C4 Photosynthesis at Barro Colorado Island, Panama

Abstract

C4 photosynthesis is one of three metabolic pathways found in the photosynthetic tissues of vascular plants for assimilation of atmospheric carbon dioxide (CO2). Like crassulacean acid metabolism (CAM) photosynthesis, C4 photosynthesis has evolved from the ancestral C3 pathway. C4 photosynthesis is an adaptation that enhances CO2 assimilation in warm, high-light environments. On Barro Colorado Island (BCI), C4 photosynthesis (including variations of it) occurs in close to 60 her?baceous species from six families: Hydrocharitaceae, Cyperaceae, Poaceae, Euphorbia?ceae, Amaranthaceae, and Portulacaceae. Plants are typically found in clearings and not inside the forest, and about one-third of the C4 species are non-native. With at least 42 C4 species, Poaceae is the greatest contributor to the C4 flora of BCI. One grass species, Homolepis aturensis (Kunth) Chase has C3–C4 intermediate characteristics termed C2 photosynthesis. Portulaca oleracea L. (Portulacaceae) is a C4 plant with the remark?able capacity to also exhibit CAM photosynthesis under conditions of water-deficit stress. Hydrilla verticillata (L.f.) Royle (Hydrocharitaceae) is a submerged species that lacks Kranz-anatomy typical of fully developed C4. The species usually exhibits C3 photosynthesis but induces a C4 CO2 concentrating pathway when CO2 concentration in the water declines.
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C4 Photosynthesis at Barro
Colorado Island, Panama
Winter
FROM
The First 100 Years of Research on Barro Colorado:
Plant and Ecosystem Science, Volume 1
WASHINGTON, D.C.
2024
Smithsonian
Scholarly
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Compilation copyright © 2024 by Smithsonian Institution
Recommended citation:
Winter, Klaus.2024. C4 Photosynthesis at Barro Colorado Island, Panama. In The First 100 Years of
Research on Barro Colorado: Plant and Ecosystem Science, Volume 1, ed. Helene Muller-Landau and S.
Joseph Wright, pp. 000–000. Washington, DC: Smithsonian Institution Scholarly Press.
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C4 Photosynthesis at Barro Colorado
Island,Panama
Klaus Winter
Smithsonian Tropical Research Institute, P.O.
Box 0843-03092, Balboa, Ancón, Republic of
Panama.
Correspondence: winterk@si.edu
https://orcid.org/0000-0002-0448-2807
Manuscript received 14 November 2022; accepted
17 July 2023.
ABSTRACT. C4 photosynthesis is one of three metabolic pathways found in the pho-
tosynthetic tissues of vascular plants for assimilation of atmospheric carbon dioxide
(CO2). Like crassulacean acid metabolism (CAM) photosynthesis, C4 photosynthesis
has evolved from the ancestral C3 pathway. C4 photosynthesis is an adaptation that
enhances CO2 assimilation in warm, high-light environments. On Barro Colorado
Island (BCI), C4 photosynthesis (including variations of it) occurs in close to 60 her-
baceous species from six families: Hydrocharitaceae, Cyperaceae, Poaceae, Euphorbia-
ceae, Amaranthaceae, and Portulacaceae. Plants are typically found in clearings and
not inside the forest, and about one-third of the C4 species are non-native. With at least
42 C4 species, Poaceae is the greatest contributor to the C4 flora of BCI. One grass spe-
cies, Homolepis aturensis (Kunth) Chase has C3–C4 intermediate characteristics termed
C2 photosynthesis. Portulaca oleracea L. (Portulacaceae) is a C4 plant with the remark-
able capacity to also exhibit CAM photosynthesis under conditions of water-deficit
stress. Hydrilla verticillata (L.f.) Royle (Hydrocharitaceae) is a submerged species that
lacks Kranz-anatomy typical of fully developed C4. The species usually exhibits C3
photosynthesis but induces a C4 CO2 concentrating pathway when CO2 concentration
in the water declines.
Keywords: Barro Colorado Island; C3 photosynthesis; C4 photosynthesis; Homolepis;
Hydrilla; photosynthetic pathway; Saccharum
INTRODUCTION
C4 photosynthesis includes biochemical and anatomical modifications of the ances-
tral C3 pathway that concentrate carbon dioxide (CO2) around the carboxylating enzyme
ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco), thereby suppressing photo-
respiration and enhancing carbon assimilation in warm climates (Hatch et al., 1971; Sage
and Monson, 1999). The principal metabolic steps of C4 photosynthesis, as compared
with C3 photosynthesis, are shown in Figure 1, and further explanations of the biochemi-
cal underpinnings of the two pathways, including C3–C4 intermediacy, are provided in
Box 1.
28

242
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WINTER
The C4 pathway has evolved independently over 60 times
(Sage, 2016), and although C4 is present in only about 3% of
angiosperm species, C4 vegetation accounts for >20% of terres-
trial gross primary productivity (Beer et al., 2010). As of 2016,
C4 photosynthesis was documented in 8,265 species (5,044
grasses; 1,322 sedges; 1,777 eudicots) from 418 genera and 19
families (Sage, 2016). C4 species are generally superior to C3
plants in terms of water- and nitrogen-use eciency, as well as
in their ability to exhibit higher photosynthesis rates at high tem-
peratures and high irradiances than C3 species. Consistent with
these physiological advantages, C4 species are predominantly,
although not exclusively, found in open habitats of the tropics
and subtropics. Widely cultivated, highly productive C4 crops
include maize (Zea mays L.) and sugarcane (Saccharum oci-
narum L.), and 8 of the world’s 10 most “aggressive” weeds
exhibit C4 photosynthesis (Edwards and Walker, 1983). Eorts
are underway to supercharge C3 plants such as rice, one of the
world’s most important food crops, by bioengineering C4 pho-
tosynthesis into them (e.g., c4rice.com), to increase plant pro-
ductivity and meet future food demand in the face of a growing
human population.
Although C4 grasses are defining components of tropical
savannas (Medina, 1996; Knapp and Medina, 1999), the signifi-
cance of C4 photosynthesis in tropical forest of Barro Colorado
Island (BCI) has not been characterized. Botanical and ecological
research on BCI has traditionally focused on trees. To the best of
the author’s knowledge, all trees on BCI are C3 plants, with the
exception of four woody species of Clusia (Clusiaceae), which
have the capacity for CAM photosynthesis (Winter, 2024; Zotz,
2024). C4 photosynthesis is rare in trees (Sage and Sultmanis,
2016). The few woody species of large stature known to exhibit
C4 include trees of Euphorbia (Chamaesyce clade; Euphorbia-
ceae) endemic to Hawaii (Pearcy and Troughton, 1975; Young
et al., 2020), and arborescent members of Haloxylon (Amaran-
thaceae) and tall shrubs of Calligonum (Polygonaceae) from the
deserts of the Middle East and Western Central Asia (Winter et
al., 1977; Winter and Troughton, 1978; Winter, 1981).
Although a highly diverse tree community dominates the
landscape of BCI, the island has also a rich flora of herbs where
C4 photosynthesis is likely to occur. The survey presented here
identifies the C4 pathway (and C3–C4 intermediacy; Box 1) in
close to 60 herbs from six plant families on BCI and highlights
an aspect of the flora of BCI that has been overlooked.
POACEAE
Of the 86 species of Poaceae on BCI (two species were merged
into one since Croat [1978]), at least 42 are C4 and at least 29 are
C3 species (Table 1). One species is a C3–C4 intermediate (Box 1).
For the remaining 14 species, specific photosynthetic pathway
information was not obtained, but considering the taxonomic
aliation of these species, a tentative photosynthetic pathway
assignment could be made for most of them. For example, species
FIGURE 1. Principal daytime carbon fluxes in
leaves of C3 and C4 plants. For simplicity, subcel-
lular compartmentation has been omitted. In C3
plants, atmospheric CO2, after entering the meso-
phyll cells, is directly fixed by the enzyme Rubisco
into the Calvin–Benson–Bassham (CBB) cycle.
In C4 plants, CO2 (following its conversion to
HCO3–) is initially fixed by the enzyme phospho-
enolpyruvate carboxylase (PEPC) in mesophyll
cells. The reaction of HCO3– with phosphoenol-
pyruvate (PEP) forms the four carbon oxalo-
acetic acid (OAA). After reduction of OAA to
malate, the latter diuses to bundle sheath cells
where it is decarboxylated. This leads to elevated
concentrations of CO2 in the vicinity of Rubisco,
suppressing photorespiration and increasing the
rate of CO2 flux into the CBB cycle. C4 photo-
synthetic reactions may include aspartate instead
of malate, but this is not shown. Acronyms:
[CH2O], sugars/carbohydrate; RuBP, ribulose
1,5-bisphosphate; Rubisco: ribulose 1,5-bispho-
sphate carboxylase-oxygenase; Triose-P, triose
phosphate (glyceraldehyde 3-phosphate).

C4 PHOTOSYNTHESIS AT BCI
•
243
BOX 1
C3 Photosynthesis and Rubisco (Ribulose 1,5-Bisphosphate Carboxylase-Oxygenase)
C3 photosynthesis is the basic route of carbon fixation in plants, with the reactions of the Calvin–Benson–Bassham (CBB)
cycle at its core. The initial step of the CBB cycle, the conversion of atmospheric CO2 and ribulose bisphosphate (a 5-carbon mol-
ecule) into two molecules of 3-phosphoglycerate (a 3-carbon molecule), is catalyzed by Rubisco. The net result of the CBB cycle
is a surplus of triose-P (glyceraldehyde 3-phosphate) which can be used to synthesize glucose and other carbohydrates (Fig. 1).
Rubisco functions as both a carboxylase and oxygenase, that is, it accepts both CO2 and O2 as substrate. Only the carboxyl-
ation reaction results in carbon gain. By contrast, the oxygenation reaction results in carbon loss through the process of photores-
piration. CO2 loss occurs when one of the photorespiratory intermediates, glycine, is decarboxylated. Despite the extremely high
O2:CO2 ratio in air (21% O2 versus 0.04% CO2), the carboxylation: oxygenation ratio in C3 plants exposed to air is 3:1 because
of the lower solubility of O2 in the aqueous phase and the higher anity of Rubisco for CO2. Increases of the concentration of
CO2 in the cellular vicinity of Rubisco, either through elevated CO2 concentrations in the atmosphere or through plant-internal
CO2 concentrating mechanisms increase the rate of carboxylation and decrease photorespiratory loss of CO2, thereby enhancing
carbon sequestration and growth.
C4 Photosynthesis
C4 photosynthesis is a modification of the ancestral C3 pathway and leads to elevated concentrations of CO2 in the vicinity of
Rubisco to increase its CO2 fixation activity (Fig. 1). A major characteristic of C4 plants is Kranz anatomy, that is, leaf vascular
bundles are typically surrounded by two distinct types of chloroplast-containing cells, an inner ring of bundle-sheath cells that
contain Rubisco, and an outer ring of mesophyll cells that lack Rubisco. Atmospheric CO2 is first incorporated by the enzyme
phosphoenolpyruvate carboxylase (PEPC) in the outer ring of mesophyll cells, leading to the formation of the 4-carbon molecules
malate or aspartate, which are then decarboxylated in the bundle-sheath cells. This releases CO2 in the bundle-sheath cells and
creates a CO2-enriched environment around Rubisco with CO2 concentrations up to 10-fold above intercellular CO2 levels, sup-
pressing photorespiration and enhancing the rate at which CO2 enters the CBB-cycle.
C2 Photosynthesis
Plants with C2 photosynthesis show C3–C4 intermediate characteristics. They have a simplified wreath-like anatomy that
looks like a precursor to C4-Kranz anatomy as chloroplast-containing mesophyll cells and bundle-sheath cells, the latter with
centripetally concentrated mitochondria and chloroplasts, are distinguishable. However, both mesophyll and bundle-sheath cells
have Rubisco, and there is no C4-type prefixation of CO2 through PEPC in mesophyll cells. Only the mitochondria of the bundle-
sheath cells have glycine decarboxylase. Photorespiratory glycine (a 2-carbon molecule) diuses from the mesophyll cells to
glycine decarboxylase localized in centripetal mitochondria in the bundle sheath. The released CO2 accumulates and enhances
CO2 fixation by Rubisco inside the chloroplasts of the inner bundle sheath. Thus, in essence, C2 photosynthesis is a photorespi-
ratory glycine shuttle that raises bundle-sheath CO2 levels. Most known C2 plants are closely related to C4 species, supporting
the evolutionary intermediacy of C2 photosynthesis, but numerous species with no C4 relatives demonstrate the C2 mechanism is
adaptive in its own right.
CO2 Compensation Point
This is the CO2 concentration at which photosynthetic CO2 uptake exactly balances respiratory CO2 loss. It can be measured
by enclosing illuminated leaves into small spaces, whereupon the CO2 concentration decreases to approximately 50 ppm for
C3 leaves, and close to 0 ppm for C4 leaves. The dierence in CO2 compensation point is an indication of photorespiration in
C3 plants and its suppression in C4 plants.
of Bambusa, Guadua, and Chusquea were all classified as C3
because they belong to the tribe Bambuseae for which C4 has
never been recorded, although slight deviations from typical C3
photosynthesis may occur with improved refixation of photores-
pired CO2 attributed to close contact between mitochondria and
chloroplasts in three bamboo species from the Brazilian Cerrado
(Peixoto et al., 2021). These considerations lead to an overall
percentage contribution of C4 grasses to the Poaceae of BCI of
roughly 50%. While 18 (43%) of the C4 species are introduced,
largely from the Old World, only 3 (9%) of the C3 species are not
native (see information on species distributions in Croat [1978]
and Plants of the World Online [POWO, 2022]).

244
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WINTER
TABLE 1. Presence of C4 photosynthesis in species from Barro Colorado Island. Families are arranged linearly according to their
phylogenetic position as suggested by Angiosperm Phylenogy Group (APG) IV (2016). Photosynthetic pathway classification in parenthesis
is tentative. Species with confirmed, fully developed C4 are in bold. Fields marked n/a indicates data were not available or not applicable.
Accepted species in Plants of the World Online
(POWO; 2022) Species in Croat (1978)
Photosynthetic
pathway Referencea
Hydrocharitaceae
Hydrilla verticillata (L.f.) Royle Hydrilla verticillata (L.f.) Royle Aquatic, C3 with
inducible C4
1
Cyperaceae
Calyptrocarya glomerulata (Brongn.) Urb. Calyptrocarya glomerulata (Brongn.) Urban C32
Cladium mariscus subsp. jamaicense (Crantz.) Kük Cladium jamaicense Crantz C32
Cyperus blepharoleptos Steud. Scirpus cubensis Kunth ? n/a
Cyperus brevifolius (Rottb.) Hassk. Cyperus brevifolius (Rottb.) Endl. ex Hassk. C43
Cyperus chorisanthos C.B.Clarkebn/a ?n/a
Cyperus conservator-davidii G.C.Tuckerbn/a ?n/a
Cyperus hortensis (Salzm. ex Steud.) Dorr Cyperus densicaespitosus Mattf. & Kuek. C42 for Kyllinga
pumila Michx.
Cyperus diusus Vahl Cyperus diusus Vahl ?n/a
Cyperus giganteus Vahl Cyperus giganteus Vahl C42
Cyperus haspan L. Cyperus haspan L. C32
Cyperus laxus Lam. Cyperus laxus Lam. C32
Cyperus luzulae (L.) Retz. Cyperus luzulae (L.) Retz. C32
Cyperus odoratus L. Cyperus odoratus L. C42
Cyperus rotundus L. Cyperus rotundus L. C42
Cyperus sesquiflorus (Torr.) Mattf. & Kük. Cyperus sesquiflorus (Torr.) Mattf. & Kuek. C44
Cyperus simplex Kunth. Cyperus simplex H.B.K. C35
Cyperus tenuis Sw. Cyperus tenuis Sw. C4 2
Eleocharis geniculata (L.) Roem. & Schult. Eleocharis caribaea (Rottb.) S. F. Blake C32
Eleocharis interstincta (Vahl) Roem. & Schult.bn/a C32
Eleocharis plicarhachis (Griseb.) Svenson Eleocharis plicarhachis (Griseb.) Svens. C32
Fimbristylis dichotoma (L.) Vahl Fimbristylis dichotoma (L.) Vahl C42
Fuirena umbellata Rottb. Fuirena umbellata Rottb. C32
Hypolytrum longifolium (Rich.) Neesbn/a C32
Hypolytrum longifolium subsp. nicaraguense
(Liebm.) T.Koyamabn/a ?n/a
Hypolytrum schraderianum Nees Hypolytrum schraderianum Nees C32
Rhynchospora cephalotes (L.) Vahl Rhynchospora cephalotes (L.) Vahl C32
Rhynchospora corymbosa (L.) Britton Rhynchospora corymbosa (L.) Britt. C32
Rhynchospora rariflora (Michx.) Elliott Rhynchospora micrantha Vahl C32
Rhynchospora nervosa (Vahl) Boeckeler Rhynchospora nervosa (Vahl) Boeck. C32
Rhynchospora pedersenii Guagl.bn/a ?n/a
Rhynchospora pura (Nees) Griseb.bRhynchospora nervosa subsp. ciliata T.Koyamab?n/a
Rhynchospora radicans (Schltdl. & Cham.) H.Pfei.bn/a C32

Accepted species in POWO (2022) Species in Croat (1978)
Photosynthetic
pathway Referencea
Rhynchospora radicans subsp. microsephala (Bertero
ex Spreng.) W.W.Thomasbn/a C32
Scleria eggersiana Boeckeler Scleria eggersiana Boeck. (C3)2
Scleria gaertneri Raddi Scleria pterota Presl C36
Scleria macrophylla J.Presl & C.Presl Scleria macrophylla Presl (C3)2
Scleria mitis P.J.Bergius Scleria mitis Bergius (C3)2
Scleria secans (L.) Urb. Scleria secans (L.) Urban C36
Poaceae
Acroceras zizanioides (Kunth) Dandy Acroceras oryzoides Stapf C37
Andropogon bicornis L. Andropogon bicornis L. C48
Andropogon glomeratus (Walter) Britton, Sterns &
Poggenb.
Andropogon glomeratus (Walt.) B.S.P. C49
Andropogon leucostachyus Kunth Andropogon leucostachyus H.B.K. C410
Andropogon virginicus L. Andropogon virginicus L. C47
Anthephora hermaphrodita (L.) Kuntze Anthephora hermaphrodita (L.) O. Kuntze C410
Axonopus compressus (Sw.) P.Beauv. Axonopus compressus (Sw.) Beauv. C47
Bambusa bambos (L.) Voss Bambusa arundinacea Retz. (C3)11
Bambusa multiplex (Lour.) Raeusch. ex Schult.f. Bambusa glaucescens (Willd.) Sieb. Ex Munro (C3)11
Bothriochloa bladhii (Retz.) S.T.Blake Bothriochloa intermedia (R. Br.) A. Camus C49
Bothriochloa pertusa (L.) A. Camus Bothriochloa pertusa (L.) A. Camus C410
Cenchrus brownii Roem. & Schult. Cenchrus brownii R. & S. C410
Chloris radiata (L.) Sw. Chloris radiata (L.) Sw. C49
Chusquea simpliciflora Munro Chusquea simpliciflora Munro (C3)11
Cynodon dactylon (L.) Pers. Cynodon dactylon (L.) Pers. C47
Digitaria bicornis (Lam.) Roem. & Schult.bn/a C412
Digitaria ciliaris (Retz.) Koeler Digitaria ciliaris (Retz.) Koel. C410
Digitaria horizontalis Willd. Digitaria horizontalis Willd. C49
Digitaria violascens Link Digitaria violascens Link C410
Eleusine indica (L.) Gaertn. Eleusine indica (L.) Gaertn. C410
Guadua amplexifolia J.Presl. Bambusa amplexifolia (Presl) Schult.f. (C3)11
Guadua maclurei R.W. Pohl & Davidsebn/a (C3)11
Gynerium sagittatum (Aubl.) P.Beauv. Gynerium sagittatum (Aubl.) Beauv. C310
Homolepis aturensis (Kunth) Chase Homolepis aturensis (H.B.K.) Chase C3–C4 13
Hymenachne amplexicaulis (Rudge) Nees Hymenachne amplexicaulis (Rudge) Nees C310
Hymenachne grandis (Hitchc. & Chase) Zuloaga Panicum grande Hitchc. & Chase C37
Hyparrhenia rufa (Nees) Stapf Hyparrhenia rufa (Nees) Stapf C410
Ichnanthus pallens (Sw.) Munro ex Benth. Ichnanthus pallens (Sw.) Munro ex Benth. C38
Ichnanthus pallens var. pallens (Sw.) Munro
exBenth.
Ichnanthus brevivaginatus Swall. C310
Ichnanthus tenuis (J.Presl) Hitchc. & Chase Ichnanthus tenuis (Presl) Hitchc. & Chase C310
Imperata contracta (Kunth) Hitchc.bn/a C410
(Continued)
TABLE 1. (Continued.)
C4 PHOTOSYNTHESIS AT BCI
•
245

246
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WINTER
Accepted species in POWO (2022) Species in Croat (1978)
Photosynthetic
pathway Referencea
Isachne polygonoides (Lam.) Döll Isachne polygonoides (Lam.) Doell ?
Ischaemum rugosum Salisb. Ischaemum rugosum Salisb. C410
Ischaemum timorense Kunthbn/a ?n/a
Lasiacis maculata (Aubl.) Urb. Lasiacis sorghoidea (Desv. ex Ham.) Hitchc. &
Chase var. sorghoidea
C310
Lasiacis oaxacensis (Steud.) Hitchc. ex Chase Lasiacis oaxacensis (Steud.) Hitchc. (C3)12
Lasiacis procerrima (Hack.) Hitchc. Lasiacis procerrima (Hack.) Hitchc. C310
Leersia hexandra Sw. Leersia hexandra Sw. C39
Leptochloa virgata (L.) P.Beauv. Leptochloa virgata (L.) P.Beauv. C410
Lithachne pauciflora (Sw.) P.Beauv. Lithachne pauciflora (Sw.) Beauv. ex Poir. C314
Luziola subintegra SwallenbC310
Megathyrsus maximus (Jacq) B.K.Simon &
S.W.L.Jacobs
Panicum maximum Jacq. C47
Ocellochloa pulchella (Raddi) Zuloaga & Morrone Panicum pulchellum Raddi C37
Olyra ecaudata Döllbn/a ?n/a
Olyra latifolia L. Olyra latifolia L. C38
Oplismenus burmanni (Retz.) P.Beauv. Oplismenus burmanni (Retz.) Beauv. C37
Oplismenus hirtellus (L.) P.Beauv. Oplismenus hirtellus (L.) Beauv. C37
Orthoclada laxa (Rich.) P.Beauv. Orthoclada laxa (L. C. Rich.) Beauv. C310
Oryza latifolia Desv. Oryza latifolia Desv. C310
Panicum trichanthum Nees Panicum trichanthum Nees C37
Panicum trichoides Sw. Panicum trichoides Sw. C37
Paspalum conjugatum P.J.Bergius Paspalum conjugatum Bergius C49
Paspalum corcovadense Raddibn/a ?n/a
Paspalum decumbens Sw. Paspalum decumbens Sw. C47
Paspalum microstachyum J.Presl Paspalum microstachyum Presl ?n/a
Paspalum notatum Flüggé Paspalum notatum Flugge C47
Paspalum paniculatum L. Paspalum paniculatum L. C49
Paspalum plicatulum Michx. Paspalum plicatulum Michx. C49
Paspalum repens P.J.Bergius Paspalum repens Bergius C47
Paspalum saccharoides Nees ex Trin. Paspalum saccharoides Nees C410
Paspalum virgatum L. Paspalum virgatum L. C49
Pharus latifolius L. Pharus latifolius L. C37, 15
Pharus parvifolius Nash Pharus parvifolius Nash (C3)11
Pharus virescens Döllbn/a (C3)11
Phragmites australis (Cav.) Trin. Ex Steud. Phragmites australis (Cav.) Trin. C312
Polytrias indica (Houtt.) Veldkamp Polytrias amaura (Buse ex Miq.) O. Kuntze
Ischaemum indicum (Houtt.) Merr.
C412
Rhipidocladum racemiflorum (Steud.) McClure Rhipidocladum racemiflorum (Steud.) McClure C312
Rottboellia cochinchinensis (Lour.) Clayton Rottboellia exaltata (L.) L.f. C410
Rugoloa pilosa (Sw.) Zuloaga Panicum milleflorum Hitchc. & Chase
Panicum pilosum Sw.
C37
TABLE 1. (Continued.)

Accepted species in POWO (2022) Species in Croat (1978)
Photosynthetic
pathway Referencea
Rugoloa polygonata (Schrad.) Zuloaga Panicum polygonatum Schrad. ex Schult. C37
Saccharum ocinarum L.Saccharum ocinarum L.C47
Saccharum spontaneum L. Saccharum spontaneum L. C49
Sacciolepis striata (L.) Nash Sacciolepis striata (L.) Nash C37
Schizachyrium brevifolium (Sw.) Nees ex Buse Schizachyrium brevifolium Nees ex Kunth C410
Schizachyrium microstachyum (Desv.) Roseng.,
B.R.Arril. & Izag.
Schizachyrium microstachyum (Desv.) Roseng.,
Arr. & Izag.
?n/a
Setaria geminata (Forssk.) Veldkamp Paspalidium geminatum Stapf C412
Setaria palmifolia (J.Koenig) Stapf Setaria paniculifera (Steud.) Fourrn. C47
Setaria parviflora (Poir.) Kerguélen Setaria geniculata (Lam.) Beauv. C48
Setaria vulpiseta (Lam.) Roem. & Schult. Setaria vulpiseta (Lam.) R. & S. C410
Sporobolus indicus (L.) R.Br. Sporobolus indicus (L.) R. Br. C49
Sporobolus pyramidalis P.Beauv. Sporobolus jacquemontii KunthbC410
Stephostachys mertensii (Roth) Zuloaga & Morrone Panicum mertensii Roth C37
Streptochaeta sodiroana Hack. Streptochaeta sodiroana Hack. C315
Streptochaeta spicata Schrad. ex Nees Streptochaeta spicata Schrad. ex Nees C315
Streptogyna americana C. E. Hubb. Streptogyna americana C.E.Hubb. C312
Urochloa fusca (Sw.) B.F.Hansen & Wunderlin Panicum fasciculatum Sw. C47
Urochloa mutica (Forssk.) T.Q.Nguyen Brachiaria mutica (Forssk.) Stapf C416
Euphorbiaceae
Euphorbia hirta L. Chamaesyce hirta (L.) Millsp. C417
Euphorbia hypericifolia L. Chamaesyce hypericifolia (L.) Millsp. C417
Euphorbia hyssopifolia L. Chamaesyce hyssopifolia (L.) Small C418
Euphorbia thymifolia L. Chamaesyce thymifolia (L.) Millsp. C418
Amaranthaceae
Alternanthera ficoidea (L.) P.Beauv. Alternanthera ficoidea (L.) R. Br. C3–C419
Alternanthera sessilis (L.) R.Br. ex DC. Alternanthera sessilis (L.) R. Br. C319
Amaranthus viridis L. Amaranthus viridis L. C419
Chamissoa altissima (Jacq.) H.B.K. Chamissoa altissima (Jacq.) Kunth C319
Cyathula prostrata (L.) Blume Cyathula prostrata (L.) Blume C319
Gomphrena serrata L. Gomphrena decumbens Jacq. C419
Iresine angustifolia Euphrasén Iresine angustifolia Euphr. C319
Iresine rhizomatosa Standl. Iresine celosia L. C319
Portulacaceae
Portulaca oleracea L. Portulaca oleracea L. C4–CAM 20
a References: 1, Reiskind et al. (1997); 2, Bruhl and Wilson (2007); 3, Carolin et al. (1977); 4, Rudov et al. (2020); 5, Martins and Alves (2009); 6, Hoss
(2013); 7, Brown (1977); 8, Klink and Joly (1989); 9, Waller and Lewis (1979); 10, Giraldo-Cañas (2010); 11, Sage et al. (1999); 12, Osborne et al.
(2014); 13, Khoshravesh et al. (2016); 14, Smith and Brown (1973); 15, Mulkey (1986); 16, Batanouny et al. (1988); 17, Hofstra et al. (1972); 18, Yang
and Berry (2011); 19, Sage et al. (2007); 20, Koch and Kennedy (1980).
b Species added to the BCI flora after Croat (1978). Thomas Croat collected most of these in the 1960s and 1970s and deposited vouchers at the Missouri
Botanical Garden Herbarium that were identified later (https://stricollections.org/portal/checklists/checklist.php?pid=20&clid=177).
TABLE 1. (Continued.)
C4 PHOTOSYNTHESIS AT BCI
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Frequently encountered C4 grasses in natural clearings and
open areas of BCI are Andropogon bicornis L., Cenchrus brownii
Roem. & Schult., Digitaria horizontalis Willd., and Setaria par-
viflora (Poir.) Kerguélen. Locally abundant in the BCI laboratory
clearing are the C4 grasses Axonopus compressus (Sw.) P.Beauv.,
Bothriochloa bladhii (Retz.) S.T.Blake, Hyparrhenia rufa (Nees)
Stapf, Megathyrsus maximus (Jacq) B.K.Simon & S.W.L.Jacobs,
Paspalum paniculatum L., Polytrias indica (Houtt.) Veldkamp,
Roettboellia cochinchinensis (Lour.) Clayton, and Sporobolus
indicus (L.) R.Br. Commonly found Poaceae within the forest are
the C3 species Chusquea simpliciflora Munro, Lithachne pauci-
flora (Sw.) P.Beauv., Olyra latifolia L., Oplismenus burmanni
(Retz.) P.Beauv., Oplismenus hirtellus (L.) P.Beauv., and Ortho-
clada laxa (Rich.) P.Beauv. Physiological studies with three C3
species of understory herbaceous bamboo, two of which (Pharus
latifolius L. and Streptochaeta sodiroana Hack.) are abundant in
the forest of BCI, demonstrated only limited potential for photo-
synthetic acclimation upon exposure of plants to elevated light
levels (Mulkey, 1986).
The most prominent “invasive” C4 grass in Panama is Sac-
charum spontaneum L. Its native range includes Sicilia, northern
Africa, southern Asia to North and Northeast Australia (POWO,
2022). S. spontaneum occupies large areas of the Panama Canal
Watershed where its presence greatly complicates eorts to
restore forest cover (Jones et al., 2004). The species, however, is
only occasionally spotted on BCI where some plants reproduce
along the margins of the island. The BCI 50-ha plot averaged
three seeds of S. spontaneum m–2 year–1, likely dispersed from
the mainland, between 1990 and 2019 (Joseph Wright, pers.
comm.). Although S. spontaneum recruits into treefall gaps on
BCI, it has never been observed to reproduce successfully in tree-
fall gaps (Joseph Wright, pers. comm.). Plants die when treefall
gaps close. Research on the origin of the species in Panama com-
pared DNA sequences and microsatellite markers of Panamanian
populations with those of modern and past sugarcane germplasm
collections and concluded that the likely starting point for the S.
spontaneum invasion in Panama was an accidental escape from a
U.S. Department of Agriculture sugarcane germplasm collection
that was brought to Panama in 1939 (Saltonstall et al., 2021).
Another notable species is Homolepis aturensis (Kunth)
Chase, a perennial grass with a native range from Mexico to
southern Tropical America that is common in clearings on BCI.
It has a C3-type δ13C value (Smith and Brown, 1973), but the
concentration of chloroplasts within large bundle-sheath cells
led Christin et al. (2013) to suggest that the species is a C3–C4
intermediate. This was confirmed by Khoshravesh et al. (2016),
who reported a CO2 compensation point (Box 1) for H. aturensis
that was in between that of C3 plants (typically 50 ppm CO2)
and that of C4 plants (close to 0 ppm CO2). Furthermore, the
authors provided evidence for the operation of a C2-metabolic
cycle in H. aturensis involving photorespiratory glycine (Lun-
dgren, 2020; Box 1). It has been argued that C2 photosynthesis
intermediacy is a distinct photosynthetic adaptation and not just
a transitional stage leading to C4 (Sage et al., 2011).
CYPERACEAE
The sedges (Cyperaceae) are another family of monocoty-
ledons in which C4 is well documented. Among the 38 sedge
species recorded from BCI, the C4 pathway is present in at least
8. All C4 sedges except one, Cyperus rotundus L., are native to
Panama. Seven are species of Cyperus. The most conspicuous
C4 species is Cyperus giganteus Vahl, an aquatic perennial that
forms dense stands up to 2.5 m tall in some places along the
shore (Croat, 1978). Cyperus rotundus L. has been called the
world’s worst weed (see Simpson et al., 2011).
EUPHORBIACEAE
Euphorbiaceae is a large family of dicotyledonous plants
containing C3, C4, and CAM species. For BCI, 33 species of
Euphorbiaceae have been described, 19 of which are shrubs and
trees. C4 photosynthesis is present in four monoecious herbs of
Euphorbia, which were formerly ascribed to the genus Chamae-
syce. They are typically found in clearings. Euphorbia hirta L. is
the most abundant of the four C4 species of Euphorbiaceae on
BCI (Croat, 1978).
AMARANTHACEAE
Of the eight species of Amaranthaceae on BCI, two are C4
and one may be C3–C4. Croat (1978) described the status of the
two C4 species, Amaranthus viridis L. and Gomphrena serrata
L., as rare on BCI, but this may have changed since then. A. viri-
dis is an annual herb, up to 1 m tall, and the prostrate G. serrata
is an invasive weed in many Central American countries.
PORTULACACEAE
There is only one genus, Portulaca, in the Portulacaceae.
The best-known species is the C4 plant Portulaca oleracea L.
(Hatch, 1975). It is a prostrate radially spreading herb with a
worldwide distribution in temperate and tropical regions. The
species occurs across Panama in disturbed areas. P. oleracea is
the only species of Portulacaceae listed in the Flora of Barro Col-
orado Island by Croat (1978), who notes on page 389 that “no
specimens from the island have been seen, but the species should
be expected there.”
What is remarkable about P. portulaca is that although C4
photosynthesis is the principal pathway of carbon assimilation,
leaves can shift from C4 to CAM photosynthesis under water-
deficit stress (Koch and Kennedy, 1980)—that is, as soil water
content decreases, C4 photosynthesis is downregulated and CAM
photosynthesis induced. Upon rewatering, plants abolish CAM
and return to solely C4 photosynthetic CO2 fixation (Winter and
Holtum, 2014). In contrast to C3 and C4 plants, which assimilate

CO2 exclusively during the daytime, CAM photosynthesis is
characterized by net CO2 uptake at night and allows plants to
gain carbon at extremely low water expenditure (for more details
on CAM, see Winter, 2024). Although C3+facultative CAM is
observed in a range of species from at least 15 families (Winter,
2019), C4+facultative CAM has been conclusively demonstrated
only for species of Portulaca, although evidence now also sup-
ports the presence of CAM in the C4 species Trianthema portu-
lacastrum, a pantropical prostrate annual herb of the Aizoaceae
(Winter et al., 2021). Unlike C3 and CAM, there are many rea-
sons to expect that C4 and CAM are mutually exclusive (Sage,
2002); however, this is evidently not so. The simultaneous opera-
tion of C4 and CAM photosynthesis within the same leaf raises
interesting questions about the compartmentalization of the two
pathways, as both are modifications of the ancestral C3 path-
way and both employ two comparable carboxylation reactions
in sequence, spatially separated in C4 and temporally separated
in CAM. The critical question of whether C4 and CAM occur
in dierent regions of the Portulaca leaf, or whether there is cell
sharing, is currently under intense investigation (Ferrari et al.,
2020; Moreno-Villena et al., 2022).
HYDROCHARITACEAE
Hydrilla verticillata (L.f.) Royle is one of three aquatic species
of Hydrocharitaceae noted for BCI. It is a perennial, submerged
freshwater herb, and is extremely abundant along the edges of
Gatun Lake (Croat, 1978). About 100 years ago, Sir Jagadish
Chandra Bose (1858–1937), an eminent Indian scientist, studied
the physiology of H. verticillata and observed anomalies pointing
to the involvement of malate in the photosynthetic process in the
light (Raghavendra and Govindjee, 2011). We now know that H.
verticillata, which lacks Kranz anatomy (Box 1), typically exhibits
C3 photosynthesis, but induces a C4 system upon exposure to low
[CO2] that involves initial CO2 fixation via PEPC (with malate as
intermediate) and concentrates CO2 from the cytosol into chloro-
plasts of single mesophyll cells, thereby suppressing photorespira-
tion (Bowes and Salvucci, 1984; Reiskind et al., 1997; Reiskind
and Maberly, 2014). This aquatic, inducible single-cell CO2 con-
centrating mechanism, with C4 and CBB cycles operating within
the same cell, likely provides an ecological advantage in densely
vegetated lake environments where daytime CO2 levels are low
and O2 levels and temperatures are high.
C4 FAMILIES WITHOUT C4
REPRESENTATIVES ON BCI
Eight angiosperm families recorded from BCI are known
to have C4 species, but lack C4 species on BCI (Boraginaceae,
Asteraceae, Caryophyllaceae, Cleomaceae, Nyctaginaceae, Polyg-
onaceae, Acanthaceae, and Scrophulariaceae). For example,
Cleomaceae contain a range of species that span a developmental
progression from C3 to C4 photosynthesis (Marshall et al., 2007).
The only species of Cleomaceae occurring on BCI, the annual
herb Cleome parviflora Kunth subsp. parviflora, is a C3 plant
(Feodorova et al., 2010).
CONCLUSION
Grasses are the most significant component of the C4 flora
on BCI. Although the C4 grasses of BCI mainly occur at open
sites and the C3 grasses in more shaded habitats, the local grass
biogeography on BCI has never been studied rigorously. State-
ments made more than 40 years ago by T. B. Croat in his Flora
of Barro Colorado Island on the abundance of some of the spe-
cies highlighted in this article may no longer be correct. More
research on distribution and dynamics of the C4 herbs on BCI
seems worthwhile, especially in the context of contemporary
atmospheric and climate change.
ACKNOWLEDGMENTS
No specific funds were received to write this article. Joseph
Wright and Rowan Sage provided useful suggestions on earlier
versions of the manuscript. Aurelio Virgo assisted with the prep-
aration of Figure 1.
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