Allelopathic interaction can be considered as a chemical-mediated interference of one species with the growth of other species (Rice 1984). Allelochemicals are basically secondary metabolites released by plants as by-products of primary metabolic processes, and their major role is to protect the plants from abiotic stress (Latif et al. 2017). Our study attempted to understand the allelopathic influence of rapidly advancing invasive weed Hyptis suaveolens that enabled its invasibility. The study revealed that the seed germination and growth of leguminous crop lentil was adversely impacted by the soils amended with PLB of this invasive weed that entered the soil system on its decomposition in the field. The adverse impact increased with increasing applied dose of the PLB, indicating the increasing amount of allelochemicals released from the higher doses of PLB. These allelochemicals, as recorded from GC-MS investigation of the methanolic leaf extract of this weed, could be derivatives of phenolic, terpenes, fatty acid, alkaloids etc. Li et al. (2019) opined that such derived compounds have been reported to have growth inhibitory effect on several crop species. In fact, the incomplete removal of weed-biomass in the fields has been reported to adversely impact the succeeding crop yield (Chikoye et al. 2000; Gupta and Narayan 2010).
The invasive weeds could be considered influencing the adjoining plant communities by altering the soil characteristics. This study showed that the PLB of this weed enhanced the soil fertility, as evinced by investigation on organic carbon and total nitrogen. This should have enhanced the crop growth. But in contrast, crop growth declined in soils amended with PLB of this weed which could be attributed to some other factors e.g. increasing secondary metabolites like phenolic content in soil that had an overriding retardatory influence on crop growth and yield. The quality and scale of weed impact was, however, dose-dependent. Lentil seed germination was adversely impacted at higher doses of PLB. However, this adverse impact was not significant at lower dose (< 2g kg− 1 soil), indicative of higher dose-dependent influence at germination stage. Such an inhibitory impact by this weed’s leaf extract has been reported on seed germination of other crops e.g. Oryza sativa by Oraon and Mondal (2021). In fact, the germination of plant seeds constitutes a primary step towards the growth and development of many plant species, thus, the result in the present study, highlights the importance of allelopathic activity (Gorai et al. 2014; Wang et al. 2019; Hussain et al. 2020b). This inhibitory impact on seed germination may be due to the release of different allelochemical constituents on decomposition of leaf-biomass of the weed, which was often too slow to affect germination during the short period for crop emergence. These allelochemicals may be released from plants by several ways including volatilization, leachation, residue degradation, and root exudation too (Hernandez-Aro et al. 2016; Latif et al. 2017; Mehmood et al. 2018; Laxman et al. 2019; Ghimire et al. 2020; Zhang et al. 2020).
The lentil growth and its yield significantly declined with increasing dose (H1-H8) of Hyptis PLB added to the soil. Similar results were also observed on the crop growth by the application of leaf-biomass and leachate of several other invasive weeds in laboratory experiments (Siyar et al. 2018; Laxman et al. 2019; Lal et al. 2021). Such an allelopathic activity depends on the concentration of allelochemicals which generally varies with species (Bari and Kato-Noguchi 2017). Sharma et al. (2019) reported the presence of various chemical compounds in the leaves of this exotic weed Hyptis suaveolens which had growth retardatory impact on rice growth. Aromatic plants of Lamiaceae to which the investigated weed belonged, has been found to exhibit phytotoxicity, inhibiting germination and seedling growth (de Almedia et al. 2010; Pinheiro et al. 2015). Another weed Salvia plebeia of Lamiaceae has also been reported to have inhibitory impact on crop seed germination, biomass and chlorophyll contents (Husna et al. 2016).
In the present study, besides allelopthic effect on growth characteristics (shoot and root length and their respective biomass) (Table 2), a significant reduction in the photosynthetic chlorophylls (a, b and total) and carotenoids (Fig. 2) and protein content (Fig. 3a) were also recorded in the leaves of the test crop plants. Batish et al. (2007b) observed the similar decline in chlorophyll content of legume crops when grown in soils amended with leaf-residue of Chenopodium murale L. (annual weed). This decline in photosynthetic pigments may be attributed to either decreased synthesis of chlorophyll or its enhanced degradation or both (Yang et al. 2004; Desai and Gaikwad 2015). Terpene-derivative chemical constituents can also reduce the amount of photosynthetic pigments in the leaves of recipient crop (Kordali et al. 2007; Kaur et al. 2010; Sharma et al. 2019). Chlorophyll and carotenoids constitute the central part of the energy manifestation of evergreen plant system and therefore, any significant alteration in their levels is likely to have a marked effect on the photosynthetic metabolism of the plant (Sun et al. 2017). Protein plays a vital role in biological processes as it regulates growth, development and reproduction of plants (Duke and Dayan 2006; Maiti et al. 2013). In the present study, it decreased significantly with increasing dose of PLB of the investigated weed. The level of protein as well as amino acids has been reported to be reduce under allopathic impact El-Shora et al. (2022). Protein degradation suppress the absorption as well as transportation of amino acids that results in interrupted protein synthesis (Ghayal et al. 2011; Thapar 2012; Huang et al. 2013; Lee et al. 2016). The allelochemicals (mainly phenolic compounds) get accumulated in the plant cell and may interfere with the cytoplasmic ribosomes, and RNA formation, which results in low protein synthesis (Hegab and Ghareib 2010).
In contrast to the observed growth retardatory impacts on the lentil plants with increasing dose of PLB, a significant stimulatory impact on proline and phenolic content was evinced in the present investigation (Fig. 3b, c). Proline, considered as a stress marker, an amino acid, is a solute that enhances tolerance, and is reported to protect against a variety of abiotic stresses (Flores et al. 1988). Increase in proline accumulation with increasing dose of weed-PLB in contrast to significant trend of reduction in the photosynthetic pigments and protein contents, indicates enhanced production of proline under stress-generated conditions (e.g. decline in synthesis of photo-pigments, protein degradation etc.) through the allelochemicals released by the investigated invasive weed Hyptis suaveolens on decomposition. El-Shora et al. (2022) reported that phenolic content increased in recipient plant after applying leaf extract of donor invasive (Rumex dentatus) plant. It is generally believed that allelochemicals enhance the content of non-enzymatic antioxidants (Garcia-Sanchez et al. 2012) like phenolics (Vinson et al. 2001). The antioxidant property of phenolic compounds has been proposed because of several mechanisms that include up-regulation of antioxidant defense, scavenging reactive oxygen species (ROS) and its formation (Dai and Mumper 2010; Swietek et al. 2019).
In this study, the allelopathic response indices (RI) were less than zero, indicate that the varying doses of applied PLB inhibited the different growth characteristics of lentil plants. The allelopathy inhibition effect increased with increasing doses of applied PLB. According to synthetical effect (SE) and their value, it is evident that varying doses of PLB inhibited the growth of lentil crop in dose dependent manner (H1 > H2 > H4 > H8). Similar dose dependent effect of weed on crops has been observed by Balah et al. (2022) and Dai et al. (2022).
In the present study, no negative impact of applied PLB doses of invasive weed Hyptis suaveolens was observed on soil resource status at least upto about three months after its incorporation in the soils. On the other hand, the applied weed-PLB into the soil in this period improved the nutrients (organic carbon and total nitrogen) (Table 5). In fact, decomposing residues of invasive species enhance soil nutrients, which are often utilized for their own propagation (Sharma et al. 2017; Kaur et al. 2019). Enhanced electrical conductivity of the amended soils also indicates greater nutrient availability. In contrast, incorporation of PLB into the soil lowered the soil pH in comparison to unamended soils. Similar alteration in soil characteristics (soil pH and electrical conductivity) has been reported by Batish et al. (2009) in soils amended with leaf debris of invasive alien weed Ageratum conyzoides. The decline in soil pH is obvious because the phenolics (that increased with increasing doses of PLB of the weed in the current investigation) make the soil acidic (Dalton et al. 1983).
It is well recognised that some weeds adversely influence the growth of neighbouring plant by releasing allelochemicals (Qasem and Foy 2001) that could typically be water-soluble and released by leaching or by microbial decomposition (Singh et al. 2005; Callaway et al. 2008). The most prevalent and extensive water-soluble allelochemicals are generally phenolics (Singh et al. 2005; Szwed et al. 2020). It is suggested that when phenols reach the dynamic soil system, they undergo a range of chemical changes, including microbial activity and adsorption onto organic matter or clay particles (Blum et al. 1999). The allelopathic effect of applied PLB on the crop lentil recorded in the present study could be attributed to the higher amount of the phenolic content as recorded in PLB amended soils. Phenolics may decrease the chlorophyll content and net photosynthetic rate (Lu et al. 2018). Phenolics have also been reported to slow down the plant development by partially inhibiting the nutrient uptake (Baziramakenga et al. 1995; Lehman and Blum 1999). Phenolics have been reported to enhance the activity of indole acetic acid (IAA) oxidase and slow down the reaction of peroxidase (POD) with IAA, bound gibberellin (GA) or IAA that affects the level of endogenous hormones (Cheng and Cheng, 2015).Such growth inhibitory impacts due to increasing levels of phenolics has been reported on other crop plants by several workers e.g. Batish et al. (2007a), Zhang et al. (2012).
The applied PLB in the present study exhibited improvement in soil nutrient status. In contrast, this study also exhibited adverse impact on the growth of associated plants ostensibly through biochemical impact of allelochemicals. These allelochemicals may adversely impact plant processes such as hormonal balance, protein synthesis, respiration, photosynthesis, chlorophyll formation and plant water relations (Fagodia et al. 2017; Mahdavikia et al. 2017; Kumar et al. 2020; Yousefi et al. 2020; Khaleghnezhad et al. 2021). This is intelligible from the GC-MS analysis that revealed presence of 35 distinct chemical compounds in the methanolic leaf extract of H. suaveolens that majorly belonged to different classes of alkaloids (41.35%), fatty acid (18.48%), alkane (15.69%), terpenes (14.47%), phenolic (2.91%) compounds, which are considered as allelopathically important phytochemicals that inhibited the growth of several crop species (Li et al. 2019; de Oliveira et al., 2021). Lomas et al. (2022) identified forty-one functional groups of different classes of compounds in the same investigated weed Hyptis, of which 71% were reported from its leaf organ alone, implying a significant role of leaf harboring the allelochemical potentials. Alkaloid compounds (highest percentage in present study) may retard the crop growth by affecting the DNA synthesis, respiration and electron transport (Hagan et al. 2013). Among the identified 35 chemical compounds present in H. suaveolens, squalene and caryophyllene oxide have been reported to have allelopathic impact on the growth of plants (Flores-Palacios et al. 2015; Abd El-Gawad 2016; Abd El-Gawad et al. 2019). Compound dibutyl phthalate has certain allelotoxicity to tabacco seedlings and the growth of Microcystis aeruginosa (Jiajun et al. 2017; Gu et al. 2017). Compound 2-hexadecen-1-ol, 3,7,11,15-tetra-methyl is a decomposition product of chlorophyll and is cytotoxic (Kumar et al. 2010). Several identified compounds (Caryophyllene; 10-Heneicosene (c,t); Eicosane; Hexadecanoic acid, methyl ester; Phytol; Cholest-22-ene-21-ol, 3,5-dehydro-6-methoxy-pivalate; Tetratetracontane etc.) showed different biological activities like antibacterial, antifungal, antimicrobial and antioxidant (Sharma and Cannoo 2016; Frank et al. 2016; Selvaraju et al. 2021; Subin et al. 2021; Albratty et al. 2021; Adelusi et al. 2022). The compound ethyl- iso-allocholate is used in medical research (Thakur and Ahirwar 2019).
Accordingly, several researchers have opined that various phenolic derivatives (Bhuiyan et al. 2010; Ghayal et al. 2011; Huang et al. 2013; Narendhran et al. 2014) and terpene derivatives (Eshilokun et al. 2005; Conti et al. 2012; Ashitani et al. 2015; Bezerra et al. 2017; Sharma et al. 2019) allelochemicals present in leaf part of the investigated weed Hyptis suaveolens may significantly impact the adverse growth of associated plants including the crops. Thus, the currently observed decline in crop growth in soils amended with different doses of PLB should rather result from allelochemicals released into the soil from the applied PLB. These results depicted stimulatory impact on soil nutrients (organic carbon and total nitrogen) with increasing dose of PLB in contrast to declining total biomass (upto 67% biomass loss at 8g/kg soil) of the crop in this study. It is opined by some workers that allelochemicals may kill different soil microorganisms, and upon decomposition such microorganism may enhance the soil nutrients (Rice 1984; Rizvi and Rizvi 1992). On the whole, invasibility of the weed Hyptis suaveolens in the newly invaded areas in Indian dry tropics, despite improving soil fertility, appears validated due to its ability to inhibit the crop seed germination and growth by adversely impacting synthesis of photo-pigments and proteins through release of allelochemicals and concomitant induction of proline and phenolics under physiologically stressed conditions (Sharma et al. 2019; Sharma and Raghubanshi 2007; Li et al. 2006; Fan et al. 2010). The results of present investigation also demonstrated that the impact of allelopathy is dose dependent. Similar observations were reported in Ageratina adenophora (Yang et al. 2016), Chromolaena odorata (Laxman et al. 2019), Paspalum commersonii (Zaman et al. 2018) and Solanum elaeagnifolium (Balah et al. 2022).