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 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.