Discussion
The Teff lines evaluated in this study exhibited a wide phenotypic
variation (Supp. Fig.7a), which is comparable to previous literature
reports (Assefa et al., 2002, Girma et al., 2019, Nigu el al., 2016).
For example, the GY range in the current field experiment was equivalent
to 0.24-0.6 t/ha, where the national average farmer’s yield is around 1
t/ha, and 2.5 t/ha under experimental conditions in Ethiopia (Berhe et
al., 2011, Girma et al., 2019, Nigu et al., 2016, Zhu, 2018). Teff has a
potential for yielding 4.6-5 t/ha if lodging can to be resolved (Hailu
et. al., 2000). Harvest Index values previously reported (Assfa et al.,
2002) were of a similar range to our field experiment (Table 3). In
addition, phenological values (Table. 6) were in accordance with
previously reported ranges for Teff cultivation (Assfa 2001 b & 2002).
Within the genetic material tested, no correlation was found between TDM
and GY, whereas in other reported positive (Braha et al , 2017) or
negative correlation (Chanyalew et al., 2010, Lule & Mengistu, 2014).
However, when analyzing the white and brown separately (and excluding
the white cultivar which appears to be much different) there appears to
be some degree of correlation (r=0.6 and 0.7 for the white and black
respectively), yet not statistically significant. We found correlation
between GY and HI (0.8***) that was in agreement with previous reports
(Lule & Mengistu, 2014).
Plant height was previously reported to range between 74 and 116 cm
(Assfa, 2002). Under our field conditions, within the growing season MPH
was 58-78 cm (Table. 8), and in the greenhouse off-season MPH was
180-200 cm (Table 1), which was more than double field growth but with
narrower range across the lines. Differences in day length and other
environmental factors may account for these differnces. Some of the
lines were ranked similarly in both experiments (Table 1 and 6) in terms
of MPH.
Since there are two duplicate genes for grain color in Teff, which are
known to be dominant (Berhe et al., 2001), the small fraction of the
brown seeds found within the seeds propagated from the initial collected
panicle (0.064, 0.105 and 0.015 for 53-1, 53-2, 53-3 respectively, Table
1, Fig. 6) can only be explained by an external foreign pollination. The
increase in brown seed ratio in 53-1 from the first collected generation
to the greenhouse next-generation, from 0.021 to 0.064 - as would be
expected from the segregation of o heterozygosity of the grain color
loci (A/a). The small fraction of A/a in the collected panicle would be
expected to triple (a total of: 1 A/A and 2 A/a) over the course of a
single generation. The data also support the hypothesis that the brown
lines are half-siblings (hybrids) to their white counterparts; these
half-siblings share the maternal side but differ in the paternal one. It
is very likely that these hybridizations were most probably wind-driven.
As oppose to 53-1 No dark brown seeds could be detected within the grain
of the collected panicles of these two lines, so there must have been
undetectable light brown seeds that were in a heterozygous state A/a and
were later segregated.
The brown lines in this study (pure and segregating) exhibited an
overall advantage over the white lines in terms of directing their
biomass towards grain production (Table 3, PCA Fig. 5, excluding the
case of the ‘white commercial cultivar’). The white commercial cultivar,
the pure brown 44A-163-B and some of the segregating brown lines were
the highest ranked for GY (Table 3, PCA Fig. 5).
A clear pattern emerges that all brown hybrids being earlier to flower
(along with the white commercial cultivar, Table. 6) as compare to their
white half-siblings. Therefore, it is possible that the pollen donor/s
was/were a relatively early flowering type . Also the significant
differences in plant height, observed between 53-2-W and 53-2-B (76 cm
vs. 58 cm, Table 7) may indicate that the pollen donor in this case has
a shorter stature than the maternal line.
53-3-B, which originated from the segregation of the mixed color line
53-3 into brown and white seeds (Table 1), was especially interesting.
This line exhibited relatively low TDM and high HI (Table 3), low EGC
(Table 4), and high Chl levels (Table 5). Contrastingly, this line was
also relatively tall as indicated by its high MPH (Table 7) and thin
stems (Table 4). In terms of sensory evaluation, 53-3-B was the most
promising line with its preferable taste, smell and appearance (Tables 9
and 10).
Our hypothesis is that each of the three half-siblings originated from a
different pollen donor was strengthened by the large variations in Chl
levels between the three half-siblings. 53-2-B had the lowest Chl
levels, and 53-1-B and 53-3-B exhibited the highest levels among the
collection (Table 6 and Fig. 3). Interestingly, 53-2 which presented the
lowest Chl levels had the highest GY among the three. Another result was
the grouping of the traits: Chl Tot, Chlb, flavor and AGW that was
negatively correlated with TDM (Fig. 5). This is in agreement with the
literature that suggests that a smaller plant may contain denser leaves
and more chloroplast and Chl per gr FW of leaf tissue (Fritschi & Ray.,
2007). The positive correlation obtained between flavor and Chl may be
indirect however, these correlations may have importance for future
breeding programs as initial phenotypes for selection.
Following the genetic model of lemma color (Berhe et al., 2001), it
appears that both the maternal line of 53-2 and 53-3 had the basic p1
and/or p2 genes in the background of the dominant C gene thus resulting
in red lemma color, and that the hybridization with an unknown brown
donor introduced a P1 or P2 thus resulting in purple lemma color. There
seems to be differences that may have come from maternal differences
between 53-2 and 53-3 in lemma color, because 53-3-W was not red as
53-2-W, but rather gray (Fig. 3).
In this work we characterize in detail the lodging phenomena in the
studied Teff lines. The simplified scoring system for lodging (Caldicott
& Nuttall, 1979), which is commonly used, does not take into account at
which growth stage lodging starts nor the uniformity of lodging within a
plot. Therefore we choose to document plot-height over the course of the
reproductive period (Fig. 2), as well as to calculate the days to 75%
lodging in a plot. This detailed inspection and documentation explains
mechanisms related to Teff lodging (Fig. 2 & Supp. Fig. 7-8), which are
very much context-dependent in terms of environmental conditions. In
that respect, the lodging resistance which was the reason for collecting
these lines to begin with, (Supp. Fig. 1). It appears that since the
collected plants growing randomly as single plants within a homogeneous
lodging inclined genetic population as well as occurring at very
low-density planted areas, showed a lodging-resistant phenotype. In
well-irrigated, well fertilized and low-stress conditions of our
experiments, completely lodging-resistant line was not found, and
lodging seems to be a flash-mob phenomenon were several plants start a
lodging movement that sweeps the reset of the field. However, we show
that our in-depth documentation and interpretation of lodging here may
be useful for future breeding of -lodging resistance characteristics in
Teff.
High yielding lines tend to lodge at harvest time (Yu et al., 2007,
Davison & Laca, 2010). The lack of variation in lodging resistance may
be a result of unfavorable associations of lodging resistance with
productivity promoting traits such as plant height, panicle length,
grain and shoot biomass (Kebebew et al., 2011, Nigus Eet al., 2016, Yu
et al., 2007). Improvement of lodging related traits, such as culm
length, overall-height and diameter of the culm internodes, through
breeding is expected to be a demanding task due to their relatively low
heritability and lack of reliable genetic advance-estimates (Assefa et
al., 2001a). Therefore, increasing our understanding of the lodging
phenomena and its phenotyping can improve our ability to breed for high
yielding lodging-resistant cultivars. This study shows that the nature
of lodging is variable in terms of timing and strength (Fig. S7a). In
terms of timing, we observed an early type of lodging which was most
likely triggered by the fast inflorescence weight increase exhibited by
the white commercial cultivar (Fig. 3 and Fig. S7b) and 53-1-B (Fig. 3).
Other lines were ‘strong’ enough to carry the inflorescence most of the
grain-filling period, such as 44A-163-B and 44B-163-W (Fig S7b).
Therefore, the rate in which panicle increases in weight, prior and
throughout grain filling appears to vary and may be important from a
breeding perspective.
Surprisingly, the white commercial cultivar which was the first to
lodge, was mostly stable in plot-height once lodged (around 35 cm above
ground surface) during the entire grain filling and was the best
yielding line. In the white commercial cultivar, despite stem weakness
and the plant being bent towards the ground, the panicles are mostly
above ground level. This pattern creates a medium level of lodging that
appears to be different from the strong lodging where the plant is
heavier and is totally bent to the ground (Fig S7b: 44B-163-W and
53-2-2-W).
Despite the large number of studies screening Teff lines (Assefa et al.,
2003, Zeid et al., 2012, Girma et al., 2019, Nigus et al., 2016) there
have been only few which include in-depth characterization of different
lines. In addition, there are hardly any studies that present agronomic
as well as sensory traits side by side to analyze possible links between
them . The processing, eating, and nutritional quality of food products
may be greatly influenced by Teff variety. Therefore, it is necessary to
assess the genetic diversity of this crop for potential improvements of
agronomic as well as edible traits. Newly breed varieties must be
subjected to sensory analysis for consumer acceptance to make the
research efforts commercially meaningful (Zhu, 2018). We report a
significant genotypic effect on most of the sensory traits evaluated
(Tables 9-10). The effectiveness of flour grinding was found to be
crucial for texture across of all market samples. The white cultivars
were ranked higher than the rest of the lines. The size of the grain may
also effect Injera acidity and odor, as AGW was found to be positively
correlated with odor intensity and negatively with acidity.
The current study growing conditions (pots and field experiments) were
not favorable to observe the lodging resistance which was observed
during original seed collection. The experiment didn’t replicate the
specific environmental context of a single seed developing in low
density planted area and/or surrounded by a homogenous population of the
plots which it was collected from. A wide genotypic variance was found
in the current study for stem width and plant height. However, under an
abundance of water and nutrient and, at high plant density, a thick stem
does not ensure lodging resistance. It appears however that under low
density and/or some sort of environmental stress may lead to increased
stem lignification and hardening allowing the plant to carry the grain
load at an erect posture. Future experimentation to test this hypothesis
may include of combinations of agro-technics implementations such as:
seed-coverage to enlarge seeds, using a mixture of Teff lines, reducing
sowing density to reduce plant density, and the introduction of
controlled stresses such as water deficiency and salinity. It is clear
that late maturing, thick stem and tall Teff varieties possess deeper
root systems than early maturing lines of shorter height (Ayele et al.,
2001). Therefore, a combination that include lines that are
characterized by thick stems with stress can improve lodging resistance.
Integration between a wide range of parameters and the correlations
obtained between agronomic and sensory traits may improve our ability to
breed towards a “real world” better end-product.