The physiological traits underlying the apparent drought resistance of 'Tomàtiga de Ramellet' (TR) cultivars, a population of Mediterranean tomato cultivars with delayed fruit deterioration (DFD) phenotype and typically grown under non-irrigation conditions, are evaluated. Eight different tomato accessions were selected and included six TR accessions, one Mediterranean non-TR accession (NTRM) and a processing cultivar (NTRO). Among the TR accessions two leaf morphology types, normal divided leaves and potatoleaf, were selected. Plants were field grown under wellwatered (WW) and water-stressed (WS) treatments, with 30 and 10% of soil water capacity, respectively. Accessions were clustered according to the leaf type and TR phenotype under WW and WS, respectively. Correlation among parameters under the different water treatments suggested that potential improvements in the intrinsic water-use efficiency (AN/gs) are possible without negative impacts on yield. Under WS TR accessions displayed higher AN/gs, which was not due to differences in Rubisco-related parameters, but correlated with the ratio between the leaf mesophyll and stomatal conductances (gm/gs). The results confirm the existence of differential traits in the response to drought stress in Mediterranean accessions of tomato, and demonstrate that increases in the gm/gs ratio would allow improvements in AN/gs in horticultural crops.

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.

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