Mesophyll photosynthetic sensitivity to leaf water potential in Eucalyptus: A new dimension of plant adaptation to native moisture supply

Photosynthetic sensitivity to drought is a fundamental constraint on
land-plant evolution and ecosystem function. However, little is known
about how the sensitivity of photosynthesis to non-stomatal limitations
varies among species in the context of phylogenetic relationships. Using
saplings of 10 Eucalyptus species, we measured maximum CO2-saturated
photosynthesis using A-ci curves at several different leaf water
potentials (PSIleaf) to quantify mesophyll photosynthetic sensitivity to
PSIleaf (MPS), a measure of how rapidly non-stomatal limitations to carbon
uptake increase with declining PSIleaf. MPS was compared to the
macroclimatic moisture availability of the species’ native habitats, while
accounting for phylogenetic relationships. We found that species native to
mesic habitats have greater MPS but higher maximum photosynthetic rates
during non-water-stressed conditions, revealing a trade-off between
maximum photosynthesis and drought sensitivity. Species with lower turgor
loss points have less MPS, indicating coordination among photosynthetic
and water-relations traits. By accounting for phylogenetic relationships
among closely related species, we provide the first compelling evidence
that MPS in Eucalyptus evolved in adaptive fashion with climatically
determined moisture availability, opening further study of this poorly
explored dimension of plant adaptation to drought.