen leaves (Fig. 1). To explain the beneficial effect of humic acid-mediated salinity stress mitigation, the current study assessed several features that could be used to improve pittosporum response under salinity conditions. It has been observed that subjecting plants to salt stress led to significant decreases in all vegetative growth parameters (plant height, lateral branches number, leaves number, stem diameter, fresh and dry weights of plant parts (root and shoot), while fallen leaves and toxic ions increased. Salinity had serious unfavorable effects on plant growth, as evidenced by the significant reduction in growth and biomass of tomato (Alam et al. 2020) and faba bean (Abdel Latef et al. 2021). In the current study, salinity resulted in an ionic imbalance, since potassium was reduced while chloride and sodium were increased under salt stress.

Due to induction of osmotic stress with over-accumulation of Na⁺ and Cl⁻ ions in cells by salinity, ionic imbalance and toxicity were obtained (Khan et al. 2018). Salinity causes handicapping the absorption of needed nutrients (Khan et al. 2019). Owing to the ionic misbalance in plants, negative changes in different morpho-physiological and biochemical were reported (Makhlouf et al. 2022; Saudy et al. 2023b). Moreover, mung bean plants suffer from water deficiency with low nutrients absorption under saline conditions, hence photosynthesis disruption and oxidative stress occurred (Rahman et al. 2019). Owing to salinity, reactive oxygen species (ROS) generated in plant cells (Khan et al. 2018; Hadid et al. 2023), causing oxidative damage expressed in lipid membranes deterioration of the plant cells (Guimarães et al. 2011). Accordingly, salinity leads to reduction of plant leaf area, gas exchange through stomata, and plant pigments concentrations (Netondo et al. 2004; Shaaban et al. 2023a). Due to high accumulation of sodium as a toxic ion and decrease in potassium, calcium and magnesium as beneficial ions, the distinctive association between weak growth of salt-stressed plants and ionic toxicity was observed (Abdel Latef et al. 2021). This might be attributed to the injuries of the cell membrane and ion leakage (Abd El-Mageed et al. 2022), and turmoil in uptake of beneficial ions (Liu et al. 2019; Mubarak et al. 2021).

For adapting to salt stress, plants synthesize specific proteins to be more tolerant under salinity situations (Bavei et al. 2011). Various plants produced certain proteins under stress conditions (Parker et al. 2006; Younis et al. 2009; Hadid et al. 2023). This effect has been proved in the current study, since protein, proline and peroxidase activity increased with increasing salinity level (Fig. 2). Plants could combat the osmotic stress of salinity by accumulating a diverse osmolytes in their cells such as proline (Hayat et al. 2012; Zulfiqar et al. 2020), hence water balance and osmotic potential maintained (Mansour and Ali 2017; Hasanuzzaman et al. 2019). The physiological upregulation of the defensive actions for oxidative pressure has been recorded in different plants under abiotic stresses (Latif et al. 2016; Tahjib-Ul-Arif et al. 2018).

Several attempts have been adopted to mitigate the impact of salinity and increase the tolerance of plants to salt stress (Wang et al. 2019). It is well known that nutrients are readily absorbed by plant roots when the soil minerals are available (Ouni et al. 2014). Therefore, as obtained in the current study, soil addition of humic acid significantly increased the beneficial nutrient contents in the leaves of pittosporum plants. This useful finding may be because humic acid participates to an enhancement in organic acids (Liu et al. 2019), while plant phytohormones released by microorganisms (Cheng et al. 2020; Sun et al. 2021) to shrink the unwholesome impacts of salts in the soil. Additionally, the generated phytohormones motivate the proliferation of plant roots (Nunes et al. 2019), promoting the nutrient absorbing surfaces and organic acids production which increase the solubility for various mineral forms, hence enhance plant responses (Rady et al. 2016; Belal et al. 2019; Saudy et al. 2020b). Humic increases accumulation of leaf nutrients and chlorophyll biosynthesis (Kaya et al. 2018). Furthermore, humic substances can display gibberellin- and cytokinin-like activities (Nardi et al. 2016), with improvement in cell membrane permeability (Osman and Rady 2014). Therefore, the saline soil fertility and plants nutrients uptake were ameliorated in favor of plant growth and yield with humic acid application (Table 1, 2 and 3).

Generally, humic acid ameliorated the negative effects of salinity on pittosporum plant through adjusting the osmolytes and nutrient homeostasis. Thus, pittosporum growth and development were relatively improved under saline conditions by application of humic acid.

Humic acid is a good anti-stress for a salinity level of 4000 mg L⁻¹ where most pittosporum traits of are improved. The beneficial effects of humic acid for coping the salinity injury are associated with its significant role in adjusting various physiological aspects expressed in nutritional status and osmo-protectant composition. Accordingly, it is advisable to introduce humic acid as a useful practice pittosporum plants cultivated under saline conditions.