Introduction
The heat tolerance of plants’ photosystem II (PSII) photochemistry may provide a useful estimate of the upper thermal limit of photosynthesis, and has the potential to explain the physiological mechanisms underlying some of the ecological responses of plants to climate change (Clark, Piper, Keeling & Clark 2003; Doughty & Goulden 2009; Mau, Reed, Wood & Cavaleri 2018; Pau, Detto, Kim & Still 2018; Feeley, Fadrique, Perez & Zuleta 2020). Higher heat tolerance of PSII photochemistry is generally assumed to allow for improved growth, reproduction, and/or survival in hot environments, presumably by allowing for photosynthesis at higher temperatures (Krause, Winter, Krause & Virgo 2015; Feeley, Martinez-villa, Perez & Duque 2020a; Perez & Feeley 2020; Tiwariet al. 2020). However, these assumptions have not been widely tested and it is unclear how PSII heat tolerance integrates with different thermal strategies that may be important for determining the impacts of climate change.
Heat tolerance of PSII is commonly measured using chlorophyll fluorescence. Early studies to adopt the use of chlorophyll fluorescence quantified PSII heat tolerance using the F0 fluorometric parameter - indicating the number of maximally open reaction centers - and found it was correlated with the temperature that caused carbon assimilation to approach zero (Tmax; Downton, Berry & Seemann 1984; Seemann, Berry & Downton 1984). However, F0 can provide biased estimates of PSII function during heat treatments that change leaf optical properties (Baker 2008), which has led many researchers to adopt the maximum quantum yield (FV/FM) fluorometric as a more robust metric for estimating PSII heat tolerance where FV = FM - F0, and Findicates closed reaction centers in saturating light (Maxwell & Johnson 2000; Baker 2008)
Although FV/FM can reliably measure PSII function under stress treatments and is commonly used to measure PSII heat tolerance, FV/FM may not be a reliable proxy for carbon assimilation under field conditions. FV/FM is only proportional to carbon assimilation under low light conditions and when photorespiration is minimized (Brooks & Farquhar 1985; Baker 2008). These conditions are not met in the field when leaves experience high light and temperatures. Few studies have tested if FV/FM heat tolerance promotes carbon assimilation in hotter environments, but empirical evidence and ecological theory generally support this assumption.
As was shown with heat tolerance estimates that used F0(Downton et al. 1984; Seemann et al. 1984), one way the PSII heat tolerance could promote photosynthesis at higher temperatures is if it is correlated with Tmax. Reported values for Tmax range from 40.1 to 41.8˚C and are comparable to the temperatures that cause the first signs of damage in FV/FM (Tcrit) for tropical species (Fig. 1a ; Slot et al. 2018; Tiwariet al. 2020; Perez and Feeley 2020). Coordination between Tcrit and Tmax would provide support for the hypothesis that PSII heat tolerance fixes the upper limit of carbon assimilation by limiting electron transport (Slot & Winter 2017a).
Another way that PSII heat tolerance could promote carbon assimilation at higher temperatures is by increasing the breadth of temperatures over which carbon assimilation can occur (Ω; Fig. 1a ; Cunningham  S. C. & Read  J. 2003; Slot & Winter 2017a). The Ω metric can be used to characterize plants as physiological thermal generalists vs. specialists, similar to what is done with animal species (Huey & Hertz 1984; Ghalambor, Huey, Martin, Tewksbury & Wang 2006; Huey 2012). A positive correlation between PSII heat tolerance and Ω would be consistent with a thermal generalist strategy of carbon assimilation and would provide a physiological explanation for why thermal specialist plant species are more susceptible to climate change than generalist plants (Janzen 1967; Ghalambor et al. 2006; Perez, Stroud & Feeley 2016). Indeed, Ω is a key trait linking leaf thermoregulation to the “fast-slow” leaf economic spectrum (Michaletzet al. 2015, 2016). Since variation in PSII heat tolerance is driven by high leaf temperature (Perez & Feeley 2020) and ‘fast’ species are expected to have high leaf temperatures and large Ω (Michaletz et al. 2015, 2016), PSII heat tolerance is expected to be proportional to Ω.
Plants with ‘fast’ resource acquisition strategies are characterized in part by their high rates of carbon assimilation (Wright et al.2004; Reich 2014). The plant economic spectrum typically proposes that ‘fast’ strategies are characterized by poor physiological tolerances (Reich 2014), such that the optimum rates of photosynthesis (Popt, Fig. 1a ) and PSII heat tolerances may be inversely proportional. Conversely, since ‘fast’ species are also characterized by high leaf temperature (Michaletz et al. 2015, 2016), PSII heat tolerance may be positively correlated to Popt. This expectation is consistent with the idea that high PSII heat tolerance is beneficial for plants growing in hot environments.
The optimum temperature for carbon assimilation (ToptFig. 1a ) is another important parameter that describes carbon assimilation as a function of temperature and is potentially coordinated with PSII heat tolerance. For example, species tend to increase in both their PSII heat tolerance and Topt when grown in hotter environments (Valladares & Pearcy 1998; Way & Yamori 2014; Zhuet al. 2018). High PSII heat tolerance may promote increases in Topt by improving electron transport or the availability of ATP and NADH at high temperatures (Genty, Briantais & Baker 1989; Maxwell & Johnson 2000; Baker 2008), in support of the assumption that PSII heat tolerance will facilitate carbon assimilation in hot environments.
In this study we measured three common metrics of PSII heat tolerance that indicate the temperatures that cause an initial, 50%, and 95% decrease in FV/FM(Tcrit, T50 and T95, respectively; Fig. 1b ). We compared these metrics of heat tolerance to Tmax, Popt, Topt, and Ω for 21 plant species grown in a quasi-common garden environment (Fairchild Tropical Botanic Garden, Coral gables FL USA; Perez et al. 2019). We tested four hypotheses consistent with the assumption that high PSII heat tolerance promotes carbon assimilation in hotter environments. Specifically, we looked at the correlations among the different metrics of heat tolerance and carbon assimilation, after controlling for any potential effect of phylogenetic non-independence, to test the hypotheses that H1) Tmaxis constrained by PSII heat tolerance; H2) high PSII heat tolerance is indicative of a thermal generalist strategy of carbon assimilation; H3) high PSII heat tolerance is characteristic of species with “fast” carbon acquisition strategies; and H4) high PSII heat tolerance promotes higher Topt (Fig. 2 ).