Agriculture Reference
In-Depth Information
5.6.1 E
xploiting
g
EnEtic
V
ariability
for
b
EttEr
c
rop
S
tand
E
StabliShmEnt
undEr
c
onSErVation
a
griculturE
p
racticES
Residue retention for mulching has often been reported to physically hinder shoot
emergence and slow down germination, as well as reduce seedling biomass in wheat,
rice, corn, and canola (Chastain et al. 1995; Swan et al. 1996; Wuest et al. 2000;
Bruce 2003; Mohanty and Painuli 2004). Coleoptile elongation is considered as an
important mechanism in overcoming mulch and seed depth barriers to seedling
emergence. Coleoptiles in cereals provide protection to subcrown internodes. The
elongation capacity of subcrown internodes generally determines the seed placement
depth in crops. Deeper placement of seeds under CA, particularly under rainfed con-
ditions, provide uniform stand establishment. Alternately, rainfed farmers have to
open “V-shape furrows” with narrow shovels of the cultivator followed by placement
of seed 5 cm deep at the sill of the furrows using inverted T-openers of the zero-
till drills. Ferguson and Boatwright (1968) have indicated that as surface residues
increased, the crown node formed farther from seed—closer to soil surface and, in
some cases, above the surface. They noted a positive variety × soil temperature ×
light intensity interaction mediated via surface mulch. Crown nodes form closer to
soil surface as the light available for seedlings or soil temperature decreased. Shorter
coleoptiles associated with the presence of the gibberellin-insensitive
RhtB1b
and
Rht-D1b
dwarfing genes in wheat can result in slow seedling emergence, reduced
tillering, and poor crop stand establishment when seed is placed deeper in mulched
no-till situations (Fick and Qualset 1976; Allan 1989; Matsui et al. 1998; Schillinger
et al. 1998; Rebetzke et al. 2001; Rebetzke et al. 2005). Wheat genotypes with the
Rht-B1b
gene for a short coleoptile emerged slower with more nonproductive tillers
with less biomass. In contrast, genotypes with longer coleoptile length due to various
gibberellin-responsive dwarfing genes, such as
Rht8
and
Rht9
, are less affected by stub-
ble mass and sowing depth. Combining alternative dwarfing genes (
Rht8
,
Rht4
,
Rht12
,
and
Rht13
) for reduced plant height with longer coleoptiles can be highly rewarding for
a large impact under rainfed CA (Hughes and Mitchell 1987; Schillinger et al. 1998;
Rebetzke et al. 1999; Botwright et al. 2001; Ellis et al. 2004). Thus, wheat genotypes
well adapted to surface residues coupled with reduced light intensities, higher soil tem-
perature, and higher soil moisture content should produce crown node well below soil
surface, and reduce root rot problems (Boosalis et al. 1981).
5.6.2 r
EprioritizE
E
mphaSiS
on
r
ESiStancE
b
rEEding
Wheat is generally planted in the first fortnight of November in South Asia to avoid
terminal heat stresses and moisture shortages resulting in shriveled grains and reduced
yields. Early planting, however, in October, generally result in stunted growth, poor
tillering, and drastic reduction in yields. Thus, heat stresses in early and late planted
wheat crop adversely affect grain yields for different reasons. CA is known to help
in early planting, and increase cropping intensity by reducing the turn-around time
between crops. However, sustainable intensification will need crop varieties having
in-built tolerance to both early and late heat stress and drought, and traits for higher
resource use efficiency and enhanced pest and disease resistance (
http://www .fao
