In this investigation,
Fabrice Martin-Laurent Foong-Yee Tham, Sing-Kong Lee, Jie He and Hoang G. Diem,
show the adventage of aeroponics growth and inoculation over other plant
culture methods. Their work (published in
1997) focused on inoculation of Acacia
mangium with selected strains of Bradyrhizobium
spp. Aeroponics was found to be the best system compared to plants grown
in liquid or sand media. Aeroponically grown plants developed a very high
number of small nodules distributed all along the root system, resulting in an
increase in nitrogen and chlorophyll content in plant tissues.
In a laterwork (published in 2000) they report that the positive effects
on growth, such as increased height and leaf area, induced by aeroponic culture
with the Bradyrhizobium strains
continued to be sustained for 4 months after plants were transferred to field
conditions. They also showed that Aust
13c, one of the Bradyrhizobium strains, continued to persist in root nodules
of inocualated plants in the field, with inoculated plants. The following text and the next page are a
resume of the original publications.
Acacia mangium Willd., has gained increasing popularity in the last 2 decades for reforestation of degraded lands in the humid tropics, particularly in Sabah, Malaysia, where it competes well with noxious grasses. Thanks to its natural nitrogen-fixing ability, A. mangium possesses the remarkable growth potential of pioneer tree legumes and will grow well even on a very acid and infertile soil. Its introduction is, however, not always successful due to growth limitations induced by detrimental factors in the soil of the site of introduction. Indeed, it needs to establish an association with symbiotic soil organisms including rhizobia and/or mycorrhiza to survive and grow in natural forest ecosystems. A. mangium is specifically associated with slow-growing rhizobium strains belonging to the Bradyrhizobium group. However, many strains naturally present in the soil have a low nitrogen fixation.
In order to improve the growth of A. mangium, the seedlings should be inoculated with the appropriate Bradyrhizobium strain, which is effective in fixing atmospheric nitrogen and aggressive enough to compete with the less efficient strains present naturally in the soils. The technique commonly used to produce nodulated seedlings is soil inoculation with large amounts of rhizobial inoculant.
Technologies
for the mass production of rhizobial inoculants are available from a number of
sources worldwide. Investment in such large quantities of inoculant is,
however, relatively costly and diminishes economic returns.
In
orther to improve the current inoculation technique, non-solid culture media
are a good alternative.
Several
approaches in which plants are grown in non-solid media have been used in
studies on nodulation. Hydroponics has been employed in evaluating the
effectiveness of plant nitrogen-fixing symbiosis. The inability to accurately study
symbiotic relationships in hydroponics led to the first significant research
with aeroponics (Zobel et al.1976).
Unlike
water culture, plants grown aeroponically always show good root hair
development due to the highly aerated environment surrounding the root system.
This is an extremely important consideration in nodulation studies.
In
their work, fourteen-day-old seedlings
were simultaneously transferred into sand, hydroponic and aeroponic systems.
Within each system, plants were either (1) inocu-lated with Bradyrhizobium strain
Aust 13c, (2) inoculated with Bradyrhizobium strain Tel 2 or (3) not
inoculated (control).
After
3 months, plants were harvested to measure shoot and root fresh and dry
weights, number and weight of nodules, total shoot nitrogen content, leaf area,
photosynthetic rate 
and
chlorophyll content.
During
the first 54 days of culture, the growth of A. mangium seedlings
expressed by plant height was not significantly affected by the culture system
used (Fig. 1). In contrast, plant development expressed by the formation of
full phyllodes on shoots was greatly influenced. Plants growing in aeroponics,
hydroponics and sand took 8, 10 and 12 weeks, respectively, to reach the mature
leaf morphology stage. Once plants had developed mature leaf morphology, their
growth was dramatically influenced by the culture system (Fig. 1). Plants grown
aeroponically were twice as high as those in hydroponics and 4 times taller
than those grown in sand.
Plants
grown aeroponically showed not only greater height but also greater root and shoot
dry weights, as well as leaf area, compared to the plants grown hydroponically
(Table 1 and Fig 2a).
The
results demonstrate that with a relatively small quantity of inoculum, plants
grown aeroponically nodulated more profusely than plants grown in liquid or
sand media. The high number of nodules observed along the root system (Fig 2)
of aeroponically grown A. mangium and 
the
increase in nitrogen content in shoots (Table 2) suggest that the establishment
and the functioning of the N2-fixation symbiosis are facilitated in
aeroponics. The development of numerous root hairs and secondary roots of
aeroponically grown A. mangium can facilitate the initial interaction
between the host plant and the microsymbiont, characterized by the attachment
of the rhizobia to root hairs all over the roots, by increasing the number of
infection sites.
A.
mangium grown in aeroponics and inoculated with the
Australian Bradyrhizobium strain Aust 13c or Malaysian Bradyrhizobium
strain Tel2 showed a significant improvement in nitrogen content (Table 3)
and growth. Indeed, the chlorophyll content and the photosynthetic rate were
significantly increased certainly due to the high nitrogen content in the
shoot.
