Evaluation of yield and some morpho-physiological characteristics of pinto bean (Phaseolus vulgaris L.) genotypes under different irrigation regimes

The pulses, including pinto bean have a great contribution to human nutrition by having a significant   protein content. Moreover, drought is one of the most important environmental stresses that negatively affects plant growth and development stages and consequently grain yield. In general, the most important effects of drought stress are the disruption of the photosynthetic apparatus, the reduction of stomatal conductance, premature leaf senescence and ultimately, the reduction of yield and yield components. However, the response of different plant genotypes to drought stress is different. About 60 percent of bean production in developing countries is done under drought stress. Iran is a country with a warm and dry climate that more than 85 percent of it located in arid and semi-arid regions. Therefore, the cultivation of pinto beans in Iran is carried out under drought stress conditions and we need to find a way to increase yield under drought stress conditions. One of the strategies to against drought stress is the introduction of tolerant and compatible plants. Accordingly, it is important to evaluate and select drought tolerant genotypes based on the traits that directly affect crop yield potential. Therefore, this study was conducted to evaluate 18 pinto bean genotypes under different irrigation regims.

This experiment was conducted in 2019 at Bean Research and Training Campus Farm, Khomein, using a 3-replicate split plot RCBD. The irrigation regimes as main plot consisted 50 (I1), 70 (I2) and 110 (I3) of cumulative evaporation using a standard class "A" evaporation pan.18 pinto bean genotypes (KS21336, KS-21359, KS-21331, KS21293, KS-920054, KS-21284, KS-21374, KS-21195, KS-21318, KS-21168, KS-21373, KS-21158, KS-21359, KS -21486, KS-21488, KS-21573, Ghaffar and Sadri) were considered as subplot. The grains of each genotype were planted in six rows with a length of three meters. Irrigation treatments were applied about 30 days after planting. Pest, disease and weed control were performed according to conventional methods during the growing period. Plants were harvested from each experimental unit at physiological maturity stage and leaf area index, leaf relative water content, plant height, grains per plant and in pod, pods number per plant, 100 grains weight, biological yield, grain yield and harvest index traits were recorded. In addition, the drought tolerance index was calculated to identify the tolerant genotypes of pinto beans. Analysis of variance (ANOVA) was performed using the GLM procedure in SAS (version 9.1; Cary, North Carolina, USA). The least significant difference test (LSD) was used to assess the significance of differences in treatment means at the 5 percent probability level.

Irrigation regimes caused a significant reduction in leaf area index, leaf relative water content, pods number per plant, grains number per pod, grains per plant, 100 grain weight, grain yield, biological yield and harvest index at I2 (27, 7, 25, 17, 37, 4, 6, 30 and 11 percent, respectively) and at I3 (41, 10, 45, 34, 60, 14, 23, 30 and 40 percent, respectively). In general, the rate of decreases was greater with increasing water stress intensity. Therefore, it can be concluded that drought stress negatively impacts plant growth and yield, with the severity of the stress playing a significant role. Drought stress can result in reduced photosynthesis, metabolic disturbances, and even plant mortality. Among the genotypes studied, KS-21318 exhibited the highest grain yield, followed by Ghaffar and KS-21158, respectively. Under normal irrigation conditions, grain yield demonstrated a positive and significant correlation with the number of grains per pod. In the I3 irrigation regime, characterized by severe stress, grain yield showed positive and significant correlations with the number of pods per plant, number of grains per pod, and number of seeds per pod. The pinto bean genotypes examined exhibited notable variations in leaf area index, plant height, number of grains per pod, number of pods per plant, grain yield, harvest index (HI), and drought tolerance index under both mild and severe stress conditions. The highest and lowest harvest index in I3 irrigation regime (severe stress) belonged to KS-21318 and Ghaffar genotypes. Among the studied genotypes, the highest and lowest mild drought tolerance index belonged to Ghaffar (1.17) and KS-21486 (0.387) genotype, respectively. As well as, the highest and lowest severe drought stress tolerances belonged to Ghaffar (1.12) and KS-21486 (0.373) genotype, respectively.

Correlation coefficients between traits showed that in order to breed pinto beans in terms of seed yield, the number of pods per plant, the number of seeds per pod and thenumber of seeds per plant should be considered. Among the studied genotypes, according to drought tolerance index in terms of grain yield, Ghaffar genotype was identified as drought tolerant genotype. In general, there was a significant variation among the studied genotypes in response to different levels of irrigation and this can be used to breed and select pinto bean for drought tolerance.