Biotic Stress and Crop Improvement: A Wheat Focus Around Novel Strategies - Crop Improvement: New Approaches and Modern Techniques - page 239

loids belong to the “primary” gene pool and the B(S) genome to the “secondary”
pool. Exploiting these diploids requires skills of developing user friendly genetic
stocks commonly known as “synthetic hexaploids (SH)”. The stocks are produced
by combining durum wheat cultivars (2n = 4x = 28) with each diploid thus generat-
ing hexaploids that are genomically AABBDD, AABBAA and AABBBB(SS). All
stocks cytologically are expected to be 2n = 6x = 42 and major resources and provide
unique allelic diversity for wheat improvement.
Biotic stresses of significance vary according to location and our major ones are
the three rusts, karnal bunt with upcoming concern prevailing for powdery mildew,
barley yellow dwarf and the new emergence of spot blotch. Progress to combat
these stresses has be driven in tandem with locational priorities and these dictates
have shifted global and national focus among the rusts to stem rust with the threat
of race UG99's spread linked with a local races presence. Thus diversity for ex-
ploitation has extended beyond the diploid relatives to include tertiary gene pool
resources where most notable mention is of the diploid Thinopyrum bessarabicum
that has the potential to address multiple stress factors and will be elucidated in an
agglomerated manner to embrace various accessional sources as they relate to the
major biotic stresses resistance management.
Introduction
Currently much of the wheat genetic variability is obtained through conventional
crop improvement methods involving land races and normal varieties. Hence, the
germplasm base available in the form of cultivars is becoming increasingly nar-
row and the need for widening the gene pool is essential in view of the emerging
biotic and abiotic stresses due to global warming and climate change. Major abiotic
constraints that have surfaced are due to increased salinity, water logging, drought
and heat (Mujeeb-Kazi et al. 2008a ). Biotic stresses of emphasis here additionally
contribute to the crops productivity situation. To counter these maladies a broad
genetic base is essential to have on hand and its implementation a dire need forming
the focus of this communication. New and useful genetic variations exist in the wild
uncultivated wheat progenitor species that can be utilized for the enhancement of
the existing wheat breeding pools and improve yield stability. These genetic varia-
tions can be harnessed through a combination of conventional breeding methods
coupled with interspecific, intraspecific and intergeneric hybridization approaches
popularly known as “wide crossing” that independently and cumulatively augment
the available genetic variability for wheat improvement.
Diploid wheat progenitors (2n = 2x = 14) A, B, and D are the constituents of bread
wheat ( Triticum aestivum L) offering extensive diversity that contributes to crop
improvement by providing novel allelic enrichment. A and D genome diploids be-
long to the “primary” gene pool and the B(S) genome to the “secondary” pool.
Exploiting these diploids requires skills of developing user friendly genetic stocks
commonly known as “synthetic hexaploids (SH)” (Mujeeb-Kazi et al. 1996a ). The