Similar temperature dependence of photosynthetic parameters in sun and shade leaves of three tropical tree species

Abstract

Photosynthetic carbon uptake by tropical forests is of critical importance in regulating the earth’s climate, but rising temperatures threaten this stabilizing influence of tropical forests. Most research on how temperature affects photosynthesis focuses on fully sun-exposed leaves, and little is known about shade leaves, even though shade leaves greatly outnumber sun leaves in lowland tropical forests. We measured temperature responses of light-saturated photosynthesis, stomatal conductance, and the biochemical parameters VCMax (maximum rate of RuBP carboxylation) and JMax (maximum rate of RuBP regeneration, or electron transport) on sun and shade leaves of mature tropical trees of three species in Panama. As expected, biochemical capacities and stomatal conductance were much lower in shade than in sun leaves, leading to lower net photosynthesis rates. However, the key temperature response traits of these parameters—the optimum temperature (TOpt) and the activation energy—did not differ systematically between sun and shade leaves. Consistency in the JMax to VCMax ratio further suggested that shade leaves are not acclimated to lower temperatures. For both sun and shade leaves, stomatal conductance had the lowest temperature optimum (~25 °C), followed by net photosynthesis (~30 °C), JMax (~34 °C) and VCMax (~38 °C). Stomatal conductance of sun leaves decreased more strongly with increasing vapor pressure deficit than that of shade leaves. Consistent with this, modeled stomatal limitation of photosynthesis increased with increasing temperature in sun but not shade leaves. Collectively, these results suggest that modeling photosynthetic carbon uptake in multi-layered canopies does not require independent parameterization of the temperature responses of the biochemical controls over photosynthesis of sun and shade leaves. Nonetheless, to improve the representation of the shade fraction of carbon uptake dynamics in tropical forests, better understanding of stomatal sensitivity of shade leaves to temperature and vapor pressure deficit will be required.

Photosynthetic plasticity of a tropical tree species, Tabebuia rosea, in response to elevated temperature and [CO2]

“…Previous studies in Panama suggested an important role for stomatal conductance in controlling the short‐term temperature response of photosynthesis in naturally growing lowland tropical trees (Hernández et al, 2020; Slot & Winter, 2017b, 2017c). The relatively small role of stomatal limitation in the control over the temperature response of photosynthesis in the current study on well‐watered potted saplings suggests that stomata in tall trees are more sensitive to changes in VPD (as shown e.g., for Mediterranean oaks by Mediavilla & Escudero, 2004), presumably due to the challenges of water delivery to the canopy.…”

“…Previous studies in Panama suggested an important role for stomatal conductance in controlling the short-term temperature response of photosynthesis in naturally growing lowland tropical trees (Hernández et al, 2020;Slot & Winter, 2017b, 2017c Kumarathunge et al (2019). Previous work found reductions (Cernusak et al, 2011) or no effect (Fauset et al, 2019) on stomatal conductance when tropical species were grown at elevated CO 2 .…”

“…The short‐term temperature response of net photosynthesis can be controlled by different factors, including the temperature sensitivities of V CMax , J Max , and respiration in the light, and by stomatal conductance (Lin, Medlyn, & Ellsworth, 2012). We have shown that the temperature response of net photosynthesis of field‐grown lowland tropical trees is largely controlled by decreases in stomatal conductance as the leaf‐to‐air vapour pressure deficit (VPD) increases with increasing measurement temperature (Slot & Winter, 2017b, 2017c; Hernández, Winter, & Slot, 2020; see also Smith et al, 2020), whereas Vårhammar et al (2015) reported significant limitations by J Max in tropical montane species in Rwanda. Growth at elevated CO 2 generally results in decreases in stomatal conductance (Ainsworth & Rogers, 2007; Saxe, Ellsworth, & Heath, 1998), potentially increasing stomatal control over net photosynthesis.…”

Experimental warming across a tropical forest canopy height gradient reveals minimal photosynthetic and respiratory acclimation

“…Smith and Dukes (2017) found that tropical seedling acclimation was more likely to occur in processes associated with the light reactions of photosynthesis (e.g., electron transport), as opposed to processes associated with the carbon reactions of photosynthesis (e.g., Rubisco carboxylation). In contrast, Hernández et al (2020) recently found no T opt differences in ETR between tropical tree sun and shade leaves, suggesting no temperature acclimation. Electron transport acclimation can occur through stabilization of the thylakoid membrane (Havaux, 1996;Neta-Sharir et al, 2005) or implementation of cyclic electron transport (Havaux, 1996;Schrader et al, 2004), which allows for maintained adenosine triphosphate (ATP) phosphorylation when photosystem II function has declined Schrader et al (2004).…”

Improved representation of plant physiology in the JULES-vn5.6 land surface model: Photosynthesis, stomatal conductance and thermal acclimation

“…S1a, the fixed Topt of photosynthesis in the Collatz scheme is ~33 o C and in Farquhar is ~34 o C. This is higher than observations from Fig. 1a of Kumarathunge et al (2019b), where the Topt for net leaf photosynthesis lies between ~29 to 32 o C, and other studies also show a lower Topt for photosynthesis of around 30 o C for mature tropical trees (Hernández et al, 2020;Mau et al, 2018). This supports our results, and suggests the fixed temperature sensitivity of photosynthesis for tropical trees in the JULES model results in a Topt of photosynthesis that is too high for current-day.…”