Agricultural soil degradation is occurring at unprecedented rates, not only as an indirect effect of climate change (CC) but also due to intensified agricultural practices which affect soil properties and biodiversity. Therefore, understanding the impacts of CC and soil degradation on plant physiology is crucial for the sustainable development of mitigation strategies to prevent crop productivity losses. The amino acid proline has long been recognized for playing distinct roles in plant cells undergoing osmotic stress. Due to its osmoprotectant and redox-buffering ability, a positive correlation between proline accumulation and plants’ tolerance to abiotic stress has been pointed out in numerous reviews. Indeed, proline quantification is used systematically by plant physiologists as an indicator of the degree of tolerance and a measurement of the antioxidant potential in plants under stressful conditions. Moreover, the exogenous application of proline has been shown to increase resilience to several stress factors, including those related to soil degradation such as salinity and exposure to metals and xenobiotics. However, recent data from several studies often refer to proline accumulation as a signal of stress sensitivity with no clear correlation with improved antioxidant activity or higher stress tolerance, including when proline is used exogenously as a stress reliever. Nevertheless, endogenous proline levels are strongly modified by these stresses, proving its involvement in plant responses. Hence, one main question arises—is proline augmentation always a sign of improved stress resilience? From this perspective, the present review aims to provide a more comprehensive understanding of the implications of proline accumulation in plants under abiotic stress induced by soil degradation factors, reinforcing the idea that proline quantification should not be employed as a sole indicator of stress sensitivity or resilience but rather complemented with further biochemical and physiological endpoints.

Currently, salinity and heat are two critical threats to crop production and food security which are being aggravated by the global climatic instability. In this scenario, it is imperative to understand plant responses to simultaneous exposure to different stressors and the cross-talk between underlying functional mechanisms. Thus, in this study, the physiological and biochemical responses of tomato plants (Solanum lycopersicum L.) to the combination of salinity (100 mM NaCl) and heat (42 °C; 4 h/day) stress were evaluated. After 21 days of co-exposure, the accumulation of Na⁺ in plant tissues was superior when salt-treated plants were also exposed to high temperatures compared to the individual saline treatment, leading to the depletion of other nutrients and a harsher negative effect on plant growth. Despite that, neither oxidative damage nor a major accumulation of reactive oxygen species took place under stress conditions, mostly due to the accumulation of antioxidant (AOX) metabolites alongside the activation of several AOX enzymes. Nonetheless, the plausible allocation of resources towards the defense pathways related to oxidative and osmotic stress, along with severe Na toxicity, heavily compromised the ability of plants to grow properly when the combination of salinity and heat was imposed.

Eucalyptus globulus Labill. is a widespread exotic species that contributes to the formation of fire-prone environments, a great concern under climate change conditions. Therefore, sustainable practices to help locals managing eucalyptus stands are needed. In this perspective, harnessing eucalyptus’ specialized metabolism as a source of allelochemicals can be a promising approach for weed control. Thus, the main goals of this work were to evaluate the herbicidal potential of post-fire regenerated E. globulus leaves against Portulaca oleracea L. and to unravel the physiological mechanisms behind this phytotoxic action. For this, aqueous extracts of fresh (FLE; 617 g FW L⁻¹) or oven-dried leaves (DLE; 250 g DW L⁻¹) were foliar-sprayed at different dilutions in purslane seedlings. After five weeks, results revealed that DLE at the highest dose detained the greatest herbicidal activity, affecting purslane growth and cellular viability. Moreover, biochemical data pointed towards an overproduction of reactive oxygen species, causing harsh oxidative damage in roots, where the upregulation of important cellular players, like sugars, amino acids, and proline, was not able to reestablish redox homeostasis. Overall, this study proved that dried leaves from young E. globulus had potent herbicidal properties against P. oleracea and can represent a feasible strategy for weed management.