Boron is important in sugar translocation and carbohydrate metabolism. It is particularly needed at the location of active cell division and plays an important role in flowering, pollen-tube growth, fruiting processes, N metabolism, and hormone activity. Boron also maintains Ca in a soluble form; thus, ensuring its proper utilization.
Native Florida soils are low in Boron, and a deficiency of this element occasionally occurs in field conditions. In acidic soil, B is more water-soluble, very mobile in the sandy soil profiles, and readily leaches by rainfall or excessive irrigation. Boron deficiency may be aggravated by severe drought, excessive lime applications, or irrigation with alkaline water that can result in reduced B availability and uptake.
Boron deficiency is known as “hard fruit,” because the fruit is hard and dry because of lumps in the rind caused by gum impregnations. The chief fruit symptoms include premature shedding of young fruits. Such fruit have brownish discolorations in the white portion of the rind (albedo), which are described as gum pockets or impregnations of the tissue with gum and unusually thick albedo. Older fruit are undersized, lumpy, and misshapen with an unusually thick albedo containing gum deposits. Seeds fail to develop, and gum deposits are common around the axis of the fruit (Figure 1). Boron is relatively immobile in plants. The first visual symptoms of B deficiency are typically the death of the terminal growing point of the main stem. Further symptoms include brittle and slightly thickened leaves, vein splitting, a tendency for the leaf blade to curl downward, and at times chlorosis (Figure 2). Boron deficiency also tends to cause corking and enlargement of the upper surface of the main veins (Figure 3). A premature shedding of leaves beginning at the treetop that soon renders the tree almost completely defoliated is also associated with this symptom. Fruit symptoms are the most consistent and reliable tool to diagnose B deficiency. Boron deficiency is associated with citrus greening (HLB) disease. It is likely caused by restrictions of nutrient uptake and/or transport.
Boron deficiency—Small size and misshapen fruit, thick albedo containing gum pockets, and aborted seeds with gum deposits around the axis of the fruit.
Boron deficiency—Thickening of the leaves, vein splitting, a tendency for the leaves to curl downward, and chlorosis.
Boron deficiency—Corking and enlargement of the upper surface of the main veins and leaf chlorosis.
This document is SL406, one of a series of the Soil and Water Science Department, UF/IFAS Extension. Original publication date March 2014. Visit the EDIS website at http://edis.ifas.ufl.edu.
Mongi Zekri, Hendry County Extension agent; and Tom Obreza, senior associate dean and professor, Office of Dean for Extension and Florida Cooperative Extension Office, UF/IFAS Gainesville, FL 32611.
Photo Credits: Jack Whiteside, UF/IFAS retired faculty member, Mongi Zekri