A greenhouse experiment was conducted to evaluate the effects of foliar application of an Ascophyllum nodosum seaweed extract (AZAL5) on the growth, nutrient uptake, and yield of winter wheat in a surface soil of the Thessaly Plain classified as TypicXerorthent. Twelve treatment combinations in a randomized complete block design with a factorial arrangement were composed of two rates of inorganic fertilizer (0 and 50 ppm N), three rates of AZAL5 (0, 1.5, and 3 % diluted extract), and two levels of water supply (75 and 45 % of field capacity). Under soil P and K sufficiency, the addition of fertilizer N greatly increased grain yield and nutrient uptake in the shoots (N and K) and grain (N, P, and K). Reduced water supply decreased grain yield and caused water stress as evidenced by decreased ?13C in the N-deficient treatments and decreased nutrient uptake. AZAL5 application caused increased grain K uptake and a 25 % increase in yield only when mineral N was added. Differences in the efficacy of the two AZAL5 concentrations indicated that optimal dilution ratios were directly or indirectly dependent on soil water content. Complex interaction effects between AZAL5 and water supply on grain ?13C could not be explained by conventional physiological response to water stress. The lack of biomass, nutrient content, and ?13C differences between AZAL5 and control treatments in the shoot indicated that the reproductive organs of wheat were the main site of biostimulant action. Overall, the results underline the potential of this product to enhance the effectiveness of inorganic N fertilizers in intensively managed cropping systems under optimal irrigation, deficit irrigation, or rainfed conditions. © 2014, Springer Science+Business Media Dordrecht.
