Introduction

In general, peanut (Arachis hypogaea L.) seed consist of approximately 52% oil, 28% protein 13% carbohydrates, 6% moisture, 2% fiber, and 2% ash (Cobb and Johnson, 1973). These authors reported a range in crude protein percentage of whole raw seed from 25-34% and a range of oil from 44-56% depending upon genotypes, year, maturity, and several other environmental factors. More recently, Davis and Dean (2016) reported that raw peanut seed contained 49.2% oil, 25.8% protein, 16.1% carbohydrate, 8.5% fiber, 6.5% moisture, 4.7% sugar, and 2.3% ash.

St. Angelo and Mann (1973) stated that most of the nitrogen in peanut seed is in the form of storage proteins and that purification of these protein has been difficult. Previous conversion factors for percent nitrogen to peanut protein has been based upon peanut protein containing approximately 16% nitrogen (100/16 = 6.25), and it is still widely used, especially in feed stock. (St. Angelo and Mann, 1973).

Jones and Horn (1930) found that the peanut protein consists almost entirely of two globulins, arachin and conarachin, and both contain approximately 18.3% nitrogen. These authors also reported about 25% arachin and 8% conarachin in an unidentified virginia-type peanut oil-free meal (Jones and Horn ,1930). Jones (1931) was also among the first to suggest a 5.46 nitrogen to protein conversion factor in peanut (100/18.3 = 5.46).

Unfortunately, these early studies did not consider possible genetic differences among peanut genotypes (landraces, genetic stock, breeding lines, and cultivars) and environmental effects across years and locations. Nearly a two-fold variation in five essential amino acids of oil-free peanut meal were found among 16 peanut genotypes (Young et al., 1973). Young et al., (1974) also reported significant location effects, irrigated vs nonirrigated effects, and genotypic differences for eight amino acids and total amino acids. In an eight-year study (1957-1964), protein and oil percentages were shown to vary significantly from year to year among 26 different peanut strains (Holley and Hammons, 1968). In this same study, significant genotypic differences were also found for protein (25.65 to 28.48%) and oil (51.40 to 55.18%), respectively.

Likewise, nitrogen percentages in peanut seed has been shown to vary significantly over three-years (1979-81) among 26 diverse germplasm lines (Branch and Gaines, 1983). In this study, the previously popular runner-type cultivar ‘Florunner’ (Norden et al., 1969) had a three-year average of 4.46% N; whereas, the highest % N was found with the large-seeded, virginia-type peanut, Jenkins Jumbo (4.98%), and the lowest % N was obtained with two non-nodulating mutants (T-2289 and T-2378). It is interesting to note that Florunner was near the overall average (4.42% N) which agrees with the previous study by Holley and Hammons (1968) regarding ‘Dixie Spanish’ and ‘Argentine’, two long-term established older spanish-type cultivars which were also near the mean.

Dwivedi et al., (1996) found that late-season drought stress also caused an increase in total protein and reduced total oil percentages. There is an inverse relationship between protein and oil percentages (Holley and Hammons, 1968; Tai and Young, 1975), however no association between 100 seed weight (mass) and oil or protein percent was found by Dwivedi, et al., (1990).

Ahmed and Young (1982) emphasized the growing demand and importance for increased percent protein throughout the world. Peanut seed are a rich source of protein and oil. So, the objective of the study was to address this continuing need for increasing protein and oil percent by screening desirable high-oleic cultivars and advanced breeding lines for potential genetic differences. Since protein and oil percent in peanut has been shown to be highly influenced by drought and other environmental factors, multiple years with maximum-input production practices and irrigation were used to uniformly screen different pod and seed size genotypes across two planting dates (April and May) each year.

MATERIALS AND METHODS

During three-years (2016-18), several different high-oleic peanut genotypes with varying seed size were grown under irrigation with maximum-input production practices at the University of Georgia, Coastal Plain Experiment Station in Tifton, GA. The same set of genotypes were planted both early in April and later in May each year for comparison. The number of genotypes varied each year from 30 in 2016 to 24 in 2017 and 2018.

A randomized complete block field design with four replications was used each year. All plots in each test consisted of two-rows, 6.1 m long x 1.8 m wide on a Tifton loamy soil-type (fine-loamy, siliceous, thermic Plinthic Kandidult). UGA extension service recommended production practices of herbicides, fungicides, and insecticides were applied as needed for control of weeds, diseases, and insects, respectively.

Each genotype was individually dug near optimum maturity based upon the hull-scrape method from adjacent border plots (Williams and Drexler, 1981). After harvest, peanut pods from each plot were dried with forced warm air to approximately 6% seed moisture. Pods were presized and shelled on Federal-State Inspection Service equipment. Samples were then screened to remove splits and immature seed. Only sound mature seed were used for protein and oil content evaluations.

One hundred sound mature kernels (SMK) riding >8.53 x 25.40 mm slotted screen were used to further reduce the influence of any maturity effects. Seed samples were sent to Waters Agricultural Laboratories in Camilla, GA for peanut protein and oil determinations. Protein percent was determined from ground samples of peanut seed by a LECO Model 828 combustion instrument to estimate percentages of nitrogen (LECO Corporation, St. Joseph, MI). The percent protein was obtained by multiplying %N x 6.25 and 5.46 (Misra, 2001). Oil percent (crude fat) was determined from these same peanut seed samples using an ANKOM^XT15 extractor (ANKOM Technology, Macedon, NY). This process is a fully automated system for Soxhlet-type extractions.

Data from each test was statistically analyzed by analysis of variance (ANOVA) using the PROC GLM procedure in SAS version 9.4 (SAS Institute, Inc., Cary, NC). Waller-Duncan’s Bayesian T-test (k-ratio = 100) was used for mean separation at P≤0.05.

RESULTS

Percentage of protein was determined by two nitrogen-to-protein conversion factors (NPCF). The first NPCF equaled previously used standard of 6.25. However, Misra (2001) suggested the use of a smaller 5.46 NPCF. Unfortunately, this later NPCF results in a much lower percent protein compared to the previous 6.25 NPCF. In this study, the amino acid profile of these genotypes are unknown and since Young et al., (1973) previously found highly significant differences among five essential amino acids, it is only logical to consider both 5.46 and 6.25 for the NPCF. Consequently, both are presented for literature comparisons. However for results and discussion, the 5.46 NPCF was used throughout this study.

Significant differences (P≤0.05) were found among the peanut genotypes for protein and oil percentages during each planting date for all three-years (Tables 1-6). Overall protein averages were 19.70%, 21.12%, and 23.37% during 2016, 2017, and 2018, respectively. Likewise, overall averages for percent oil were 52.66%, 52.44%, and 50.57% during 2016, 2017, and 2018, respectively. In this study, there was also an inverse relationship between protein and oil percentages as previously reported by Tai and Young (1975).

Percent Protein

During 2016, ‘Georgia-19HP’ (Branch and Brenneman, 2020) had the highest percent protein for both the April and May planting dates (Table 1). However, it was not significantly higher than GA 942009 in the April planting and GA 122707 in the May planting date.

Table 1

Average percent protein across two planting dates and 30 high-oleic peanut genotypes at the Coastal Plain Experiment Station, Tifton, GA, 2016.^†

In 2017, GA 152704 had the highest percent protein for both April and May planting dates (Table 2). However, it was not significantly higher than GA 942009, Georgia-19HP, and GA 122703-10 in April planting date and GA 942009, Georgia-19HP, and ‘Georgia-09B’ (Branch, 2010) in the May planting date.

Table 2

Average percent protein across two planting dates and 24 high-oleic peanut genotypes at the Coastal Plain Experiment Station, Tifton, GA, 2017. ^†

Likewise in 2018, Georgia-19HP again had the highest protein percentage in both April and May planting dates (Table 3). However, Georgia-19HP was not significantly higher than ‘TifNV-High O/L’ (Holbrook et al., 2017) in percent protein during the May planting date.

Table 3

Average percent protein across two planting dates and 24 high-oleic peanut genotypes at the Coastal Plain Experiment Station, Tifton, GA, 2018. ^†

During all three years, Georgia-19HP had a consistently high percentage of protein across both April and May planting dates. Even though the actual protein percentages varied during these three years, the protein ranking for Georgia-19HP remained high compared to these other peanut cultivars and advanced Georgia breeding lines. The percent coefficient of variation (2-5%) was also low for both planting dates and across each of the three years (2016-18) which suggest good stability and consistency throughout these tests for percent protein.

Percent Oil

During 2016, GA 142721 and TUFRunner ‘727’ had the highest percentage of oil averaged over April and May planting dates (Table 4). However, these two genotypes were not significantly higher than several other genotypes in the April planting date and GA 132706, GA 142722, ‘Georgia-20VHO’ (Branch, 2021), GA 142712, GA 132712, and GA 142725 in the May planting date. Conversely, Georgia-19HP had the lowest percentage of oil averaged across both planting dates, but it was not significantly lower than Georgia-09B in the April planting date and GA 122706, GA 942009, GA 122707, and GA 142726 in the May planting date.

Table 4

Average percent oil across two planting dates and 30 high-oleic peanut genotypes at the Coastal Plain Experiment Station, Tifton, GA, 2016.

In 2017, Georgia-20VHO had the highest percentage of oil in the April planting date (Table 5). However, it was not significantly higher in oil percent than TUFRunner ‘727’, GA 132705, TUFRunner ‘511’ (Tillman and Gorbet, 2017), ‘Georgia-16HO’ (Branch, 2017), ‘Georgia-11J’ (Branch, 2012), and GA 132712 at the May planting date.

Table 5

Average percent oil across two planting dates and 24 high-oleic peanut genotypes at the Coastal Plain Experiment Station, Tifton, GA, 2017.

In 2018, FloRun ‘331’ (Tillman, 2021) had the highest percentage of oil at both April and May planting dates (Table 6). However, it was not significantly higher than ‘AU-NPL 17’ and Georgia-20VHO and several advanced Georgia breeding lines in both planting dates.

Table 6

Average Percent Oil across Two Planting Dates and 24 high-oleic Peanut Genotypes at the Coastal Plain Experiment Station, Tifton, GA, 2018.

Overall, oil percentages were consistent with coefficients of variation ranging from 1.36% and 1.87% in April 2017 and 2018, respectively to 4.28% in April 2016. Averaged across both planting dates and genotypes, coefficient of variation were also low with 4.26%, 2.50%, and 2.60% in 2016, 2017, and 2018, respectively. These results also suggest good stability and consistency throughout these tests for oil similar to protein percentages.

DISCUSSION

As previously mentioned, significant differences were found among several peanut cultivars and advanced Georgia breeding lines for percent protein and oil in this study. During each of the three years and averaged over two planting dates, Georgia-19HP was consistently high in percentage of protein. It was also found to be higher than ‘Bailey’ (Isleib et al., 2011) and ‘Georgia-06G’ (Branch, 2007) across each of three southeast states (GA, FL, and AL) during 2017 (Branch and Brenneman, 2020).

In addition to having high protein and high-oleic fatty acid percentages, Georgia-19HP has a very high level of root-knot nematode (RKN) resistance caused by Meloidogyne arenaria (Neal) Chitwood; resistance to early and late leafspot caused by Passalora arachidicola (Hori). U. Braun. syn. Cercospora arachidicola Hori and Nothopassalora personata (Berk & M. A. Curtis) U. Braun, C. Nakash., Videira & Craus syn. Cercosporidium personatum (Berk and Curt.) Deighton, respectively; and resistance to tomato spotted wilt disease caused by Tomato spotted wilt virus. Georgia-19HP is a new virginia-type peanut cultivar that has many desirable traits for growers, shellers, manufacturers, and consumers of peanut and peanut products.

High oil percentage is also currently in demand in the U. S. (Ledbetter, 2022; Parker, 2022). Some in the peanut industry are considering expanding oil production for use in the domestic and export markets. Thus, the combination of high-oleic and high percent oil peanut genotypes would be desirable for such utilization, especially when combined with yield, grade, and dollar value return. Georgia-20VHO was among the highest in oil percent for both planting dates over each of the three-years of this study. It is a new high-yielding, TSWV-resistant, high-grading, very high-oleic (O) to linoleic (L) fatty acid ratio, runner-type peanut cultivar that would be a good option for this potential new market.

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