PHYSIOLOGICAL-MECHANISMS OF DROUGHT RESISTANCE IN 4 NATIVE ORNAMENTALPERENNIALS

Understanding physiological drought resistance mechanisms in ornamenta ls 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 pu rpurea (L.) Moench], orange coneflower [Rudbeckia fulgida var. Sulliva ntii (Beadle and Boynt.) Cronq.], beebalm (Monarda didyma L.), and swa mp sunflower (Helianthus angustifolius L.). We measured a) stomatal co nductance of leaves of drying plants, b) lethal water potential and re lative water content, and c) leaf osmotic adjustment during the lethal drying period. Maintenance of stomatal opening as leaves dry, low let hal water status values, and ability to osmotically adjust indicate re lative drought tolerance, with the reverse indicating drought avoidanc e. Echinacea purpurea had low leaf water potential (psi(L)) and relati ve water content (RWC) at stomatal closure and low lethal psi(L) and R WC, results indicating high dehydration tolerance, relative to the oth er three species. Rudbeckia fulgida var. Sullivantii had a similar low psi(L), at stomatal closure and low lethal psi(L) and displayed relat ively large osmotic adjustment. Monarda didyma had the highest psi(L) and RWC at stomatal closure and an intermediate lethal psi(L), yet dis played a relatively large osmotic adjustment. Helianthus angustifolius became desiccated more rapidly than the other species, despite having a high psi(L) at stomatal closure; it had a high lethal psi(L), and d isplayed very little osmotic adjustment, results indicating relatively low dehydration tolerance. Despite differences in stomatal sensitivit y, dehydration tolerance, and osmotic adjustment, all four perennials fall predominantly in the drought-avoidance category, relative to the dehydration tolerance previously reported for a aide range of plant sp ecies.