Thermal tolerance at the hottest Amazonian forest
site
Tree species measured in Nova Xavantina showed a very high
thermotolerance, with one species, Amaioua guianensis, recorded
the highest T50 documented for any tropical
evergreen tree thus far (52.7 ± 1.05 °C). The data reported here
represent the first F v/F m measurements made on adult Amazon trees. Thus, it is not possible to
directly compare these results with published data from other Amazonian
sites. However, the T50 values reported here are
similar to values reported for four Panamanian species (Slot et al.,
2018) and also to T crit values reported for two
Amazonian sites(O’Sullivan et al., 2017). However, although F v/F m and F 0 are related, they are not equivalent. Hence,
care must be taken when making comparisons across metrics. However, the
species in Nova Xavantina (Table 1) were considerably more
thermotolerant than the Western Ghats forests in India, where the most
extensive datasets by far are available for leaf thermotolerance (Sastry
& Barua, 2017).
The differences in the long-term Tmax mean values
for the site across the two seasons was ~2.5°C
(Tmax = 35.7±0.37°C for October and 33.19 ±0.23°C
for the months March-April). Consistent with the literature,
T50 measurements across seasons showed significant
differences, with T50 values for the hot/dry
season being ~1.6 °C greater than in the cooler/wetter
season. A. guianensis , a characteristic mid-storey species found
across a wide range of ecosystems in the region (Morandi et al., 2016),
had the highest T50 and the highest seasonal
plasticity in T50 . A. guianensis is the
slowest growing species among the species sampled (Mews, Marimon, Pinto,
& Silvério, 2011). These results are consistent with studies that
indicate that slow-growing species have greater capacity for flexible
heat dissipation and thermal protection mechanisms than fast-growing
plants (Adams & Demmig-Adams, 1994). Maintaining high thermal tolerance
during dry periods could be energetically expensive (Wahid, Gelani,
Ashraf, & Foolad, 2007), thus requiring down-regulation during wet
periods. Stomatal regulation varies across seasons and species.
Moreover, lower water availability limits the transpiration-dependent
cooling capabilities in dry periods.
The observed leaf level differences could be due to small variations in
the prehistory of the leaves within the canopy for example in terms of
light or temperature exposure (Colombo & Timmer, 1992) during the peak
dry period, prior to sampling. For example, the acquired tolerance
induced by a pre-exposure to heat is associated with the synthesis of
heat shock proteins (HSPs) and with other low molecular weight proteins
that protect PSII (Gifford & Taleisnik, 1994). The variations inT95 values were higher than those observed inT5 or T50 . This may
indicate that there is greater variation in the high temperature
threshold for loss of photosystem integrity (T95 )
between species than in the initiation temperature of sensitivity
(T5 ) (Figure 6).
The mechanisms that underpin the thermal stability of photosynthesis in
tropical trees have not been fully characterised. The data presented
here clearly show that species such as A. guianensis are able to
maintain photosystem II functions up to high temperatures (53.9±0.75°C
during the end of dry period i.e., the highest reported in literature
for C3 plants). These trees could have mechanisms to not only protect
the photosystem II from irreversible thermal inactivation, but also have
a high degree of plasticity in relation to temperature fluctuations. In
such trees, loss of photosynthetic functions may be prevented by a more
rapid repair cycle than that occurring in other species. However, the
precise nature of the specific mechanisms involved remains unclear.
Further molecular and metabolic studies on tropical tree species are
required to understand how photosynthesis can withstand high
temperatures. Some studies indicate that redundancy and diversity of
light harvesting complexes (Tang et al., 2007) could play a role in
buffering the high light, high temperature-induced changes in
photosystem functions. Hence, heat sensitive systems could be replaced
with more thermally stable forms. Similarly, large variations in heat
shock factors/proteins, oxidative stress/signalling and thermal energy
dissipation could exist across taxa, hinting at differential thermal
sensitivity of tropical evergreen trees to high-temperature stress.