Agriculture Reference
In-Depth Information
4   Boron Toxicity
4.1   Symptoms
Visual symptoms of B toxicity include inhibition of root and shoot growth, and
chlorosis and necrosis of shoots (Lovatt and Bates 1984 ; Nable et al. 1990a ). The
underlying cause of these developmental changes may be linked to the disruption
of a range of physiological processes, including inhibition of photosynthesis, lower
stomatal conductance (Lovatt and Bates 1984 ), increased peroxidation of lipids, and
alterations in enzymes within antioxidation pathways, increased membrane leaki-
ness (Karabal et al. 2003 ), and reduced proton extrusion from roots (Roldán et al.
1992 ). Increased deposition of suberin and lignin has also been reported (Ghanati
et al. 2002 ).
Toxicity symptoms are generally correlated with the accumulation of high con-
centrations of boron in shoots, which is related to the soil boron concentrations
and the length of exposure. Early observations by Oertli and Kohl ( 1961 ) of 29
plant species, including grasses, citrus, vegetables and flower crops established that
in general, chlorosis of the leaves occurred at approximately 1,000 mg kg −1 DW
and necrosis between 1,500-2,000 mg kg −1 DW. The pattern of necrosis was cor-
related with venation, such that symptoms developed first at the ends of the veins.
From this, it was concluded that excess boron remained in the xylem and therefore
accumulated where the xylem vessels terminated. This means for grasses, which
have parallel venation, toxicity will first be observed at the leaf tip, whereas for
dicots, which generally have reticulate venation, toxicity is observed around the
leaf margins.
4.2   Causes
The actual cause of boron toxicity still remains a mystery. Compared to other es-
sential plant nutrients, boron is relatively unreactive, which in theory should limit
the possible targets for toxicity. Complexation with boron is mainly restricted to
those compounds possessing two hydroxyls in the cis-conformation, known as cis-
diols. The most stable complexes occur with cis-diols attached to a furanoid ring
Hunt ( 2002 ). The only certain role for boron in plants is as a component of the
rhamnogalacturonan II (RGII) complex in cell walls where boron is bound to the
cis hydroxyl groups of the furanoid ring of the sugar apiose (Matoh 1997 , 2000 ).
Plant boron requirements closely reflect cell wall content of 2-keto-3-deoxysugars,
which includes apiose (Matoh and Kobayashi 1998 ). This largely explains the lower
requirement for boron in monocots compared to dicots. However, there is no strong
evidence of disruption of cell wall structures under high boron conditions.
The other possible target for boron complexation is the sugar ribose which occurs
in a number of key metabolites such as ATP, NADH, NADPH and in nucleic acids.
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