Effects of spelt husk extracts on growth and stress-related biochemical and physiological parameters in salt-stressed maize (Zea mays L.)

In the natural environment, plants are exposed to a complex set of biotic and abiotic stresses that can negatively influence their survival and growth. Salinity is a major abiotic stress that affects plant growth and biomass production as well as yields of staple food crops. Biostimulants are heterogeneous products obtained from different natural sources representing an innovative solution to address the challenges of a sustainable agriculture, not only to ensure optimal nutrient uptake, crop yield and quality, but also tolerance to abiotic stresses. During my PhD, I explored the effect of plant by-product extracts on plant responses to abiotic stresses, in particular to salt stress. Two polyphenol- enriched extracts were obtained from einkorn (Triticum monococcum L.) husks, the soluble- conjugated (SC) and the insoluble-bound (IB). The free and bound polyphenolic (PP) content of methanolic chaff extracts was determined by HPLC. Moreover, a more specific GC-MS analysis has been carried out for a better characterisation of the two extracts. Maize plantlets, grown under semi- hydroponic conditions, were treated with different concentrations of NaCl. The extracts (0.1 and 1 mL L-1) were added to the nutrient solution in the presence or absence of NaCl and their effect was evaluated by measuring: plant growth (fresh and dry weight), photosynthetic pigments, lipid peroxidation, hydrogen peroxide production, content of proline, glutathione and ions (Na+ and K+). In addition, since we used methanolic extracts, the effect of methanol alone was also investigated. The results indicate a different effect of the two extracts, in particular the IB, which has a higher concentration of phenolic and hydroxycinnamic acids, shows a stronger protective effect from the salt stress that in some cases is equivalent to the control level. The methanol doesn’t exhibit a significant protective effect against salinity and also, in some parameters, exacerbates the negative response of this stress. ORAC, DPPH and ABTS assays confirmed that the IB extract has a much greater antioxidant activity compared to SC, in agreement with a higher polyphenol content. Based on previous results, the 0.1 mL L-1 concentration was selected and both extracts were tested as biostimulants in seed priming to increase maize seed germination and seedling tolerance to salt stress. Evaluation of germination percentage, fresh and dry weight, leaf chlorophyll content index, leaf stomatal conductance, leaf water potential, osmotic potential indicated that the extracts are ineffective in priming the seeds. Overall, results indicate that a polyphenol-containing extract obtained from spelt by-products can behave as biostimulant in maize plants and can mitigate their response to salt stress, by acting on different biochemical targets.