Variation in root traits
No significant mean square differences were observed between the soil water treatment regimes for all the root variables except for HRL (P<0.05) (Table 2). However, there were highly significant (P<0.01) mean square differences among the genotypes and the genotype × water treatment regime interactions for all the root traits measured and seed weight. The effect of soil moisture stress on expression of root traits and seed weight varied and that was probably due to genetic effects which is an indication that selection can be done for genotypes with tolerance to drought conditions utilising root traits. The genotype × water treatment regime interactions observed suggest that the phenotypic root and seed weight characters of the genotypes across the two main water treatment regimes were adaptable to specific environment. The findings of this study agree with those of
Asfaw and Blair (2014) and
Rezene et al., (2011) who reported variations in common bean roots response to low and optimum soil moisture conditions.
The HRN ranged from 3.5 to 27 in the optimum water treatment and from 7.5 to 48.5 under water stress treatment (Table 3). The effect of water stress increased HRN by 127%. Genotypes BFS-29 and DOR-364 had the lowest HRN, while SER-124 had the highest HRN after exposure to low soil moisture. While the effect of low soil moisture increased HRN by 127%, the mean HRL was slightly reduced by only 7%. The HRL ranged from 2.1 to 27.3 cm with mean of 8.2 cm under optimum water treatment and from 1.7 to 25 cm with mean of 7.6 cm under water stress treatment. The increase in HRL and HRN observed under low soil moisture contributed to increased root surface area. This is crucial in enabling tolerant genotypes explore and access soil moisture resource under limited conditions. For example, genotype SER-124 had the highest HRN and longest HRL after exposure to low soil moisture and this genotype is known to be drought tolerant. Contrary to the findings of this study,
Amane et al., (2016) reported more hypocotyl roots of the bean genotypes after exposure to unlimited soil moisture conditions.
Low soil moisture effect reduced BRWN by 1.9% (Table 3). Under low soil moisture, genotypes CAL-143 and BC-265 had one basal root whorl, while genotypes that had the highest number of basal root whorls include Tepary-22, CL-43, the checks Kambidzi-A286 and Kabalabala-UBR (92) 25-LF. Exposure to low soil moisture increased the BRN and BRGA (Table 3). Genotypes SER-124 had the lowest number of basal roots (3.5), while Kabalabala-UBR (92) 25-LF (check) had four times more basal roots than genotype SER-124 under low soil moisture conditions. Genotypes Qwimbaya followed by Inter-lab-31 and SER-118 were among the genotypes with the deepest basal root growth angle (> 45°) after exposure to low soil moisture conditions. The study revealed genotypes with deeper basal root angles from the horizontal axis, longer roots and high number of root whorls under low soil moisture. Genotypes with such root systems are expected to explore and access soil moisture from the deep soil profile, relatively grow well and yield high under limited soil moisture conditions (
Lynch, 2014). Similarly,
Amane et al., (2016) reported an increase in basal root growth angles after exposure of the genotypes to low soil moisture conditions. The findings agree with
Miguel et al., (2013) that drought tolerant landraces or adapted cultivars tend to have basal root whorl number reaching four and sometimes more. Similarly,
Comas et al. (2013) screened and identified common bean genotypes tolerant to drought which had 16 basal roots from four distinct root whorls.
Comas et al., (2013) further reported that drought tolerant genotypes preferentially increase elongation of roots and suppress basal roots branching in response to drought conditions.
Broad sense heritability ranged from 0.02 for BRN to 0.99 for HRL under low soil moisture (Table 3). The genetic advance (%) ranged from 10.37 for BRN to 106.84 for HRL under limited soil moisture conditions. The variables that exhibited high broad sense heritability and genetic advance can be used in the selection of genotypes with tolerance to drought conditions. Variables with high broad sense heritability and genetic advance are expected to be controlled by additive genes and are less influenced by the environment
(Panes and Sukhatme, 1995).
Variability in seeds weight and indices for tolerance to low soil moisture
100 seeds mean weight (SdWt) ranged from 15.8 to 55 grams under low soil moisture (Table 4). The effect of low soil moisture reduced SdWt by 9.4%. Under low soil moisture, the genotype Bonus had the lowest SdWt, while Tio-Canela had the highest SdWt. Genotypes of Andean gene pool, including SBA-686 and EAL-14, performed well in terms of SdWt contrary to what was reported by
Hayes and Singh (2007) that Andean gene pool is not a good source of drought tolerance compared to the Meso-American gene pool.
Under low soil moisture, twenty-five genotypes outperformed the checks having lower values for percent reduction (%) in SdWt and seeds weight stability index (SSI) values equal or greater than 0.90 (Table 4). Genotypes CER-78, SAB-560 and SER-125 of Meso-American origin had the lowest SdWt reduction and SSI values equal or greater than 1.0. The three genotypes, CER-78, SAB-560 and SER-125, are considered tolerant to low soil moisture conditions. These genotypes (CER-78, SAB-560 and SER-125) had high SdWt and the tolerance could also be attributed to the highest number of hypocotyl roots for CER-78 and SAB-560, the longest hypocotyl roots for CER-78 and SER-125 and a deeper Basal Root Growth Angle for CER-78. Genotypes CER-78 and SER-125, outperformed all other genotypes with regard to percent reduction in SdWt and SSI (Table 4) and therefore, considered as the most drought tolerant genotypes. Similarly,
Golabadi et al., (2006) identified genotypes with tolerance to soil moisture stress in wheat based on tolerance indices.