Paulownia - Bioenergy Crop Plants - page 674

latitudes of 40° north and 40° south and at altitudes of up to 2000 m (El-Showk and El-Showk 2003).
P. elongata grows faster and is suitable for intercropping with arable crops, whereas P. catalpifolia
and P. tomentosa grow slower but have better wood quality. The annual mean temperature in the
Paulownia growing areas varies a great deal, ranging from 10 to 22°C. The optimal temperature
range for Paulownia growth is 24-29°C. The lowest temperature that Paulownia can resist is -20°C
for P. tomentosa , -16°C for P. elongata , and -13°C for P. fortunei , suggesting that some of these fast-
growing species can be grown in large, unused tracts of land in the United States (Zhu et al. 1986).
Geographical areas suitable for P. elongata cultivation are shown in the Figure 27.1a. Paulownia
can adapt to a wide range of precipitation ranging from 500 to 2500 mm. The suitable edaphic
conditions for Paulownia growth are fertile sandy loam to heavy loam soils with loose structure and
salt content less than 0.05%. Paulownia species respond very well to fertilizer application but have
little tolerance to water-logging. In a comparative study to assess nutrient requirements and stress
response of Populus simonii and P. tomentosa , it was found that the maximum weight increase was
19% and over 25% per day, respectively. Further, the nitrogen retention in the above-ground parts of
P. tomentosa was very high in comparison to P. simonii [0.26 g dry weight (gN) -1 h -1 vs. 0.16 g dry
weight (gN) -1 h -1 ], establishing Paulownia sp. as one of the fastest growing tree species for biomass
production (Hui-Jun and Ingestad 1984).
The leaf, flower, fruit, and bark of Paulownia have been extensively used in Chinese medicine
to treat bronchitis, especially on relieving the cough and reducing phlegm, enteritis, tonsillitis, and
dysentery (Jiang et al. 2004). Further, chemical analysis of flowers revealed high quantity of bioactive
flavonoid apigenin that has been found to show various pharmacological activities, including anti-
inflammatory, antispasmodic, antidiarrheal, vasorelaxant, and an antibacterial activities (Jiang et al,
2004). Antiviral furanoquinone and antimicrobial phenylpropanoid glycosides have been isolated
from P. tomentosa (Kang et al. 1994, 1999).
Except for P. tomentosa (Miller 2004), most Paulownia species grown in the United States are
noninvasive. Although there is little doubt that it is an exotic genus, the question of its invasiveness
is open to conjecture. The prolific small seeds of Paulownia are windblown. However, the seeds
do not germinate and survive unless they fall on soils with low pathogen load. Young Paulownia
seedlings have a high rate of mortality because of damping-off disease caused by various soil
fungi. Generally, Paulownia does not colonize in open areas. Requiring full sunlight for continued
development, it is often overtopped by other species and succumbs. Paulownia is usually found on
the edge of a forest where sunlight is more available rather than in the interior forest. Because of the
strict sunlight and soil requirements, the number of Paulownia plants appears to have declined in
the recent years. Seed dispersed from Paulownia plantings does not appear to establish and colonize
outside of Paulownia plantations.
27.3.1 r Eproduction and E arly g rowth
27.3.1.1 Flowering and Fruiting
The perfect flowers of Paulownia are borne in terminal panicles up to 25 cm (10 in) long in April and
May. Their violet, lavender, or blue appearance before the leaves emerge is quite striking (Figure
27.1e). The fruits are ovoid, pointed, woody capsules approximately 30-45 mm long. Immature
green fruits from the current year and mature, dehisced, brown to black capsules from the previous
year can be spotted on a tree at the same time (Figure 27.1g, h). Capsules turn brown as they mature
in September or October and persist on the tree through the winter (Bonner and Burton 1974).
27.3.1.2 seed Production and dissemination
Numerous seeds are borne in fruits called “capsules.” Each capsule contains up to 2000 seeds, and a
large tree may produce as many as 20 million seeds a year. The tiny, flat, winged seeds (Figure 27.1i)