Fig. 6 Germination probability in an incubator (at 35/20 °C,
light/dark, 18/6 hours cycled) of seeds buried and then exhumed from the
soil for up to three months. Seeds were buried in the exposed Spring
compartment.
Discussion
- Use of semi-natural and simulated habitats for germination
ecology experiments
In the present study we used a Semi-NH from the native range of M.
balbisiana , and several Simulated-NHs in glass houses located in a
temperate region, to examine wild banana seed germination ecology. We
established that with such approximations of NHs, it is possible to link
germination responses to ecological factors such as foliar-shading and
burial-depth. Hence with this approach we overcame limitations when
access to experimental NHs was not possible, this allowed for greater
ecological interpretation than with LCs alone.
Temperature
We found that Musa seed germination is stimulated by exposure to
the sun. It was the maximum part of the temperature fluctuation, in our
results, that is most closely associated with germination. Above the
threshold of 23°C, germination increased to a maximum at 35°C (in
Simulated-NHs and 42°C in Semi-NHs) in exposed conditions. Our findings
are broadly consistent with previous LC results, where optimal maximal
and minimal temperature for germination were diurnal cycles of
35/18-20°C respectively, for both M. acuminata and M.
balbisiana (Stotzky & Cox 1962; Kallow et al. 2021).
Interactions between the elements of warming and cooling cycles play an
important role in simulating germination.
Light
One might think that as germination responses were directly associated
with soil exposure to sun and, in simulated-NHs, light intensity,
germination is stimulated by light. Additionally, we also found that
seeds germinated to a greater extent from 1 cm compared to 7 cm.
However, light waves cannot usually penetrate the soil to greater than
4-5 mm depth (depending on soil moisture and particle size), and not to
any amount that can illicit germination responses to light sensitive
seeds (Woolley & Stoller 1978; Tester & Morris 1987), but in our
experiment, seeds germinated from a depth of 7 cm. We therefore infer
that whilst Musa seed germination may be correlated with factors
associated with light (light intensity, shallow burial) theses are
correlations rather than causal, and it is temperature that regulates
germination.
Gap detection
Musa seed germination in response to sun exposure demonstrates
adaptation to detect suitable niches for seedling establishment
following disturbance in forest NHs. Conversely, inhibition of
germination in shade is also an adaptation for seedling survival (Kos &
Poschlod 2007; Poschlod et al. 2013). Musa germination
responses were sensitive to sun/shade even when microclimates were very
similar. For instance, mean temperatures were the same and range
differed by only a few degrees in sun and shaded semi-NHs. Germination
is therefore finely tuned to respond to microclimate such as that which
would occur when a forest gap is formed (Pearson et al. 2002;
Pearson et al. 2003). The effect of forest disturbance on
temperature dynamics was studied by Harwick et al . (2015). The
authors measured soil (10 cm depth) and air temperature (1.5m height) at
three levels of forest disturbance in Borneo. Diurnal
temperatures in oil palm plantations (formerly forested) were around 7°C
greater at the hottest part of the cycle compared to old growth forests
and soil temperatures were around 3 °C warmer at this point and around 1
°C cooler in the night; these were similar to our results in semi-NHs.
Temperatures in logged forests were somewhat in between these two, but
more similar to old growth forest. One could imagine that even in small
forest gaps, microclimates could therefore also vary considerably
(Pearson et al. 2002). These responses are in line with
adaptations of other disturbance-adapted species that also require
alternating temperature cycles rather than constant temperatures to
germinate (Vázquez‐Yanes & Orozco‐Segovia 1982; Vázquez-Yanes &
Orozco-Segovia 1993; Pearson et al. 2002; Seiwa et al.2009).
Seed burial-depth
When seeds were in the shade, burial-depth made no difference to
germination, they did not germinate irrespective of burial-depth. In
exposed sites, shallow buried seeds were more likely to germinate than
deeply buried seeds. This was because temperature dynamics are likely
buffered by burial depth. For some species, sensitivity to alternating
temperatures is an adaptation to detect burial depth (Thompson, Grime &
Mason 1977; Thompson & Grime 1983). Pearson et al. (2002) found, for
large-seeded species, diurnal temperature sensitivity was more likely to
related to forest gap size than seed burial-depth. This was also the
case for large-seeded Musa , in that exposure was by far the most
significant factor in simulated-NHs, and burial-depth was only
secondary. For small seeds it is important to detect burial-depth as
seedlings must reach the surface with small endosperm reserves; for
larger seeds with greater nutrient reserves this is less of a limiting
factor for survival.
Survival and dormancy loss in the soil
We found seeds can persist and remain viable buried in the soil for at
least two years. In fact, there was no loss of viability after one year,
there was however loss of actual seeds. Seed loss was more pronounced
with shallow burial, suggesting it is the result of predation or perhaps
splashing during watering, rather than decomposition. This is also
supported by the fact that accessions with less viable seeds were more
likely to be found i.e., seeds were not lost by decomposition of dead
seeds.
For seeds adapted to disturbance, seed persistence in the soil seed bank
is important. Musa clearly invest considerably in seed coat
defenses (Graven et al. 1996), to survive the intense pressure
present in the soil community (Dalling et al. 2011).
Not only do seeds persist in the soil, but dormancy is reduced during
this process. In our results, when seeds were stratified for three
months, or when they were in the soil for a year, germination increased.
A stratification requirement is in keeping with results from our
previous study (Kallow et al. 2021). Although, stratification was
not required for freshly extracted M. balbisiana seeds in
Semi-NHs, implying drying induces secondary dormancy in Musaseeds, as proposed by Chin (1996).
There was greater germination synchronization in cooler Simulated-NHs as
seeds responded to threshold temperatures when sun was stronger during
the summer. When temperatures were consistently warmer, synchronicity
was reduced - this may again be a disturbance adaptation. We found this
response more evident in M. balbisiana than in M.
acuminata, suggesting it may also relate to seasonality as M.
balbisiana has a large distribution that includes subtropical seasonal
climates (Mertens et al. 2021).
Conclusions
Studying germination ecology has intrinsic challenges, possibly the
biggest being access to seeds and experimental set-ups in suitable
conditions and timeframes. In the present study we demonstrate an
approach for dealing with such difficulties in studying tropical seed
germination ecology, which is a challenge when researchers are outside
of a plant’s native region. Using Semi-NHs and Simulated-NHs we found:
(1) foliage-shading inhibits germination of non-dormant seeds, and
exposure to sun stimulates germination; this response is most closely
associated with maximum temperature variation found under direct
sunlight; this effect is marginally buffered by deep burial in the soil;
(2) freshly extracted seeds are non-dormant, but stored seeds lose their
dormancy during burial in the soil; (3) Musa seeds remain viable
in the soil for at least a year without any loss in viability. Thus,
wild banana species are well adapted to exploit canopy gaps following
disturbance.
Acknowledgments
We thank Isla Kallow, Jasmin Kallow, Manuela Garcia Zuluaga and Kevin De
Pauw for assisting with the setting up the glass house experiments. We
gratefully acknowledge the help of John Mark Barios, Lyka Yanos and
Paulo Jerome Lopez at NPGRL for setting up and monitoring the
germination experiments in the Philippines and for Michelle Lyka V.
Descalsota for assistance in viability assessing seed collections at
NPGRL. We also thank the late Daniele Roques (CIRAD, Centre de
Ressources Biologiques Plantes Tropicales (CRB-PT), Guadeloupe),
Josephine Agogbua (International Institute of Tropical Agriculture
(IITA), Nigeria) and Zhiying Li (Institute of Tropical Crop Genetic
Resources, CATAS Tropical Crops, Danzhou, China) for providing seeds to
use. This work was funded as a sub-grant from the University of
Queensland from the Bill & Melinda Gates Foundation project ‘BBTV
mitigation: Community management in Nigeria, and screening wild banana
progenitors for resistance’ [OPP1130226]. The authors thank all
donors who supported this work also through their contributions to the
CGIAR Fund (http://www.cgiar.org/funders), and in particular to the
CGIAR Research Program Roots, Tubers and Bananas (RTB-CRP). This study
was supported by a bilateral grant between the Research Foundation -
Flanders (FWO-Vlaanderen) and the Vietnamese National Foundation for
Science and Technology Development (NAFOSTED) under grant number
G0D9318N.
Author contributions
SK: conceptualization, methodology, software, formal analysis,
investigation, data curation, writing-original draft, writing-review and
editing, visualization; KQ: investigation; BP: conceptualization,
methodology, resources, writing-review and editing, supervision, funding
acquisition; SBJ: conceptualization, methodology, resources,
writing-review and editing, supervision, funding acquisition; JD:
writing-review and editing, supervision, funding acquisition; LG:
resources; RS: writing-review and editing, supervision; FV:
conceptualization, methodology, resources, writing-review and editing.
Conflict of interest
The authors declare that there is no conflict of interest associated
with this article and research.
Data availability
All data available at Kallow, Simon (2021): Using semi-natural and
simulated habitats for seed germination ecology. figshare. Dataset.
https://doi.org/10.6084/m9.figshare.14884470.v1
Supplementary figures and
tables
Fig. S1. Average and standard deviations of temperature and
light intensity during cue periods of exposed and shaded environments,
temperatures are °C, light intensity is lux.
Fig. S2. Light intensity and temperature measured at simulated
natural environments in glass houses during germination tests;
asymptotic non-linear regression shown (residual standard error 4.119 on
417,661 degrees of freedom).