Effect of boron deficiency on boron distribution in different plant parts and boron concentration in leaf cell wall components in navel orange plants

Objectives Boron (B) plays an important role in maintaining the normal structure of cell walls. The previous results showed that the cell wall structure of leaves with severe B deficiency had a lot of change, but it was not clear whether this change was related to the change of B concentration in the cell wall components. This paper aims to clarify the relationship between B deficiency symptoms and changes of cell wall structure and B content in cell wall component.

Methods In this study, the changes of B distribution in different plant parts and B concentrations in leaf cell wall and cell wall components of navel orange plants were measured using hydroponics method.

Results The symptoms of leaf chlorosis and curling of the leaves were observed only in the upper leaves of B-deficient plants. The lower leaves did not show any visible symptoms. The B concentration and B accumulation in different parts of the boron-deficient navel orange plants were significantly decreased. Boron deficiency reduced the relative distribution ratio of B in the shoots and the greater impact on the upper leaves. There was no evident difference in concentrations of free B, protoplast B and cell wall B in leaves between the two types of leaves under normal B condition. However, the water-soluble B (including free B and protoplast B) decreased to extremely low levels in the upper and lower leaves of navel orange under B deficiency, especially the protoplast B in the lower leaves was even lower than that in the upper leaves. In control plants, the upper and lower leaves contained only about 22% of cellular B in the cell wall, whereas in B-deficient plants the cell walls of the upper and lower leaves contained >80% of total cell B. Furthermore, the concentrations of B in lower leaves and their cell walls were much higher than those in the upper leaves and their cell walls under B-deficient conditions. Further analysis of the changes of B content in cell wall components showed that B deficiency significantly reduced the boron content in the ion-bound pectin in the upper leaf cell wall and had no significant effect on that in the lower leaf cell wall. In contrast, no significant difference was found in changes of B content in other wall components due to B deficiency between the upper leaves and lower leaves. In addition, the structural changes of the upper leaf cell wall were much larger than those of the lower leaf cell wall under B deficiency.

Conclusions Boron concentration in the protoplast was not the main factor determining the symptoms of B deficiency, while the binding capacity of ion-bound pectin to B was crucial to the structure of cell wall and the occurrence of B deficiency symptoms in navel orange leaves.