Boron is thought to play important roles in cell wall synthesis and sugar transport. As such, it affects cell wall strength and resistance to disease, as well as the efficiency of nutrient movement between sources (such as leaves) and sinks (either roots or grain). The latter has important implications for maintaining sufficient sugar movement into embryos to avoid abortion during grain fill. It may also explain why boron availability is associated with nodule formation in legumes.
Like potassium and manganese, boron is severely affected by drought conditions. Dry soils fail to support mass flow, whereby boron flows with moving soil water to the roots. Secondly, boron is not plant mobile meaning the plant cant move the nutrient from a leaf of higher concentration to new growth that may have a limited supply.
The dependency of boron on xylem transport may also explain why shortages are often more pronounced in flower and seeds than leaves and roots. Because reproductive structures are more poorly supported by vascular tissues and have lower transpiration rates, supply of boron may be more limited.
Foliar applications of boron are the most reliable form of application as the nutrient is used in very small quantities. With most soils receiving a variable rate P and K application it is very hard to adequately include Boron as a soil broadcast partner.
Because of its critical role in cell wall and cell membrane integrity and function, boron availability is also associated with other plant functions. This includes the establishment of nodules in legume roots, which requires the successful construction of cell walls and membranes around nodules following Rhizobium infection.
Cell wall and membrane integrity also affects uptake and retention of nutrients. Healthy membranes allow nutrient uptake and retain nutrients once absorbed. Adequate boron is associated with improved soil phosphorus uptake. Conversely, boron deficiency is associated with potassium leaching from excessively wetted leaves.
These characteristics, combined with the general challenges of efficiently delivering micronutrients to plants through the soil, shows how effective foliar boron can be, especially when used in combination with manganese and potassium.
