Amphistomy increases leaf photosynthesis more in coastal than montane plants of Hawaiian ʻilima

Premise of the study The adaptive significance of stomata on both upper
and lower leaf surfaces, called amphistomy, is unresolved. A widespread
association between amphistomy and open, sunny habitats suggests the
adaptive benefit of amphistomy may be greatest in these contexts, but this
hypothesis has not been tested experimentally. Understanding amphistomy
informs its potential as a target for crop improvement and
paleoenvironment reconstruction. Methods We developed a method to quantify
"amphistomy advantage", AA, as the log-ratio of photosynthesis
in an amphistomatous leaf to that of the same leaf but with gas exchange
blocked through the upper surface (pseudohypostomy). Humidity modulated
stomatal conductance and thus enabled comparing photosynthesis at the same
total stomatal conductance. We estimated AA and leaf traits in six coastal
(open, sunny) and six montane (closed, shaded) populations of the
indigenous Hawaiian species ʻilima (Sida fallax). Key results Coastal
ʻilima benefits 4.04 times more from amphistomy than montane leaves.
Evidence was equivocal with respect to two hypotheses – that coastal
leaves benefit more because 1) they are thicker and have lower conductance
through the internal airspace, and 2) they benefit more because they have
similar conductance on each surface, as opposed to most conductance being
through the lower surface. Conclusions This is the first direct
experimental evidence that amphistomy increases photosynthesis, consistent
with the hypothesis that parallel pathways through upper and lower
mesophyll increase CO2 supply to chloroplasts. The prevalence of
amphistomatous leaves in open, sunny habitats can partially be explained
the increased benefit of amphistomy in 'sun' leaves, but the
mechanistic basis remains uncertain.