Agriculture Reference
In-Depth Information
B deficiency or toxicity affects various metabolic and physiological processes
(Blevins and Lukaszewski
1998
; Bola˜os et al.
2004
). Less than 10 mg kg
1
Bin
the soil is considered to be a B deficient soil (Woods
1994
). Moreover, most of this
B is in a bound form in rocks and is not readily available to plants. Boric acid is the
most common form of B liberated during weathering of rocks (Nable et al.
1997
)
and is easily absorbed by plant roots, but this represents only 10 % of the total B in
the soil (Power and Woods
1997
).
Soil pH, texture, temperature, and organic matter affect soil B availability;
among these the soil pH is one of the most significant characteristics for B
availability (Goldberg
1997
). Boric acid is a very weak acid and when the pH is
below 7, it appears in its undissociated form; at alkaline pH, boric acid dissociates
to form the borate anion:
ð
pKa
9
:
25
Þ
Therefore, at common soil pH values (5.5-7.5), B exists mainly as soluble
uncharged boric acid (B(OH)
3
), and in this form B is absorbed by plant roots
(Hu and Brown
1997
; Power and Woods
1997
; Sathya et al.
2013
). In flooded
conditions, B is easily solubilised and leached resulting in a deficiency of B for
growing plants.
A plant requirement for B varies between species and is dependent on the
conditions in which they grow. The B requirement for one plant species may be
toxic or deficient for other species (Blevins and Lukaszewski
1998
). There are three
main groups of plants based on B requirements, one consists of graminaceous
species, a second are monocotyledons and a third are dicotyledons and lactifers,
having minimum, moderate and high B demands, respectively (Blevins and
Lukaszewski
1998
; Goldbach et al.
2001
). It is necessary to study mechanisms of
B uptake and transport, as well as translocation in plant systems, to optimise
agricultural production. For example, B deficiency results in major disorders,
which may decrease plant growth in soil with excess water. Molecular mechanisms
involved in B transport from the soil to root cells and xylem have two different
routes. Transport of B is dependent mainly on its availability and it is mostly
transported into the plant from the soil by passive diffusion, which is possible
when adequate B is available. When B availability is low or under deficiency
conditions, uptake is facilitated by active transport, which requires energy.
B absorbed by roots must be delivered to the xylem for further transport around
the plant. When there is sufficient B availability, absorbed B moves by passive
diffusion involving MIPs channels (Dannel et al.
2002
; Miwa and Fujiwara
2010
).
In limited or deficient B conditions, transport of B towards the xylem is facilitated
via a specific B transporter (BOR), which is an energy dependent transport process
(Fig.
6.1
). Takano et al. (
2002
) had identified such a transporter of B under limiting
conditions, known as BOR1 in
Arabidopsis
. Subsequently, BOR1-like genes have
been reported in
Eucalyptus
(Domingues et al.
2005
) and in rice (Nakagawa
et al.
2007
). Expression of two genes, NIP5;1 and BOR1, are decreased by
transcriptional and post-translational regulation respectively, under sufficient B
supply (Miwa and Fujiwara
2010
; Yang et al.
2013
).
