The diurnal variation in leaf stomatal conductance (gs) of three xerophylus species (Buddleia cordata HBK, Dodonanea viscosa L. Jacq. and Senecio praecox L.) was measured over a 10-month period during the dry and wet season. These measurements were made in four sites of a xerophylus shrubland which is developing in a lava substratum in the south of Mexico City. Averaged stomatal conductances were 147 and 60.2 (B. cordata); 145 and 24.8 (D. viscosa) and 142.8 and 14.1 mmol m-2 s-1 (S. praecox) during the wet and dry season, respectively. Measured leaf water potential varied from -0.6 to -1.2 (S. praecox), -0.6 to -1.8 (B. cordata) and -0.9 to -3.4 MPa (D. viscosa) in the measurements period. Stomatal conductance was dominantly controlled by irradiance (Q), air temperature (Ta) and leaf-air vapor pressure difference (VPD), and there was not evidence that stomata were sensitive to leaf water potential in B. cordata and D. viscosa, during the dry or wet season. However, gs for S. praecox showed a significant response to leaf water potential and DPV in the dry season and Ta and VPD in the wet season. Nevertheless, in annual analyses the effect produced on gs by leaf water potential in the three studied species was significant. A model relating gs to environmental and physiological variables accounted for 79-83% of the variation of gs in two sites (pooled data), however the performance of the model was lower (60-76%) for individual species from other two sites not included in the pooled data.

Key words: Buddleia cordata, Dodonaea viscosa, leaf water potential, Senecio praecox, shrubs, stomatal conductance