Genetic diversity for grain nutrients and its stability across years
The genetic diversity for grain protein and mineral nutrient concentrations was studied across two years in a large collection of wild emmer wheat accessions and a set of durum wheat and bread wheat cultivars. Frequency distributions of grain nutrients, with domesticated wheat indicated in grey, are presented in Fig. (for detailed data see Table S1, available online
). The wild emmer accessions exhibited normal distributions for most variables in both years, with the exception of S, Ca and Fe, which did not distribute normally in 2007, and Ca and Mn, which showed nearly significant normal distributions in 2008.
Fig. 1. Frequency distribution of 145 wheat accessions in the 2007 experiment and 91 accessions in the 2008 experiment, for grain concentrations of the following nutrients: (A) protein, (B) zinc, (C) iron, (D) copper, (E) manganese, (F) calcium, (G) magnesium, (more ...)
A comparison between the wild emmer accessions and the domesticated lines by Student's t-test revealed significantly greater values for the wild accessions for all minerals in both years (data not shown), with the exception of Ca in 2008. For example, the average values of GPC were 12·0 and 22·2 % in 2007 and 12·5 and 23·9 % in 2008 for the domesticated and wild genotypes, respectively (Fig. A). The highest values of grain protein, Zn and Fe concentration in the wild accessions were about two-fold greater than the highest values in the domesticated genotypes (Fig. A–C).
Grain protein and macronutrients (Ca, Mg, K, P and S) as well as one micronutrient (Mn) distributed across similar ranges in both years, whereas most micronutrient (Zn, Fe and Cu) concentrations showed different ranges in each year (Fig. ). Regardless of the range of distribution, the 73 wild emmer accessions tested in both experiments exhibited a significantly positive correlation (P ≤ 0·001) between years for each of the grain nutrients tested (Fig. ), with correlation coefficients being 0·62 ≤ r ≤ 0·83, with the exception of Fe (r = 0·47).
Correlations between grain nutrient concentrations in the 2007 vs. the 2008 experiments.
Associations among grain nutrients and productivity traits
As grain nutrient concentrations of the domesticated wheat cultivars were significantly lower than those of the wild emmer accessions, only the latter were used to study the association between grain nutrients. PCA of the 73 wild emmer accessions, tested across the 2 years, extracted two major principal components (Eigenvalues > 1), which collectively accounted for 57·0 and 59·9 % of the variation for 2007 and 2008, respectively (Fig. ). In 2007 (Fig. A), principal component 1 (PC1, x-axis) explained 37·4 % of the dataset variation, and was loaded positively with GPC, Zn, Fe, P, Mg and S; principal component 2 (PC2, y-axis) explained 19·6 % of the variation, and was positively loaded with spike DM and total DM. In 2008 (Fig. B), PC1 explained 37·5 % of the dataset variation, and was loaded positively with GPC, Zn, Fe, P, Mg, S and Mn; PC2 explained 22·4 % of the variation, and was positively loaded with spike DM and total DM and negatively loaded with Mg, K, P and Ca.
Fig. 3. Principal components analysis (based on correlation matrix) of continuous plant traits recorded on 73 wild emmer wheat accessions in (A) 2007 and (B) 2008 field experiments. Biplot vectors are trait factor loadings for principal component (PC) 1 and PC2. (more ...)
A closer look at the PCAs (Fig. ) shows that in both years, there were two major clusters of minerals: one consisting of the micronutrients (Zn, Fe, Cu and Mn) and the other consisting of the macronutrients (Ca, Mg, K, P and S), with the GPC located between the two groups, somewhat towards the micronutrients. This was further supported by significant positive correlations among all the micronutrients (except Cu and Fe in 2007), and among all macronutrients (except K and Ca in 2007), but fewer significant correlations between macro- and micronutrients, usually involving Zn and Fe (Table ). Also in line with the PCA, GPC exhibited significant positive correlations with all micronutrients, but only with three (Mg, P, and S) of the five macronutrients.
Coefficients of correlation (r) between plant productivity (spike DM and total DM per plant) and concentrations of grain protein (GPC) and mineral nutrients in a collection of 73 wild emmer wheat accessions tested across two years
Another interesting aspect is the association between grain nutrients and plant productivity. It is of interest that spike DM and total DM (indicators of the plant's reproductive and overall productivity, respectively) did not show any significant negative correlation with grain nutrient constituents, with only one exception: K vs. total DM in 2008 (Table ). Moreover, plant productivity exhibited positive and significant correlations with grain Zn and Mn (consistent across the two years), as well as with Fe in 2008.
Association between eco-geographical origin and grain nutrients
To investigate the relationships between the origin of the wild emmer accessions and their grain protein and mineral concentrations, a collection of 128 wild emmer wheat accessions, tested in 2007, was divided into two geographical groups as suggested by Luo et al. (2007)
. These included the southern Fertile Crescent (Israel, Jordan, Lebanon and Syria) with 101 accessions and the northern Fertile Crescent (Turkey, Iran and Iraq) with 27 accessions. Analysis of variance revealed significant effects of geographical group and accessions (nested within geographical group) for GPC, Zn and Fe (Table ), as well as for most other mineral nutrients (data not shown). Grain Zn and Fe showed significantly higher values for the southern group, whereas GPC was higher for the northern group (Table ). Nevertheless, for all mineral nutrients, differences between groups were of small magnitude (≤5 %).
Table 2. Analysis of variance for the effect of geographical origin in the Fertile Crescent and accessions (nested within geographical group) on concentrations of grain protein (GPC), zinc (Zn) and iron (Fe) in 128 wild emmer wheat accessions grown in the 2007 (more ...)
Dissecting the overall genetic diversity into its components (between and within populations) is of critical importance when devising an optimal strategy for further exploration of potential genetic resources. The genetic diversity between and within populations was examined using ten and 12 (2007 and 2008, respectively) wild emmer populations from Israel. Analysis of variance revealed highly significant (P ≤ 0·001) effects of populations and accessions (nested within populations) for grain protein, Zn and Fe concentrations (Table ), as well as for most other mineral nutrients (data not shown).
Analysis of variance for the effect of population and accessions (nested within population) on concentrations of grain protein (GPC), zinc (Zn) and iron (Fe) in populations of wild emmer wheat from Israel, grown in the 2007 and 2008 experiments
Soil samples collected from the populations' natural habitats (Table S2
) were analysed to examine the association between their characteristics and mineral nutrient concentrations in grain produced under our experimental conditions. Among all of the soil and grain variables tested, associations between any two pairs of variables were consistent across the two years. Negative associations were found between soil clay percentage and GPC (%), with the 2008 data showing high significance (r
= –0·72, P
< 0·01) and the 2007 data being nearly significant (r
= –062, P
= 0·054) (Fig. A). Significant negative correlations (r
= –0·68, P
= 0·042, and r
= –0·58, P
= 0·045, for 2007 and 2008, respectively) were also found between soil Zn concentration in the habitats and grain Zn concentration, with the exception of a single population in 2007 (Fig. B).
Fig. 4. Correlation between soil properties of wild emmer wheat collection sites (= populations) and grain nutrient compositions. (A) Soil clay percentage vs. grain protein concentration and (B) zinc concentration in soil vs. grain, in the 2007 (excluding Beit-Oren (more ...)