EVALUATION OF DROUGHT TOLERANCE MECHANISMS IN MESOAMERICAN DRY BEAN GENOTYPES
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Abstract

Drought is the major abiotic constraint to common bean (Phaseolus vulgaris L.) production globally. It results into yield losses of over 60% and less than 900 kg/ha of grain per season. In the face of climate change and variability, droughts have become frequent and severe leading to a reduction in the productivity of land, loss of productive land under dry bean production and threatening malnutrition especially among the poor in developing countries. It is therefore important to curb further decline in dry bean production and enhance food security by developing drought tolerant varieties of dry bean. Of the two major dry bean gene pools, the small seed Mesoamerican beans are considered to be more drought tolerant than the large seeded Andean beans. The objective of this study was to identify physiological and phenological traits associated with enhanced drought tolerance in Mesoamerican dry bean genotypes. On-farm and on-station field experiments were conducted in Mwea and Kabete respectively. The on-farm trial at Mwea site was conducted on a farmer’s field and was used for screening of genotypes whose selection of the best drought tolerant materials was done through a participatory varietal selection involving well qualified dry bean farmers, traders and consumers. The on-station field trial at Kabete Field Station was used to conduct studies on physiological mechanisms of adaptation to drought stress including stomatal control and assimilate partitioning among Mesoamerican dry bean genotypes. Both trials were conducted during the dry periods of the year. Eighty four Mesoamerican dry bean genotypes from  three market classes namely drought tolerant navy beans, drought tolerant small reds, drought tolerant mixed colours and their local and international checks with contrasted drought responses were tested under drought stress and non-stress conditions. Irrigation was withheld at the initiation of flowering to induce the stress treatment. Nondestructive sampling of plants for canopy temperature, leaf chlorophyll and stomatal conductance measurements were conducted at three stages beginning from the time of stress induction, full flowering and mid-pod filling. Also, destructive sampling of plants on a 0.5 m row of every genotype for partitioning of dry matter in the stem, leaves and pods was done at mid pod filling. Data on phenology including days to flowering and days to physiological maturity, yield and yield components, pest and disease incidence, plant vigour and growth habit were also recorded. At physiological maturity, seed yield and shoot biomass were measured. Soil moisture at soil depths of 0 to 80 cm was monitored every week from the time of stress induction to physiological maturity in both treatments in order to indicate the level of stress and need for irrigation on the non stressed plots. Harvest index, Drought intensity index, pod partitioning index, grain filling index, stem reduction biomass among other measurements were calculated for both drought stress and non stress treatments. The results indicated that under stress some genotypes such as DSR11-02, DSR11-21, DMC11-10, DMC11-11, DNB11-03, DNB11-07 as well as checks like SEA 15, KAT B1 among others exhibited a tendency to escape drought effects through accelerated reproductive development. Drought stress reduced grain yield and stomatal conductance for most of the dry bean genotypes. The grain yield ranged between 200 and 800 kg/ha, while the stomatal conductance varied between 55 to 194 mm/m2/s for the drought stress treatment.  Significant differences in dry matter partitioning among genotypes were observed with high yielding drought tolerant genotypes such as DSR11-08, DMC11-10, DNB11-10 and SEA15 having higher harvest indices than the susceptible genotypes like DMC11-14, DMC11-20, DNB11-13 and GLP585. There was significant genotypic variation in stomatal conductance under water stress and non-water stress conditions. It was concluded that the best drought tolerant materials possessed a combination of mechanisms enhancing adaptation to drought stress including stomatal control and remobilization of photosynthates towards the developing grain. There is need for studies on root related drought tolerance mechanisms in Mesoamerican dry bean genotypes.