Abiotic constraints, such as drought and heat driven by climate change, negatively impact the production of the common bean (Phaseolus vulgaris L.), an essential grain legume worldwide. The ability to tolerate drought and heat stress in common bean can be improved by introducing genetic variation from related species, such as tepary bean (Phaseolus acutifolius A. Gray), which has recently gained attention because of its adaptation to drought and heat stresses and potential use as a genetic resource and alternative crop. To better understand the phenotypic response of tepary bean to drought and heat stress in multiple environments and trials and to select highly adapted tepary beans, we conducted two field experiments. In Experiment 1, we compared the adaptation to drought stress of tepary bean (n = 10), common bean (n = 10), and Lima bean (Phaseolus lunatus L.; n = 9) by assessing the reduction in grain yield under terminal drought compared to well-irrigated conditions in two California locations with arid summer conditions. Of the three species, tepary bean showed the statistically strongest adaptation to terminal drought, followed by Lima bean and common bean. In Experiment 2, we evaluated a set of 22 tepary beans from contrasting origins for drought and heat stresses across multi-environment trials (METs), in California, Nebraska, and Colombia, with common bean as a control. We found a considerable variation in the tepary bean phenotypic response to these MET conditions, as a result of a strong genotype x environment (G x E) interaction. Also, we identified tepary bean accessions adapted to drought, heat, and well-irrigated conditions across multiple climate zones. Understanding the performance of tepary bean across multiple environments and identifying tepary beans with broad and target-specific adaptation will maximize the potential use of the species.

Experiment 1: Drought evaluation of three domesticated Phaseolus species

To compare the drought stress response among key domesticated Phaseolus species, ten genotypes of each of the common bean and tepary bean and nine genotypes of the Lima bean (Supplemental Table S1) were evaluated under well-irrigated and terminal drought conditions during the summer of 2016 in two locations in California (Table 1). These 29 genotypes are representative of the genetic diversity in these three species as they include representatives of the two domestications in common and Lima beans (e.g., Kwak and Gepts, 2009; Garcia et al., 2021) and their breeding status (landraces vs. improved varieties). Field trials were conducted at the University of California, Agricultural and Natural Resources West Side Research and Extension Center (WSREC) (Bsk climate, i.e. arid, cold steppe type climate, according to the Köppen-Geiger classification: Peel et al. 2007; Rubel & Kottek, 2010; Beck et al., 2018), and the University of California, Davis, Plant Science Field Facility (UC Davis) (Csa climate: temperate with hot, dry summer), which differ in altitude, soil type, and average weather data such as precipitation, temperature, and relative humidity (Table 1). The plant material was obtained from the seed inventory of the Bean Breeding Program, Department of Plant Sciences, University of California, Davis.

Drought/irrigated field trials in both locations were planted side by side in a row-column randomized design with three replications. Individual plots consisted of two 3 m-rows (50 seeds/row), spaced 0.76 m apart. The drought stress was terminal, with irrigation reduced by 50% at WSREC and 75% at UC Davis between flowering and harvesting compared with the well-irrigated trial. Trials were agronomically maintained using standard common bean commercial procedures (Long et al., 2010). To control for spatial variability in each trial, best linear unbiased estimators (BLUEs) and best linear unbiased predictors (BLUPs) of yield (kg/ha) were fitted using the SPATs package (Rodríguez-Álvarez et al., 2018) in the Mr. Bean App (Aparicio et al., 2019) (Figures 3 and 4). Genetic correlations of the locations were obtained using the statistical program EchidnaMMS (Gilmour, 2020). The analysis of variance (ANOVA) was conducted using the lme4 package (Bates et al, 2015) in R (R Core Team, 2023). The model, which was fitted with the lmer function (Bates et al., 2015), included species, genotype, locations, and irrigation/drought treatment as fixed effects.  ANOVA calculations utilized the Kenward and Roger (1997) method for determining the denominator degrees of freedom to enhance the accuracy of F-tests and t-tests for fixed effects.  Mean comparisons were performed with the emmeans function using the Tukey test to compare the estimated marginal means (Bates et al. 2015).

Experiment 2: Evaluation of tepary beans to drought and heat stress throughout multi-environment trials (MET) study
Drought stress trials

To determine the drought tolerance of tepary beans across different environments, we evaluated 22 tepary bean genotypes (Table 2) in three drought-stressed and well-irrigated trials in three locations in 2017 and 2018. These 22 lines were chosen to represent germplasm accessions or landraces (G entries) and improved cultivars (TARS-Tep entries) (Table 2) and were selected from a wide range of annual mean temperature and annual rainfall (Figure 2). For the purpose of comparison, four P. vulgaris breeding lines were also added (DOR, SEN, and SEF lines (Table 2). Field trials were conducted in a humid tropical environment at the International Center for Tropical Agriculture (now called Alliance Bioversity & CIAT), Palmira, Colombia (DrPAL; Af climate); a semi-arid temperate environment at the University of California, Davis, CA (DrUCD); and a semi-arid temperate climate at the University of Nebraska, Panhandle Research and Extension Center, Scottsbluff, NE (DrUNL; Bsk climate) (Table 1). Trials were planted during the summer seasons in the two temperate locations and during the low precipitation season in the tropical location. Environmental conditions such as altitude, soil type, and weather data such as precipitation, temperature, and relative humidity differed across the three locations (Table 1). Drought stress in Palmira was intermittent and moderate, with water supply reduced by 28% compared to the well-irrigated trial. Drought stress in Davis was terminal and severe, with water supply to the crop reduced by ~75% compared to the well-irrigated trial. Drought stress in Scottsbluff was terminal and severe, with water supply reduced by 74% compared to the well-irrigated trial (Table 1). Drought and well-irrigated trials were planted side-by-side in a randomized complete block design with three replicates in Palmira, Colombia (PAL), five in Davis, CA (UC Davis), and two in Scottsbluff, NE. Individual plots in Davis consisted of two rows (50 seeds/row), 3 m-long, and 0.76 m apart; in Palmira, the trials were planted in plots 4-row, 3-m-long plots, spaced 0.6 m apart; Scottsbluff trials were planted in 2-row, 3.65 m-long plots, 0.6 m apart. Trials were agronomically maintained using local, standard common bean commercial procedures.

High heat trials

To evaluate the heat tolerance of tepary beans across different environments, we evaluated the same 22 drought-tolerance-tested tepary bean genotypes (Table 2) under heat stress in three distinct locations in 2017 and 2018 (Table 1). It has been reported that minimum temperatures (i.e., night temperatures) above 20˚C and maximum temperatures (i.e., day temperatures) above 30˚C negatively impact the yield production in the common bean (Porch & Jahn, 2001; Porch, 2006). Thus, locations above the 20/30˚C night/day threshold were selected to conduct the heat stress trials. Field trials were planted in two hot and humid tropical locations in Alvarado (HtALV; Department of Tolima, Colombia; Af climate) and Caribia (HtCAR; Department of Magdalena, Colombia; Af climate), and one desert climate at the University of California, Desert Research and Extension Center, Holtville, CA (HtDREC; Bwh climate) (Table 1). Reduction of water supplied (by rainfall and irrigation, Table 1) provided an additional environment in Holtville that combined heat and drought stress.

High heat trials were planted at low altitudes in tropical locations and during the summer season in the desert. Environmental conditions such as altitude, soil type, and weather data such as precipitation, temperature, and relative humidity differed across the three locations (Table 1). Field trials were planted in a randomized complete block experimental design, with three replicates in all three locations. Plots in Alvarado consisted of two rows (50 seeds/row), 3 m long, and 0.6 m apart; in Caribia, the plots consisted of 4 rows (50 seeds/row), 3 m long, and 0.6 m apart; in Holtville, California, plots included 2 rows (50 seeds/row), 3 m long, and 0.76 m apart. Trials were agronomically maintained using standard common bean commercial procedures.