Effects of A. nodosum seaweed extracts on spinach growth, physiology and nutrition value under drought stress

Seaweed extracts (SWE) are biodegradable and non-polluting to the environment, and have become popular as biostimulants. Their effects on spinach growth, physiology and nutrition value under drought stress were evaluated in a growth chamber study. Differential irrigations (100 and 50% evapotranspiration for full irrigation and drought stress, respectively) and Ascophyllum nodosum SWE application methods (water foliar, SWE foliar, SWE drench, SWE foliar and drench) were applied every 4 d. After 35 d, leaf growth, chlorophyll content and fluorescence, gas exchange, and nutrition value were analyzed. Under drought stress, leaf area, fresh and dry weight (FW and DW) were only 42, 42 and 60% of these under full irrigation, respectively. Drought stress also decreased leaf relative water content (RWC) and specific leaf area (SLA) from 89 to 76% and 179 to 139 cm2 mg−1, respectively. Gas exchange was reduced by drought stress although chlorophyll content and fluorescence were not altered. Drought stress had no direct effects on the leaf nutrition quality except for reduced ferrous ion chelating ability (FICA). Under full irrigation SWE had no effects on leaf growth, physiology and nutrition value but under drought stress SWE improved plant growth. Under drought stress A. nodosum SWE application increased leaf RWC from 76 to approximately 82% and leaf area by 16, 21 and 38% with SWE foliar spray, drench or both, respectively. Also, SWE increased leaf FW, DW and SLA by 20 to 32%, 11 to 22%, and 5 to 19%, respectively. The inhibition of gas exchange and increases of stomatal limitation induced by drought stress were reduced by SWE, irrespective of application methods. SWE application had no effects on phenolic, flavonoid, carotenoid, and antioxidant capacity, but reduced FICA. These results indicated that under mild drought stress A. nodosum SWE enhanced spinach growth by improving leaf water relations, maintaining cell turgor pressure and reducing stomatal limitation, which in turn led to large leaf area and high photosynthetic rate. A. nodosum SWE application negatively impacted nutritional quality by reduced FICA under drought stress.