Allocation Pattern, Nutrient Partitioning, Sugar Metabolism, and Pigment Composition in Hydroponically Grown Loquat Seedlings Subjected to Increasing Boron Concentrations

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 μΜ) 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, Β 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.