Effect of Ecklonia maxima seaweed extract on yield, mineral composition, gas exchange, and leaf anatomy of zucchini squash grown under saline conditions

The presence of NaCl in soil or water is one of the most critical environmental stresses limiting crop productivity worldwide. Seaweed extract (SWE) represents an important category of plant biostimulants able to improve crop tolerance to abiotic stresses, such as salinity. The current research aimed at elucidating the physiological and anatomical effects as well as the changes in mineral composition of greenhouse zucchini squash (Cucurbita pepo L.) treated with Ecklonia maxima SWE. Plants were sprayed every 2 weeks with a solution containing 3 mL L-1 of SWE. Zucchini squash plants were supplied with four nutrient solutions: 1 (non-salt control), 20, 40, or 60 mM NaCl. Increased salinity in the nutrient solution triggered a decrease in marketable yield, shoot biomass, Soil Plant Analysis Development (SPAD) index, net CO2 assimilation rate (A (CO2)), transpiration rate (E), and leaf macronutrient concentration (P, K, Ca, and Mg), whereas it augmented Na and Cl concentrations in leaf tissue without altering PSII photochemistry. Anatomical changes in leaves, including an increase in lamina, palisade, spongy parenchyma thickness, and intercellular spaces, were recorded under saline conditions. Foliar application of SWE increased yield and shoot biomass by 12.0 and 17.4 %, respectively, as well as fruit dry matter and total soluble solid contents in comparison to untreated plants. This was associated with an improvement in A (CO2) (+14 %), chlorophyll content (+8 %), and nutritional status (high K and low Na accumulation) in SWE-treated plants. The size of stomata was influenced by foliar application of SWE, since the smallest cell guard length and width were recorded in the leaves of SWE-treated plants. Finally, the phenolic compounds in both palisade and spongy parenchyma were higher in untreated than in SWE-treated plants.