Biology Reference
In-Depth Information
to confirm whether or not their epistatic inter-
actions are of economic importance. Currently
a few selected best-performing stay-green QTL
ingtrogression lines are being used in microarray
assay and proteomics studies seeking up- and/or
down-regulated genes and to identify gene prod-
ucts specific to these gene combinations.
from the N-rich leaf tissues (Sinclair and Vadez
2002). As rubisco, a central enzyme for the con-
version of CO
2
into carbohydrates, accounts for
about half the nitrogen in leaves of C3 plants and
about 25% of the leaves of C4 plants, remobiliz-
ing N from rubisco and photosynthetic pigments
implies that the photosynthetic rate is bound
to decrease during grain filling. For instance,
Borrell and Hammer (2000) showed that senes-
cent and stay-green sorghum hybrids differed
in the supply-demand balance for N, with stay-
green having a shortfall in N that is about 25%
lower than that in senescent hybrids, and explain-
ing a slower rate of leaf senescence in the stay-
green genotypes. A similar case was reported in
maize, where a stay-green hybrid acquired up
to 60% of its N supply during the grain-filling
period, whereas a senescent hybrid acquired only
40% of its total N during the same period (Rajcan
and Tollenaar 1999a). This showed the impor-
tance of maintaining N uptake during grain fill-
ing in staygreen lines across different species.
Subedi and Ma (2005) also clearly showed that
in both stay-green and senescent maize hybrids,
stopping the supply of N from V8 to maturity dra-
matically accelerated the decrease in leaf green-
ness, measured by SPAD readings, compared to
a treatment in which N supply was maintained.
Another study also showed that under low-N
conditions, there were genotypic differences in
sorghum in the capacity to extract N from the
soil profile (Nakamura et al. 2002). For these rea-
sons, the N status of a plant is still considered an
important factor in the expression of stay-green.
Among the five cases of stay-green reviewed by
Thomas and Howarth (2000), the type E stay-
green is a case where senescence initiates at a
similar date and follows a similar rate to a senes-
cent type, but the higher initial N content in the
leaves buffers the grain-filling-induced decline
in leaf-N. That is, the current view is that an
increased N uptake by roots during grain-filling
leads to longer duration of leaves, and the higher
specific leaf N (SLN) levels maintains the pho-
tosynthesic activity of these leaves at high levels
for a longer period.
Mechanisms Explaining
Stay-Green
The Nitrogen and Carbohydrate Route
The potential benefit of stay-green was ini-
tially viewed from the angle of the main-
tenance of photosynthetic activity (Rosenow
et al. 1983; Thomas and Smart 1993; Borrell
et al. 2000). Results showed that indeed delayed
senescence of fully-irrigated
Lolium temulentum
leaves would increase the carbon fixation by 11%
over the entire life of the leaf, simply by delaying
senescence by two days (Thomas and Howarth
2000). Other results also showed that levels of
basal stem sugars (Duncan 1984) or carbohy-
drate contents (McBee et al. 1983) were higher in
stay-green sorghum genotypes. Sanchez and col-
leagues (2002) also made the assumption that the
delayed leaf senescence from stay-green would
sustain photosynthetic activity. Hence, a number
of studies have documented the worthiness of
maintaining photosynthetic activity of the leaf
for more time. While this may be true in situ-
ations where there is no water limitation, and
where there is a light-capture interest of delay-
ing leaf senescence, this may be less of a value in
situations where water is limited and photosyn-
thetic activity is bound to be regulated by stomata
opening. Therefore, we would argue that the con-
tribution of stay-green in terms of carbon fixation
under water-stress conditions may likely be very
limited.
Another approach to explaining stay-green
differences has been to assess their role in the
nitrogen balance of the plant. In crops produc-
ing grain, the most important nutrient required
to fill up grain is nitrogen and it is remobilized
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