In a previous study, important acclimation to water stress was observed in the Ramellet tomato cultivar (TR) from the Balearic Islands, related to an increase in the water-use efficiency through modifications in both stomatal (gs) and mesophyll conductances (gm). In the present work, the comparison of physiological and morphological traits between TR accessions grown with and without water stress confirmed that variability in the photosynthetic capacity was mostly explained by differences in the diffusion of CO2 through stomata and leaf mesophyll. Maximization of gm under both treatments was mainly achieved through adjustments in the mesophyll thickness and porosity and the surface area of chloroplasts exposed to intercellular airspace (Sc). In addition, the lower gm/Sc ratio for a given porosity in droughtacclimated plants suggests that the decrease in gm was due to an increased cell wall thickness. Stomatal conductance was also affected by drought-associated changes in the morphological properties of stomata, in an accession and treatmentdependent manner. The results confirm the presence of advantageous physiological traits in the response to drought stress in Mediterranean accessions of tomato, and relate them to particular changes in the leaf anatomical properties, suggesting specific adaptive processes operating at the leaf anatomical level.

Theory predicts that natural selection should favor coordination between leaf physiology, biochemistry and anatomical structure along a functional trait spectrum from fast, resource-acquisitive syndromes to slow, resource-conservative syndromes. However, the coordination hypothesis has rarely been tested at a phylogenetic scale most relevant for understanding rapid adaptation in the recent past or for the prediction of evolutionary trajectories in response to climate change. We used a common garden to examine genetically based coordination between leaf traits across 19 wild and cultivated tomato taxa. We found weak integration between leaf structure (e.g. leaf mass per area) and physiological function (photosynthetic rate, biochemical capacity and CO diffusion), even though all were arrayed in the predicted direction along a 'fast-slow' spectrum. This suggests considerable scope for unique trait combinations to evolve in response to new environments or in crop breeding. In particular, we found that partially independent variation in stomatal and mesophyll conductance may allow a plant to improve water-use efficiency without necessarily sacrificing maximum photosynthetic rates. Our study does not imply that functional trait spectra, such as the leaf economics spectrum, are unimportant, but that many important axes of variation within a taxonomic group may be unique and not generalizable to other taxa.

No abstract

15Natural selection may often favor coordination between different traits, or phenotypic 16 integration, in order to most efficiently acquire and deploy scarce resources. As leaves are the 17 primary photosynthetic organ in plants, many have proposed that leaf physiology, biochemistry, 18 and anatomical structure are coordinated along a functional trait spectrum from fast, resource-19 acquisitive syndromes to slow, resource-conservative syndromes. However, the coordination 20 hypothesis has rarely been tested at a phylogenetic scale most relevant for understanding rapid 21 adaptation in the recent past or predicting evolutionary trajectories in response to climate change. 22To that end, we used a common garden to examine genetically-based coordination between leaf 23 traits across 19 wild and cultivated tomato taxa. We found surprisingly weak integration between 24 photosynthetic rate, leaf structure, biochemical capacity, and CO 2 diffusion, even though all were 25 arrayed in the predicted direction along a 'fast-slow' spectrum. This suggests considerable scope 26 for unique trait combinations to evolve in response to new environments or in crop breeding. In 27 particular, we find that partially independent variation in stomatal and mesophyll conductance 28 may allow a plant to improve water-use efficiency without necessarily sacrificing maximum 29 photosynthetic rates. Our study does not imply that functional trait spectra or tradeoffs are 30 unimportant, but that the many important axes of variation within a taxonomic group may be 31 unique and not generalizable to other taxa.