Ponderosa pine (Pinus ponderosa Dougl. ex Laws.) is widely distributed in the western USA. We report the lack of stomatal closure at night in early summer for ponderosa pine at two of three sites investigated. Trees at a third site with lower nitrogen dioxide and nitric acid exposure, but greater drought stress, had slightly open stomata at night in early summer but closed stomata at night for the rest of the summer. The three sites had similar background ozone exposure during the summer of measurement (2001). Nighttime stomatal conductance (g_s) ranged from one tenth to one fifth that of maximum daytime values. In general, pole-sized trees (< 40 years old) had greater nighttime g_s than mature trees (≫ 250 years old). In late summer, nighttime g_s was low (< 3.0 mmol H₂O m⁻² s⁻¹) for both tree size classes at all sites. Measurable nighttime g_s has also been reported in other conifers, but the values we observed were higher. In June, nighttime ozone (O₃) uptake accounted for 9, 5 and 3% of the total daily O₃ uptake of pole-sized trees from west to east across the San Bernardino Mountains. In late summer, O₃ uptake at night was < 2% of diel uptake at all sites. Nocturnal O₃ uptake may contribute to greater oxidant injury development, especially in pole-sized trees in early summer.

• Morphological and physiological attributes were assessed to elucidate the underlying mechanisms of ozone (O₃) sensitivity in a highly sensitive species, cutleaf coneflower (Rudbeckia laciniata var. digitata). • Foliage at the same height in the canopy on paired O₃-sensitive and -insensitive cutleaf coneflowers was assessed for level of foliar symptoms, stomatal density, stomatal responsiveness to dynamic changes in light and leaf-to-air vapor pressure deficit (VPD), steady-state responses to light and CO₂, intrinsic transpirational efficiency, and plant water balance. • There were no morphological differences between the sensitivity types that might have contributed to greater O₃ uptake in sensitive individuals. Stomata of sensitive plants were less responsive than those of insensitive plants to experimentally increased and decreased light intensities, and to increased VPD. O₃-insensitive plants had greater intrinsic transpirational efficiencies, greater maximum assimilation rates under saturating CO₂ and light, and greater carboxylation rates. • Different physiological attributes vary independently within an individual plant, which collectively confer sensitivity or insensitivity to O₃ injury.