Agriculture Reference
In-Depth Information
Other potential roles for this plant nutrient have been suggested based on the binding
to cellular components of boronic acids which compete with boric acid for binding.
Bassil et al. (
2004
) found that boronic acids interfered with cytoskeletal elements
and disrupted cell to cell wall adhesion in cultured cells. Wimmer et al. (
2009
) iden-
tified various boron-binding proteins from microsomes by boronate affinity chro-
matography and proposed that boron interacts with the sugar moiety of membrane
glycolipids, thereby increasing membrane stability (Wimmer et al.
2009
).
3 Boron in Soils
Naturally, boron-toxic soils appear in pockets around the world and are much less
common than boron-deficient soils. Low boron soils are loosely considered as those
with less than about 10 mg kg
−1
and high boron soils are those with more than
this value up to very high levels of 100 mg kg
−1
(Power and Woods
1997
). How-
ever, prediction of toxicity to plants based on soil boron levels can be problematic.
Mertens et al. (
2011
) found that the boron concentration required to inhibit root
growth of barley by 10 % varied from 5-52 mg B kg
−1
depending on the soil type.
A large proportion of this variability could be explained by differences in soil mois-
ture content.
The boron concentration of soils is mainly known for areas in which agriculture
is practiced, because of the effects on crop yields. Regions with high soil boron
include Israel (Ravikovitch et al.
1961
), Turkey (Avci and Akar
2005
), Syria (Ryan
et al.
1998
), Malaysia (Shorrocks
1964
), the southwest of the USA (Ashworth et al.
1985
; Chesworth
1991
), and large tracts of southern Australia (Cartwright et al.
1986
).
Naturally, high concentrations of boron are commonly found in soils derived
from marine sediments (Erd
1980
) but in many areas, it is high boron concentra-
tions in irrigation water that create toxicity problems.
Boric acid has a pK around 9.2 and therefore exists in soils mainly as the neutral
boric acid. In high pH soils, a proportion of boric acid will be present as the borate
anion which causes it to be adsorbed to soil particles (Goldberg
1997
). In soil of
neutral or acidic pH, the lack of charge predisposes boric acid to leaching by rainfall
or irrigation, and high concentrations can therefore be found in the subsoil but not in
the topsoil.This situation favours the early growth of plants but restricts exploration
by roots deeper into the profile where boron concentrations are higher. If the pat-
tern of rainfall during the growing season is regular, and moisture is retained in the
topsoil, then plants can avoid toxicity by exploiting nutrients in the topsoil. How-
ever, in many areas, most of the rainfall occurs in winter and early spring but can
be highly variable late in the growing season. This can have negative impacts if the
topsoil moisture is depleted and roots are forced to explore the high boron subsoils
for water. Under low rainfall conditions, the lack of leaching allows boron to remain
in the topsoil, which presents challenges for seed germination and establishment,
unless the species has developed tolerance mechanisms to combat boron toxicity.
