Agriculture Reference
In-Depth Information
platform. In the
field, evaluations focus on agronomic characteristics
such as cycle time, plant height, and yield components under normal
and stress conditions. Evaluations using the SITIS platform focus on the
root system and physiological traits. The root system is characterized by
a scanner taking 360
pictures of the whole root system. Total root length,
surface, volume, diameter, and density are scored for each genotype
through this nondestructive method. The physiological traits evaluated
during the stress period are mainly photosynthetic and transpiratory
rates, stomatal conductance, leaf temperature, osmotic adjustment, rel-
ative water content, and water potential. Low P tolerance is evaluated in
°
field trials where agronomic characteristics as well as P concentration
in plant leaves and grains are measured.
The screening methodology performed by FLAR for tolerance of cold
includes evaluation at three important stages of crop establishment and
development. At the germination stage, cold stress is applied by low-
ering the air temperature in a cool chamber to 14
C. At seedling stage,
21 days after sowing, the seedlings are exposed to 5
°
C for 32 h, and
damage is evaluated using a visual scale 7 days after the cold treatment.
Cold tolerance at
°
flowering stage is scored on plants having the main
tiller at 25%of panicle exsertion and exposed to 5
°
C for 24 h. The labeled
panicle is harvested and
filled grains are weighed in order to calculate an
index of tolerance. Because of the limitations for looking at a large
number of entries at the reproductive stage, CIAT and FLAR are seeking
molecular tools to increase screening ef
ciency.
B. Breeding for Adaptation to Abiotic Stresses
1. Conventional Crossbreeding. Line development through pedigree
breeding traditionally starts from a wide source of variability that
can be generated from a germplasm improvement approach or from
crosses between selected divergent parental lines. CIAT, Embrapa, and
NARES in LAC have improved rice for tolerance of abiotic stresses
through population improvement using the recurrent selection (RS)
breeding method as well as by exploiting crosses using germplasm
collections of broad genetic diversity, sometimes expanding to wild
species. The principle of RS and population improvement activities in
LAC are further described in Section IV of this chapter.
In Colombia, CIAT developed an RS breeding scheme for tolerance
of acid soils and Al toxicity (Châtel et al. 2005). Various populations
were improved by performing cycles of RS under acid soils with high
Al toxicity. Headlines were extracted that combined various traits of
interest for developing new cultivars as well as for identifying parental
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