IR64 plants accumulate sodium in the shoot while
exhibiting low levels of shoot senescence
The concentrations of Na+ and K+ in the youngest,
fully expanded leaf of both IR64 and Fatmawati were
measured after 20 d of salt application. Leaf Na+ concentrations
of the stressed plants increased as expected
with increasing external NaCl concentration
(Tables 1 and 2). The amount of Na+ in the leaf was
cultivar dependent, with IR64 accumulating more leaf
Na+ than Fatmawati (Tables 1 and 2). However, despite
accumulating lower concentrations of Na+ in the
leaf, Fatmawati was found to have higher levels of
shoot senescence (Table 2 and Figure 5).This suggests
that IR64 may have more efficient Na+
tissue tolerance
mechanisms, such as the ability to accumulate
Na+ in the vacuoles of leaf cells (Garbarino and DuPont
1989; Blumwald et al. 2000). Sub-cellular measurements
of ion accumulation and whole plant/tissue ion fluxes will
help elucidate the mechanisms involved (James et al.
2006; Møller et al. 2009; Plett et al. 2010). Interestingly,
the concentrations of shoot K+ in the stressed plants
were higher than those of the control plants, perhaps
suggesting that rice attempts to maintain as high a K+
concentration as possible during salt stress. However,
shoot K+/Na+ ratios in the stressed plants were significantly
lower than those of the non-stressed plants
(Tables 1 and 2).
It was not possible to measure Na+ and K+ concentrations
in the last fully expanded leaf of Fatmawati plants
grown in 200 mM NaCl due to the high level of senescence
(Tables 2 and Figure 5).
A role for non-destructive phenotyping in screening for
rice salinity tolerance
Automation of the phenotyping process in combination
with automated plant handling and watering allows large
numbers of plants to be screened efficiently with limited
handling. Entire populations of plants can be grown in soil
media, emulating field conditions (at least for the earlier
stages of growth), thus facilitating the transfer of knowledge
from controlled environment to growth conditions in
the field. An increasing number of phenotyping facilities
are now accessible globally, such as the Australian Plant
Phenomics Facility (http://www.plantphenomics.org.au),
used in this study, or the centres of the European Plant
Phenomics Network (http://www.plant-phenotyping-network.
eu). The process described here has the potential to be scaled
to phenotype large numbers of rice breeding lines and
mapping populations allowing the evaluation of the effect
of different salt tolerant mechanisms on plant growth
and yield. Screening of hundreds of mapping lines and/or
rice accessions for bi-parental or association mapping
studies can now be done relatively quickly for traits that
require time course measurements of growth. The use
of such populations has the potential to identify the
underlying genetic mechanisms of salinity tolerance in a
forward genetics screen.
The main limitation for the widespread adoption of this
approach is the cost, but this is decreasing very rapidly as
imaging technologies decrease in price and, as knowledge
of phenotyping improves, short-cuts and pragmatic compromises
can be more confidently undertaken. This will
be accompanied by an increasing ability to phenotype
cheaply in the field – although this inevitably comes with