Agriculture Reference
In-Depth Information
Fig. 15.5
Correlation
between root boron concen-
tration and shoot DW (
a
) and
shoot boron concentration
(
b
) in eight cultivars of wheat
grown in solution containing
5 mM boron. (Reprinted from
Reid (
2007
) with permission)
toxicity. An experiment with eight varieties of wheat all grown at the same solution,
boron concentration illustrates this relationship (Fig.
15.5
). In these varieties, the
root boron concentrations ranged from 4-6 mM. Both shoot boron concentration and
shoot dry weight were linearly related to the root boron concentration.
From this research, it became clear that tolerant cultivars were able to efflux
boron from roots but how this was achieved at the molecular level was not known.
This led to a search for genes-encoding membrane transporters capable of pumping
boron out of cells, as described in the next section.
Although reduced tissue boron concentration as a result of efflux pumping seems
to be a common feature of tolerant cultivars, there is also evidence in support of
two other mechanisms. Choi et al. (
2007
) reported morphological changes in the
root tip of tolerant species following exposure to high boron concentrations, which
were associated with increased concentrations of reducing sugar. It was thought
that these changes allowed osmotic balance to be maintained so that root elongation
could continue.
The second mechanism relates to a transcription factor that in rice makes plants
sensitive to high boron concentrations. Ochiai et al. (
2008
) investigated the reasons
behind the greater boron tolerance of japonica rice varieties in comparison to indica
varieties and located a QTL associated with tolerance. Map-based cloning eventu-
