Plant nutrition:  Boron

Boron (B) is an essential micronutrient in plants that is often deficient in most soils because most of the boron in the soil is adsorbed to clay minerals, hydrous metal oxides, and organic matter in soils.  In addition, boron can be co-precipitated with calcium carbonate making it unavailable to the roots.  Once inside the plant, B moves with the transpiration stream and uptake is increased with increasing temperatures.  B deficiency symptoms are often seen in actively growing tissues indicating that B is a highly immobile element because it forms complexes with cell wall constituents.  In most plant species, foliar applied B is rather immobile.  However, in species such as apples, pears and peaches, it has been determined that foliar applied B is translocated from treated leaves to adjacent fruit (Brown and Hu, 1996) .  In apple, a study by Hanson (1991) demonstrated that the levels of foilar applied B decreased to levels similar to non-treated leaves within nine days of application.
The major role of B in fruit trees involves fruit set.  Apple, pear and cherry flowers are very high in B.  If the B content is deficient, the flowers die.  The B needed in the flower is transported mainly from the reserves in the adjacent branches and not from the roots during the development flower.  Close attention to B levels is important because both low and high concentrations cause poor fruit quality.  Low B results in short storage life with the fruit having a higher susceptibly to storage breakdown and fruit deformities.  High B results in a higher incidence of internal disorders such as watercore and internal breakdown.

Soil B availability decreases with:

  • Decreasing total soil B.
  • Increasing clay minerals, hydrous metal oxides, organic matter, and lime contents in the soil
  • Increasing pH levels above 6.5
  • Very wet or very dry soils
  • Increased leaching
  • Cold soil temperatures

The role of B in plants
  • Essential for reproduction, aids in the formation of pollen germination and pollen tube growth
  • Aids in the metabolism of hormones
  • Required for protein synthesis
  • Aids in the translocation of  calcium, sugars and growth regulators
  • Important for early growth, flowering and fruit set
  • Maintains balance between sugar and  starch
  • Aids in auxin regulation
  • Necessary for cell division and differentiation, and root tip development
  • Phenolic metabolism
  • May increase flower bud retention in apple
Boron deficiency symptoms
Symtoms of B are often noticed in the fruit before symptoms become visible in the shoots. 
Fruit symptoms
  • Small, flatten or misshapen fruit  (see Fig. 1)
  • Drought spot
  • Internal cork
  • Cracking and russet
  • Premature ripening
  • Increased fruit drop
  • Seed count may be low

Vegetation symptoms
  • Internal bark necrosis (bark measles)
  • Dead terminal shoots and dieback., sometimes one may see a witch's-broom effect
  • Yellowing and red veins on the terminal shoots
  • Shorten internodes
  • Smaller, stiff, thick, brittle leaves with smooth margins
  • Death of small areas of the bark the tip of the shoots followed by progressive death of the inner bark and cambium

Blossom symptoms
  • Blossom blast (flowers wither but remain on the tree)

Boron Toxicity
Fruit symptoms
  • Reduced or no yield
  • Increased internal breakdown after harvest
  • Increased watercore development after harvest
  • Premature ripening

Vegetation symptoms
  • Dead terminal; buds and dieback
  • Marginal leaf chlorosis and necrosis; defoliation (see Fig. 2)

Time of B application
The timing for B application does not appear to be critical for apple trees.  In a study conducted in Poland (Wojcik et al., 1999) four  B applications were compared.  (1) Soil application (2 g / tree); (2) Spray application 3 x before bloom (0.67 g per tree); (3) Spray application after bloom 3 x (0.67 g per tree); and (4)  No B, the results indicated that all treatments except for control that had no B added  increased in B concentration in apple fruit.  Highest concentration was found in fruits sprayed after bloom and soil applications. 
Ground:  The preferred time to apply B to the soil is about silver tip to green tip stage
Foliar : Prebloom to bloom at petal fall  or 1st or 3rd cover sprays. 

Application rates

Application rates should be based on both foliar and leaf analysis (Table 1).  General recommendations where B is deficient are to apply 0.8 to 1.6 pounds per acre of actual B in two separate sprays, prebloom to bloom at petal fall or 1st-3rd cover sprays, or a single post-harvest  foliar spray at 1.6 pounds per acre actual B.  Ground application  should be an option for trees older than 4 years.  Apply 0.12 pound of actual B per acre (see Table 2 for sources of B and their content).

Literature cited:
Brown, P. H. and H. Hu.  Phloem mobility of boron is species dependent:  evidence for phloem mobility in sorbitol-rich species.  Ann. Bot. 77(5): 497-505
Hansen, E. J. 1991.  Movement of boron out of fruit leaves.  HortScience.  26(3): 271-273.
Wojcik, P., G. Cieslinski and A. Mika.  Apple yield and fruit quality as influenced by boron applications.  J. plant nutrition.  22(9): 1365-1377.

Other resources
Boron nutrition in deciduous tree fruit.  In Tree Fruit Nutrition. 

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