Chemistry Reference
In-Depth Information
The lower threshold PGRs for kernel set was associated also with an improved tolerance to
high plant density in newer maize hybrids [10]. Greater tolerance to high plant density was
reported for hybrids released during different decades in the US, Canada and Argentina [3;
18; 32; 36; 40; 46]. The response of grain yield to plant density was curvilinear in
Argentinean maize hybrids released between 1965 and 1993 [19] and between 1965 and 1997
[40], in agreement with the generally reported grain yield response to plant density for
maize [47; 48]. Grain yield response to plant density was mostly associated with number of
kernels per unit area [19], in accordance with other works [18; 47; 49]. In general, differences
in kernel number m
-2
among hybrids released in different decades increased with plant
density [19]. Figure 6 shows that kernel number m
-2
of a hybrid released in 1965 increased
with plant density up to 8 pl m
-2
; whereas, kernel number of a newer hybrid released in 1993
increased with plant density up to 14.5 plants m
-2
.
A recent study demonstrated that kernel
number of current Argentinean maize hybrids (i.e. released in 2010) is consistently higher
than that of an hybrid released in 1993 at high plant densities [50]. Greater tolerance to other
stresses like weed competition (51), low night temperatures [16; 52], low soil nitrogen [17;
53; 54] and drought [55] were reported for hybrids released during different decades in the
US and Canada. It was demonstrated that the nature of the environmental stress (e.g., plant
density, nitrogen, water) causing variations in PGRs did not influence the KNP-PGRs
relationship [56; 57]. Therefore, it is likely that a lower threshold PGRs is the underlying
feature contributing to explain the greater general stress tolerance in newer than in older
maize hybrids.
The greater kernel number at low plant density in newer compared with older maize
hybrids (Figure 6) is another distinctive trait improved in Argentinean maize hybrids; since
no grain yield improvement at very low plant densities was reported for US and Canadian
hybrids [3; 37]. Moreover, although newer Argentinean hybrids released in 2010 yielded
more than hybrids released in 1993 in a range of plant densities between 5 to 14.5 plants m
-2
,
the greatest grain yield improvement during the 1993-2010 period occurred at the lowest
plant density (i.e. 5 plants m
-2
; 49).
8000
6000
4000
2000
older hybrid (1965)
newer hybrid (1993)
0
0
5
10
15
20
Plant density (plants m
-2
)
Figure 6.
Number of kernels m
-2
as a function of plant density for an older (DKF880) and a newer
(DK752) maize hybrid released in Argentina (year of release between brackets). Adapted from Echarte
et al. (19).
