Experiments were conducted to determine the effect of the biostimulant amino levulinic acid (5-ALA) on canopy and root competition of transplanted sweet and purple basils with the weed slender amaranth (Amaranthusviridus). Before transplanting, basil plants were sprayed with an aqueous solution of 5-ALA (0 and 15 mg·L-1 a.i.). Basil and amaranth were grown in plastic 19-L containers either: 1) individually (one plant per container = no interference); 2) one basil plant and one amaranth plant together in the same container (= full interference); 3) one basil plant and one amaranth plant together in the same container, training the shoots apart to avoid canopy interference (= below ground interference); or 4) basil and amaranth grown in different containers set side by side (= above ground interference). When 5-ALA was not applied, full-interference from slender amaranth reduced sweet basil shoot yield by 33%, and purple basil shoot yield by 48%. Above ground interference from slender amaranth was about 65% of the total interference effect. Basil plants treated with 5-ALA were less affected by amaranth interference than untreated basil plants, but the magnitude of the 5-ALA effect was greater in sweet basil than in purple basil. 5-ALA increased the yields of weed-free sweet basil and purple basil by about 15% and 10%, respectively.
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