Diurnal variation in leaf stomatal conductance (gs) of three xerophilous species (Buddleia cordata, Senecio praecox and Dodonaea viscosa) was measured over a 10-month period during the dry and wet seasons in a shrubland that is developing in a lava substratum in Mexico. 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. Leaf water potential (Psi) varied in a range of -0.6 to -1.2 (S. praecox), -0.6 to -1.8 (B. cordata) and -0.9 to -3.4 MPa (D. viscosa) during the same measurement periods. Stomata were more sensitive to changes in irradiance, air temperature and leaf-air vapour pressure difference in the rainy season than the dry season. Although stomatal responses to Psi were difficult to distinguish in any season (dry or rainy), data for the entire period of measurement showed a positive correlation, stomata tending to open as Psi increased, but there is strong evidence of isohydric behaviour in S. praecox and B. cordata. A multiplicative model relating gs to environmental variables and to Psi accounted for 79%-83% of the variation of gs in three sites (pooled data); however, the performance of the model was poorer (60%-76%) for individual species from other sites not included in the pooled data.

Precipitation seasonality and substratum in Pedregal de San Angel can play a key role on plant water status. Therefore, stomatal conductance (g), water potential (Ψ h), photosynthetically photon flux density (Q), air temperature (Ta) and leaf-air vapour pressure difference (VPD), were measured on leaves of Buddleia cordata H.B.K., because its perennial character and its dominance in the Pedregal. These measurements were carried out during the wet and dry seasons in the Pedregal which is a plant community developing in a lava substratum. The highest values of stomatal conductance were registered in the wet season (330 mmol m-2 s-1), however during the dry season, stomatal conductance was reduced by 54%. Upper limit enveloping curve technique and multiple regression analysis were performed to the data in order to study how g responded to the microenvironment and (Ψ h). Results clearly showed that there was a significant effect of Q, Ta and VPD on g and stomata were more sensitive on humid season than during dry season. (Ψ h), effect was not significant during the humid season, nor the dry season, but annually. Results from these kind of analysis may be very useful during the formulation of mathematics models to simulate or predict stomatal conductance.