A Light and Electron Microscopic Study of the Palo Verde Tree of the Desert Southwest

Introduction

The palo verde tree (well known to residents of Texas, Arizona, New Mexico, Mexico, as well as Central and South America) has bright green trunks, stems, and leaves and is perfectly fitted for life in the harsh Southwest. The bright green nature of the trunk, stems, and petioles of Parkinsonia is undoubtedly the reason why this tree is known as the palo verde (“green stick” in Spanish). It is extremely drought tolerant, thrives on little water, and is even considered a weed in many communities. It is drought deciduous, meaning that it drops its leaflets during certain, particularly dry periods, and it also employs stem photosynthesis, which means that most photosynthesis takes place in the petioles, stems, and trunk, and not in the leaves.

Palo verde has variants found on Hawaii, the Galapagos Islands, and southern Africa, as well as in India. In Australia, where it was introduced as an ornamental and shade tree in the year 1900, it has become a widely spread invasive, requiring the introduction of several insects to serve as biological controls to limit further spread. Palo verde has also been reported from the Caribbean as well as Central and South America. In Florida, it is prized as an ornamental. In 1966, it was named the city tree of Miami.

The palo verde was also made the official state tree of the state of Arizona in 1954, although two distinct species were lumped together by the state as its tree: Parkinsonia microphylla and P. florida. Both trees have green trunks and stems but the blue palo verde has more of a blue-green tint. The Arizona state website continues to refer to P. florida as Cercidium foridum, an earlier genus and species name which persists in the literature.

It may be that the widespread usage of Parkinsonia as a landscape ornamental in Arizona (Dimmit 1987, 2014; Schuch and Kelly 2008, 2012), New Mexico, Florida and outside of the United States has contributed to its familiarity. However, little besides the technical aspects of landscape management is known about the unique characteristics of this remarkable plant. These characteristics reported here are, in our minds, indicators of planning and purpose. This unique collection of characteristics allows it to thrive in environments which might kill many other well-known ornamental plants. The only alternative to a purposeful explanation of its origin is, of course, descent with modification, which we find lacking as a reasonable explanation for the collection of characteristics found in palo verde.

Materials and Methods

Stems and petioles of Parkinsonia florida (blue palo verde tree) were collected from sites in Arizona, fixed in 2% glutaraldehyde, postfixed in osmium, dehydrated through a graded series of acetone and embedded in Embed 812 (Electron Microscopy Sciences, Hatfield, Pennsylvania). Leaves were not studied. Tissue fixation, postfixation, and dehydration were performed in a laboratory microwave (Pelco Biowave, Ted Pella, Inc., Redding, California).

Embedded stems were selected for light microscopy (thick) sectioning and transmission electron microscopy (TEM) thin sectioning. Thick sections were collected on glass slides and stained with Epoxy Tissue Stain (EMS), which imparts blue and blue-related colors to the tissues and to the embedding polymer. Gold sections were collected with copper grids, stained in lead citrate and uranyl acetate, and
imaged in an AEI 801 TEM. Other sections were
affixed to aluminum stubs, sputter coated with gold
and examined on a Hitachi S 2500 scanning electron
microscope.

Here we explore some of the unique anatomical
features of the palo verde stem. We intend to discuss
morphology and fitness of leaves and petioles in a
future report.

Results

There is a paucity of anatomical reports on
Parkinsonia and Cercidium, thus a need for further
work on the anatomy of this interesting plant exists.
We could find only one study which elucidates the
ultrastructure of leaves, stems, roots, and seeds
of the palo verde (Scott 1935), but it lacks any
study of petioles, which perform a good deal of the
photosynthesis. Furthermore it lacks the necessary
micrographs for a complete understanding of
anatomy. It focused on only two of the 15 or so
varieties of the palo verde.

Palo verde is commonly used as an ornamental
tree in housing and city land improvement in the
southwest (figs. 1 and 2), thus it represents a plentiful
source of specimen material. The leaves of P. florida
are doubly compound and many leaflets and sub
leaflets are attached to each petiole (figs. 3 and 4). All
are the same asparagus green color. Hair cells (fig. 3)
are abundant on stems and leaves but rarely survived
processing for microscopy. Stem ultrastructure
agrees somewhat with results published by Scott
(1935), although she studied analogs of P. florida.
crystals were found as deep as secondary xylem in
contrast to Scott’s report. Interestingly the calcium
oxalate crystals observed in palo verde stems took the
druse form. No needle-like crystals were observed in
palo verde stems.

The sunken guard cell pairs, idioblasts, cutined
cells, and cortex are well defined (figs. 5 and 6); however, cambium, phloem, and xylem (primary and
secondary) were not as well defined as hoped. Large
slabs of cutin are visible in figs. 6–8. Xylem tissues were
thickly lignified. Starch, stained darkly with iodine,
was present in abundance, in agreement with Scott
(1935) and Scott, Bystrom, and Bowler (1962), and was
well distributed across phloem and xylem; however
boundaries between these tissues were difficult to
identify. Starch grains were highly concentrated in
association with a circumferential ring of phloem (fig.
9). Many oil droplets were found deposited within
photosynthetic cells and especially in chloroplasts
(fig. 10). Large amounts of calcium oxalate were also
deposited throughout the stem (figs. 5, 8, 10, and 11),
and of note, between stoma and the outer border of the cortex (fig. 8, oblique angle). Photosynthetic cells
usually contained crystals of calcium oxalate or lipids,
or sometimes both, (fig. 10). Many chloroplasts were
often bulging with oil droplets (fig. 10). Of note, spines
also had sunken stomata (fig. 12).

Discussion

Palo verde is generally found as a small tree
or shrub, which lives across the dry Sonoran and
Arizona deserts. The tree has a very deep root
system, which allows it to locate and use deep
groundwater and to survive extended periods of
drought. The generous root system also allows it to
survive intermittent flash floods, which are common
in the southwestern deserts during summer months.
Trunks may approach 10 in (25 cm) in diameter
and may be decumbent or vertical. Most palo verde
plants are bushes of approximately 3 ft (1 m) in
height. One early report from 1856 indicates that
“. . . this tree is not particularly useful. Its chief
purpose is to ornament the arroyos and flood basins
of the desert regions and to furnish brake blocks for
desert freight-wagons.” (Rose and Standley 1912).
While that observation seems to restrict palo verde
to flood basins, another early report indicated that
Parkinsonia preferred rocky or well-drained slopes
(Cannon 1905). Perhaps early writers had not yet
distinguished between Parkinsonia microphylla and
P. florida which thrive in different locales.

Two major genera have been erected to encompass
the different species of the palo verde tree, but there is
confusion (Lersten and Curtis 1995; Polhill and Vidal
1981). There seem to be four to six species assigned
to each group, or alternately, circa 13–15 species are
lumped together under Parkinsonia. In their study of
leaf anatomy, Lersten and Curtis (1995) offered some
morphological (and therefore taxonomic) differences
between Parkinsonia and Cercidium, but it is unclear
if the taxonomic confusion had been resolved.

P. microphylla (the yellow or foothill palo verde)
has large spines (mostly at branch terminations)
which make it undesirable as a walkway or
ornamental planting. It features very small leaves,
which are often shed soon after the spring and winter
rains subside. This variety, the slowest growing of
the group, is widespread across southern Arizona
and along the Colorado River in California. Small
flowers exhibit five petals, mostly yellow, but some
petals are white, making it a paler yellow during
spring blooms (Johnson 1996; Schuch and Kelly
2008). This species of palo verde does not tolerate
well-watered conditions and as a drought-deciduous
tree, it drops stems as well as leaves with petioles
when dry conditions persist.

P. florida, known as the blue palo verde, is often
referred to as Cercidium floridum. It is faster
growing than P. microphylla and is more decumbent,
having trunks which often lie some distance parallel
to the ground before they ascend. It has deep-yellow
flowers as opposed to the paler ones exhibited by P.
microphylla. Its flowers also bloom earlier than other
species, and it has smaller branch thorns. P. florida
appears to have a wider range than P. microphylla,
but there seem to be conflicting reports of distribution
in the literature. It is, however, found at non-desert
elevations of 1200 m (4000 ft) and is cold tolerant to
50°F. Finally, USGS distribution charts show that
P. florida (as well as P. aculeata) are native to areas
mostly outside of Arizona, which is curious, as P.
florida is the celebrated state tree of Arizona.

P. aculeata, also known as the Mexican palo verde,
is similar to the aforementioned species. Because it is
very well armed, it is no longer considered a desirable
landscape tree. It develops double or triple shaped,
sharp spines, and it bears large, multi-colored flowers.
It is distributed across west Texas, southern Arizona,
southern New Mexico, and northern Mexico. It has
become invasive in Australia, forming large thickets,
and has also been found in Europe, but it is sensitive
to temperatures below 20°F.

There has been some success at developing
hybrids, such as P. praecox, which is a three-way-cross
of P. aculeata, P. microphylla, and P. florida.
It is a hardy plant with no thorns and can tolerate
cold temperatures down to 15°F.

For the purposes of this paper, the various
members of this hybrid group will be referred to as
palo verde.

Palo verde was used by native Americans as food
(parched seeds) and in some instances, the extracts
of bark, leaves, flowers, and fruits have been used in
herbal medicines to treat arthritis and to stimulate
nerves (Liogier 1990).

Little work has been done on palo verde tree
anatomy/ultrastructure or otherwise describing stem
features and other characteristics. Few papers of it
exist in the literature. The notable exception that
it is often featured in horticultural and ornamental
landscape magazines, thus it has received some
interest for xeric use in Arizona, New Mexico, and
west Texas neighborhoods.

Drought-Deciduous Behavior

Of interest to botanists is the fact that Parkinsonia
is a drought-deciduous tree and performs a great
deal of photosynthesis in its stems and petioles.
Photosynthesis is conducted in leaves when they are
present, however it is most efficient at photosynthesis
after losing its leaves.

As a result of dry, hot conditions, Parkinsonia
sheds all leaves (and in some instances petioles
and young stems) during extended periods of stress
and yet remains healthy and productive. Fewer
resources are devoted to leaf maintenance during
stress, allowing for health and stability while needed
metabolites are produced in stems.

Drought-deciduous behavior may be changed by
routine watering. It has been reported that when
irrigated, Parkinsonia initially produces larger
leaves than non-watered specimens. These large
leaves remain highly functional, persisting for longer
periods while water is available (Cannon 1905).

There is considerable variation among different
species of plants that are drought-deciduous. Some
plants, which are somewhat like Parkinsonia drop
their leaves yet continue to perform photosynthesis
in well-watered conditions. All photosynthetic
activity is stopped, however, during drought. For
example the shallow-root summer deciduous shrub
Anthyllis (throughout Spain) performs most of its
photosynthesis and vegetative growth during cool
wet seasons (even if leaves have been dropped). With
the onset of the dry, warm season, however, leaves
are shed and photosynthetic rates decline to zero
by mid summer (Haase et al. 2000). Interestingly,
some drought-deciduous plants can perform stem
photosynthesis while heavily lignified (Kocurek and
Pilarski 2012).

Shallow-rooted plants such as Grayia (the
saltbush) and Tetradymia (the short spine
horsebush) drop leaves during summer drought, but,
because of anatomical differences in xylem, they are
able to retain and transport water during dry periods
allowing for photosynthesis in stems (Hacke, Sperry,
and Pitterman 2000). Often drought declining
photosynthesis is due to cavitation (air embolisms)
within columns of water retained in xylem tubes,
interrupting water delivery. Many shallow-root,
drought-deciduous plants suffer less from drought
due to heavily lignified xylem tissues (Hacke, Sperry,
and Pitterman 2000), which resist cavitation. Palo
verde, a deep-rooted, drought-deciduous plant also
features heavily lignified xylem (Scott 1935), adding
another fitness feature to its list.

Lycium, (the boxthorn), a drought-deciduous
shrub common in the Mojave Desert, produces
short shoots from which microphyllous leaves are
expressed early in the spring. These leaves have
higher initial productivity than do leaves on long
shoots. They are only produced during heavy rain
years and supply much of the metabolites for
annual growth (Hamerlynck et al. 2002). Thus in
boxthorn, dropping leaves during periods of drought
is complemented by the production of short shoots to
continue photosynthesis. Productivity becomes much
lower, however, after leaves are shed.

Still other drought-deciduous plants take
advantage of intermittent rainfall during dry
periods by retaining or producing non-dormant buds
throughout the year. This allows the plant to rapidly
flush or produce leaves following rainfall events
(Jolly and Running 2004), only to drop them after dry
periods return.

Drought-deciduous behavior in palo verde adds to
its fitness for desert survival.

Stem Photosynthesis

Concurrent with drought-deciduous stress/survival
behavior, much photosynthesis can take place in
trunks, stems, and petioles of trees before and after
their leaves are dropped. Many plants perform stem
photosynthesis in a wide variety of habitats including
Parkinsonia (or Cercidium as reported by Adams,
Strain, and Ting, 1967). The majority of such plants
seem restricted to desert or Mediterranean climates.
As reported here, photosynthetic cells extended from
the outer cortex of palo verde, well into vascular
tissues, thus the stem anatomy of palo verde is fitted
largely for photosynthesis. Stem photosynthesis
is considered a significant alternative leaf
photosynthesis. In fact 45% of total tree chlorophyll
has been found in twig and branch chlorenchyma in
some trees (Berveiller, Kierzkowski, and Damesin
2007; Kharouk et al. 1995).

It is certainly intuitive to expect that herbaceous
or succulent plants with green stems and trunks
perform photosynthesis in those organs as they do
(Kocurek and Pilarski 2012; Nilsen and Sharifi 1997).
Herbaceous green stems are also almost as efficient
as leaves in the absorption of light for metabolic
work (up to 90%—Kocurek and Pilarski 2012).
Recent work shows that herbaceous plant stems do
more than simply assist in light-related metabolic
activities. Some herbaceous plants and trees capture
light via stems and trunks and distribute the light
internally through the vascular system—even to the
roots (Sun et al. 2003; Sun, Yoda, and Suzuki 2005).
It has also been reported that flowers and fruits
perform photosynthesis as well (Aschan et al. 2005;
Berveiller, Kierzkowski, and Damesin 2007).

There is no question that stem photosynthesis
provides benefits for palo verde in stressed biomes.
It extends the growth season for this plant in the
absence of leaves and it efficiently recaptures and
fixes carbon. Moreover, it lessens plant stress by
providing a continuous supply of carbohydrates
while minimizing the use of water during times of
drought (Nilsen and Sharifi 1997; Pfanz 1999; Pfanz
et al. 2002).

It is surprising, however, that highly lignified,
wood plants such as alder, ash, aspen, beech, birch,
oak, eucalyptus, ginkgo, pine, and certain other trees
with seemingly opaque, hard bark perform stem
photosynthesis even when the tree is not stressed
at all by drought, pests, or pollutants (Cernusak
and Hutley 2011; Kharouk et al. 1995; Kocurek and
Pilarski 2012; Pfanz 1999; Pfanz et al. 2002; Wittman
and Pfanz 2014). This is remarkable because these
stems are not specialized for photosynthesis. Yet, as
mentioned, stems in these lignified plants contain
upwards of 45% of total tree chlorophyll (Berveiller,
Kierzkowski, and Damesin 2007; Kharouk et al.
1995). We conclude that stem photosynthesis
confers a fitness characteristic to palo verde in arid
environments.

Other Anatomical Features

Sunken guard cells present on palo verde stems
are thought to provide an advantage here for two
reasons. Most plants feature guard cells on the
surfaces of leaves which assist in gas exchange and
reduction of water loss. But stem stoma are rare.
(Lenticels take the place of stomates on stems of
most woody plants.) The fact that guard cells are
present and sunken into the stems of palo verde can
be considered a fitness feature, because even on most
leaves guard cells are not sunken in this way.

Hair cells (trichomes), present in significant
numbers on palo verde leaves (figs. 3 and 4), petioles,
and stems, are thought to interrupt air-flow over
plant surfaces, to allow water absorption via contact,
and to reduce loss of water in arid and other biomes
(Wagner, Wang, and Shepherd 2004; Werker 2000).
Many plants feature hair cells, so hair cells on palo
verde stems might not be considered unique to plants
in arid regions. Nevertheless they must certainly
assist in water retention for palo verde and are
considered a fitted feature for palo verde here.

Photosynthetic Efficiency of
Two Distinct Photosynthetic Cycles

An exhaustive discussion of C₃ and C₄
photosynthesis systems exceeds the purpose of this
paper, but it will be briefly covered here because
of its presence in palo verde plants. Most plants,
particularly the 165,000 dicot species, and a great
many of the monocot grasses (Ehleringer, Cerling,
and Helliker 1997), are known as C₃ plants because
they use atmospheric CO₂ to manufacture a
3-carbon compound in leaves during photosynthesis.
C₄ terrestrial plants instead produce a 4-carbon
compound within stems via an alternate pathway.
It is considered a relatively new photosynthetic
pathway evolutionarily speaking. It is generally
accepted that C₃ photosynthesis became the
dominant terrestrial photosynthetic system before
the C₄ system developed, yet C₄ metabolism has
been found in unicellular oceanic algae and other
less advanced [and ”earlier” plants], (Hibberd and
Quick 2002; Reinfelder, Kraepiel, and Morel 2000).
In any event, palo verde is a C₄ photosynthetic plant.

C₃ photosynthesis has been described as inefficient
(Hibberd and Quick 2002; Moore 1982; Waller and
Lewis 1979) because excess atmospheric CO₂, unused
during daytime photosynthesis, is later lost during
photorespiration. Instead, C₄ plants more efficiently
capture and use chemically bound CO₂ extracted
from xylem and phloem tissues and produce more
carbon compounds than do C₃ plants. They also
employ striking ultrastructural (anatomical)
differences, which make remarkably efficient use of
C₄ metabolic pathways (Berveiller, Kierzkowski, and
Damesin 2007; Gowik and Westhoff 2011; Muhaidat,
Sage, and Dengler 2007; Waller and Lewis 1979).
Some work has been done to support the idea that
C₄ photosynthesis developed in response to evergrowing
arid environments (Gowik and Westhoff
2011; Hobbie and Werner 2004; Muhaidat, Sage, and
Dengler 2007; Riebesell 2000). Gowik and Westhoff
supply a creative stepwise chart of C₄ evolution, but
supporting data are absent within that, as well as
the reports of others. Therefore arguments that C₄
photosynthesis in most monocots and some dicots
is due to convergent descent remain unconvincing.
Moreover, the existence of many intermediates, which
simultaneously employ C₃ and C₄ photosynthetic
systems has been well established, (Gowik and
Westhoff 2011; Hibberd and Quick 2002; Moore
1982; Muhaidat, Sage, and Dengler 2007). Reporting
of empirical evidence demonstrating multiple
convergences of the C₄ photosynthetic pathway from
the more established C₃ pathway (32 independent
convergences in dicots alone, i.e. Muhaidat, Sage,
and Dengler 2007) would be useful. Regardless of
such arguments, palo verde is convincingly fitted for
success in arid conditions with C₄ photosynthesis.

Moreover, plants subjected to drought have been
shown to become severely limited in solar-induced
chlorophyll fluorescence (SIF), using satellite imagery
(Sun et al. 2015). (Chlorophyll autofluorescence, is a
property of the molecule to give off infrared energy in
the presence of sunlight.)

This diminished fluorescence is measurable on
demand from space and is employed by workers to
track pigment loss, thus monitoring drought severity
across the country. A drop in SIF is considered to be
an indicator of diminished chlorophyll production by
plants under drought stress, particularly in summer
months, when plants generally replenish their low
chlorophyll supply. However, palo verde, like some
other drought-resistant plants, maintains chlorophyll
levels during drought periods, translating into
high productivity in photosynthesis during drought
(Arunyanark et al. 2008). This characteristic of palo
verde we consider to be a compelling fitness feature
during drought, which does cripple plants that
cannot maintain levels of photosynthetic pigments
under drought stress.

Stem Metabolic Deposits

Idioblasts are unusual plant cells, which have a
secretory function and often store plant metabolites
like oils, starches, minerals such as calcium oxalate,
waste products, pigments, and other compounds
in plant tissues. They reportedly have been found
in all types of plant tissues (Coté 2009). They were
also described in Parkinsonia leaves (Lersten and
Curtis 1995, 2001), and we found them in abundance
in Parkinsonia stems. Lerston and Curtis observed
a lack of oils and crystals of calcium oxalate within
idioblasts of palo verde leaves (Lersten and Curtis
1995), and they only saw small amounts of oil, and
no starch or calcium oxalate in Parkinsonia stems,
yet we found these also in abundance. Scott (1935)
reported the presence of calcium oxalate crystals but
only “at certain times.”

Calcium uptake by plants in excess of what is
necessary and the deposition of calcium oxalate
crystals has been widely reported (Franceschi 2001;
Franceschi and Horner 1980; Franceschi and Nakata
2005; Lersten and Curtis 1995; Prychid, Jabaily,
and Rudall 2008; Prychid and Rudall 1999; Webb
1999). It is largely unknown at present if calcium
oxalate production and storage is solely a defense
mechanism to inhibit herbivory or predation by other
organisms (some needle like calcium oxalate crystals
can even be ejected when disturbed, which might be
painful when pushed into oral or other tissues [Coté
2009]). The formation of these crystals has been
noted to play an important role in calcium regulation
(Prychid, Jabaily, and Rudall 2008) and can also stiffen
plant tissues (Coté 2009). It is proposed that they are
useful in distribution of light within leaves because
they are found in the palisade photosynthetic cells of
leaves just under the epidermis (Franceschi 2001).
Their use as a systematic guide is complicated by
the fact that their presence is distributed among all
taxonomic levels of plants from small algae to giant
gymnosperms (Franceschi and Nakata 2005).

A significant note is that crystal growth is not
controlled by random precipitation of oxalic acid
with calcium ions; rather their size, formation, and
distribution within plant tissues appears to be species
specific and mediated by cellular control (Franceschi
and Nakata 2005; Webb 1999).

The plentiful deposits of oil and starch in idioblasts
of palo verde allows them to survive dry periods,
another fitness character for the desert biome. The
fact that druse and/or rosette crystals of calcium
oxalate are the only forms observed in stems of this
plant warrants further study.

Conclusions

The fitness characters of this interesting
plant which thrives in hot, dry biomes are many.
They include (but may not be restricted to) deep
root penetration, microphyllous leaves, hair cell
adornment, decumbent trunks, drought-deciduous
behavior, petiole, stem, and trunk photosynthesis,
thick cuticular covering on petioles, stems and trunks,
sunken stoma, C₄ photosynthesis, deep penetration of
solar radiation into lignified stems, and trunks, (and
vascular systems, which transmit the light deep into
trunk tissues), lignified xylem to prevent cavitation,
and production and storage of oils and starch as well
as druse crystals of calcium oxalate. All these fitness
characters in our estimation, serve palo verde well
as it thrives in some of the most inhospitable areas
on earth. We conclude that these characters are
present due to planning and purpose of the Creator.
However, well-vetted empirical data to the contrary
are welcomed.

It also must be said that we heartily endorse
the publication of novel field and laboratory-based
research here. We believe that the reporting of
design found within the tiniest structures of the
creation is sufficient to point hearts to God and to
lead to repentance (Romans 1:18–20). Our answer
to the question, “Of what significance to the creation
message is the science you are doing?” can be found
in Paul’s exemplary pattern while he was at the
Areopagus (Acts 17:16–ff). The creation message
delivered by Paul from the steps of the highest court
in Greece ended with the obvious: God’s will that “all
people everywhere must repent.” Teaching creation
should lead to repentance and submission to Christ
even in our technologically minded society. Teaching
creation must include novel research, produced by
believers, and must speak to God’s invisible qualities.
Lab and field research, from our own efforts (and not
the work of others of the opposing worldview), carries
authority and commands respect (especially among
our detractors). These qualities are sadly missing (or
severely minimized) in our work today.

People listening to Paul who were ignorant of
God and His creation, were unaware (or at the least
willfully ignorant) that articles made from “gold,
silver, or stone” were not real gods. Tech oriented
people today are smarter than that. But they respect
novel science constructed using costly materials like
world-class laboratories. The future of the creation
movement lies not in museums or attractions or
the library, but in the laboratory. The future of the
creation movement lies in preaching the Gospel to
the lost, those fields “white unto harvest” (John 4:35)
who must be persuaded to repent and trust Jesus as
Savior.

Acknowledgments

The authors are indebted to donors of the
microscopy lab, including Per Larssen, Brent Fjarli,
Matt Pierce, and George Howe. We also thank the
anonymous reviewers and Andrew Snelling, all who
made this a better manuscript.

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