Evaluation of morpho-physiological responses to drought and salt stress in two ornamental alternatives to Invasive Ligustrum sinense Lour..

Urban environments are increasingly subject to anthropogenic pressures and climate change-induced stresses such as salinity and drought. Urban greening initiatives require the selection of ornamental species that are increasingly adapted to the unique challenges of cities. This study evaluates the response to salt and drought stress of two potential alternatives to the non-native invasive species Ligustrum sinense Lour.: Ligustrum vulgare L. (European native), and Ligustrum japonicum Thunb. ‘Texanum’ (non-invasive ornamental cultivar). A total of 270 plants, approximately 25 cm in height, including 90 plants each of L. sinense. L. japonicum ‘Texanum’, and L. vulgare were placed in a growth chamber and grown under controlled conditions from March 2024 to June 2024. The plants were subjected to one of two NaCl treatments, corresponding to 150 mM (moderate salt stress) and 300 mM (severe salt stress) or deionized water (control). Additionally, two irrigation levels were implemented, specifically 30% (severe drought stress) and 60% of the pot capacity (moderate drought stress), while the control group received 90% of the pot capacity. All three species showed intolerance to severe drought stress and its combined effect with moderate and severe salt stress. Ligustrum sinense survived only to moderate drought stress (60%). Moderate drought stress negatively affected 3D leaf area, digital biomass, and chlorophyll content, as well as dry biomass of shoots and roots, and compromised root development. In L. japonicum ‘Texanum’, drought and salinity stresses, applied individually or in combination, led to a significant reduction in 3D leaf area and digital biomass. Salinity significantly affected water band index, particularly at high levels and under combined stress. Shoot dry biomass declined under all stresses, while dry root biomass was unaffected. In L. vulgare, salinity was the main limiting factor for 3D leaf area and digital biomass, particularly under combined salt and drought conditions, causing reductions in NDVI, chlorophyll content, leaf water status, and pigment balance, indicating accelerated leaf senescence. Salinity and combined stresses most strongly affected shoot dry biomass, and also decreased root dry biomass and architecture. These findings support the use of these species as alternatives in European urban greening efforts.