Biology Reference
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situations of mild water stress or no water stress,
where indeed maintaining longer leaf life could
be beneficial.
Surprisingly, the past 25 years of research
on the stay-green phenotype have only lately
led to examination of the possible association
between stay-green expression and plant water
status, although some relation between stay-
green expression and plant water status had
been hypothesized early (Tuinstra et al. 1998).
More surprising is the fact that several reports
had shown that stay-green was likely associ-
ated with maintenance of root growth (Hatlitligil
et al. 1984; MacKay and Barber 1986), with the
hypothesis that enhanced root growth would con-
tribute to enhanced N absorption. As we saw
in the previous example, the maintenance of a
functional stay-green under water-limited con-
ditions, that is, a plant type having both green
leaf area remaining
and
active photosynthetic
activity, depends on having water available in
the soil profile at the time of leaf senescence.
The difficulty in testing this hypothesis is con-
cerned with methods that can be precise enough
to assess plant water extraction at a fairly late
stage of plant growth when stay-green expres-
sion is at its maximum. Recently, a lysimet-
ric system has been developed at ICRIDSAT-
Patancheru (Vadez et al. 2008), which consists of
long and large plastic tubes in which plants are
grown with the spacing and soil exploration vol-
ume they would have in a natural field conditions.
This system has allowed the measurement of the
pattern of water uptake to support transpiration
in several crops, including sorghum (Vadez et al.
2011), chickpea (Zaman-Allah et al. 2011), and
peanut (Ratnakumar and Vadez 2011). Using this
system, a set of pearl millet topcross hybrids con-
trasting for their level of terminal-drought toler-
ance were assessed under conditions of terminal-
drought stress, imposed by stopping irrigation at
flowering time. The results clearly showed that
hybrids differed in their stay-green expression as
the stress developed and showed highly signifi-
cant correlation (R
2
(Vadez et al. unpublished). These results have
been confirmed in several experiments of pearl
millet and offer an outstanding demonstration
that stay-green directly relates to the water avail-
ability during the grain-filling period. One of the
exciting challenges of the coming year, using that
system, is to test the hypothesis, which could not
be tested before, that maintaining water uptake
during the grain-filling period would also indi-
rectly drive N uptake during the same period.
As seen above, several stay-green genotypes in
different species have been shown to enhance N
uptake during the post-anthesis period. Since N
uptake requires that this nutrient be dissolved in
water to be taken up, what remains to be estab-
lish is whether the higher N uptake could be a
consequence of a higher water uptake.
Water in the soil profile can become avail-
able during the grain-filling period through sev-
eral possible mechanisms. The most immediate
one is the capacity to extract water. This has
been shown in wheat (Manshadi et al. 2006),
where a stay-green wheat genotype extracted
more water from deeper layers of the soil pro-
file than did a senescent line. Recently, the stay-
green QTL
Stg1
in sorghum has also shown its
capacity to enhance water uptake in senescent
S 35 background (Vadez et al. 2011b). How-
ever, the effect of
Stg1
was not visible in the R
16 background. The likely explanation for this
is the higher “baseline” capacity for extracting
water in R 16 than in S 35. This highlights the
importance for future research on stay-green to
precisely decipher the mechanisms involved, and
to determine whether any of these mechanisms
are already available in intended target recurrent
parent genotypes.
The case of pearl millet described above is
interesting because the materials that differed in
stay-green (Vadez et al. in preparation) did
not
differ in the total water extracted from the soil
profile. In other words, stay-green in this case
was not related to an effect on rooting. By con-
trast, other studies have showed that these mate-
rials vary in constitutive water-saving traits, that
is, through a lower leaf conductance (Kholova
0.76-0.79) with the water
extracted three weeks after panicle emergence
=
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