This paper reports the results of work done to determine if
the positive effects on A. mangium induced by aeroponic culture with Bradyrhizobium
strains Tel 2 and Aust 13c are sustained after transplanting
to poor soil in the field. The following text is a resume of this work.
For each treatment,
aeroponically grown seedlings of A. mangium that were 3 months old, were
transferred to autoclaved soil and acclimatised in the greenhouse for 2 weeks.
The plants were then transplanted into a site with very poor soil at the new
extension plot of the Singapore Botanical Gardens, Singapore. Before planting,
the site was colonised by the highly competitive weed Imperata cylindrica which
grows well on poor soil.
After 4 months of growth in the
field, the growth parameters such as height, diameter (DBH) and leaf area, as
well as physiological para-meters, were determined.
In addition, identification of Bradyrhizobium
was carried out by using polymerase chain reaction–restriction fragment
length polymorphism (PCR–RFLP) techniques.
Among the aeroponically cultured plants, after 4 months in
the field, those inoculated with Aust 13c or Tel 2 were
significantly taller than the controls which were not inoculated with any Bradyrhizobium
(Table 1). The mean leaf area of Aust 13c and Tel 2 nodulated
plants was also greater than that of the control. However, there was no significant
increase in stem diameter compared to the control.
Photosynthetic
light response curves (Fig. 1) indicated that the saturating irradiances for
photosynthesis were similar in all plants. There was also no difference in
quantum yield of photosynthetic oxygen evolution, further confirming that all
plants grown in the field were not under photo-inhibition. Thus, any difference
between plants is likely to be due to differences in treatment and not to
variable light or stress conditions on the site.
Also chlorophyll content and maximum photosynthetic rate of the plants inoculated with Aust 13c were significantly higher than in the control plants after 4 months in the field (Table 2; Fig. 1). Surprisingly, statistical analysis did not show any significant difference in maximum photosynthetic
rate between Tel 2-inoculated plants and
controls in the field; distinctly positive effects of Tel 2 inoculation
over the controls were observed when plants were grown in aeroponics
(Martin-Laurent et al. 1997). Similarly, there was no significant difference in
N, P and chlorophyll contents between Tel 2-inoculated plants and
controls after 4 months in the field.
Aust 13c-inoculated plants had
significantly higher maximum photosynthetic rates than the controls. The
improved effects observed in aeroponics as a result of inoculation and
nodulation continued in the field. Fast-growing aeroponically grown and
inoculated seedlings can thus grow and perform equally well under both
green-house and field conditions.
The results from molecular
identification of bradyrhizobia present in the nodules of the aeroponically
grown A. mangium transferred to the field (Fig 2) suggests that Aust
13c may be more competitive than Tel 2. Plants inoculated with Aust
13c had nodules containing only Aust 13c bacteria.
In contrast, plants inoculated with Tel 2 had
nodules which contained either Tel 2 or another previously unrecorded
bradyrhizobial strain. This strain had apparently infected the plants after
their transfer to the field.
This may explain why there were no significant
differences in N, P and chlorophyll contents or maximum photosynthetic rates
between Tel 2-inoculated and control plants when they were transferred
to the field. It may be that Tel 2 is not as competitive as other
strains of bradyrhizobia
already present in the local soil in the field site.
Plants inoculated with Tel
2 in aeroponics were gradually being infected with the new strain of
bradyrhizobia and this strain, although more competitive than Tel 2, may
not be as efficient in nitrogen fixation.
These results lead them to
speculate that the best root nodule bacterium–host tree combination may have
been naturally selected for in the course of evolution of the two symbiotic
partners in the
region of origin. An
association formed between A. mangium and a Bradyrhizobium strain
from Australia (e.g. Aust 13c) may thus actually be stronger and more
specific than between A. mangium and a Bradyrhizobium strain from
a non-native site (e.g. Tel 2, a Malaysian). Such local Bradyrhizobium
strains may compete well with Tel 2 strain but not with Aust 13c strain,
which may be highly ‘adapted’ and naturally selected to form a symbiotic
relationship with A. mangium.
The results of the nodulation
experiments serve to emphasise that although enhancement of growth and
development of host plants in response to inoculation with a symbiont such as
bradyrhizobia is relatively easily observed under controlled laboratory or
greenhouse conditions, it is more difficult to observe an effect resulting from
inoculation of a symbiont and a host in the field. There is the complication of
interaction of numerous factors such as soil chemical and microbiological
composition, light, rainfall and drainage.
