SUMMARY
Drought is a limiting factor for plant survival and food production. Plants have developed various strategies in response to this stress. The immediate response to drought is the closure of the stomata. However, it prevents the transpiration and the photosynthesis. From the point of view of crops, it is interesting the ability of the plants to keep photosynthesizing under drought. Leaf water content is affected by the difference among leaf and soil water potentials (Ψleaf-Ψh) and by the xylem-specific hydraulic conductivity (Ks). It has been hypothesized that the number and diameter of the xylem vessels would affect the drought tolerance. In this experiment the influence of the xylem anatomy in the tolerance to water stress of three Capsicum sp accessions: Bol 58, Piquillo, Chimayo was studied. Plants were grown in plastic pots filled with soil:sand mixture in a glasshouse at UPV, during spring season. Two different treatments were used in the assay: normal irrigation and restricted irrigation (50% less irrigated than the control). Three and a half months after transplantation plants were harvested; roots, stems and fruits were weighted. Portions of the stem bases were fixed in FPA and rehydrated for the analysis of the xylem total area. Material was sectioned with a freezing microtome and dyed with phloroglucinol. Vessel element area was calculated with Autocad software.There were significant differences of biomass among genotypes and treatments. Piquillo showed lowest reduction of biomass under stress. The genotypes studied showed differences in the xylem structure. In addition, there were changes in the vessel diameter pattern from control to drought conditions. Generally, plants grown under water deficit reduce the number of bigger vessels (>7500µm2). Chimayo was the accession with bigger vessels, in normal and stress conditions, whereas Piquillo had smaller vessels. Therefore results showed a negative correlation between vessel size and production under stress conditions. Big vessels are prone to cavitation and a high proportion of small vessel may contribute to maintain Ks without collapse risk.