Evaluation of morphological aspects of common bean ( Phaseolus vulgaris L . ) genotypes for post-flowering drought resistance in Rift Valley of Ethiopia

The development of bean varieties adapted to drought situations is a key strategy to minimize crop failure and improve food security. In this study, 25 genotypes of common beans (Phaseolus vulgaris L.) were grown under post flowering drought stress and non-stress conditions to evaluate their performance at Melkassa Agricultural Research Center during the off-season months (from December to May) in 2011/2012. The treatments were laid out on a triple lattice design with three replications. A number of plant attributes were measured at mid-pod fill and harvesting stages. Under drought stress, the highest seed yield (125.3 gm -2 ) was recorded for a Dimtu variety, while the lowest (72.5 gm -2 ) for SB15945-17. Therefore, Dimtu was the most drought tolerant genotype under drought stress. On the contrary, SB-15945-17 had the lowest seed yield under drought stress and drought-induced seed yield reduction of 50.8%. Seed yield showed significant and positive correlation with number of pods per plant (r =0. 39), number of seeds per pod (r =0. 32) and hundred seed weight (r =0. 41) under drought stress. The study demonstrated the existence of genetic variability among the common bean genotypes when subjected to post-flowering drought stress and such variability could be utilized in the development of common bean genotypes suitable for drought prone-areas.


INTRODUCTION
Drought is the most important limiting factor for crop production and it is becoming an increasingly severe problem in many regions of the world.In addition to the complexity of drought itself (Passioura, 2007), plant responses to drought are complex and different mechanisms are adopted by plants when they encounter drought (Hinkossa et al., 2013).Drought can be defined as a state where a dry soil (due to lack of rain or delayed irrigation) causes a substantial reduction in crop performance in terms of plant survival, economic yield or crop quality (Muñoz-Perea et al., 2007).Common bean (Phaseolus vulgaris L.) performance is severely constrained by periodic water deficits in most production areas (Beebe et al., 2013;Hinkossa et al., 2013;Yaqoob et al., 2013).Frequency of occurrence of water deficits, severity of stress, timing of stress relative to plant age, E-mail: alemnehmideksa@gmail.com.
Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License and sensitivity of the plant at different stages of growth interact to determine yield loss associated with water deficits (Beebe et al., 2013;Hinkossa et al., 2013).
Post-flowering drought responses have been identified in common bean (Hinkossa et al., 2013).Post-flowering response is observed when water limitation occurs during the grain-filling stage (Subbarao et al., 1995).The most sensitive stages to grain filling in common bean were at flowering and ten days prior to flowering (Hinkossa et al., 2013;Yaqoob et al., 2013).It was noted that postflowering heat stress caused yield losses up to 50% due to reduced seed filling duration (Bernier et al., 2007).
Common bean is probably native to the tropical parts of South America (Onwueme and Sinha, 1991), from where it was perhaps introduced to Africa and other continents (Baudoin et al., 2001;Beebe et al., 2013).After its dissemination, it is grown extensively in five major continental areas: Africa, North and Central America, South America, Eastern Asia and Western and Southeastern Europe (Adams et al., 1985;Beebe et al., 2013).Moreover, common bean is a non-centric crop, with multiple domestication sites throughout the distribution range.Even though the exact time of its introduction is controversial, it is generally believed that common bean was probably brought to Ethiopia in the 16 th century (Gepts, 1990).
In common bean, four different drought scenarios have been identified in the growing region (Subbarao et al., 1995;Tilahun et al., 2004).The first scenario represents terminal drought where there could be enough moisture for early establishment and growth, but later phonological stages are exposed to moisture deficit (Beebe et al., 2013).The second scenario represents intermittent drought where dry spells happen any time during the growing period and, the third scenario represents predictable drought where common bean plants could be exposed to stress at an early stage of growth but could receive enough water at later stages.The last scenario represents dry, semi-arid climate where the amount of rainfall is relatively low to cover the physiological demand of the crop at any stage of growth (Bernier et al., 2007;Beebe et al., 2013).
Even though the previous research efforts made by the national program in Ethiopia and by other international research institutions have resulted in a release of a number of improved common bean varieties with resistance to drought, the evaluation of the morphophysiological attributes to drought tolerance in these varieties still remain the subject of investigation.Information on genetic progress achieved over time from breeding efforts for drought tolerance attributes since the early inception of common bean breeding and the morpho-physiological factors of genetic improvement achieved so far from the same efforts have also not been systematically studied.The study was to evaluate genotypic differences in growth, physiological response and yield of the bean genotypes when subjected to post-Mideksa 3021 flowering drought stress.

Description of the study area
The

Parameters measured
Five plants were randomly sampled from the useful area of each split plot, and the following parameters were assessed: (1) Above ground biomass weight: was determined by adding up various plant part (2) Leaf area (cm 2 ): was estimated by measuring the maximum length (ML) and width (MW) of leaves and multiplying these by a correction factor of 0.6 derived from the actual leaf area determined by leaf area meter (Setegn, 2006).
(3) Days to 50% flowering (DFF): number of days was taken by each genotype from the day of planting for the day on which 50% of the plants in a plot opened at least one flower per plant.(10) Seed yield (gm -2 ): Seed yield was determined as: Seed yield (gm -2 ) = (Seed weight/plot area) (11) Harvest Index (%): the ratio of seed yield to the above ground dry weight (stem + leaves + pods + seed) at harvest.( 12) Drought susceptibility index (DSI) for seed yield: DSI = (1-Yds/Yns)/DII, where Yds and Yns mean yields of a given genotype in drought stress and non-stress, respectively (Fisher and Maurer, 1978).( 13) Geometric mean (GM): the GM was determined for seed yield as GM = (ns x ds) 1/2 where ns and ds are mean of a given genotype in drought stress and non-stress, respectively.

Statistical analysis
The analysis of variance was computed for all parameters considered using SAS (v 9.1.3)GLM procedure (SAS Institute, 2004) software to demonstrate the existence of differences among the genotypes under the two growth conditions.Means of the parameters that exhibited significant differences were separated using Duncan's Multiple Range Test (DMRT).The nature and magnitude of associations among the quantitative traits were analyzed using simple correlation test at 5 and 1% probability.

Effect of drought stress on biomass production
The effect of drought stress was highly significant for above ground fresh and dry biomass weights at mid podfill and harvesting stages (Table 1).The water regime x genotypes interactions term was also significant for both fresh and dry weights at both mid-pod fill and harvesting stages.Relative to non-stress, drought stress caused significant reductions in above ground biomass weight in the range of 12.0 (Gofta) to 41.4% (IBADO) for fresh weight, 7.3 (SB-15945-19) to 58.6% (IBADO) for dry weight at mid pod-fill stage.Higher biomass accumulation in legumes is positively correlated with higher seed yield, while negatively with drought tolerance since genotypes that are water saving are commonly lowyielding.This implies that above ground biomass determines sink establishment and economic yield.These results are in agreement with those obtained by Setegn (2006); Subbarao et al. (1995); Tilahun et al. (2004) and Hinkossa et al. (2013).In addition, common bean genotypes under drought stress responded to drought by leaf movement, leaf shedding, reducing leaf area and inhibition of the expansion of younger leaves (Acosta-Gallegos and Adams, 1991).

Effect of drought stress on growth
Effect of drought stress on leaf area and days to 90% maturity were highly significant, whereas differences among genotypes were significant for all the four traits (Table 2).Drought induced reduction in leaf area and days to 90% maturity ranged from 7.9% (IBADO) to 54.7% (Atndaba), and 3.5% (Mexican-142) to 9.4% (Dinknesh), respectively.The highest leaf area (790.3 cm 2 ) was recorded for Cranscope, while the lowest (350.0 cm 2 ) for SB-15945-17 under drought stress.The highest number of days to maturity was recorded by Zebra and the lowest by Argene under both growth conditions.Adams et al. (1985) and Hinkossa et al. (2013) similarly reported that under drought stress common bean genotypes respond to drought by leaf movement, leaf shedding, reducing leaf area and inhibition of the expansion of younger leave.

Effect of drought stress on seed yield and yield components
Although genotypic differences were significant only for harvest index, effect of drought stress was significant on seed yield, as well as harvest index (Table 3).Seed yield of all twenty five genotypes under drought stress were significantly lower than their corresponding non-stress growth conditions.Drought stress caused a significant reduction in seed yield that ranged from 22.6 (Dimtu) to 56.3% (Gofta) (Table 4).Under drought stress, the highest seed yield(125.3g m -2 ) was recorded for Dimtu, while the lowest (72.5 g m -2 ) for SB-15945-17.Drought events during the seed filling stage can cause major reduction in yield by reducing starch accumulation as a result of limited assimilate partitioning to the developing grain (Hinkossa et al., 2013).Gofta had the highest (1.13) DSI for seed yield, whereas Dimtu had smallest (0.45) DSI for seed yield.Geometric mean (GM) was the highest for Dinknesh (147.5)followed by Dimtu (142.4) and the least was for SB-15945-17 (103.4) (Table 4).These results were similar with those reported by Setegn (2006), Tilahun et al. (2004), Hinkossa et al. (2013) and Yaqoob et al. (2013).
Differences between the watering regimes, among the genotypes and the water regime x genotypes interactions were significant for all seed yield components (Table 5).Drought stress induced reduction in number of pods per plant ranged from 2.4% (Cranscope) to 41.8% (Beshbesh) and number of seeds per pod within a range of 6.0 (Melka dima) to 40.6% (Mexican-142) (Table 6).Drought stress also caused a reduction in hundred seed weight that ranged from 3.3 % (Nazareth-2) to 36.5% (Argene) (Table 6).The amount of yield reduction depends not only on the timing of stress, but also on the severity of the stress (Passioura, 2007).Number of pods per plant is the most variable trait to affect yield in common beans.The studies shown that terminal drought could reduce pod formation, seed setting and seed filling by affecting the source-sink relationships.Tilahun et al. (2004) observed that pods per plant, seeds per pod and hundred seed weight are crucial for producing economic yield, and vary in time scale.Similar results were reported by Hinkossa et al. (2013) and Yaqoob et al. (2013).

Correlation coefficient analysis
The correlation coefficient among most of the quantitative characters was highly significant under both growth conditions (Table 7).Seed yield was significantly and positively correlated number of pods per plant (r = 0.39), number of seeds per pod (r = 0.32) and hundred seed  weight (r = 0.41) under stressed condition, while seed yield was positively correlated with days to 90% maturity (r = 0.19), under non-stressed condition (Table 7).Similarly, Szilagyi (2003) reported that correlation coefficients between the non-stress and drought stress were positive and highly significant for seed yield, number of pods per plant, 100-seed weight, and days to maturity.This indicated that number of pods per plant, number of seeds per pod, hundred seed weight and days to maturity have positive effect on seed yield.

Conclusion
Accurate identification and detection of drought tolerance common beans genotype is the cornerstone for droughtprone areas.The study demonstrated that Dimtu was the most drought resistant genotype, produced the highest seed yield under drought stress.In contrast, SB-15945-17 had the lowest seed yield under drought stress, and this genotype can be considered the most droughtsusceptible of all the genotypes.Although evaluation of drought resistance in common bean using the qualitative and quantitative traits remains an effective method, morphological comparisons have some limitations, including the influence of environment, subjectivity in the character evaluation and management practice.Therefore, complementary evaluation using appropriate molecular markers such as Amplified Fragment Length Polymorphism (AFLP) and Simple Sequence Repeat Means within the same column followed by similar letters are not significantly different according to DMRT at 5% level of probability, DSI = drought susceptibility index, GM =geometric mean.(SSR) markers is needed to identify better varieties suitable for drought-prone areas.Understanding the genetics of drought tolerance, and having DNA markers linked to drought tolerance genes will help plant breeders to combine drought tolerance with other traits desired by farmers.Means within the same column followed by similar letters are not significantly different according to DMRT at 5% level of probability.
Table 7. Correlation coefficient among eight quantitative traits of common bean genotypes grown under non-stress (above the diagonal) and drought stress (below the diagonal) conditions at Melkassa Agricultural Research Center.

( 4 )
Days to 90% maturity (DM): Determined as the number of days from date of planting to the date when 90% of the plants in each plot attained physiological maturity.(5) Pod Harvest Index (PHI %) = [pod weight] / [leaf weight + stem weight +pod weight] x 100 (6) NPPP: Number of pods per plant (7) NSPP: Number of seeds per pod (8) 100-seed weight (HSW) (g): weight of hundred randomly sampled seeds from all plants harvested per plot.

Table 1 .
Analysis of variance of above ground dry and fresh weights of common bean genotypes grown under two water regimes and harvested at two developmental stages.

Table 2 .
Analysis of variance for growth and phenological parameters of 25 common bean genotypes grown under two watering regimes.

Table 3 .
Mean square of yield and harvest index of 25 common bean genotypes grown under two water regimes harvested at maturity stage.

Table 4 .
Effect of drought stress on seed yield and harvest index and seed yield based DSI and GM in 25 common bean genotypes grown at Melkassa.

Table 5 .
Mean squares of seed yield components of 25 common bean genotypes grown under two water regimes at Melkassa.

Table 6 .
Effect of drought stress on seed yield components of common bean genotypes grown at Melkassa.