Boron (B) is generally present in both the soil solution and plant tissue water, as either boric acid (B(OH)3) or borate (B(OH)4–). Boric acid has a pKa of 8.92-9.24. Hence, boric acid is the major form of B in neutral and acidic soils and borate is found in appreciable amounts only in alkaline soils. In the acidic conditions of the apoplast and vacuole almost all soluble B is present as boric acid, whilst in the cytoplasm about 98% is present as boric acid. Borate forms diester bonds with molecules containing cis-diols, such as sorbitol, mannitol, mannans, fructose, apiose, ribose, phenolics, glycoproteins and glycolipids.
Soil B concentrations vary widely from about 1-250 mg kg-1. The main factors affecting B availability to plants include the B content of the soil’s parent material, soil texture, soil organic matter content, clay mineralogy and pH. Boron deficiency is one of the most widespread micronutrient deficiencies in crops and occurs on deep sandy soils in regions of high rainfall, through which B readily leaches. Boron deficiency in crops can be addressed through the application of soil or foliar B fertilisers or the development of crops that acquire or utilise B more effectively. Boron toxicity to plants occurs in arid regions where the B content of the soil’s parent material is high. It is also prevalent when irrigation water contains high B concentrations. Boron toxicity can be addressed by improved soil management, particularly liming of acid soils, and through the development of crop genotypes with greater abilities to restrict B uptake or tolerate large tissue B concentrations.
Boron can cross biological membranes either (1) by diffusion of uncharged boric acid through the lipid membrane, (2) by facilitated diffusion of uncharged boric acid through protein channels of the aquaporin family, particularly its Nodulin26-like Intrinsic Protein (NIP) transporter subfamily, or (3) by the transport of borate through borate exporter (BOR) proteins driven by the electrochemical gradient. Plant root cells can control B uptake by regulating the entry of boric acid and the efflux of borate through the expression of genes encoding B transport proteins of their plasma membranes. Boron is relatively immobile in the phloem of most plant species, with the exception of plants that transport sugar alcohols (polyols) in the phloem. Thus, a constant B supply is required to prevent the occurrence of B deficiency symptoms, which will occur first in developing tissues. There is considerable variation among species, and among genotypes of a species, in the ability to control B uptake, the efficiency by which B is utilised physiologically, and the ability to tolerate large tissue B concentrations. Chromosomal loci (QTL) and beneficial alleles of genes impacting these processes have been identified and are being used to develop crops for soils with either low or high B phytoavailability.