Comparison of Selection Indices for Tolerance to Water Stress in Common Beans(Phaseolus Vulgaris L) for the Dry Environment of Tanzania

Abstract

Beans are an important crop in Tanzania grown by a wide range of farming communities. Its production, however, is very low due to various factors of which drought is becoming very important due to its frequency in the recent past. Provision of bean varieties that would withstand drought is important to contribute to food security in the farming communities. This study was conducted with the overall aim of selecting a simple selection index for drought tolerance. Performance of the bean genotypes under water stress conditions was assessed and drought tolerant varieties were identified. Fourteen genotypes, MN 14059-4-4P, RWR 109, MR 14000-2-IP, MR 14144, MMS 243, MN 14059-4-4P, MRl4000-2-1 OP, MR 14140-45-4P, MR 14215-9-8P, MR13944-14-9P, MR 14153-3-2P, MR 14198-13-lP, CNF5547 (Control), MR 13095-6-IP and DOR 390 were planted in a Split-Plot design with water stress period as main plot and bean genotypes as subplots at Sokoine University of Agriculture horticulture unit in Tanzania. Grain yield, seeds/pod; pods/plant, 100-seed weight, root length, root weight, days to physiological maturity, days to 50% flowering and dry matter content were determined. Drought Susceptibility Index (DSI), Sensitivity Index (SI) and Drought Intensity Index (DII) were derived from the grain yield data under the three water stress treatments. The water stress imposed at different periods of plant growth and development significantly influenced the phenotypic expression of the bean line in morphological traits and grain yield and its components. Days to 50% flowering dropped from 43 in the no water stress treatment to 41 days when water stress was imposed from flowering to mid-pod filling. The early maturing lines however showed less sensitivity to water stress with regards to days to 50% flowering. Root weight and dry matter weight were reduced under water stress while root elongation was enhanced from 27 cm to 29 cm when stress was early. Water stress imposed during any period of growth and development negatively affected grain yield. In this study yields dropped from 1188 kg/ha (stress O) to 720 kg/ha (stress I) and 432 kg/ha (stress II). Yields of early maturing genotypes were relatively less affected by water stress at any of the periods due to the earlier flowering, thereby escaped the full effects of the stress. Medium maturing lines, gave the lowest yield reduction from stress 0 to stress I and this was associated with relatively high retention of root weight, dry matter accumulation and enhanced root elongation under water stress. DSI was found to be the most reliable index to identify the drought tolerant genotypes because it identified specific drought tolerant genotypes while DII and SI related intensity of drought at a location and grouping of drought tolerant genotypes, respectively. In this regards the current study recommends DSI to be used further in breeding programme. Based on DSI MR 14154-4-6P, MR 14153-4-2P, MR 14000-2-1 OP and MR 14144-11-5 were the most tolerant genotype.