Physiological Mechanisms of Drought Resistance in Four Native Ornamental Perennials

Understanding physiological drought resistance mechanisms in ornamentals may help growers and landscapers minimize plant water stress after wholesale production. We characterized the drought resistance of four potted, native, ornamental perennials: purple coneflower [ Echinacea purpurea (L.) Moench], orange coneflower [ Rudbeckia fulgida var. Sullivantii (Beadle & Boynt.) Cronq.], beebalm ( Monarda didyma L.), and swamp sunflower ( Helianthus angustifolius L.). We measured a) stomatal conductance of leaves of drying plants, b) lethal water potential and relative water content, and c) leaf osmotic adjustment during the lethal drying period. Maintenance of stomatal opening as leaves dry, low lethal water status values, and ability to osmotically adjust indicate relative drought tolerance, with the reverse indicating drought avoidance. Echinacea purpurea had low leaf water potential (ψ L ) and relative water content (RWC) at stomatal closure and low lethal ψ L and RWC, results indicating high dehydration tolerance, relative to the other three species. Rudbeckia fulgida var. Sullivantii had a similar low ψ L at stomatal closure and low lethal ψ L and displayed relatively large osmotic adjustment. Monarda didyma had the highest ψ L and RWC at stomatal closure and an intermediate lethal ψ L , yet displayed a relatively large osmotic adjustment. Helianthus angustifolius became desiccated more rapidly than the other species, despite having a high ψ L at stomatal closure; it had a high lethal ψ L and displayed very little osmotic adjustment, results indicating relatively low dehydration tolerance. Despite differences in stomatal sensitivity, dehydration tolerance, and osmotic adjustment, all four perennials fall predominantly in the drought-avoidance category, relative to the dehydration tolerance previously reported for a wide range of plant species.