Chemistry Reference
In-Depth Information
works [64-68]. As such, ear demand was greater in newer than in older hybrids by means of
a greater kernel number per plant or a large potential kernel weight [60].There was not a
clear trend with the year of the hybrid release in source-sink ratio in non-limiting
environments (i.e. optimum resources availability; 59). An enhanced source-sink ratio (i.e.
calculating the sink as kernel number alone) has been indicated for Argentinean maize
hybrids released between 1965 and 1997 [40]. However, kernel weight reductions in
response to source reductions due to defoliation during grain filling were greater in newer
than in older hybrids (Figure 8a; 60). This response was associated with the greater ear
demand relative to the source capacity in newer Argentinean maize hybrids (Figure 8b).
Thus, if breeding for high yield potential continue increasing the ear demand without a
proportional increment in total source capacity, kernel weight would be source limited and
it will be more affected by source variations during the grain filling period in the newer
maize hybrids. In agreement, ear demand of current Argentinean maize hybrids (i.e.
released in 2010) was greater than that of maize hybrids released in 1993 [72]. As such, ear
demand increased at a rate of 1.13% year
-1
during the last 45 years in Argentina; and kernel
number was the main component influencing this increment rather than kernel growth rate
[72]. In contrast, source-sink ratios were greater for newer than for older Ontario maize
hybrids for the 1959-2007 period [8]. The increased functional “stay green” (i.e. capacity of a
leaf to retain its photosynthetic rate during the grain filling period; 8) was the main factor
underlying the larger source during the grain filling period in newer maize hybrids of the
US corn belt and Ontario, Canada [1; 8; 17].
50
50
a
b
40
40
30
30
20
20
10
10
y = 0.15x - 9.3
R² = 0.93
0
0
1960
1980
2000
0
200
400
Year of release
Ear demand
(mg ºC
-1
d
-1
)
Figure 8.
Kernel weight reduction (%) due to full defoliation during the grain filling period as a
function of (a) year of hybrid release and (b) ear demand (mg ºC
-1
d
-1
) for 5 hybrids released in
Argentina from 1965 to 1993. Adapted from Echarte et al. [60].
The greater ear demand along with the genotypes used in the Argentinean maize breeding
programs influenced the grain chemical quality of hybrids released in different decades [11].
Protein concentration decreased with the year of the hybrid release in an environment
without nitrogen (N) fertilization but it was not modified when N was applied (Table 1); soil
N-NO
3
level at V6 stage in this experiment was higher than the minimum required for
maximum yield achievement (i.e. 27 ppm in this experiment versus a threshold of 24 ppm
