Boron (B) is an essential micronutrient for plant growth and development, but in soil of arid and semiarid environments, B frequently exceeds the plant requirements. B toxicity hampers plant performances and productivity even though there is a lack of information about changes in leaf sugar metabolism and nutrient partitioning provoked by B excess, especially in tree sugar alcohol-producing species where B is highly mobile. In current experiment, hydroponically grown loquat seedlings were subjected to increasing B levels (25, 50, 100, 200, and 400 mu M) in the nutrient solution for 69 days. B excess caused visible symptoms in the upper part of loquat shoots (leaves and stem), typical symptoms usually found in species where B is highly phloem mobile. Furthermore,. excess caused significant (i) reduction of plant growth, leaf number, and stem diameter; and (ii) alterations in macro- and micronutrient allocation patterns in different plant organs, e.g., decrease of K, P, Mn, and Mg concentration in roots. Younger fully expanded leaves of B-treated seedlings showed a decline of sucrose paralleled by increments of glucose and fructose concentration in leaves, alteration of leaf pigment composition, and increased peroxidation of lipid bilayers (higher malondialdehyde by-products). Our observations suggest that loquat is very sensitive to B excess and B toxicity can affect dramatically the plant physiology and biochemistry, thus leading to changes in sugar patterns, a reduced growth and, eventually, a reduced productivity of this species.