Investigating the Drought and Salinity Effect on the Redox Components of Sulla coronaria (L.) Medik

For the Mediterranean region, climate models predict an acceleration of desertification processes, thus threatening agriculture. The present work aimed to investigate the effect of drought and salinity on Sulla coronaria (L.) Medik., a Mediterranean forage legume, for understanding plant defence systems activated by these stressors. In detail, we focused our attention on the variations on the plant redox status. Plants were subjected to suboptimal watering and irrigation with sodium chloride (NaCl) solutions. The same salt treatment was applied for in vitro tests on seedlings. Water content did not change after treatments. Salt negatively influenced seed germination and seedling development, but it did not affect photosynthesis parameters, contrary to what was observed in adult plants. Proline concentration increased in all samples, while abscisic acid level increased exclusively in seedlings. NaCl caused accumulation of superoxide anion in plants and seedlings and hydrogen peroxide only in seedlings; nevertheless, lipid peroxidation was not detected. Total phenolics, glutathione, expression level, and activity of antioxidant enzymes were assayed, revealing a complex antiradical molecular response, depending on the type of stress and development stage. Our results confirm Sulla as a drought- and salt-tolerant species and highlight its ability to counteract oxidative stress. This evidence suggests a key role for the redox components, as signal transduction messengers, in Sulla acclimation to desertification. Finally, plants and seedlings showed different acclimation capacity to salinity, revealing a greater genomic plasticity for seedlings.

Inhibitory effect of aqueous extracts of Centaurea solstitialis subsp. schouwii on seed germination and growth of Sulla coronaria

Biodiversity has been confronted with anthropogenic threats and several natural threats such as biological invasions. The success of these invasions involves phytotoxic products released by invasive plants that can exhibit allelopathic effects on target species. Thus, aqueous extracts from different parts of the Mediterranean yellow star-thistle [Centaurea solstitialis subsp. schouwii (DC.) Gugler], were tested for their allelopathic effects on seed germination and seedling growth of Sulla coronaria (L.). Bioassays were conducted in vitro to test the effects of the aqueous extracts of shoot, basal and root parts of C. solstitialis subsp. schouwii at two different concentrations (50 g·L−1 and 10 g·L−1). The concentrations of total polyphenols, flavonoids, and tannins of the extracts were also evaluated. Our results showed inhibitory effects on the germination and seedling growth of S. coronaria seedlings, particularly with the extract form the basal part, reaching 84%. This study confirms the linear relationships between the allelopathic effects of C. solstitialis subsp. schouwii and the polyphenol and flavonoid contents. However, further experiments are needed under field conditions to confirm the results obtained under laboratory conditions.

Potential of Rhizobium sullae–Sulla coronaria Symbiotic Biological Nitrogen Fixation to Supplement Synthetic Mineral Nitrogen in Olive Tree Fertilization

The aim of the present work is to compare olive tree nitrogen fertilization over two years of trials, using synthetic chemical fertilizers along with organic fertilizers composed of the green manure of sulla (Sulla coronaria) inoculated with the symbiont Rhizobium sullae or left uninoculated. The tests indicated that symbiotic nitrogen fixation promoted by the sulla–R. sullae symbiosis represents an important source of nitrogen that can replace or supplement synthetic nitrogen fertilizers for olive tree cultivation when sulla is inoculated with R. sullae in a soil already populated by the symbiont. Integration of the indigenous population of R. sullae via sulla inoculation with a selected strain yielded nodule formation in 100% of plants and produced a sufficient amount of biomass rich in nitrogen with a low C/N ratio. On the contrary, olive tree fertilization using the green manure of sulla that was not inoculated with the symbiont supplied significantly less organic nitrogen in 2017 and 2018, respectively, compared to the control. Optimal management of the multi-factorial approaches involved in green manure olive fertilization are also reported.

Cropping systems sustainability: Inoculation and fertilisation effect on sulla performances in a new cultivation area

To assess the feasibility of the sulla [Sulla coronaria (L.) Medik] forage legume in a new agroecosystem, its host-specific symbiotic interaction needs to be taken into account. This study aimed to investigate the effects of inoculation and nitrogen (N) fertiliser on productive performances and N-fixation ability of sulla established in a new habitat within a Mediterranean agropastoral area. Sulla plants, previously inoculated (with peat-based, liquid inoculants, and using soil from an existing sulla field) and unfertilised or N fertilised were evaluated in Sardinia (Italy). During 2013-2014, sulla plants were sampled at four growing stages, from vegetative stage to seed set, and shoot length, shoot dry matter (DM) yield and N content were monitored. Moreover, atom% 15N isotopic excess, proportion of N derived from the atmosphere and fixed N of sulla shoots were quantified. Inoculation and N fertilisation both affected growth, DM and N yields, and N-fixation of sulla. Compared to the best inoculated treatment, the DM yield and fixed N of the control only represented 10 to 22% and 2 to 11%, respectively. Nitrogen fertilisation caused temporary decreases in the N fixing ability of sulla. Results pointed out that rhizobial inoculation is essential for the exploitation of sulla outside its traditional cropping area.