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 nonstomatal limitations varies among species in the context of phylogenetic relationships. Using saplings of 10 Eucalyptus species, we measured maximum CO2-saturated photosynthesis using A-c(i) curves at several different leaf water potentials (psi(leaf)) to quantify mesophyll photosynthetic sensitivity to psi(leaf) (MPS), a measure of how rapidly nonstomatal limitations to carbon uptake increase with declining psi(leaf). 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 lower 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 an adaptive fashion with climatically determined moisture availability, opening the way for further study of this poorly explored dimension of plant adaptation to drought.

Automated summary

Photosynthetic sensitivity to drought is a key constraint on plant evolution and ecosystem function. A study on saplings of 10 Eucalyptus species found that species native to mesic habitats have higher drought sensitivity, but those with lower turgor loss points exhibit lower sensitivity. This study provides evidence that photosynthetic sensitivity in Eucalyptus evolved adaptively with macroclimatic moisture availability.