Relationship between leaf chlorosis and different boron forms in Trifoliate orange seedlings under excessive boron supply

Boron (B) is an essential micronutrient element for plants, and it exists in many forms in plants. B toxicity is common in arid and semiarid regions, and inappropriate application of B fertilizer can lead to B toxicity. Trifoliate orange [Pincers trifoliata (L.) Raf] is a main citrus rootstock of the world, and is sensitive to excess B. Although it is known that leaf chlorosis is the typical visible symptom of B toxicity, it is still unclear how leaf chlorosis is related to different forms of B in Trifoliate orange seedlings. A hydroponic experiment was carried out with four B levels: B5 (5 mu mol L-1), B10 (10 mu mol L-1), B20 (20 mu mol L-1) and B100 (100 mu mol L-1). After a 117-d culture, free B, semi-bound B and bound B in root, stem and leaves were extracted and measured. The results showed that the seedling height, the number of leaves and the dry weight decreased with increasing B supply. Leaf B concentration was generally higher than those of roots and stems. New leaf B concentration showed no difference in different B treatments, but was lower than that of older leaves. B in root and stem was mostly in the form of bound B, and B in leaves was mainly in free B form. Moreover, the free B, semi-bound B and bound B had little changes in roots and stems with various B treatments, but free B in leaves significantly increased as leaf chlorosis was aggravated, indicating that leaf chlorosis is closely related with free B in leaves.