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Bedding plants are at increased risk for exposure to drought stress during production because they are grown in small containers. Physiological mechanisms of bedding plants at leaf and cellular scales that regulate whole-plant photosynthesis under drought conditions are not well understood. This information can be useful for screening bedding plant cultivars with improved drought-tolerance and generate guidelines to mitigate drought stress during production. We subjected drought-sensitive salvia ( Salvia splendens  ‘Bonfire Red’) and drought-tolerant vinca ( Catharanthus roseus  ‘Cooler Peppermint’) to gradual drought stress inside whole-plant gas exchange chambers. Substrate water content (Θ), whole-plant net photosynthesis (P n _avg  ), whole-plant respiration (R d _avg  ), and daily carbon gain (DCG) were measured continuously, whereas stomatal conductance ( g  S ) to water, leaf water (Ψ L ), osmotic (Ψ S ), and turgor (Ψ P ) potentials were measured at the start and end of the drought phase. In addition, Ψ S  was measured before exposure to stress and after thoroughly rehydrating plants. Dark-adapted quantum efficiency (dark-adapted Φ PSII ) was measured after rehydrating plants. The results indicated that, at whole-plant scale, vinca continued to uptake water at lower Θ levels than the Θ level that resulted in wilting of salvia. There were no differences in R d _avg  ; however, P n _avg   and DCG of salvia decreased at a higher Θ level than that of vinca. This indicated that salvia experienced drought stress at a higher Θ level than did vinca. At the leaf scale, there were no differences in Ψ L ; however, a more negative Ψ S  ( P  = 0.06) and significantly higher Ψ P  were observed in vinca (compared to salvia) under drought conditions. In addition, Ψ S  was not different between species before exposure to drought, whereas Ψ S  of rehydrated leaves after exposure to drought in vinca was significantly lower than that in salvia. Moreover, Ψ S  of rehydrated leaves after exposure to drought was significantly lower than that observed before exposure to drought in vinca. This indicated osmotic adjustment (OA) in vinca under drought conditions. Dark-adapted Φ PSII  was lower in salvia than in vinca after exposure to drought, indicating damage to photosynthetic mechanisms. Our results suggested that increased OA likely helped to maintain higher Ψ P  under drought conditions and continuation of water uptake at lower Θ in vinca compared to salvia. In addition, healthier photosynthetic mechanisms of vinca (compared to salvia) under drought conditions likely resulted in its higher P n _avg   and DCG at lower Θ. Screening for OA and dark-adapted Φ PSII  may be useful for developing drought-tolerant bedding plant cultivars.

Relating Whole-plant Photosynthesis to Physiological Acclimations at Leaf and Cellular Scales under Drought Stress in Bedding Plants