This study contributes to deepening the knowledge of the mechanisms of tolerance to adverse environmental conditions that operate in the woody plant P. praecox. Woody plants exhibit prolonged juvenile phases and often undergo reproduction across multiple years, a characteristic notably distinct from herbaceous plants. The developmental changes and growth of long-lived woody species are governed by complex dynamic mechanisms, contrasting with relatively simpler growth patterns of short-lived herbaceous plants. Woody species possess various traits, being more resilient to habitat fragmentation or climate changes, including high salinity and drought conditions (Ma et al. 2018; Winkler et al. 2019; Llanes et al. 2021). Upon salinity and drought perception, multiple cell signaling pathways are triggered, leading to gene expression reprogramming in response to adverse environmental conditions. Nevertheless, woody plant species exhibit varying degrees of tolerance (Munns et al. 2020). In this study, P. praecox can tolerate high concentrations of salt in the medium and drought. Interestingly, growth parameters were not affected by 250 mM NaCl or 70% FC.
The adaptive success of P. praecox could involve different mechanisms or differential responses; there are no significant modifications in growth parameters, such as root length, total height, leaf area and number of leaves, in plants grown at low salt concentrations. (250 mM NaCl) and those exposed to irrigation at 70% FC. The increase in the R/S ratio and in the length and biomass of the root system, from 700 mM NaCl and irrigation at 30% FC of P. praecox, represents a mechanism to explore areas and ensure the acquisition of water to maintain the plant water status compared to the aridity and salinity conditions present in its habitat. Water limitation conditions produce, among other effects, an increase in the weight ratio between the root and the aerial part (the root maintains its growth speed, while the shoot decreases it). This coincides with studies on roots of different clones of Populus exposed to salinity and drought stress, which increased due to hormonal changes that promote root growth and change the root/shoot ratio (Junghans et al. 2006; Zhou et al, 2020).
It is widely known that an increase in salinity or water deficit conditions in cells generates both primary and secondary effects (Sheldon et al. 2017), which involve not only the biomass level, but also numerous compounds. They are synthesized as mechanisms to face adverse environmental conditions. These compounds include the synthesis of compatible and osmoprotective solutes, carbohydrates and specific proteins, alterations in hormonal levels, among others (Sankari et al. 2019; Kotula et al. 2020).
Proline functions as a vital compatible osmolyte in plants, synthesized during exposure under stressful conditions (Ghosh et al. 2022). It serves various purposes, including stabilizing membranes, maintaining protein conformation, and mitigating damage to thylakoid membranes by scavenging reactive oxygen species (ROS) (Hosseinifard et al. 2022). Elevated levels of proline, coupled with increased activities of ascorbate peroxidase, catalase, and superoxide dismutase, can bolster antioxidant defense mechanisms (Zulfigar et al. 2023). In this study, the high accumulation of proline observed only in roots and shoots of plants with an irrigation of 30% FC could be related to numerous functions of proline to cope with this adverse condition. Dikilitas et al. (2020) reported that as the drought stress level increases, the content of proline and other osmolytes increase, helping to keep the relative water content in plants. However, in P. praecox plants exposed to drought simulated conditions the high proline content could be associated not only with its osmotic function but also with its proposed multifaceted signalling molecule in plant responses to abiotic stress (Ghosh et al. 2022).
Among the various quaternary ammonium compounds, glycine betaine (GB) is typically highlighted in contexts of salinity and water stress. Synthesized within chloroplasts, GB serves to safeguard thylakoid membranes, thereby aiding in the maintenance of photosynthetic efficiency (Gupta and Thind 2018). Primarily, GB plays a pivotal role in shielding plant cells by stabilizing proteins, facilitating osmotic adjustments, and neutralizing reactive oxygen species (ROS) (Roychoudhury and Banerjee 2016). Several studies have indicated that even at low concentrations, GB can safeguard macromolecules such as nucleic acids, proteins, and lipids, which are rich sources of nitrogen and carbon, thus preserving them for energy utilization (Umezawa et al. 2006). Furthermore, accumulation of GB have been linked to stress resilience through heightened catalase (CAT) and superoxide dismutase (SOD) activities, alongside mitigating cell membrane damage via regulation of lipid peroxidation and ion homeostasis pathways (Alasvandyari et al. 2017). The findings of this study revealed that under salinity conditions, P. praecox plants do not accumulate this compound, as evidenced by the absence of differences compared to control plants. However, exposure to 30% FC irrigation led to the accumulation of this compound in both leaves and roots.
The synthesis and accumulation of soluble carbohydrates play a direct role in radical scavenging, osmotic adjustment, carbon storage, and the stabilization of protein structures (Afzal et al. 2021). Carbohydrates serve as crucial components for osmotic adjustment across various plant species, acting as osmoprotectants, substrates for growth, and regulating gene expression during abiotic stress (Alagoz et al. 2023). The accumulation of total soluble carbohydrates in plants experiencing salinity and drought conditions represents vital defense strategies to cope with these conditions. In this study, P. praecox accumulated soluble carbohydrates when plants were exposed to salinity and drought. Interestingly, by the end of the experiment, plants subjected to 30% FC exhibited notable increases in soluble carbohydrates, alongside elevated levels of proline and glycinebetaine, particularly concentrated in the roots. This pattern suggests significant metabolic adaptations to address the water deficit. Conversely, in saline conditions, such dramatic metabolic shifts were not observed in the roots, mirroring the behavior seen in plants exposed to 70% FC. These findings imply that significant accumulation of osmoprotectants like proline, glycinebetaine, and soluble carbohydrates might signify stress-induced damage in plants experiencing severe water deficit (such as 30% FC). Contrarily, moderate increases in these compounds, as observed under high NaCl concentrations, may indicate an effective osmoregulatory mechanism to contend with saline stress.