Commercial Extracts of Ascophyllum Nodosum Increase Growth and Improve Water Status of Potted Citrus Rootstocks Under Deficit Irrigation

Water availability is a pressing concern of modern agriculture on a global basis.  Greenhouse trials have shown that vegetable, bedding and turf plants treated with commercial extracts of Ascophyllum nodosum have increased resistance to drought stress; however less work has been done on woody crops.   Potted citrus rootstocks ‘Carrizo’ and ‘Swingle’ were grown for four weeks with or without seaweed extract (Stimplex, Acadian Seaplants Limited), then subjected to drought stress (50% evapotranspiration (ET) , or remained under full irrigation (100% ET).  Seaweed extract treatments were applied either foliar or as a soil drench, and continued through the drought stress period.  After 4 weeks of seaweed extract treatment (prior to drought stress), extract treatment resulted in increased shoot length, which carried through the drought stress period.  Both foliar and soil drench extract-treated plants had significantly increased shoot dry weight as well as leaf area and leaf dry weight.  At the end of the experiment, no differences in root dry weight between treatments or stress levels were detected.  Before drought stress was imposed, ‘Carrizo’ plants that had drench applications of seaweed extract had higher stomatal conductance than control plants, however ‘Swingle’ did not.  Prior to drought stress there were no differences in stem water potential between treatments.  After drought stress plants treated with an extract drench had less negative stem water potentials than did the foliar treated or the controls.  Stem water potentials were similar between all treatments on plants that were not drought stressed.  After 8 weeks of drought stress, photosynthesis was reduced similarly in all extract treatments.  After 12 weeks of extract treatment (without drought), both drench and foliar treatments resulted in reduced stomatal conductance of ‘Carrizo,’ however ‘Swingle’ had increased stomatal conductance.  Photosynthesis was reduced with drought, however after 12 weeks there were no differences based on extract treatment .  After 12 weeks, ‘Carrizo’ showed no differences in water use efficiency (WUE) based on extract treatment or water stress, but ‘Swingle’ plants under drought stress with drench treatments of seaweed extract showed WUE similar to control plants without drought stress.  Foliar treated plants under drought stress showed intermediate WUE compared to the drench and no extract treatments.  These results show the potential for seaweed extracts to increase growth and improve stem water potential in citrus rootstocks under full irrigation and drought stress, as well as affect photosynthesis, stomatal conductance, and WUE, however these results appear to be cultivar specific.