Genetic analysis for drought tolerance and biomass allocation in newly developed populations of bread wheat (triticum aestivum L.).
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Date
2023
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Abstract
Bread wheat (Triticum aestivum L., 2n = 6x = 42, AABBDD) is the most lucrative commodity
crop cultivated worldwide. Wheat productivity is crucial for economic gains and food security to
the growing global population. Global wheat production is affected by recurrent droughts that are
further exacerbated by a changing climate characterized by rising temperatures and erratic rainfall.
In response to these challenges, most wheat breeding programs have focused on increasing harvest
index to improve grain yield and drought adaptation without considering below-ground root
biomass. In recent years there has been a growing interest in using crops such as wheat to store
some of the atmospheric carbon previously lost from soils due to past agricultural practices to
sustain soil quality and to mitigate climate change. Increasing biomass allocation of new wheat
genotypes to the root system may enhance carbon (C) extraction from the atmosphere and transfer
to crop tissues and to soils through carbon sequestration while increasing resilience to drought
stress by improving water and nutrient uptake. Therefore, this study aimed at improving drought
tolerance and C sequestration ability of wheat for production under dryland farming systems. The
specific objectives were:
to provide information based on a retrospective quantitative genetic analysis on combining
ability studies of wheat for yield and yield-related traits to predict potential genetic gains
achievable in improving biomass allocation for drought tolerance and soil carbon storage;
to determine the extent of genetic variation present in wheat germplasm collections for
biomass allocation and drought tolerance based on complementary phenotypic and root
attributes and high-density single nucleotide polymorphism (SNP) markers to select
breeding parents;
to estimate the magnitude of the relationships between root biomass and yield components
and to identify influential traits to optimise genotype selection for enhanced biomass
allocation, drought tolerance and carbon sequestration potential in bread wheat (Triticum
aestivum L.);
to determine the general and specific combining ability, maternal effects and the mode of
gene action controlling the major yield-related traits and biomass allocation in wheat to
identify good combiners for breeding and enhanced carbon sequestration, and;
to determine the genetic variability of newly developed wheat populations for grain yield
and biomass allocation under different water stress conditions to select the best-performing
families for advancement.
The first study compared data on the general combining ability (GCA) and specific combining
ability (SCA) effects of wheat for yield and related traits under optimum and drought-stressed
conditions from 40 studies worldwide. Days to heading (DTH), plant height (PH), number of tillers
per plant (TN), kernels per spike (KPS), 1,000-kernel weight (TKW), shoot biomass (SB), and
grain yield (GY) exhibited wide variation for GCA and SCA effects. Progeny performance
increased by 14.30 and 4.04% for SB and GY, respectively, compared with parental values under
optimum water conditions. The number of tillers and SB exhibited positive associations with GY
(0.45 ≤ r ≤ 0.85, p < 0.05) under both water conditions. Meta effect sizes for drought stress were
negative. The highest meta-effect sizes were calculated for DTH (−4.5) followed by SB (−2.0),
whereas KPS (−1.25) had the lowest. The genetic gains for PH, SB, and other yield components
showed that divergent crosses involving complementary parents could enhance biomass allocation
patterns in wheat. This could be used as a basis for improving biomass allocation to roots.
In the second study, a total of 97 bread wheat genotypes were evaluated in field and greenhouse
trials under drought-stressed and non-stressed conditions and genotyped using 16 382 high-density
single nucleotide polymorphism (SNP) markers. The analysis of molecular variance showed that
the intrapopulation variance was very high at 99%, with a small minimal inter-population variance
(1%). The genetic distance, polymorphic information content and expected heterozygosity varied
from 0.20 to 0.88, 0.24 to 1.00 and 0.29 to 0.58, respectively. The cluster analysis based on SNP
data showed that 44% and 28% of the assessed genotypes maintained their genetic groups
compared to hierarchical clusters under drought-stressed and non-stressed phenotypic data,
respectively. The joint analysis using genotypic and phenotypic data resolved three heterotic
groups and allowed the selection of genotypes BW140, BW152, BW157, BW162, LM30, LM47,
LM48, LM52, LM54 and LM70. The selected genotypes were the most genetically divergent, with
high root biomass and grain yield and are recommended for production or breeding.
The third study evaluated 100 wheat genotypes consisting of 10 parents and 90 derived F2 families
under drought-stressed and non-stressed conditions at two different sites. Data were collected for
DTH, days to maturity (DTM), PH, TN, spike length (SL), spikelets per spike (SPS), KPS, TKW,
SB, root biomass (RB), total plant biomass (PB), root-to-shoot ratio (RS) and GY. There was
significant (p < 0.05) genetic variation in most assessed traits except TN and RS. Root biomass
had significant positive correlations with grain yield under drought-stressed (r = 0.28) and nonstressed
(r = 0.41) conditions, but a non-significant correlation was recorded for RS and grain
yield. Notably, both root and shoot biomass had significant positive correlations under both water
regimes, revealing the potential to increase both traits with minimal biomass trade-offs. The
highest positive direct effects on grain yield were found for KPS and PB under both water regimes.
The present study demonstrated that selection based on KPS and PB rather than RS will be more
effective in ideotype selection of segregating populations for drought tolerance and carbon
sequestration potential.
In the fourth study, the above dataset from the ten parental lines and their F2 progeny were
subjected to combining ability analysis using a full-diallel mating design. Significant differences
were recorded among the tested families revealing substantial variation for PH, KPS, RB, SB, PB
and GY. Additive gene effects conditioned PH, SB, PB and GY under drought, suggesting the
polygenic inheritance for drought tolerance. Strong maternal and reciprocal genetic effects were
recorded for RB across the testing sites under drought-stressed conditions. The parental line LM75
maintained the GCA effects in a positive and desirable direction for SB, PB and GY. Early
generation selection using PH, SB, PB and GY will improve drought tolerance by exploiting
additive gene action under drought conditions. Higher RB production may be maintained by a
positive selection of male and female parents to capture the significant maternal and reciprocal
effects found in this study.
The fifth study showed higher phenotypic coefficient of variation (PCV) than the genotypic
coefficient of variation (GCV) for PH, KPS, SB, RB, PB and GY. Moderate heritability of 41.61%
and 45.14% and genetic advance as a percentage of the mean (GAM) of 3.49% and 3.58% were
observed for RB under drought and for KPS under non-stressed conditions, respectively. Based on
correlation and principal component analysis, geometric mean productivity (GMP) and stress
tolerance index (STI) were identified as the most efficient drought tolerance indices for selecting
drought-tolerant families with high RB. Direct crosses such as BW162 × LM75, BW152 × LM75,
LM70 × LM75, LM71 × LM75 and LM26 × LM75 and reciprocal crosses LM48 × BW140, LM71
× LM26, LM70 × BW152, LM70 × BW141 and LM75 × LMBW152 were identified as drought
tolerant and are recommended for genetic advancement. The high root biomass production of these
families will contribute to carbon inputs through rhizodeposition in agricultural soils. Further
research studies should investigate the link between changes in biomass allocation and
atmospheric carbon transfer to soils for improving soil quality and mitigating climate change.
The present study revealed that maternal and reciprocal effects should be considered when
selecting root biomass and biomass allocation traits. Also, the study identified drought tolerant
genotypes and developed new families with high biomass production for enhanced carbon
sequestration. The identified families should be advanced for variety development and further
evaluated for their net carbon contribution to the soil.
Description
Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.