Agriculture Reference
In-Depth Information
rainfall during the early stages of growth of barley plants by spraying the leaves at
regular intervals.They were able to show that more than 50 % of shoot boron could
be washed out in this way, resulting in substantial improvements in growth of not
only shoots but also roots.
6 Boron-Salinity Interactions
A confounding factor in assessing boron toxicity is the co-occurrence with salinity
toxicity. Saline soils often contain high levels of both salts and boron (Nable et al.
1997
) while in many areas of the world, saline groundwater used for irrigation also
contains high levels of boron. Increasingly, recycled wastewater is being used for
irrigation and this can represent an additional source of both boron and salinity
(Feigin et al.
1991
; Tsadilas
1997
). Although both salts and boron can be leached
from the root zone in areas receiving high rainfall, in semi-arid regions they tend to
remain in the topsoil (Keren and Bingham
1985
).
Where two toxicity stresses are present simultaneously, the effect of one toxicity
can intensify the other, ameliorate the other, or simply be additive. There is no clear
consensus of which of these processes applies in the case of boron and salinity since
studies done under different conditions or with different crops tend to yield conflict-
ing conclusions (Bingham et al.
1987
; Mikkelsen
1988
; Grattan et al.
1997
; Shani
and Hanks
1993
; Holloway and Alston
1992
; Grieve and Poss
2000
; Ben Gal and
Shani
2002
; Alpaslan and Gunes
2001
; Ferreyra et al.
1997
; Yermiyahu et al.
2008
).
Much of the research on boron tolerance mechanisms is targeted at improving
crop productivity in high boron soils.Cultivars with high levels of expression of
boron-efflux transporter genes showed improved growth and yield at high boron
levels in solution culture, or in glasshouse trials with boron uniformly distributed
through soil and with adequate watering and low salinity. However, field trials in
southern Australia have been disappointing, generally showing little or no improve-
ment (Emebiri et al.
2009
; McDonald et al.
2009
). In reality, high boron soils invari-
ably contain other abiotic stress factors such as high salinity or variable moisture
(Fig.
15.7
) and the inability of the plant to deal with these stresses may outweigh
any advantage gained by tolerance to high boron (Nuttall et al.
2006
). Further-
more, where these stresses are heterogeneous in soil, avoidance by plasticity of root
growth may be more important than tolerance (Choi et al.
2006
).
7 Concluding Remarks
The past two decades have seen some important discoveries concerning the role of
boron in plants and the consequences of having too little or too much of it. Despite
considerable effort, there is still much that we do not really understand about bo-
ron, most significantly, the chemistry of its toxicity. Even its essential role in plant
