Applications of a Commercial Extract of the Brown Seaweed Ascophyllum nodosum Increases Drought Tolerance in Container-grown ‘Hamlin’ Sweet Orange Nursery Trees

‘Hamlin’ sweet orange trees on ‘Carrizo’ citrange and ‘Swingle’ citrumelo rootstocks were treated weekly with a commercial extract of the brown seaweed Ascophyllum nodosum at 5 and 10 mL·L −1 as either a soil drench or foliar spray. Half of the trees in each treatment were subjected to drought stress [irrigated at 50% of evapotranspiration (ET)], whereas the other half remained fully irrigated (100% ET). Drought stress reduced shoot growth and leaf photosynthesis but increased root and total plant growth relative to the amount of water applied, thus increasing whole plant water use efficiency. Trees treated with seaweed extract and drought-stressed had significantly more total growth than untreated drought-stressed trees for both rootstocks. The maintenance of growth by the seaweed extract under drought stress conditions was unrelated to photosynthesis. However, the seaweed extract treatment did have a significant effect on plant water relations. Soil drench-treated trees had more growth and higher stem water potential than foliar-treated or control trees after 8 weeks of drought stress. These results indicate that seaweed extract may be a useful tool for improving drought stress tolerance of container-grown citrus trees.