Photosynthetic heat tolerances and extreme leaf temperatures

Photosynthetic heat tolerances (PHTs) have several potential applications
including predicting which species will be most vulnerable to climate
change. Given that plants exhibit unique thermoregulatory traits that
influence leaf temperatures and decouple them from ambient air
temperatures, we hypothesized that PHTs should be correlated with extreme
leaf temperatures as opposed to air temperatures. We
measured leaf thermoregulatory traits, maximum leaf
temperatures (TMO) and two metrics of
PHTs (Tcrit and T50) quantified using the
quantum yfield of photosystem II for 19 plant species growing in
Fairchild Tropical Botanic Garden (Coral Gables, FL, USA).
Thermoregulatory traits measured at the Garden and microenvironmental
variables were used to parameterize a leaf energy balance model that
estimated maximum in situ leaf temperatures (TMIS)
across the geographic distributions of 13 species.
T MO and TMIS were positively
correlated with T50 but were not correlated
with Tcrit. The breadth of species' thermal safety margins
(the difference between T50 and TMO) was
negatively correlated with T50. Our results provide observational
and theoretical support based on a first principles approach for the
hypothesis that PHTs may be adaptations to extreme leaf temperature, but
refute the assumption that species with higher PHTs are less susceptible
to thermal damage. Our study also introduces a novel method for studying
plant ecophysiology by incorporating biophysical and species distribution
models, and highlights how the use of air temperature versus leaf
temperature can lead to conflicting conclusions about species
vulnerability to thermal damage.