This study assessed the response to irrigation rate and the minimum irrigation rate for optimum growth of seedlings of five tropical tree species used in urban landscaping in Ondo, Nigeria. The study also evaluated the effects of irrigation rate and sawdust biochar on growth attributes and biochemical constituents of the seedlings and the changes in the physical and chemical properties of the soil on which the tree seedlings were grown. Seedlings of five tree species, Bauhinia monandra, Delonix regia, Terminalia catappa, Dypsis lutescens, and Veitchia merrillii, were subjected to six treatments as follows; The first three treatments comprised of seedlings grown in soil without biochar application and subjected to 80%, 60%, and 35% FC irrigation rates while the last three treatments were seedlings grown in the soil-biochar mixture and subjected to 80%, 60%, and 35% FC irrigation rates. Seedlings grown in soil irrigated at 80% FC served as control. Relative to 80% FC treatment, soil bulk density increased, moisture content decreased significantly, and plant available K, Ca, and Mg and percentage N decreased transiently in the soil subjected to 60% and 35% FC irrigation rates. There were no significant differences between the measured growth attributes and malondialdehyde content of the seedlings irrigated at 80% FC and those irrigated at 60% and 35% FC. In addition, seedlings in the 35% and 60% FC treatments deployed non-enzymatic and enzymatic antioxidant machinery to overwhelm the oxidative burst in their chloroplasts. Conclusively, the minimum irrigation rate for optimum growth of the tree seedlings was 35% FC. The practice of a 60% and 35% FC irrigation rate for tree production in the nursery was a viable water-saving strategy, and a 35% FC watering rate was beneficial for enhancing the tolerance of the tested trees. Irrespective of irrigation rate, biochar application improved soil's physical and chemical properties, moisture content, and plant-available macronutrients. Biochar amendment also produced the most vigorous seedlings at an 80% FC irrigation rate, enhanced the measured attributes of seedlings irrigated at 60% FC, alleviated the adverse effects, and strengthened the tolerance of tree seedlings in the 35% FC watering rate.



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