Nutrient Management Research Database

It has been accepted that the uptake of boron is a passive (non-metabolic) process and that it is a phloem immobile element, so once incorporated into tissue, it cannot be remobilized to supply the needs of other plant tissue. Recent work by Brown and co-workers, however, has now demonstrated that the physiology (mobility) of boron varies dramatically between plant species and that our current knowledge of the symptoms and management of boron nutrition must be re-examined on a species by species basis. To properly address boron toxicity and deficiency, knowledge of the processes of uptake, remobilization, and distribution in the plant is needed to inform boron sampling, diagnoses, and fertilization management. Boron in almond trees is thought to be mobile, meaning that: nutrient status of the hulls are tested to determine boron toxicity, foliar sprays are an effective means of addressing boron deficiencies at any time, and boron toxicity is mainly exhibited as die back.

This work presents the results of studies on multiple crops.

• The work suggests that the mobility of boron is different across plant species
• gives examples for boron management including:
  • symptoms of toxicity
  • sampling protocols
  • and fertilizer applications based on the mobility of boron.
• Boron in almond trees is mobile.

• Analysis of almond trees indicates that boron distribution is likely phloem mobile, with the highest concentrations of boron in the hull (fruit tissue) and lower boron concentrations in the leaves.
• Differences in boron mobility and accumulation result in different expressions of boron toxicity among trees.
• In almonds, where boron is mobile, boron toxicity is exhibited as:
  • die back in young shoots
  • gumming in the leaf axil
  • and brown lesions along the stems and petioles.
• Because boron is mobile in almonds trees, hulls should be tested to determine boron status of almonds
  • boron does not accumulate in the older leaves of species were boron is mobile, meaning that leaf samples cannot be used to determine toxicity.
• Foliar sprays can be used at any time when functional leaves are present to address boron deficiency in almonds;
  • the phloem mobility of boron in almonds means that the boron will be translocated from the leaves to the growing organs.