Climate change and limited water are significant challenges to the future of peanut production. In the southwestern U.S. peanut production area, an estimated 35% of the southern High Plains will not be able to support irrigated agriculture within 30 years at current depletion rates of the Ogallala Aquifer (Scanlon et al., 2012). In addition, global warming is expected to add additional demands on the aquifer (Rosenberg et al., 1999). Recurring and severe droughts have also occurred in the southeastern U.S. (Branch and Kvien, 1992; Strzepek et al., 2010), which produced 65% of U.S. peanuts from 1980-2019 (USDA-ERS, 2021). The importance of drought-tolerant peanuts also extends beyond producing profitable yields under limited water. Higher levels of aflatoxin contamination are observed under drought conditions (Sanders et al., 1985; Holbrook et al., 2000, 2009; Arunyanark et al., 2009, 2010; Girdthai et al., 2010). Despite this, few drought-tolerant peanut cultivars are available.

In peanut, up to 153 marker-trait associations and quantitative trait loci associated with drought-tolerance have been identified, but many of them have minor effects (Ravi et al., 2011; Gautami et al., 2012; Pandey et al., 2014, 2021). Multiple physiological processes mediate drought tolerance, making breeding and phenotyping for this trait especially difficult (Cattivelli et al., 2008; Ravi et al., 2011). Yield and seed quality traits, such as shelling percentage and 100-seed weight, are frequently used to assess drought resistance (Nageswara Rao et al., 1989; Branch and Kvien, 1992; Faye et al., 2015; Pandey et al., 2021). However, yield data are difficult to procure for large mapping populations (Nigam et al., 2005) and may be less reliable as an indicator for multiple locations due to significant genotype x environment interactions (Arunyanark et al., 2008). As a result, surrogate physiological traits associated with drought tolerance are now used to screen larger numbers of genotypes (Pandey et al., 2014, 2021).

Soil plant analysis development measurement (SPAD) or SPAD chlorophyll meter readings (SCMR) are one of the most widely measured physiological traits, perhaps due to the ease of data collection (Upadhyaya, 2005; Rowland and Lamb, 2005; Lal et al., 2006; Arunyanark et al., 2008; Songsri et al., 2008; Balota et al., 2012; Singh et al., 2014; Luis et al., 2016). Several studies with peanut have demonstrated SCMR as a relatively stable trait that correlates well with transpiration efficiency, a trait generally regarded as a reliable indicator of drought tolerance, but one that is difficult to measure on a large scale (Nageswara Rao et al., 2001; Upadhyaya, 2005; Sheshshayee et al., 2006; Arunyanark et al., 2008). Canopy temperature, canopy temperature depression (CTD), and normalized difference vegetation index (NDVI) have also been used for phenotyping drought tolerance (Rucker et al., 1995; Holbrook et al., 2000; Jongrungklang et al., 2008; Nautiyal et al., 2008; Balota et al., 2012; Singh et al., 2014; Luis et al., 2016). Visual indicators of drought stress, such as wilting and leaf color were significantly correlated with aflatoxin contamination (Holbrook et al., 2000; Luis et al., 2016) and are also promising as surrogate traits. Lastly, another visible trait, leaf folding or paraheliotropism, is an adaptation present in some peanut genotypes to reduce solar radiation under drought stress (Matthews et al., 1988; Chapman et al., 1993).

To be useful for peanut breeding, genotypes at the phenotypic extremes of these surrogate traits need to be identified (Shekoofa et al., 2013). Preliminary work identified candidate accessions with divergent phenotypes from the U.S. mini-core. The goal of this project was to evaluate the accessions in three peanut production environments, spanning subhumid southeastern Virginia to semiarid southwestern Oklahoma and the southern Texas High Plains.

A total of 21 core collection (CC) accessions from the U.S. mini-core collection, in addition to seven reference genotypes, were evaluated in 2018 and 2019 in Oklahoma, Texas, and Virginia (Table 1). The 21 accessions were selected from a 2017 study of the broader mini-core because they exhibited divergent phenotypes for traits associated with drought tolerance (data unpublished). The seven reference genotypes included the following: runner breeding line C76-16 (Holbrook et al., 2013) and cv. Tamrun OL11 (Baring et al., 2013); Spanish cv. Tamspan 90 (Smith et al., 1991); Valencia cvs. New Mexico Valencia C (Hsi, 1980) and TamVal OL14 (Burow et al., 2019); and Virginia cvs. Walton (Balota et al., 2021; tested as 08X09-3-14-1) and Wynne (Plant Variety Protection 201500288). C76-16 (Holbrook et al., 2013; Luis et al., 2016; Bhogireddy et al., 2020; Wang et al., 2021) and Walton (Balota et al., 2021) have demonstrated drought tolerance, and New Mexico Valencia C (Bhogireddy et al., 2020) was recently shown to have poor drought tolerance.

In Oklahoma, the experiments were conducted at Oklahoma State University’s Caddo Research Station in Fort Cobb. In 2018, entries were planted in a randomized complete block design (RCBD) with four replications/blocks on 18 May. Half of the field was harvested at 134 days after planting (DAP) on 28 September; the remainder was harvested two weeks later on 12 October. In 2019, entries were planted on 17 May using a RCBD, however, three blocks were in a water-restricted field, and two blocks were in a field with normal irrigation. The irrigated field was harvested 141 DAP, but due to logistical and weather delays, the water-restricted field was harvested 172 DAP. In Texas, the experiments were conducted at the USDA-ARS Cropping Systems Research Laboratory in Lubbock. Entries were planted using a RCBD with four blocks in 2018 and 2019 under water-deficit conditions. Planting and harvest dates were 17 May and 8 November (176 DAP) in 2018 and 14 May and 17/18 October (157-158 DAP) in 2019, respectively. In Virginia, the experiment was conducted at Virginia Tech’s Tidewater Agricultural Research and Extension Center (TAREC) in Tidewater. Three blocks arranged in a RCBD were each planted in an irrigated field and a water-restricted field. Rain-out shelters covered the water-restricted fields from 16 July 2018 and 15 July 2019 for 6 weeks and were removed on 30 August 2018 and 27 August 2019. In 2018, the irrigated plots were planted on 17 May and harvested on 9 September (116 DAP), and the water-restricted plots were harvested at 125 DAP. Planting and harvest dates in 2019 were 30 April and 10 September (134 DAP). Plots in all states were managed for weeds and diseases following their respective University Extension recommendations. In all states, water-restricted plots were irrigated to 25% evapotranspiration (ET) replacement from approximately 40 to 105 DAP (weather permitting in Oklahoma and Texas). This growth period includes the pod-filling stage (Boote, 1982), a time that is especially susceptible to water stress (Nageswara Rao et al., 1985, 1989).

Ten traits associated with drought resistance were measured, but the specific traits measured varied among states depending on equipment availability and labor (Table 2). SCMR readings were taken by recording the mean from four representative plants in each plot. The 4th fully expanded leaf from the top of the mainstem of each of four plants was measured in Oklahoma and Virginia; the 4th fully expanded leaf from lateral branches were measured in Texas. SCMR readings were taken with a SPAD 502 Plus (Spectrum Technologies, Aurora, IL) in Oklahoma, and a SPAD-502 (Minolta, Tokyo, Japan) in Texas and Virginia. In Oklahoma and Texas, fresh, non-wilted flowers were counted in the morning within a representative 1.86-m section inside each plot (Oklahoma) or within the entire plot (Texas). An Agri-Therm II (model 100L, Everest Interscience, Chino Hills, CA) was used to measure afternoon (13:00 to 16:00) canopy temperature and canopy temperature depression (CTD). Normalized difference vegetation index (NDVI) was measured in Texas and Virginia with the GreenSeeker Handheld Crop Sensor (Trimble Agriculture, Sunnyvale, CA). In Oklahoma and Texas, paraheliotropism was rated in the afternoon using the following scale: 0, no folding; 1, 10% leaves folded; 2, 30% leaves folded; 3, 50% leaves folded; 4, 70% folded; 5, 90% folded; and 6, 90% folded but with leaves closed tightly and puckered around leaf margins. Plant wilting was also rated in the afternoon with the following scale in Oklahoma and Texas: 1, no wilting; 2, 25% of plants wilted; 3, 50% wilted; 4, 75% wilted; and 5, 100% leaves wilted and ca. 50% leaves brown for >90% plants within plot. In Virginia, a wilting scale from 0-5 was used (Sarkar et al., 2021). After harvest, pods were dried in a forced-air dryer to approximately 7% moisture by weight and separated from soil and plant debris with a peanut cleaner. One 200-g sample of pods from each plot was used to determine the percentage of total sound mature kernels (TSMK) following USDA-AMS (2019) guidelines. To simplify grade collection from diverse genotypes, TSMK for all entries was determined using the smallest screen for Spanish/Valencia kernels (5.94-mm x 19.05-mm (15/64 x 3/4 inch) slots) in Oklahoma and Virginia.

Temperature and rainfall were monitored using the Fort Cobb Oklahoma Mesonet station at the Caddo Research Station (McPherson et al., 2007), the Plant Stress & Water Conservation Meteorological Tower at ARS-Lubbock (https://www.ars.usda.gov/plains-area/lubbock-tx/cropping-systems-research-laboratory/wind-erosion-and-water-conservation-research/docs/pswc-met-tower-data/), and the WeatherStem system at TAREC (http://vt-arec.weatherstem.com/arec115). Degree day heat units were calculated using the West Texas method 1 formula for peanut (Rowland et al., 2006)(Figure 1):

Degree Day = (AT_max35 + AT_min12.8)/2 – 12.8

where AT_max35 and AT_min12.8 are the maximum and minimum air temperatures capped at 35C and 12.8C, respectively.

Data were analyzed with SAS version 9.4 (SAS Institute, Cary, NC). To account for environmental differences between years and among states (environments), cumulative degree day was tested as a covariate in all analyses. Differences among genotypes and environments for drought-associated traits were estimated using mixed-model ANOVA with PROC MIXED. Fixed effects included state, genotype, year, and block; year*block was used as the random effect. The SLICE option was used to examine the effects of state and genotype. For flower count data, the sum of flowers counted each season per plot was used for the analyses to minimize the effect of genetic differences in flowering times among genotypes. Similarly, differences among genotypes and water treatments (irrigated and water-deficit) in Oklahoma and Virginia were also examined with PROC MIXED. Pearson’s correlations among traits collected from the water-restricted plots in all states and years were calculated using the raw data with PROC CORR.

Since rainout shelters were not available in Oklahoma and Texas, water-restricted plots were exposed to rain. However, rainfall events greater than 20 mm were relatively rare between 40 and 105 DAP in both years, when deficit irrigation was imposed (Figure 1). The following range of cumulative degree days between 40 and 105 DAP were observed: 561-1493 and 429-1374 in Oklahoma, 582-1529 and 420-1384 in Texas, and 426-1269 and 428-1277 in Virginia in 2018 and 2019, respectively (Figure 1). In 2018 in Virginia, irrigated plots were harvested early (116 DAP) due to severe disease outbreaks of late leaf spot, southern blight, and Sclerotinia blight (caused by Nothopassalora personata, Athelia rolfsii, and Sclerotinia minor, respectively). In 2019, plants in the water-restricted plots in Oklahoma were killed by an early freeze (22 October) 13 days before digging.