Pod harvest index as a selection criterion to improve drought resistance in white pea bean

Highlights

• Eighty-one genotypes of common bean were evaluated for their adaptation to drought stress.

• Seed yield, seed and pod number per area and seed weight were reduced by drought stress.

• Six genotypes had better drought yield compared with a standard check.

• Pod harvest index was reduced in sensitive genotypes under drought stress.

• Pod harvest index could be an effective selection criterion for evaluating drought resistance.

Abstract

Common bean (Phaseolus vulgaris L.) is an important food legume grown in Africa and Latin America, where water deficits frequently reduce grain yield. The objectives of this study were to identify advanced lines of common bean with superior seed yield under drought, and to identify plant traits that could serve as selection criteria for evaluating drought resistance. Seventy-eight advanced inbred lines (genotypes) were generated by single seed descent and evaluated with the two parents (ICA Bunsi, a white pea bean variety and SXB 405, a breeding line) and a standard check (Awash melka) under drought and irrigated field conditions in 2008 and 2009 at Melkassa, Ethiopia. Seed yield, seed number per m², pod number per m² and 100 seed weight were reduced by 65%, 34%, 29% and 12%, respectively under drought stress compared to irrigated conditions. Two genotypes (G87, G80) had better drought yield compared with a standard check, and several also responded to irrigation. Pod harvest index (PHI; [dry weight of seed/dry weight of pod at harvest] × 100) was reduced in sensitive genotypes and increased in resistant genotypes under drought stress conditions indicating the importance of remobilization of photosynthates from pod wall to seed. Principal component analysis indicated that the first component including five traits (grain yield, seed number per m², pod number per m², 100 seed weight and PHI) explained 35.7% of the total variation under drought stress. Correlations of PHI with yield (0.43***) and fair heritability (0.48) in drought suggest that PHI would be an effective selection criterion for identifying genotypes with improved drought resistance. Correlated gain in drought yield from selection for PHI would be greater than direct yield selection, due to much better heritability of PHI, and would also contribute to irrigated yield.

Introduction

The global average grain yield of common bean is less than 900 kg ha⁻¹ (Beebe et al., 2010). As much as 60% of the common bean production area in the developing countries is estimated to suffer from moderate to severe drought, making drought stress the second most important yield constraint next to diseases (Rao, 2001). In some semi-arid areas of Eastern and Southern Africa, drought is the major problem reducing grain yield by 50% or more (Wortmann et al., 1998). Intermittent and terminal droughts are the major types of drought stress and are endemic to many regions in Africa and Latin America where beans are major crops for local consumption. The level of yield reduction is determined by the type, intensity and duration of drought stress (Thung and Rao, 1999, Beebe et al., 2010).

Improving yield under drought is a major goal of plant breeding (Cattivelli et al., 2008). While natural selection under drought stress conditions has favored mechanisms for adaptation and survival, plant breeding efforts have directed selection toward increasing the economic yield of field crops (Blum, 2010). For grain legumes such as common bean, a genotype that produces higher grain yield per unit area under drought stress is considered to be drought resistant. Early work on drought stress in common bean mainly focused on grain yield to identify drought resistant genotypes, using both drought and non-stress conditions (White and Singh, 1991, Thung and Rao, 1999, Terán and Singh, 2002a, Beebe et al., 2010). Seed yield differences among genotypes under drought stress conditions have been reported for common bean (White et al., 1994, Terán and Singh, 2002a, Beebe et al., 2008). Drought stress significantly reduces the individual yield components (number of pods per plant, number of seeds per plant, number of seeds per pod and 100 seed weight) (White and Izquierdo, 1991, Acosta-Gallegos and Kohashi-Shibata, 1989, Terán and Singh, 2002b, Beebe et al., 2008, Beebe et al., 2010).

Progress in breeding using physiological traits or mechanisms for improving drought resistance has been slow for most field crops (Rao, 2001, Blum, 2005). In common beans, different adaptive morphological and physiological traits and mechanisms may be associated with growth, biomass partitioning and yield under different drought stress conditions (Beebe et al., 2010). A better understanding of these traits and mechanisms may contribute to development of rapid and reliable selection criteria to identify drought resistant genotypes. Mechanisms of drought resistance include both avoidance and tolerance (Blum, 2005). Deep rooting ability was identified as a key mechanism for improving drought resistance in a number of field crops including common beans (Sponchiado et al., 1989, Rao, 2001, Beebe et al., 2010). Canopy biomass (CB) accumulation and leaf area index (LAI) were also considered as plant traits that are useful to identify drought resistant genotypes (White et al., 1994, Ramirez-Vallejo and Kelly, 1998, Rosales-Serna et al., 2004, Rao et al., 2009). Positive associations have been confirmed between CB and seed yield under drought conditions (White et al., 1994, Ramirez-Vallejo and Kelly, 1998, Polania et al., 2008, Rao et al., 2009, Blum, 2010). Recently, biomass partitioning from vegetative structures to pod and from pod wall to seed was found to vary among genotypes and to be a useful approach to identify drought resistant genotypes (Rao et al., 2006, Rao et al., 2007, Rao et al., 2009, Polania et al., 2008, Beebe et al., 2009, Beebe et al., 2010). Pod harvest index (PHI) is considered as one of the key partitioning indices that indicate the extent of remobilization of photosynthates to seed. Strong positive associations have been reported between PHI and grain yield under drought stress and non-stress conditions (Rao et al., 2007, Rao et al., 2009, Polania et al., 2008, Beebe et al., 2009).

Although there is significant progress in improving drought resistance of different commercial classes of small seeded Mesoamerican beans (reds, blacks, carioca), the progress with small white pea beans is limited (Assefa et al., 2006). Like other bean market classes, the white pea bean is also produced in drought affected parts of Africa, mainly for industrial canning. The objectives of this study were to: (i) identify the most promising advanced lines (genotypes) of small white seeded common bean with superior seed yield under drought; and (ii) identify plant traits that could serve as selection criteria for evaluating drought resistance.