Biology Reference
In-Depth Information
Given the tight N supply-demand balance, it
has been shown that de-topping plant panicles
can indeed delay leaf senescence. For example
Rajcan and Tollenaar (1999b) showed that green
leaf area maintenance was higher in situations
of high source:sink ratios, achieved by partial or
full prevention of maize cob fertilization. A sim-
ilar phenomenon occurs in the case of genotypes
having a poor grain yield potential and there-
fore a poor sink for N, leading to the expres-
sion of a “yield-resistant” stay-green phenotype.
This association of stay-green expression with a
low grain-yield trend has been one of the main
criticisms of the use of the stay-green trait in
crop improvement (Ludlow and Muchow 1990).
However, Borrell and colleagues (2000) and
Haussmann and colleagues (2002) showed a pos-
itive correlation between stay-green expression
and grain yield under terminal drought-stress
conditions; although in the latter study, one of the
two RIL populations that was used showed no
correlation between grain yield and stay-green
expression. Tuinstra and colleagues (1997) also
identified two QTLs for sorghum stay-green that
co-mapped with grain-yield QTLs. So it appears
that the relationship between stay-green expres-
sion and grain yield under terminal drought-
stress conditions depends both on the environ-
ment and on the background that are considered.
One of the future challenges will surely be to
identify the genotypes and the environments in
which stay-green expression is not at the expense
of grain and/or stover yield potential. The poten-
tial of stay-green genotypes to accumulate N dur-
ing the grain-filling period, provided that grain
yield potential is not compromised, is the most
promising hypothesis to explain an N effect on
the expression of stay-green that could have
agronomic relevance. Further work is needed to
understand the processes that allow N absorption
to be sustained during grain-filling, especially
under water-limited conditions. As discussed in
the next section, the capacity to absorb N during
grain-filling under such conditions is bound to
be closely related to water status issues (having
water left in the profile to allow N absorption).
Addressing the Symptoms or
Addressing the Causes?
Thomas and Howarth (2000) reviewed different
modalities resulting in the display of a stay-green
phenotype. They have suggested five ways to
stay green. Of these, four were concerned with
the rate and onset of pigment decline. Pigment
degradation is a self-programmed process in
plants during maturation and there are a number
of factors affecting N remobilization that alter
that natural process. For example, de-topping the
panicle delays leaf senescence. However, even
after removing the effect of N-status-altering
processes, pigment degradation continues its nat-
ural way. Research initiated to counter that pro-
cess found that enhancing cytokinin production
delayed pigment degradation in tobacco leaves
(Gan and Amasino 1995; Roitsch and Ehne
β
2000). A number of studies have followed in
which transgenics were developed, which con-
tained a gene contributing to enhanced cytokinin
production to retard pigment degradation (Rivero
et al. 2007; Peleg et al. 2011), and which were
reported to be drought tolerant. While the main-
tenance of green leaf area may be beneficial
in situations when water is not limiting, as
earlier shown in
Lolium temulentum
(Thomas
and Howarth 2000), the overall approach and
hypothesis of its value under water-limited sit-
uation remains questionable. Plants exchange
carbon dioxide for water through the stomata
and under water-limited situations the degree of
stomatal opening is what sets the photosynthetic
rate, because of the absolute necessity to match
stomatal opening to the limited water available.
Therefore, the cytokinin-related maintenance of
green pigmentation under water-limited condi-
tions may be assigned to a “type-C” stay-green as
defined by Thomas and Howarth (2000), that is,
a type that stays green but in which the photosyn-
thetic functionality is equivalent to a senescent
line because of the effects of water limitation
on stomatal opening. Although the work on the
overexpression of cytokinins to retard leaf senes-
cence is intriguing, we would argue that it may fit
Search WWH ::
Custom Search