Optimum Boron Supplementation Enhances Growth and Nutrient Uptake of Broad-Leaved Tree Seedlings

Boron (B) is a well-known essential element for plant growth, but B supplementation is not widely considered for broad-leaved seedlings because of the lack of B deficiency symptoms. The effects of B on various broad-leaved seedlings in tree nurseries need to be discovered and studied. We selected five native hardwood species commonly used in plantation forestry in Taiwan. Moreover, five concentrations of B supplementation were designed to determine the growth response and nutrient uptake of these seedlings. Supplementation with 10 mg L-1 B nutrient solution, corresponding to 2 kg ha(-1) B, in the growing season significantly increased the growth of most species. Excluding Zelkova serrata, which was confirmed as a B-sensitive species in our study, the seedling height, dry weight, and total leaf area of the other species were increased by 21.2-30.3%, 23.2-41.3%, and 14.2-37.9%, respectively, by B supplementation versus the control, suggesting the existence of a hidden hunger effect. Additionally, moderate B supplementation increased the levels of other nutrients, such as nitrogen, phosphorus, magnesium, sodium, and potassium, whereas decreases of calcium levels were observed under the highest B supplementation. Moreover, visible toxic symptoms such as chlorosis and necrosis of leaves and growth inhibition were observed and enhanced by increasing the concentration of B. Our results demonstrated that the B management in the tree nurseries of broad-leaved species is a key factor for obtaining high-quality seedlings, as optimum B supplementation helps improve their growth and nutrient uptake.