American Journal of Plant Sciences, 2013, 4, 2070-2082
Published Online November 2013 (http://www.scirp.org/journal/ajps)
http://dx.doi.org/10.4236/ajps.2013.411259
Open Access AJPS
A Review of Paulownia Biotechnology: A Short Rotation,
Fast Growing Multipurpose Bioenergy Tree
Niraj Kumarmangalam Yadav1*, Brajesh Nanda Vaidya1*, Kyle Henderson1, Jennifer Frost Lee2,
Whitley Marshay Stewart1, Sadanand Arun Dhekney3, Nirmal Joshee1#
1Agricultural Research Station, Fort Valley State University, Fort Valley, USA; 2Wesleyan College, Macon, USA; 3Department of
Plant Sciences, University of Wyoming, Sheridan Research & Extension Center, Sheridan, USA.
Received August 5th, 2013; revised September 5th, 2013; accepted October 1st, 2013
Copyright © 2013 Niraj Kumarmangalam Yadav et al. This is an open access article distributed under the Creative Commons Attribu-
tion License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
Paulownia is a genus of fast-growing and multipurpose tree species that is native to China. Due to their rapid growth
and value in the timber market, many Paulownia species are cultivated in several temperate zones worldwide. Eco-
nomic importance of Paulownia is increasing as new uses and related products are developed. It is also suitable as a
lignocellulosic feedstock crop for the bioethanol industry in the Southeastern USA. A number of Paulownia species are
valuable sources of secondary metabolites including flavonoids with high antioxidant activities. A high demand for
planting material in domestic and international markets for afforestation and bioenergy production has necessitated the
development of efficient micropropagation protocols for rapid and mass propagation of Paulownia. Over the past sev-
eral decades, research on Paulownia species has been conducted to develop micropropagation, somatic embryogenesis
and genetic transformation protocols for use in agroforestry and reforestation programs. Given the economic importance
and current and potential future uses of Paulownia, this paper reviews the development of biotechnological approaches
for plant propagation and genetic improvement, and antioxidant potential of secondary metabolites occurring in species.
Keywords: Micropropagation; Biofuel; Plant Growth Regulator; Regeneration; Somatic Embryogenesis;
Transformation; Antioxidant Potential
1. Introduction
Paulownia is a deciduous, fast growing, hardwood tree
(family Paulowniaceae, previously in the family Scro-
phulariaceae) comprised of nine species and a few natu-
ral hybrids that are native to China [1]. Important species
in this genus include P. albiphloea, P. australis, P.
catalpifolia, P. elongata, P. fargesii, P. fortunei, P. ka-
wakamii, and P. tomentosa [2]. Paulownia species are
found growing naturally and under cultivated conditions
in several parts of the world including China, Japan and
Southeast Asia, Europe, north and central America, and
Australia. Species in the genus are extremely adaptive to
wide variations in edaphic and climatic factors, and grow
well on lands deemed marginal. In China, Paulownia
grows in regions from the plains to elevations up to 2000
feet [2]. It exhibits a number of desirable characteristics
such as rot resistance, dimensional stability and a high
ignition point [3], which ensures the popularity of its
timber in the world market [4,5]. For decades, Japanese
craftsmen have utilized it as revered wood in ceremonial
furniture, musical instruments, decorative moldings,
laminated structural beams and shipping containers. The
tree made its way to the United States during the mid-
1800 s in the form of seed, which was as packaging ma-
terial for delicate porcelain dishes [5]. Once unpacked,
the tiny seeds were dispersed by wind and naturalized
throughout the eastern states. Paulownia cultivation for
timber production is an unorganized but emerging enter-
prise in the US and gaining importance because of the
strong demand in Japan and some other countries. The
total consumption of Paulownia wood in Japan was ap-
proximately 17 million board feet (MBF) during 1971-
1973. In a few years imported Paulownia wood volume
increased from 16% to 60% of total consumption [6].
With a shift in paradigm in favor of alternative fuels, a
*These authors contributed equally to the manuscript.
#Corresponding author.

A Review of Paulownia Biotechnology: A Short Rotation, Fast Growing Multipurpose Bioenergy Tree 2071
change from food and feedstock to non-food and feed-
stock sources for cellulosic ethanol is mandatory. The US
has already mandated a goal of producing 36 billion gal-
lons of biofuels by 2022 via the Renewable Fuels Stan-
dard, RFS2 [7].
In recent years, molecular and genetic engineering
techniques have gathered momentum in forest tree re-
search for industrial use and assistance with reforestation
and forest management programs. Paulownia species
have received due attention in tree-tissue culture owing
to their multifaceted significance. In vitro propagation
has ensured that the growing demand for superior planting
material, biomass and forest products is met. A number
of factors including explant selection, macro- and mi-
cronutrient composition, incorporation of plant growth
regulators, antioxidants, additives and adsorbents during
in vitro culture have been optimized to develop success-
ful regeneration protocols for several Paulownia species.
This review attempts to highlight the current procedures
available for in vitro propagation of Paulownia species
and their applications in genetic engineering for crop
improvement. Additionally, antioxidant capacity of leaf
extract from two Paulownia species for their potential
medicinal value is also reviewed.
2. A Multipurpose Tree
Under natural conditions a 10 year old Paulownia tree
measures 30 - 40 cm diameter at breast height (dbh), and
contains a timber volume of 0.3 - 0.5 m3 [2]. Paulownia
timber is lightweight, yet strong, dries rather rapidly and
has an aesthetically pleasing light colored grain that does
not warp, crack, or deform easily. In addition, the wood
is easily worked, suitable for carving and has excellent
insulation properties [2]. Several species have been
planted extensively in Australia to meet demand for tim-
ber [8]. Due to its rapid growth and high cellulose con-
tent (440 g·cellulose/kg) studies have been conducted to
evaluate its suitability for solid biofuel and cellulose pulp
industry [9]. P. elongata wood fillers are successfully
employed to create biodegradable bio-composite with
poly lactic acid (PLA), introducing a new product in the
market [10]. Paulownia wood flour filler produced com-
posites that had comparable or superior mechanical,
flexural, and impact strength properties to composites of
pine wood flour filler [11]. A recent study showed that P.
elongata wood flour could be utilized in the production
of the filled polypropylene composites [11]. Paulownia
flowers and leaves are a good source of fat, sugar and
protein, and utilized as fodder for pigs, sheep and rabbits
[2]. The nitrogen content in Paulownia leaves can be
compared favorably with some leguminous plants. Pau-
lownia leaves are used as a green manure crop by farm-
ers in Kwangsi, China. Paulownia is used to treat various
ailments in traditional Chinese medicine due to medicinal
compounds it contains [2]. Paulownia inflorescences are
large in size and a good source of honey [2]. Paulownia
has been capitalized for agroforestry [12,13], biomass
production [14], land reclamation [15], and animal waste
remediation [16]. Various attributes of Paulownia are
summarized in the Figures 1(A)-(I ).
3. Potential Biomass Crop
Greater than 30 million acres of woodland, and idle pas-
ture and cropland exist in the southeast United States,
and much of this land could potentially be used to pro-
duce valuable tree-crops, Paulownia being one of them
[17]. Due to the fast growth and coppicing property (Fig-
ure 1(D)), its potential as a biofuel crop has been exten-
sively studied [10,18,19]. A major advantage of using
biomass as a source of fuels or chemicals is its renew-
ability. Wood from forest trees modified for greater cel-
lulose or hemicelluloses could be a major feedstock for
fuel ethanol. In a biomass comparison study performed in
Germany, P. tomentosa (12.7 tons·ha−1) out-produced
Salix viminalis (8.2 tons·ha−1) on short rotation coppice
under dry land conditions [20].
An evaluation of Paulownia wood revealed the com-
position to be 14.0% extractive, 50.55% cellulose,
21.36% lignin, 0.49% ashes, 13.6% hemi-cellulose [19].
Ongoing research at Fort Valley State University (FVSU)
has determined that the harvestable biomass of Paulownia
elongata after 30 months (after three growing seasons) is
almost 92 kg/tree (unpublished results). Under favorable
conditions, an intensive plantation of 2000 trees per ha
can yield up to 150 - 300 tons wood annually, only 5 - 7
years after planting [21]. Additional studies are required
to study biomass potential in variable soils and climates.
Paulownia produces many fine winged seeds (up to
2000 seeds per fruit) weighing about 5000 seeds per
gram [21]. Scanning electron microscopy of seeds reveal
an extensive network of fine tubes that may play an im-
portant role in maintaining structural integrity of the
wing to assist with wind dispersal, and create water
channels for promoting seed germination (Figures 1(A)
and (B)). Seedling development studies indicate that a 16
h photoperiod is optimum for leaf production, stem
elongation, root elongation, and total dry mass accumu-
lation [22]. Seeds sown in a greenhouse germinated in as
little as one week, with true leaf emergence within two
weeks after germination. Lipids in P. tomentosa seed
extract consist of linoleic (64.1%), oleic (21.2% and
palmitic acids (7.3%). γ-Tocopherol (approx. 100.0%)
predominated in the tocopherol fraction, and in the sterol
fraction—β-sitosterol (79.2%), campesterol (10.3%) and
stigmasterol (7.7%) were the dominant components.
hough lipid profile of seeds suitable for biodiesel Ts i
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A Review of Paulownia Biotechnology: A Short Rotation, Fast Growing Multipurpose Bioenergy Tree
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2072
Figure 1. Paulownia elongata as a multipurpose tree for SE USA. (A) Scanning electron micrographs (SEM) of a seed, (B)
SEM of Paulownia elongata seed (wing) exhibiting structural organization to support dispersal, (C) SEM of unaltered ground
wood, (D) One-year-old tree exhibiting coppicing potential after cutting, (E) and (F) Micropropagation protocols resulting in
multiple adventitious shoot generation for commercial mass production, (G) A seven year old tr ee bloo ming in Central Geor-
gia, USA, (H) and (I) Scope for monofloral honey production as trees bloom for 4 - 6 weeks.
production, it would potentially be labor-intensive due to
high number of small seed production. Seeds contain
10.6% protein, 9.5% cellulose and 38.2% hydrolysable
carbohydrates [23]. Research addressing development of
a suitable machine for fruit/seed collection and further pro-
cessing would be required to make this aspect a reality.
4. Plant Regeneration and Genetic
Transformation in Paulownia
Documented proof of plant tissue culture work on Pau-
lownia spans little over three decades and are presented
in the Table 1. Various aspects of plant tissue culture
dealing with callus induction, micropropagation, proto-
plast culture, somatic embryogenesis and the need of a
reproducible regeneration system for genetic transforma-
tion of the plant is discussed below.
4.1. Organogenesis and Micropropagation
Paulownia is conventionally propagated using seed and
root cuttings. Seed propagation is not reliable due to
presence of seed borne pathogens and pests, poor seed
germination and altered growth habit. Additionally, seed-
ling growth is slow compared to root cutting-derived
plants [4,24]. Propagation by root cuttings pose limita-
tions as these can be a potential source of pathogens.
An efficient plant regeneration system (Figures 1(E)
and (F)) is a prerequisite for transgenic plant production
and further studies. Micropropagation protocols have
been established for a number of Paulownia species
(Table 1) including P. catalpifolia [25,26], P. elongata
[27,28], P. fortunei [29-34], P. kawakamii [35-38], P.
taiwaniana [39-41] and P. tomentosa [29-31,37,38,42-
45]. Most protocols developed to date predominantly use
nodal explants. Another explant used successfully,
though to a lesser extent, is petiole with cut leaf [46].
Callus production and plant regeneration via organo-
genesis was observed in P. fortunei shoot tip explants
cultured on MS medium containing 4.4 mg·L−1 thidiazuron
(TDZ) for 4 weeks followed by transfer to MS medium

A Review of Paulownia Biotechnology: A Short Rotation, Fast Growing Multipurpose Bioenergy Tree 2073
Table 1. A chronological summary of Paulownia biotechnology research.
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A Review of Paulownia Biotechnology: A Short Rotation, Fast Growing Multipurpose Bioenergy Tree 2074
Continued
MS: Murashige and Skoog (1962), 1/2 MS: 1/2 strength Murashige and Skoog, WPM: Woody Plant Medium (Lloyd and McCown, 1981), B5: Gamborg’s B5 (1968), N6: Chu’sN6 medium (1975), BTM: Broad Leaf Medium (Chalupa, 1981), Kinetin, BAP/BA: 6-Benzylaminopurine, TDZ: Thidiazuron, AdS: Adenine sulfate, IBA: Indole-3-Butyric Acid, IAA: Indole-3-Acetic Acid, NAA: 1-Naphthalene Acetic Acid, 2,4-D: 2, 4-Dichlorophenoxyacetic Acid.
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A Review of Paulownia Biotechnology: A Short Rotation, Fast Growing Multipurpose Bioenergy Tree
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2075
containing 1.0 mg·L−1 activated charcoal [47]. Other ex-
plants including petioles, stems and leaves failed to pro-
duce an organogenic response on the same medium
thereby underlying the importance of explant selection.
Current research at FVSU includes shoot induction from
leaf and seed explants. In addition, studies are underway
to optimize thin cell layer (TCL) culture protocols for
leaf and nodal explants. Murashige and Skoog (1962) [48]
medium (MS) and woody plant medium (WPM) [49]
have been widely used for Paulownia micropropagation
with considerable success. Among various growth regu-
lators studied, benzyl amino purine (BAP) is effective in
most Paulownia species; however, specific species may
exhibit variable responses. For instance, TDZ in combi-
nation with IAA produced the best regeneration response
from P. tomentosa mature leaf explants [43]. Rooting in
Paulownia, in general, is fairly easy as microcuttings can
root efficiently in MS basal medium [50] without inclu-
sion of an auxin. Production of hyperhydric shoots has
been frequently observed during in vitro culture of sev-
eral Paulownia species. A significant reduction in shoot
hyperhydricity was achieved by adjusting culture me-
dium conditions, especially gelling agent and sucrose
concentrations [51]. Paulownia species is amenable to
photoautotrophic culture exhibiting satisfactory shoot
growth and spontaneous rooting [52,53]. The system
takes advantage of chlorophyllous explant by placing it
an environment conducive to photosynthesis by cutting
down carbon source (sucrose) with CO2 enrichment. An
exposure of P. tomentosa and P. fortunei nodal cultures
to magnetic field enhanced adventitious shoot bud multi-
plication and improved plant regeneration while signifi-
cantly reducing the amount of time required for obtaining
whole plants [30,54].
Molecular studies aimed at understanding the mecha-
nism of organogenesis in P. kawakamii reported differ-
ential expression of a cDNA encoding a putative bZIP
transcription factor during multiple shoot proliferation
[55]. Molecular dissection of the chronology of gene
expression using Quantitative PCR revealed that only
basal levels of transcripts were present in callus-forming
tissues at day 0 and day 10, whereas a six-fold increase
in gene expression was seen in shoot-forming tissues at
day 10, suggesting that PKSF1 is associated with adven-
titious shoot bud development in Paulownia. Induction
of polyploidy to obtain phenotypes with enlarged mor-
phology and lower fertility has been successfully em-
ployed in Paulownia tomentosa using colchicine [56,57].
4.2. Callus Initiation and Somatic
Embryogenesis
The first report of somatic embryogenesis in Paulownia
was reported [58] using placental tissue of fertilized
ovules as an explant. Zygotic embryos cultured on me-
dium containing MS macro-, micronutrients and vitamins,
200 mg·L−1 casein hydrolysate, 100 mg·L−1 myoinositol,
10 mg·L−1 pantothenic acid, 0.7% agar and IAA pro-
duced embryogenic callus; medium containing other
growth regulators including 2,4-D and NAA produced
non-embryogenic callus. Callus induced from fertilized
ovular explants showed a persistent embryogenic capac-
ity, eventually differentiating into embryos and plants.
Callus induction was optimized in five Paulownia spe-
cies (P. tomentosa, P. australis, P. fortunei, P. elongata,
P. tomentosa × P. fortunei) using leaf explants [31]. MS
medium [48] containing varying levels of NAA (0.1 - 1.1
mg· L −1) and BAP (2 - 12 mg·L−1) were tested for em-
bryogenic culture induction and plant regeneration. Al-
though five different media formulations were used, MS
medium supplemented BAP and NAA produced an em-
bryogenic response. Other Paulownia species tested
elsewhere failed to produce an embryogenic response on
the same medium composition suggesting a strong media
× genotype interaction [40]. Direct and indirect somatic
embryogenesis was observed from leaf and internode
explants in P. elongata [59,60]. Somatic embryos were
found suitable for synthetic seed production [59]. The
technology assumes significance in terms of its utility for
large-scale plant propagation in bioreactors, short- and
long-term germplasm conservation, and easy transporta-
tion of planting stock material. Other reports on somatic
embryogenesis and plant regeneration in P. elongata
have been described [4,27,31,61], but have been difficult
to replicate using stated procedures.
Suspension cultures have great potential for screening
and production of secondary metabolites in light of nu-
merous medicinal phytochemicals occurring in Pau-
lownia. Callus cultures of P. taiwaniana were obtained
from leaf explants on MS medium supplemented with
multiple plant growth regulators [37]. Histology and
morphology analyses distinguished cultures as nodular
cell aggregated of three distinct sizes, which eventually
exhibited plant regeneration. Suspension cultures were
produced by transferring such nodular aggregated to liq-
uid medium with the same composition and maintained
for over a year.
The protocol was modified to establish suspension
cultures of four Paulownia species (P. fortunei, P. ka-
wakamii, P. tomentosa, and P. taiwaniana). Seeds, ger-
minated seedlings, shoot tips and leaves were directly
cultured in liquid MS medium containing 1.0 mg·L−1
2,4-D and 0.1 mg·L−1 kinetin [37]. Suspension cultures
were obtained from all species tested but long-term
maintenance (over one year) with frequent transfers to
fresh medium was feasible only for P. taiwaniana and P.
tomentosa.

A Review of Paulownia Biotechnology: A Short Rotation, Fast Growing Multipurpose Bioenergy Tree 2076
4.3. Genetic Transformation
Technology advances for in vitro propagation and im-
provement in genetic transformation protocols have ac-
celerated the development of genetically engineered trees
during the past 15 years. Targeted traits include herbicide
tolerance, pest resistance, abiotic stress tolerance, modi-
fied fiber quality and quantity, and altered growth and
reproductive development [62].
Genetic transformation of Paulownia has been
achieved using both Agrobacterium-mediated transfor-
mation and biolistic bombardment [61,63,64]. In vitro
shoots used for co-cultivation with Agrobacterium tume-
faciens produced transgenic callus after transfer to induc-
tion medium. Hairy root production occurred in 33%
shoot explants that were wounded by nicking and then
co-cultivated with A. rhizogenes strain R1601. Hairy
roots were observed from the site of explant wounding.
Opine analyses demonstrated transgene expression in
proliferating galls/hairy roots shortly after emergence
from wound sites and in callus and roots after 12 weeks
of in vitro culture [63]. Agrobacterium rhizogenes (LBA
9402 and A4 strains) was successfully used to induce
eleven independent hairy root lines of Paulownia tomen-
tosa by infecting stem explants. Transformation effi-
ciency was dependent on explant age. Hairy root cultures
grew rapidly in hormone-free Woody Plant liquid me-
dium. Depending on the line used, the level of verbasco-
side varied from 1.7 to 8% of dry weight. Experiments
with a high producing line PT-3 showed maximum ver-
bascoside yield was achieved in half-strength Gamborg’s
B5 liquid medium being approximately double compared
with the roots of 4.5-month-old-plants grown outside.
Biolistic bombardment of P. elongata leaf explants
with a construct pBI121 harboring the β glucuronidase
(GUS) and neomycin phosphotransferase (NPT II) genes
produced transgenic plants via organogenesis. Transgene
insertion was demonstrated using PCR while expression
was studied using fluorometric assay for GUS and paper
chromatographic assay for NPT II. Optimizing transfor-
mation systems will greatly aid in the development of
improved genotypes with rapid growth habits for ligno-
cellulosic feedstock in future [61].
A MADS box transcription factor PkMADS1 regulat-
ing adventitious shoot bud induction was constitutively
expressed in P. kawakamii using Agrobacterium-medi-
ated transformation (strain GV3850). Transgenic plants
were obtained by selection of proliferating shoots on
medium supplemented with 10 mg·L−1 kanamycin [35].
Paulownia shoots cultured in vitro exhibited a high sen-
sitivity to kanamycin, thereby making it an effective se-
lection agent for screening transgenic plants. Transgenic
Paulownia plants expressing an antimicrobial Shiva-1
lytic peptide that encodes cecropin were produced using
Agrobacterium-mediated transformation [65]. Enhanced
resistance to a mycoplasms causing Witches’ Broom dis-
ease was observed in transgenic plants. Another useful
application of A. tumefaciens mediated gene transfer was
the introduction of shiva-1 gene that produces cecropin
peptides imparting increased resistance to mycoplasma
causing Paulownia Witches’ Broom disease [65].
Following development of genetic engineering proto-
cols for tree species such as Paulownia, the next step in
determining acceptability of transgene technology for
forest tree improvement is to assess the adverse envi-
ronmental impacts, if any, from field-release of geneti-
cally modified species. Ecological risks associated with
commercial release range from transgene escape and
introgression into wild gene pools, to the impact of
transgene products on other organisms and ecosystem
processes. Evaluation of those risks is confounded by the
long life span of trees, and by limitations of extrapolating
results from small-scale studies to larger-scale planta-
tions.
5. Medicinal Properties of Paulownia
Polyphenolic compounds produced by plants as secon-
dary metabolites exhibit high antioxidant activity [66].
Free radicals are implicated in several disorders in hu-
man body including atherosclerosis, central nervous sys-
tem injury and gastritis [67,68]. Flavonoids with strong
antioxidant properties inhibit hydrolytic and oxidative
enzymes, prevent decomposition of peroxides into free
radicals, act in anti-inflammatory pathways and minimize
damage caused at the cellular level [69,70]. Plant-based
antioxidants are used as therapeutics to supplement the
human body’s immune system [71]. A number of plant
species in the Lamiaceae family including Basil (Oci-
mum spp.), Mint (Mentha spp.), Rosemary (Rosmarinus
officinalis), Lavender (Lavandula spp.) and Baikal
skullcap (Scutellaria baicalensis) [72,73] exhibit high
total phenols and antioxidant activity [74,75]. Research
on antioxidant properties and medicinal value of second-
dary metabolites of Paulownia is gaining importance and
lately many publications have appeared. Paulownia spe-
cies are rich in phenolic substances distributed in differ-
ent parts and tissues of the tree [76,77]. P. tomentosa
leaves contain ursolic acid, and matteucinol. Xylem ves-
sels contain paulownin, and d-sesamin while syringin
and catalpinoside occur in bark extracts. Phytochemical
screening of P. tomentosa var. tomentosa bark resulted in
eight phenolic compounds through spectroscopic analysis
[77]. Analysis of P. tomentosa fruits indicated presence
of numerous C-geranyl compounds in ethanol fraction
[78] that exhibited antiradical and cytoprotective activity
when tested on Alloxan-induced diabetic mice [79].
Aqueous extract of Paulownia leaves and silage exhibit
pronounced inhibitory activity against the Gram-negative
bacteria in vitro [80]. Tablets and injections derived from
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A Review of Paulownia Biotechnology: A Short Rotation, Fast Growing Multipurpose Bioenergy Tree 2077
Paulownia leaf, fruit and wood extracts are effective for
bronchitis, especially relieving cough and reducing
phlegm. Pharmacological experiments demonstrate util-
ity of fruit extracts to relieve cough and asthma, and
cause a reduction in blood pressure [2].
Fresh and dry leaves of P. elongata and P. fortunei,
from FVSU experimental plots were used to conduct a
comparative study on the antioxidant potential of leaf
extracts. The colorimetric Folin-Ciocalteu reagent me-
thod [81] with modification [82] was used to measure
total polyphenol (TPP) contents with gallic acid as a
standard. Average TPP contents of fresh leaf extracts was
144.28 mg/g Gallic Acid Equivalent (GAE) (for P. elon-
gata) and 207.53 mg/g GAE (P. fortunei) respectively,
while the average TPP content ranged from 94.15 mg/g
GAE (P. elongata) to 266.74 mg/g GAE (P. fortunei)
(Figure 2).
5.1. Antioxidant Capacity Measurement
Additional studies were conducted to determine antioxi-
dant capacity of leaf extracts of P. elongata and P. for-
tunei. ABTS (2, 2’-azinobis (3-ethylbenzothiazoline-6-
sulfonic acid) diammonium salt) is the chemical of
choice used to measure the antioxidant capacity by the
food industry and agricultural researchers. The ABTS
radical cation is reactive towards most antioxidants in-
cluding phenolics, thiols and Vitamin C. This assay is
often referred to as the TROLOX (6-hydroxy-2, 5, 7,
8-tetramethychroman-2-carboxylic acid) equivalent an-
tioxidant capacity (TEAC) assay. The reactivity of the
various antioxidants in the leaf extracts are compared to
that of TROLOX, which is a water-soluble analog of
vitamin E. Antioxidant capacity calculation is based on
inhibition exerted by TROLOX compared with sample
mixed with diluted ABTS. A standard curve was gener-
ated for percent inhibition of TROLOX concentration in
the range 300 - 1500 µM with R2 = 0.9935. TEAC assay
was carried out following the original protocol with mi-
nor modifications [82,83].
The percent inhibition exhibited against TROLOX by
these two species ranged from 48.84% to 97.53%. P.
elongata’s fresh and dry leaf extract average inhibitions
were 50.21% and 63.88% respectively and for P. for-
tunei it recorded 61.03% for fresh and 95.09% for dry
extract in average inhibition against TROLOX. P. for-
tunei’s average inhibition was among the highest of all
percent inhibition. TEAC was calculated using a
TROLOX standard curve. The TEAC values obtained
ranged from 1255.30 µmol/g (dry extract of P. elongata)
to 2377.10 µmol/g for P. fortunei dry extract (Figure 2).
The percent inhibition against TROLOX by two leaf
extracts ranged between 48.84% to 97.53%. P. elon-
gata’s fresh and dry leaf extract average inhibitions were
50.21% and 63.88% and for P. fortunei it recorded
61.03% for fresh and 95.09% for dry extracts. P. for-
tunei’s average inhibition for both fresh and dry leaf ex-
tracts were higher than P. elongata. The average TEAC
values of fresh and dry leaf extracts of P. elongata was
1255.30 µmol/g, and 1596.33 µmol/g respectively
whereas for P. fortunei it was 1525.66 µmol/g and
2377.10 µmol/g (Figure 2).
5.2. Estimation of Total Flavonoid Contents
Estimation of total flavonoid content was conducted us-
ing aluminum chloride colorimetric method [84]. Quer-
cetin dihydrate was used as a standard to make the cali-
bration curve with concentrations ranging from 10 - 125
µg/mL. The test solutions were prepared mixing fresh
and dry leaf extracts with 10% aluminum chloride, 1 M
potassium acetate, 95% ethanol and distilled water, then
absorption of the test solutions (for dry and fresh leaf
extracts) were recorded using a spectrophotometer at 415
nm. Once the readings were obtained, total flavonoid
content was calculated and absorbance was plotted
against µg/mL where the relationship is linear and re-
gression equation is determined by y = mx + b. For each
species, three independent replications were done and
averaged. Flavonoid content in the fresh leaf extracts of
P. elongata and P. fortunei were 102.58 µg/mL and
157.53 µg/mL, respectively. In the extracts derived from
dried leaves, flavonoid content of P. elongata was 104
µg/mL whereas in case of P. fortunei it was at 158.45
µg/mL.
Out of three assays conducted, the results from total
polyphenol concentration measurement and total flavonoid
content exhibited a strong correlation (Figure 2(A)). As
the TPP value increased, flavonoid content also increased.
In addition, an increase in TPP caused a subsequent in-
crease in TEAC values as well as flavonoid contents.
Though results obtained in this study only establish the
quantities available in fresh and dry extracts for TPP,
TEAC and estimation of total flavonoid in general, it
corroborates previous evidence for the antioxidant prop-
erties of specific bioactive compounds occurring in P.
tomentosa [78].
6. Future Prospects and Conclusions
Forest biotechnology is an emerging field of interest.
Existing protocols make it possible to genetically im-
prove existing varieties using biotechnology. With the
current economic importance and future uses of Pau-
lownia species being developed, it is clear that plant re-
generation using micropropagation is of paramount im-
portance. Paulownia plants are now produced through
tissue culture and shipped to domestic and international
destinations. The potential for plant regeneration via or-
ganogenesis, from cotyledons and hypocotyls, as well as
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2078
Figure 2. Paulownia leaf extract as a source of polyphenols with high antioxidant activity. (A) Correlation between total poly-
phenol and flavonoid content for fre sh and dry extracts of P. elongata and P. fortunei, (B) TEAC value of extracts from fresh
and dry leaf samples of P. elongata and P. fortunei. Values represent means of three replicates. The analysis of variance
(ANOVA) single factor for fresh and dry extracts were performed to compare the means with significant differences between
treatments at p < 0.05 level.
determined requirements for rapid adventitious shoot
generation from shoot tips, needs additional studies [85].
Although several techniques for plant regeneration have
been described here, protocols for culture induction and
plant regeneration from a number of explants including
seed and roots are still lacking. Culture induction via
Thin Cell Layer techniques also has not been widely
documented. Lack of a reliable tissue culture would hin-
der downstream research applications such as genetic
transformation, genomic and transcriptome analysis. A
variety of techniques described in this review is a guide-
line, which lays a solid groundwork for additional studies
by researchers.
The introduction of genes into plant cells and recovery
of stable fertile transgenic plants is a viable alternative to
conventional breeding techniques for making desired
modifications in existing varieties. As gene isolation,
characterization and genetic engineering technology be-
come routine procedures, forest-tree species are becom-
ing a major target for genetic improvement using mo-
lecular breeding [62]. The first genetically modified tree
(Populus) was produced 20 years ago [85]. Despite this
advance, the number of forest tree species for which
transformation and regeneration techniques have been
optimized remains low; they include aspen, cottonwood,
eucalyptus and walnut. Recently, transformation and
regeneration protocols have been developed for several
gymnosperms, mostly species within the genera Pinus,

A Review of Paulownia Biotechnology: A Short Rotation, Fast Growing Multipurpose Bioenergy Tree 2079
Larix and Picea. For each of these species, only a few
genotypes are known to be amenable to the recovery of
transgenic plants. In general, effective plant regeneration
has been more difficult to achieve through organogenesis
than through somatic embryogenesis. Plant regeneration
via somatic embryogenesis is a reliable method for ge-
netic transformation since it has a single cell origin. The
potential of cell suspension cultures for the production of
somatic embryos offers a possibility of reducing time in
obtaining synthetic seeds especially in case of woody
tree species with a long seed generation time. Several
forest trees (poplar, conifers) have been used for genetic
manipulation and transgenic plants are being tested under
field conditions. Ultimately, the real value of in vitro
techniques lies in their extension and applications in for-
est-tree improvement programs.
Development of new drug is a complex, time-con-
suming, and an expensive process. The time taken from
discovery of a new drug to its reaching the clinic is more
than a decade, involving high financial commitment.
Essentially, the new drug discovery involves the identi-
fication of new chemical entities (NCEs), having the re-
quired characteristic of druggability and medicinal
chemistry. Many of these NCEs can be sourced through
natural products (plants). Paulownia trees have been ex-
tensively used in many traditional medical systems in the
East and a systematic study on their medicinal compo-
nents and their activity holds promise for the health sec-
tor.
7. Acknowledgements
Paulownia research at Fort Valley State University
(FVSU) is funded through Evans Allen grant (GEOX
5213) to NJ.
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