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Stable carbon isotope values (δ13C) of purslane (Portulaca oleracea) and their archaeological significance
Journal of Archaeological Science: Reports 7 (2016) 189–194
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Stable carbon isotope values (δ13C) of purslane (Portulaca oleracea) and their archaeological significance Kenneth Barnett Tankersley a,b,⁎, Denis G. Conover c, David L. Lentz c a b c
Department of Anthropology, University of Cincinnati, Cincinnati, OH 45221, USA Department of Geology, University of Cincinnati, Cincinnati, OH 45221, USA Department of Biology, University of Cincinnati, Cincinnati, OH 45221, USA
a r t i c l e
i n f o
Article history: Received 18 January 2016 Received in revised form 12 April 2016 Accepted 19 April 2016 Available online xxxx Keywords: Stable carbon isotope value Photosynthesis Archaeobotany Paleobotany Paleodiet Common purslane (Portulaca oleracea) Maize (Zea mays) Eastern North America Late Holocene Bioarchaeology
a b s t r a c t Elemental Analyzer Isotope Ratio Mass Spectrometry was used to determine the δ13C values of common purslane (Portulaca oleracea), a highly edible and nutritious annual succulent and member of the Portulacaceae family, which uses both C4 fixation and Crassulacean acid metabolism (CAM) photosynthesis. The δ13C values for the plant range between −11.2‰ and −20.5‰ (C4 −11.2‰ to −13.9‰, CAM −17.6‰ to −20.5‰), which overlaps with δ13C values for maize (Zea mays) −9.1‰ to −17.3‰. Both plants occur on late Holocene archaeological sites in eastern North America and likely contributed to the δ13C ratios reported for ancient human collagen and hydroxyapatite. Taphonomically, P. oleracea has a lower archaeological visibility because it is completely edible and the seeds are tiny (0.02 to 0.76 mm) in comparison to maize kernels and cobs. Therefore, we can no longer assume that maize was the only significant plant food in the late Holocene diet of eastern North America, which elevated δ13C ratios in ancient human tissues. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction The photosynthetic pathways of plants vary in their biochemical processes to fix carbon. C3 plants may be hydrophytic, mesophytic, or xerophytic and are common in temperate environments. They have a normal leaf anatomy with photoactive stomata, a high rate of photorespiration, and a single CO2 fixation with a CO2 compensation point between 30 and 70 ppm (Govindjee et al., 2006). C4 plants are mesophytic and are common in warm weather tropical environments. The C4 pathway occurs in several thousand species of tropical and subtropical plants, including the economically important crops of corn, sugarcane, and sorghum. By avoiding photorespiration the C4 pathway ensures a more efficient delivery of CO2 for fixation and greater photosynthetic rates than C3 plants under conditions of high light intensity, high temperature, and low CO2 concentrations (Levetin and McMahon, 2016). C4 plants typically have a Kranz leaf anatomy with photoactive stomata, a much lower rate of photorespiration, and a double CO2 fixation with a CO2 compensation point at 10 ppm (Govindjee et al., 2006). ⁎ Corresponding author at: Department of Anthropology, PO Box 210380, 481 Braunstein Hall, University of Cincinnati, Cincinnati, OH 45221-0380, USA. E-mail address: [email protected] (K.B. Tankersley).
http://dx.doi.org/10.1016/j.jasrep.2016.04.009 2352-409X/© 2016 Elsevier Ltd. All rights reserved.
CAM (Crassulacean acid metabolism) plants include xerophytic plants such as cacti and are common in dry lands and semiarid and arid climates. CAM plants typically have a xeromorphic leaf anatomy with scotoactive stomata, a reduced rate of photorespiration, and a double CO2 fixation with a CO2 compensation point at 5 ppm (Govindjee et al., 2006). By opening their stomata at night instead of the daytime, CAM plants avoid the higher evaporative demand of the atmosphere that occurs during the daytime. Purslane, Portulaca oleracea, uses C4 photosynthesis under well-watered conditions, but has the unusual ability to switch to the CAM pathway under drought conditions (Koch and Kennedy, 1980, 1982). It has been long presumed that the consumption of the C4 cultigen maize (Zea mays) was the sole source of 13C enrichment in late Holocene age human collagen and hydroxyapatite (i.e., bone, hair, teeth) obtained from archaeological sites in eastern North America. Late Holocene archaeobotanical records for this region are dominated by C3 silvicultural masts and herbaceous plant foods, which were gathered or cultivated (Table 1). Additionally, there is also a correlation between an increase in δ13C values in human collagen and hydroxyapatite and the increase in the size and quantity and of maize recovered from late Holocene archaeological sites (van der Merwe, 1982; Vogel and van der Merwe, 1977; van der Merwe and Vogel, 1978). Consequently, δ13C values have been used as an isotopic fingerprint for the
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consumption of maize (Beehr and Ambrose, 2007; Dong et al., 2010; Emerson et al., 2005; Hedman, 2006; Hedman and Emerson, 2008; Hedman et al., 2002; Rose, 2008; Schoeninger, 2008; Tykot, 2006). Because human paleodietary reconstructions for the late Holocene of eastern North America have focused exclusively maize in the interpretation of δ13C values for human collagen and hydroxyapatite, other C4 and CAM wild plant foods such P. oleracea have been ignored. The aim of the present study is to ascertain the photosynthetic nature of P. oleracea, obtain δ13C values for the plant, and discuss the implications for archaeological interpretations of late Holocene diets in eastern North America (i.e., east of the Mississippi River).
2. Purslane Common purslane (P. oleracea), colloquially known as duckweed, fatweed, pigweed, little red root, moss rose, pursley, and pussley, is an annual succulent and a member of the Portulacaceae family of flowering plants. While several species of terrestrial plants that have the ability to switch between C3 and Crassulacean Acid Metabolism (CAM) photosynthesis, P. oleracea is one of the few species with the ability to switch between C4 and CAM (Kluge and Ting, 1978; Koch and Kennedy, 1980, 1982; Lara et al., 2004). Physiologically, the shift from C4 to CAM results from stress after an increase in CO2 uptake, a decrease in water, or both (Cushman, 2001; Taiz and Zeiger, 2006). Cross-culturally, P. oleracea is consumed as a delicious and nutritious food (Mohamed and Hussein, 1994). Leaves and stems can be harvested before the plant comes into flower, eaten raw or cooked, and they can be stored (Yanovsky, 1936). The leaves and stems provide an important nutritional wild pant food that is rich in heart-healthy omega-3 fatty acids. They also provide about 271 cal, 49 g of carbohydrate, 26 g of protein, 12 g of fiber, 4 g of fat, 1813 mg of potassium, 1488 mg of calcium, 547 mg of phosphorus, 251 mg of ascorbic acid, 55 mg sodium, 29 mg of iron, 6 mg of niacin, 1 mg of riboflavin, 0.36 mg thiamine, and 15,280 μg of β-carotene per 100 g (Mohamed and Hussein, 1994). While the plant can be harvested and consumed prior to seeding, the seeds may also be consumed and also have a high degree of nutritional value. P. oleracea seeds are extremely small (0.20 to 0.76 mm) (Fig. 1), a single plant can yield upwards of 240,000 of them, which can always be eaten raw or ground and cooked as bread, flat-cake, or as gruel (Yanovsky, 1936). There are approximately 2765 purslane seeds per gram, which contain about 21 g of protein and 19 g of lipids, including
behenic, linoleic, linolenic, oleic, palmitic, and stearic, fatty acids (Mohamed and Hussein, 1994). P. oleracea is also an important medicinal plant. The leaves and stems are used as an antibacterial, antiscorbutic, anti-inflammatory, antibiotic, demulcent depurative, diuretic, and febrifuge. It is used in the treatment of burns, caterpillar stings, coughs, earaches, headaches, insect stings, skin diseases, and stomach aches. The seeds are also used for dyspepsia and opacities of the cornea (Mohamed and Hussein, 1994). 2.1. Origin of P. oleracea in eastern North America In 1672, the British Massachusetts's colony recognized P. oleracea in eastern North America, but they assumed that an earlier European population had introduced the species to the region. However, P. oleracea has been documented from late Holocene archaeological contexts across eastern North America dating as early as 2500 and 3000 years ago (Byrne and McAndrews, 1975; Chapman and Stewart, 1974). The presence of P. oleracea at Archaic, Woodland, Fort Ancient, and Mississippian sites in Eastern North America as well as Mesoamerica (e.g., Byrne and McAndrews, 1975; Chapman and Stewart, 1974; Drooker, 1997; Lentz et al., 2014; Lopinot, 1997; Slotten, 2015) sites suggest that it is an indigenous species. The widespread geographic distribution of P. oleracea in the late Holocene is likely anthropogenic. An increase in the number and size of disturbed habitation areas in warm and moist environmental settings resulting from plant domestication and a semi-sedentary livelihood would have created ideal growing conditions. P. oleracea was likely an invasive species in late Holocene garden plots growing alongside chenopod (Chenopodium berlandieri), gourd (Lagenaria siceraria), sumpweed (Iva annua), squash (Cucurbita pepo), sunflower (Helianthus annuus), and more recently in the agricultural fields of maize and beans (Phaseolus vulgaris) of more complex polities (Chapman and Stewart, 1974). Given the pleasant taste and exceptionally nutritious and medicinal value of P. oleracea, it was likely consumed as an uncultivated food throughout the late Holocene but has largely gone unnoticed because the entire plant could be consumed. P. oleracea has a very low archaeological visibility when compared to cultigens with larger seeds such as squash, gourds, beans, and maize. Therefore, it is not surprising that
Table 1 C3 plant foods identified in the late Holocene archaeobotanical records of Eastern North America (Fritz, 2000, 2014; Lentz, 2000; Munson, 1984; Smith, 1986; Watson, 1974, 2001). Common name
Species
Plant food
American chestnut Beans Bitternut hickory Black walnut Bottle gourd Butternut Chenopod Cushaw squash Erect knotweed Giant ragweed Gourd-like squash Little barley Maygrass Mockernut hickory Pignut hickory Shagbark hickory Shellbark hickory Sumpweed Sunflower White oak
Castanea dentata Phaseolus vulgaris Carya cordiformis Juglans nigra Lagenaria siceraria Juglans cinenea Chenopodium berlandieri Cucurbita argyrosperma Polygonum erectum Ambrosia trifida Cucurbita pepo Hordeum pusillum Phalaris caroliniana Carya tomentosa Carya glabra Carya ovate Carya laciniosa Iva annua Helianthus annuus Quercus alba
Silvicultural mast Cultivated domesticate Silvicultural mast Silvicultural mast Cultivated domesticate Silvicultural mast Cultivated domesticate Cultivated domesticate Herbaceous Herbaceous Cultivated domesticate Herbaceous Herbaceous Silvicultural mast Silvicultural mast Silvicultural mast Silvicultural mast Cultivated domesticate Cultivated domesticate Silvicultural mast
Fig. 1. ESEM of a purslane seed.
K.B. Tankersley et al. / Journal of Archaeological Science: Reports 7 (2016) 189–194 Table 2 Stable carbon isotope values for P. oleracea using C4 photosynthesis.
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Table 3 Stable carbon isotope values for P. oleracea using CAM photosynthesis.
Sample
δ13C1 (‰)
Reference
Sample
δ13C1 (‰)
Error (1 σ)
Range (‰)
Leaves Leaves Leaves Leaves Leaves Leaves Range mean
−11.80 −13.60 −13.00 −13.90 −13.60 −11.40 −11.40 to −13.90 −12.88 −11.60 −11.23 −12.56 −11.23 to −12.56 −11.80
Canti, (2009) Guralnick et al., (2008) Martin et al., (1982) Voznesenskaya et al. (2010) Voznesenskaya et al. (2010) Wang (2007)
Seeds Seeds (water stressed) Leaves Leaves (water stressed) Stems Stems (water stressed) Roots Roots (water stressed)
−18.39 −18.15 −19.75 −18.91 −18.68 −18.59 −20.06 −18.57
0.25 0.51 0.20 0.02 0.04 0.11 0.39 0.04
−18.14 to −18.64 −17.64 to −18.66 −19.55 to −19.95 −18.89 to −18.93 −18.64 to −18.72 −18.48 to −18.64 −19.67 to −20.45 −18.53 to −18.61
Plant matter Plant matter Plant matter Range mean
Canti (2009) Canti (2009) Canti (2009)
of H2O and the other half of the plants received 9 l of H2O. Following maturation, the plants were frozen at −20 °C to halt metabolism. 3.2. Stable carbon isotope analysis
P. oleracea has not been considered as a contributing plant food in paleodietary studies based on stable carbon isotope analyses of human collagen and hydroxyapatite in eastern North America. 3. Methods Although δ13C values for P. oleracea using a C4 photosynthetic pathway are well documented (Canti, 2009; Guralnick et al., 2008; Martin et al., 1982; Voznesenskaya et al., 2010; Wang, 2007), there is a paucity of comparable data associated with CAM photosynthesis. An initial survey of the published δ13C values for P. oleracea shows a range from − 11.2‰ to − 13.9‰, all of which are well within the range of plants using C4 photosynthesis (range − 16.0‰ to − 10‰, mean − 12.0) (Table 1). In order to determine the stable carbon isotope values for P. oleracea using CAM photosynthesis, plants were grown under controlled laboratory conditions. 3.1. Methods for inducing CAM photosynthesis Managing nutrients, light, temperature, water, and CO2 levels initiated and maintained P. oleracea CAM photosynthesis during growth and maturity. Seeds were planted in potting soil (Miracle Grow Moisture Control, pH 7.0) to prevent nutrient stress. Plant grow lights (2–75 W Philips Reflector Agro and 2-General Electric 15 W fluorescent bulbs) provided sufficient lighting 14 h per day for 112 days. Growing temperature ranged between 20 °C and 22 °C. In order to experimentally induce the CAM pathway, the concentration of CO2 in the laboratory was elevated to 155 ppm and maintained at 565 ppm (external CO2 concentration, 410 ppm). Half of the plants received a total of 32 l
Fig. 2. Stable carbon isotope values of purslane.
Air-dried samples (20–50 mg) of seeds, leaves, stems, and roots were hand-selected from well-watered and water stressed P. oleracea plants for stable carbon isotope analysis following the procedures outlined by Tankersley et al. (2015). As a control, we also analyzed two samples of Flint maize kernels (Z. mays indurate). All non-plant particles, such as soil, were physically removed using tweezers and a scalpel. Between 200 and 600 μg aliquots were loaded in a small tin capsule and placed into the zero-blank autosampler on a Costech 4010 EA (Elemental Analyzer) along with Vienna Pee Dee Belemnite (VPDB) standard (13C/12C ratio = 0.0112372). Data were normalized to the standard value and reported as an average. A standard deviation was calculated on duplicate analyses of the aliquots to the standard. The EA was connected to a Thermo ConFlo 3 for reference gas addition and He dilution, which in turn was connected to Thermo Delta XL plus Isotope Ratio Mass Spectrometer for measurement in continuous mode in He. Stable 13C was calculated in parts per thousand (per mil, ‰) using the following equation: 0 13
C C sample
B δ13 C ¼ @13 12
12
C C standard
1 C −1A 1000‰:
4. Results All of the δ13C values obtained for the experimentally grown P. oleracea aliquots are within the range of plants using CAM
Fig. 3. Stable carbon isotope values of maize.
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photosynthesis (Table 2, Fig. 2). The elevated CO2 concentration in the laboratory produced δ13C values that are more negative (lower) in all of the samples (range −17.6‰ to −20.5‰, mean and weighted mean −18.9‰) at 1 σ. Water stress produced δ13C values that are less negative, especially in the leaf and root aliquots (see Table 2). The δ13C values of watered stressed P. oleracea aliquots ranged from − 17.6‰ to
Table 4 Stable carbon isotope values for maize (Zea mays). Sample
δ13C1 (‰)
Reference
Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Range mean
−11.36 −11.42 −11.10 −13.20 −9.30 −12.10 −10.30 −11.10 −9.10 −10.80 −10.10 −10.80 −11.60 −14.30 −12.00 −11.70 −11.80 −11.60 −11.20 −9.30 −12.10 −10.30 −11.10 −9.10 −10.80 −10.10 −10.00 −9.80 −10.60 −9.80 −9.10 to −14.30 −10.93 −12.40 −12.60 −12.40 −11.40 −11.60 −12.20 −10.30 −11.40 −11.30 −10.90 −9.80 −10.20 −12.80 −13.90 −15.10 −10.50 −10.30 −10.20 −14.00 −10.70 −12.00 −10.10 −9.80 to −15.10 −11.64 −12.66 −12.90 −12.81 −13.00 −12.50 −10.70 −17.30 −12.70 −10.70 to −17.30 −13.07
This paper This paper Bender (1968) Bender (1968) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985)
Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Range mean Husk Husk Husk Husk Husk Husk Husk Husk Range mean
Bender (1968) Bender (1968) Bender (1968) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985)
− 18.9‰ (mean − 18.6‰, weighted mean − 18.7‰) at 1 σ. The δ13C values of well-watered P. oleracea aliquots ranged from − 18.1‰ to −20.5‰ (mean −19.2‰, weighted mean −19.1‰) at 1 σ. Stable carbon isotope values obtained for maize range from −9.1‰ to −17.3‰ with the edible kernel portion of the plant ranging between − 9.1‰ and − 14.3‰ (Table 3, Fig. 3). The δ13C values for P. oleracea range from − 11.2‰ and − 20.5‰ with the highly edible leaves (C4 and CAM) ranging between −11.4‰ and −19.6‰ (Table 2, Fig. 2). In other words, the δ13C values for P. oleracea overlap with the δ13C values for maize. The δ13C values for P. oleracea do not overlap with those obtained for domesticated and wild C3 plant foods recovered from late Holocene archaeological contexts in eastern North America (see Table 4, Fig. 4). Indeed, the δ13C values for P. oleracea are more than 4‰ greater than plant foods using C3 photosynthesis (see Table 5). 5. Discussion For more than 40 years, archaeologists have assumed that maize was assumed to be the only plant food that contributed significantly to the late Holocene diets of eastern North America that followed the C4 photosynthetic pathway (Tykot, 2006). Consequently, δ13C values in ancient human collagen and hydroxyapatite, in combination with radiocarbon dating, were used to investigate the timing of the initial introduction of maize into this region (van der Merwe, 1982; van der Merwe and Vogel, 1978; Vogel and van der Merwe, 1977). These studies correlated elevated δ13C values in ancient human collagen and hydroxyapatite with an increase in the relative abundance of maize remains on late prehistoric archaeological sites to suggest an increasing economic dependence on maize produce. Their findings also suggested that the percentage of maize in prehistoric diets in eastern North America could be quantified (Tykot, 2006). Most archaeologists assume that δ13C values obtained on collagen represent the average diet of an individual (Hedman, 2006; Hedman and Emerson, 2008; Hedman et al., 2002; Tykot, 2006). There have been attempts to document the exact relationship between the quantity of maize in the diet and δ13C values obtained from living human populations and their dogs (e.g., Tankersley and Koster, 2010; Koster and Tankersley, 2012). These studies suggest that the δ13C value of human collagen characterizes the entire diet, that is, the protein-portions and mixtures of C3, C4, and CAM plant foods. While it is possible to use δ13C values obtained on human collagen to quantitatively compare the consumption of C4 plants of one individual to another or one group of individuals to another (Tykot, 2006), it is impossible to discern the consumption of one C4 plant species from another. In situations where multiple species of C4 plants were consumed, we can at best assume that δ13C values represent a dietary
Canti (2009) Canti (2009) Canti (2009) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) Fig. 4. Isotope values of purslane, maize, and C3 plant foods.
K.B. Tankersley et al. / Journal of Archaeological Science: Reports 7 (2016) 189–194
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Table 5 Stable carbon isotope value ranges for purslane, maize, and exemplary C3 plant foods. Sample
Photosynthetic pathway
δ13C1 (‰)
Reference
Purslane leaves (Portulaca oleracea) Purslane leaves (Portulaca oleracea) Maize kernels (Zea mays) Gourd (Lagenaria siceraria) Squash (Cucurbita pepo) Sumpweed (Iva annua) Sunflower (Helianthus annuus) Chenopod (Chenopodium berlandieri) Giant ragweed (Ambrosia trifida) Common bean (Phaseolus vulgaris)
C4 CAM C4 C3 C3 C3 C3 C3 C3 C3
−11.4 to −13.9 −18.9 to −19.6 −9.1 to −14.3 −23.0 to −25.3 −24.0 to −27.3 −25.0 to −25.8 −28.2 to −28.5 −24.7 to −25.4 −30.7 to −31.6 −22.2 to −25.9
This paper This paper This paper DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) Winemiller et al. (2007) Zhou & Cheng (2011) DeNiro & Hastorf (1985) Hobbie & Werner (2004) DeNiro & Hastorf (1985)
mixture. Different species of C4 plant foods contribute to both human collagen and hydroxyapatite in the proportions in which they were consumed (Tykot, 2006). This situation is further complicated by the fact that plants such as amaranth and some species of panic grasses using C4 photosynthesis have δ13C values which overlap with plants, which use CAM photosynthesis (C4 plants − 10‰ to − 16‰, CAM plants −10‰ to −20). The late Holocene archaeobotanical record of eastern North America includes both maize and P. oleracea, which suggests that both plant foods were consumed. Today, the southwestern Tewa harvest maize and P. oleracea together from cultivated fields and both are important plant foods in their diets (Tankersley and Thress, 2016). It is likely that a similar practice occurred in eastern North America during the late Holocene. Unlike maize, where only the kernel is consumed, the entire P. oleracea plant is edible. Taphonomically, this situation greatly reduces the archaeological visibility of P. oleracea, which is further complicated by the minute size of seeds when compared to maize kernels and cobs. In other words, the reported quantity of P. oleracea is likely underrepresented in the late Holocene archaeological record of eastern North America. It is likely that the quantity of P. oleracea consumed during the late Holocene changed through time and across space. It is probable, however, that less negative δ13C values for late Holocene human bone collagen and hydroxyapatite represents a dietary mixture of maize, P. oleracea, and other foods, as well. 6. Conclusion Our economic interpretations of maize consumption are likely inflated because we can no longer assume that maize was the only significant C4 plant food, which contributed to the late Holocene diet of eastern North America. The caveats presented in this study not only apply to eastern North America, but they are germane to all places and time where P. oleracea was part of the human diet. Sampling of other C4 and CAM plants foods that have δ13C values, which overlap with maize will be useful to future archaeological studies and paleodietary interpretations. Additionally, it will be beneficial if future archaeobotanical data is used to model what percentage of maize and other important C4 and CAM plant foods such as P. oleracea were consumed in the past. Acknowledgements Funding from the Charles Phelps Taft Foundation and the Court Family Foundation supported this study. Aaron Diefendorf's advice and suggestions were insightful and much appreciated. References Beehr, D., Ambrose, S.H., 2007. Were They What They Cooked? Stable isotopic analysis of Mississippian pottery residues. In: Twiss, K. (Ed.), We Are What We Eat: Archaeology, Food, and Identity. Center for Archaeological Investigations, Southern Illinois University, Carbondale, pp. 171–191.
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Journal of Archaeological Science: Reports journal homepage: www.elsevier.com/locate/jasrep
Stable carbon isotope values (δ13C) of purslane (Portulaca oleracea) and their archaeological significance Kenneth Barnett Tankersley a,b,⁎, Denis G. Conover c, David L. Lentz c a b c
Department of Anthropology, University of Cincinnati, Cincinnati, OH 45221, USA Department of Geology, University of Cincinnati, Cincinnati, OH 45221, USA Department of Biology, University of Cincinnati, Cincinnati, OH 45221, USA
a r t i c l e
i n f o
Article history: Received 18 January 2016 Received in revised form 12 April 2016 Accepted 19 April 2016 Available online xxxx Keywords: Stable carbon isotope value Photosynthesis Archaeobotany Paleobotany Paleodiet Common purslane (Portulaca oleracea) Maize (Zea mays) Eastern North America Late Holocene Bioarchaeology
a b s t r a c t Elemental Analyzer Isotope Ratio Mass Spectrometry was used to determine the δ13C values of common purslane (Portulaca oleracea), a highly edible and nutritious annual succulent and member of the Portulacaceae family, which uses both C4 fixation and Crassulacean acid metabolism (CAM) photosynthesis. The δ13C values for the plant range between −11.2‰ and −20.5‰ (C4 −11.2‰ to −13.9‰, CAM −17.6‰ to −20.5‰), which overlaps with δ13C values for maize (Zea mays) −9.1‰ to −17.3‰. Both plants occur on late Holocene archaeological sites in eastern North America and likely contributed to the δ13C ratios reported for ancient human collagen and hydroxyapatite. Taphonomically, P. oleracea has a lower archaeological visibility because it is completely edible and the seeds are tiny (0.02 to 0.76 mm) in comparison to maize kernels and cobs. Therefore, we can no longer assume that maize was the only significant plant food in the late Holocene diet of eastern North America, which elevated δ13C ratios in ancient human tissues. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction The photosynthetic pathways of plants vary in their biochemical processes to fix carbon. C3 plants may be hydrophytic, mesophytic, or xerophytic and are common in temperate environments. They have a normal leaf anatomy with photoactive stomata, a high rate of photorespiration, and a single CO2 fixation with a CO2 compensation point between 30 and 70 ppm (Govindjee et al., 2006). C4 plants are mesophytic and are common in warm weather tropical environments. The C4 pathway occurs in several thousand species of tropical and subtropical plants, including the economically important crops of corn, sugarcane, and sorghum. By avoiding photorespiration the C4 pathway ensures a more efficient delivery of CO2 for fixation and greater photosynthetic rates than C3 plants under conditions of high light intensity, high temperature, and low CO2 concentrations (Levetin and McMahon, 2016). C4 plants typically have a Kranz leaf anatomy with photoactive stomata, a much lower rate of photorespiration, and a double CO2 fixation with a CO2 compensation point at 10 ppm (Govindjee et al., 2006). ⁎ Corresponding author at: Department of Anthropology, PO Box 210380, 481 Braunstein Hall, University of Cincinnati, Cincinnati, OH 45221-0380, USA. E-mail address: [email protected] (K.B. Tankersley).
http://dx.doi.org/10.1016/j.jasrep.2016.04.009 2352-409X/© 2016 Elsevier Ltd. All rights reserved.
CAM (Crassulacean acid metabolism) plants include xerophytic plants such as cacti and are common in dry lands and semiarid and arid climates. CAM plants typically have a xeromorphic leaf anatomy with scotoactive stomata, a reduced rate of photorespiration, and a double CO2 fixation with a CO2 compensation point at 5 ppm (Govindjee et al., 2006). By opening their stomata at night instead of the daytime, CAM plants avoid the higher evaporative demand of the atmosphere that occurs during the daytime. Purslane, Portulaca oleracea, uses C4 photosynthesis under well-watered conditions, but has the unusual ability to switch to the CAM pathway under drought conditions (Koch and Kennedy, 1980, 1982). It has been long presumed that the consumption of the C4 cultigen maize (Zea mays) was the sole source of 13C enrichment in late Holocene age human collagen and hydroxyapatite (i.e., bone, hair, teeth) obtained from archaeological sites in eastern North America. Late Holocene archaeobotanical records for this region are dominated by C3 silvicultural masts and herbaceous plant foods, which were gathered or cultivated (Table 1). Additionally, there is also a correlation between an increase in δ13C values in human collagen and hydroxyapatite and the increase in the size and quantity and of maize recovered from late Holocene archaeological sites (van der Merwe, 1982; Vogel and van der Merwe, 1977; van der Merwe and Vogel, 1978). Consequently, δ13C values have been used as an isotopic fingerprint for the
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consumption of maize (Beehr and Ambrose, 2007; Dong et al., 2010; Emerson et al., 2005; Hedman, 2006; Hedman and Emerson, 2008; Hedman et al., 2002; Rose, 2008; Schoeninger, 2008; Tykot, 2006). Because human paleodietary reconstructions for the late Holocene of eastern North America have focused exclusively maize in the interpretation of δ13C values for human collagen and hydroxyapatite, other C4 and CAM wild plant foods such P. oleracea have been ignored. The aim of the present study is to ascertain the photosynthetic nature of P. oleracea, obtain δ13C values for the plant, and discuss the implications for archaeological interpretations of late Holocene diets in eastern North America (i.e., east of the Mississippi River).
2. Purslane Common purslane (P. oleracea), colloquially known as duckweed, fatweed, pigweed, little red root, moss rose, pursley, and pussley, is an annual succulent and a member of the Portulacaceae family of flowering plants. While several species of terrestrial plants that have the ability to switch between C3 and Crassulacean Acid Metabolism (CAM) photosynthesis, P. oleracea is one of the few species with the ability to switch between C4 and CAM (Kluge and Ting, 1978; Koch and Kennedy, 1980, 1982; Lara et al., 2004). Physiologically, the shift from C4 to CAM results from stress after an increase in CO2 uptake, a decrease in water, or both (Cushman, 2001; Taiz and Zeiger, 2006). Cross-culturally, P. oleracea is consumed as a delicious and nutritious food (Mohamed and Hussein, 1994). Leaves and stems can be harvested before the plant comes into flower, eaten raw or cooked, and they can be stored (Yanovsky, 1936). The leaves and stems provide an important nutritional wild pant food that is rich in heart-healthy omega-3 fatty acids. They also provide about 271 cal, 49 g of carbohydrate, 26 g of protein, 12 g of fiber, 4 g of fat, 1813 mg of potassium, 1488 mg of calcium, 547 mg of phosphorus, 251 mg of ascorbic acid, 55 mg sodium, 29 mg of iron, 6 mg of niacin, 1 mg of riboflavin, 0.36 mg thiamine, and 15,280 μg of β-carotene per 100 g (Mohamed and Hussein, 1994). While the plant can be harvested and consumed prior to seeding, the seeds may also be consumed and also have a high degree of nutritional value. P. oleracea seeds are extremely small (0.20 to 0.76 mm) (Fig. 1), a single plant can yield upwards of 240,000 of them, which can always be eaten raw or ground and cooked as bread, flat-cake, or as gruel (Yanovsky, 1936). There are approximately 2765 purslane seeds per gram, which contain about 21 g of protein and 19 g of lipids, including
behenic, linoleic, linolenic, oleic, palmitic, and stearic, fatty acids (Mohamed and Hussein, 1994). P. oleracea is also an important medicinal plant. The leaves and stems are used as an antibacterial, antiscorbutic, anti-inflammatory, antibiotic, demulcent depurative, diuretic, and febrifuge. It is used in the treatment of burns, caterpillar stings, coughs, earaches, headaches, insect stings, skin diseases, and stomach aches. The seeds are also used for dyspepsia and opacities of the cornea (Mohamed and Hussein, 1994). 2.1. Origin of P. oleracea in eastern North America In 1672, the British Massachusetts's colony recognized P. oleracea in eastern North America, but they assumed that an earlier European population had introduced the species to the region. However, P. oleracea has been documented from late Holocene archaeological contexts across eastern North America dating as early as 2500 and 3000 years ago (Byrne and McAndrews, 1975; Chapman and Stewart, 1974). The presence of P. oleracea at Archaic, Woodland, Fort Ancient, and Mississippian sites in Eastern North America as well as Mesoamerica (e.g., Byrne and McAndrews, 1975; Chapman and Stewart, 1974; Drooker, 1997; Lentz et al., 2014; Lopinot, 1997; Slotten, 2015) sites suggest that it is an indigenous species. The widespread geographic distribution of P. oleracea in the late Holocene is likely anthropogenic. An increase in the number and size of disturbed habitation areas in warm and moist environmental settings resulting from plant domestication and a semi-sedentary livelihood would have created ideal growing conditions. P. oleracea was likely an invasive species in late Holocene garden plots growing alongside chenopod (Chenopodium berlandieri), gourd (Lagenaria siceraria), sumpweed (Iva annua), squash (Cucurbita pepo), sunflower (Helianthus annuus), and more recently in the agricultural fields of maize and beans (Phaseolus vulgaris) of more complex polities (Chapman and Stewart, 1974). Given the pleasant taste and exceptionally nutritious and medicinal value of P. oleracea, it was likely consumed as an uncultivated food throughout the late Holocene but has largely gone unnoticed because the entire plant could be consumed. P. oleracea has a very low archaeological visibility when compared to cultigens with larger seeds such as squash, gourds, beans, and maize. Therefore, it is not surprising that
Table 1 C3 plant foods identified in the late Holocene archaeobotanical records of Eastern North America (Fritz, 2000, 2014; Lentz, 2000; Munson, 1984; Smith, 1986; Watson, 1974, 2001). Common name
Species
Plant food
American chestnut Beans Bitternut hickory Black walnut Bottle gourd Butternut Chenopod Cushaw squash Erect knotweed Giant ragweed Gourd-like squash Little barley Maygrass Mockernut hickory Pignut hickory Shagbark hickory Shellbark hickory Sumpweed Sunflower White oak
Castanea dentata Phaseolus vulgaris Carya cordiformis Juglans nigra Lagenaria siceraria Juglans cinenea Chenopodium berlandieri Cucurbita argyrosperma Polygonum erectum Ambrosia trifida Cucurbita pepo Hordeum pusillum Phalaris caroliniana Carya tomentosa Carya glabra Carya ovate Carya laciniosa Iva annua Helianthus annuus Quercus alba
Silvicultural mast Cultivated domesticate Silvicultural mast Silvicultural mast Cultivated domesticate Silvicultural mast Cultivated domesticate Cultivated domesticate Herbaceous Herbaceous Cultivated domesticate Herbaceous Herbaceous Silvicultural mast Silvicultural mast Silvicultural mast Silvicultural mast Cultivated domesticate Cultivated domesticate Silvicultural mast
Fig. 1. ESEM of a purslane seed.
K.B. Tankersley et al. / Journal of Archaeological Science: Reports 7 (2016) 189–194 Table 2 Stable carbon isotope values for P. oleracea using C4 photosynthesis.
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Table 3 Stable carbon isotope values for P. oleracea using CAM photosynthesis.
Sample
δ13C1 (‰)
Reference
Sample
δ13C1 (‰)
Error (1 σ)
Range (‰)
Leaves Leaves Leaves Leaves Leaves Leaves Range mean
−11.80 −13.60 −13.00 −13.90 −13.60 −11.40 −11.40 to −13.90 −12.88 −11.60 −11.23 −12.56 −11.23 to −12.56 −11.80
Canti, (2009) Guralnick et al., (2008) Martin et al., (1982) Voznesenskaya et al. (2010) Voznesenskaya et al. (2010) Wang (2007)
Seeds Seeds (water stressed) Leaves Leaves (water stressed) Stems Stems (water stressed) Roots Roots (water stressed)
−18.39 −18.15 −19.75 −18.91 −18.68 −18.59 −20.06 −18.57
0.25 0.51 0.20 0.02 0.04 0.11 0.39 0.04
−18.14 to −18.64 −17.64 to −18.66 −19.55 to −19.95 −18.89 to −18.93 −18.64 to −18.72 −18.48 to −18.64 −19.67 to −20.45 −18.53 to −18.61
Plant matter Plant matter Plant matter Range mean
Canti (2009) Canti (2009) Canti (2009)
of H2O and the other half of the plants received 9 l of H2O. Following maturation, the plants were frozen at −20 °C to halt metabolism. 3.2. Stable carbon isotope analysis
P. oleracea has not been considered as a contributing plant food in paleodietary studies based on stable carbon isotope analyses of human collagen and hydroxyapatite in eastern North America. 3. Methods Although δ13C values for P. oleracea using a C4 photosynthetic pathway are well documented (Canti, 2009; Guralnick et al., 2008; Martin et al., 1982; Voznesenskaya et al., 2010; Wang, 2007), there is a paucity of comparable data associated with CAM photosynthesis. An initial survey of the published δ13C values for P. oleracea shows a range from − 11.2‰ to − 13.9‰, all of which are well within the range of plants using C4 photosynthesis (range − 16.0‰ to − 10‰, mean − 12.0) (Table 1). In order to determine the stable carbon isotope values for P. oleracea using CAM photosynthesis, plants were grown under controlled laboratory conditions. 3.1. Methods for inducing CAM photosynthesis Managing nutrients, light, temperature, water, and CO2 levels initiated and maintained P. oleracea CAM photosynthesis during growth and maturity. Seeds were planted in potting soil (Miracle Grow Moisture Control, pH 7.0) to prevent nutrient stress. Plant grow lights (2–75 W Philips Reflector Agro and 2-General Electric 15 W fluorescent bulbs) provided sufficient lighting 14 h per day for 112 days. Growing temperature ranged between 20 °C and 22 °C. In order to experimentally induce the CAM pathway, the concentration of CO2 in the laboratory was elevated to 155 ppm and maintained at 565 ppm (external CO2 concentration, 410 ppm). Half of the plants received a total of 32 l
Fig. 2. Stable carbon isotope values of purslane.
Air-dried samples (20–50 mg) of seeds, leaves, stems, and roots were hand-selected from well-watered and water stressed P. oleracea plants for stable carbon isotope analysis following the procedures outlined by Tankersley et al. (2015). As a control, we also analyzed two samples of Flint maize kernels (Z. mays indurate). All non-plant particles, such as soil, were physically removed using tweezers and a scalpel. Between 200 and 600 μg aliquots were loaded in a small tin capsule and placed into the zero-blank autosampler on a Costech 4010 EA (Elemental Analyzer) along with Vienna Pee Dee Belemnite (VPDB) standard (13C/12C ratio = 0.0112372). Data were normalized to the standard value and reported as an average. A standard deviation was calculated on duplicate analyses of the aliquots to the standard. The EA was connected to a Thermo ConFlo 3 for reference gas addition and He dilution, which in turn was connected to Thermo Delta XL plus Isotope Ratio Mass Spectrometer for measurement in continuous mode in He. Stable 13C was calculated in parts per thousand (per mil, ‰) using the following equation: 0 13
C C sample
B δ13 C ¼ @13 12
12
C C standard
1 C −1A 1000‰:
4. Results All of the δ13C values obtained for the experimentally grown P. oleracea aliquots are within the range of plants using CAM
Fig. 3. Stable carbon isotope values of maize.
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photosynthesis (Table 2, Fig. 2). The elevated CO2 concentration in the laboratory produced δ13C values that are more negative (lower) in all of the samples (range −17.6‰ to −20.5‰, mean and weighted mean −18.9‰) at 1 σ. Water stress produced δ13C values that are less negative, especially in the leaf and root aliquots (see Table 2). The δ13C values of watered stressed P. oleracea aliquots ranged from − 17.6‰ to
Table 4 Stable carbon isotope values for maize (Zea mays). Sample
δ13C1 (‰)
Reference
Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel Range mean
−11.36 −11.42 −11.10 −13.20 −9.30 −12.10 −10.30 −11.10 −9.10 −10.80 −10.10 −10.80 −11.60 −14.30 −12.00 −11.70 −11.80 −11.60 −11.20 −9.30 −12.10 −10.30 −11.10 −9.10 −10.80 −10.10 −10.00 −9.80 −10.60 −9.80 −9.10 to −14.30 −10.93 −12.40 −12.60 −12.40 −11.40 −11.60 −12.20 −10.30 −11.40 −11.30 −10.90 −9.80 −10.20 −12.80 −13.90 −15.10 −10.50 −10.30 −10.20 −14.00 −10.70 −12.00 −10.10 −9.80 to −15.10 −11.64 −12.66 −12.90 −12.81 −13.00 −12.50 −10.70 −17.30 −12.70 −10.70 to −17.30 −13.07
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Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Cob Range mean Husk Husk Husk Husk Husk Husk Husk Husk Range mean
Bender (1968) Bender (1968) Bender (1968) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985)
− 18.9‰ (mean − 18.6‰, weighted mean − 18.7‰) at 1 σ. The δ13C values of well-watered P. oleracea aliquots ranged from − 18.1‰ to −20.5‰ (mean −19.2‰, weighted mean −19.1‰) at 1 σ. Stable carbon isotope values obtained for maize range from −9.1‰ to −17.3‰ with the edible kernel portion of the plant ranging between − 9.1‰ and − 14.3‰ (Table 3, Fig. 3). The δ13C values for P. oleracea range from − 11.2‰ and − 20.5‰ with the highly edible leaves (C4 and CAM) ranging between −11.4‰ and −19.6‰ (Table 2, Fig. 2). In other words, the δ13C values for P. oleracea overlap with the δ13C values for maize. The δ13C values for P. oleracea do not overlap with those obtained for domesticated and wild C3 plant foods recovered from late Holocene archaeological contexts in eastern North America (see Table 4, Fig. 4). Indeed, the δ13C values for P. oleracea are more than 4‰ greater than plant foods using C3 photosynthesis (see Table 5). 5. Discussion For more than 40 years, archaeologists have assumed that maize was assumed to be the only plant food that contributed significantly to the late Holocene diets of eastern North America that followed the C4 photosynthetic pathway (Tykot, 2006). Consequently, δ13C values in ancient human collagen and hydroxyapatite, in combination with radiocarbon dating, were used to investigate the timing of the initial introduction of maize into this region (van der Merwe, 1982; van der Merwe and Vogel, 1978; Vogel and van der Merwe, 1977). These studies correlated elevated δ13C values in ancient human collagen and hydroxyapatite with an increase in the relative abundance of maize remains on late prehistoric archaeological sites to suggest an increasing economic dependence on maize produce. Their findings also suggested that the percentage of maize in prehistoric diets in eastern North America could be quantified (Tykot, 2006). Most archaeologists assume that δ13C values obtained on collagen represent the average diet of an individual (Hedman, 2006; Hedman and Emerson, 2008; Hedman et al., 2002; Tykot, 2006). There have been attempts to document the exact relationship between the quantity of maize in the diet and δ13C values obtained from living human populations and their dogs (e.g., Tankersley and Koster, 2010; Koster and Tankersley, 2012). These studies suggest that the δ13C value of human collagen characterizes the entire diet, that is, the protein-portions and mixtures of C3, C4, and CAM plant foods. While it is possible to use δ13C values obtained on human collagen to quantitatively compare the consumption of C4 plants of one individual to another or one group of individuals to another (Tykot, 2006), it is impossible to discern the consumption of one C4 plant species from another. In situations where multiple species of C4 plants were consumed, we can at best assume that δ13C values represent a dietary
Canti (2009) Canti (2009) Canti (2009) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) Fig. 4. Isotope values of purslane, maize, and C3 plant foods.
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Table 5 Stable carbon isotope value ranges for purslane, maize, and exemplary C3 plant foods. Sample
Photosynthetic pathway
δ13C1 (‰)
Reference
Purslane leaves (Portulaca oleracea) Purslane leaves (Portulaca oleracea) Maize kernels (Zea mays) Gourd (Lagenaria siceraria) Squash (Cucurbita pepo) Sumpweed (Iva annua) Sunflower (Helianthus annuus) Chenopod (Chenopodium berlandieri) Giant ragweed (Ambrosia trifida) Common bean (Phaseolus vulgaris)
C4 CAM C4 C3 C3 C3 C3 C3 C3 C3
−11.4 to −13.9 −18.9 to −19.6 −9.1 to −14.3 −23.0 to −25.3 −24.0 to −27.3 −25.0 to −25.8 −28.2 to −28.5 −24.7 to −25.4 −30.7 to −31.6 −22.2 to −25.9
This paper This paper This paper DeNiro & Hastorf (1985) DeNiro & Hastorf (1985) Winemiller et al. (2007) Zhou & Cheng (2011) DeNiro & Hastorf (1985) Hobbie & Werner (2004) DeNiro & Hastorf (1985)
mixture. Different species of C4 plant foods contribute to both human collagen and hydroxyapatite in the proportions in which they were consumed (Tykot, 2006). This situation is further complicated by the fact that plants such as amaranth and some species of panic grasses using C4 photosynthesis have δ13C values which overlap with plants, which use CAM photosynthesis (C4 plants − 10‰ to − 16‰, CAM plants −10‰ to −20). The late Holocene archaeobotanical record of eastern North America includes both maize and P. oleracea, which suggests that both plant foods were consumed. Today, the southwestern Tewa harvest maize and P. oleracea together from cultivated fields and both are important plant foods in their diets (Tankersley and Thress, 2016). It is likely that a similar practice occurred in eastern North America during the late Holocene. Unlike maize, where only the kernel is consumed, the entire P. oleracea plant is edible. Taphonomically, this situation greatly reduces the archaeological visibility of P. oleracea, which is further complicated by the minute size of seeds when compared to maize kernels and cobs. In other words, the reported quantity of P. oleracea is likely underrepresented in the late Holocene archaeological record of eastern North America. It is likely that the quantity of P. oleracea consumed during the late Holocene changed through time and across space. It is probable, however, that less negative δ13C values for late Holocene human bone collagen and hydroxyapatite represents a dietary mixture of maize, P. oleracea, and other foods, as well. 6. Conclusion Our economic interpretations of maize consumption are likely inflated because we can no longer assume that maize was the only significant C4 plant food, which contributed to the late Holocene diet of eastern North America. The caveats presented in this study not only apply to eastern North America, but they are germane to all places and time where P. oleracea was part of the human diet. Sampling of other C4 and CAM plants foods that have δ13C values, which overlap with maize will be useful to future archaeological studies and paleodietary interpretations. Additionally, it will be beneficial if future archaeobotanical data is used to model what percentage of maize and other important C4 and CAM plant foods such as P. oleracea were consumed in the past. Acknowledgements Funding from the Charles Phelps Taft Foundation and the Court Family Foundation supported this study. Aaron Diefendorf's advice and suggestions were insightful and much appreciated. References Beehr, D., Ambrose, S.H., 2007. Were They What They Cooked? Stable isotopic analysis of Mississippian pottery residues. In: Twiss, K. (Ed.), We Are What We Eat: Archaeology, Food, and Identity. Center for Archaeological Investigations, Southern Illinois University, Carbondale, pp. 171–191.
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