Best of both worlds: Gas exchange and hydraulics of shade and drought tolerant species Ruscus aculeatus and R. microglossum (Ruscaceae)

Background/Question/Methods The genus Ruscus (Ruscaceae) consists of evergreen, woody monocot shrubs with phylloclades. Stem photosynthetic plants are usually found in arid environments with high light availability; Ruscus, however, is found in dry, shaded woodland area of the Mediterranean Basin and northern Europe. The drought and shade tolerance of Ruscus challenges the “tradeoff model,” which suggests that plants can be either drought adapted or shade adapted, but not both. In order to understand the mechanisms that enable Ruscus to survive in shaded-drought environments, we selected two species, R. aculeatus and R. microglossum, in the Mildred E. Mathias Botanical Gardens at the campus of UCLA. Both species grew adjacent to each other under similar environmental conditions. We studied form-function relations, which included over 35 traits, focusing on plant morphology (including specific leaf area), gas exchange measurements (including CO2 assimilation, respiration, stomatal conductance, water-use efficiency, light response curves, and CO2 response curves), and hydraulics (including cuticular conductance, pressure volume curves, and leaf hydraulic conductance). We then compared these traits to other phylloclade bearing or stem photosynthetic plants and woody evergreen shade tolerant and intolerant plants found in published data to determine to what degree Ruscus shows combined shade and drought tolerant traits. Results/Conclusions R. aculeatus and R. microglossum showed both clear drought and shade tolerant characteristics that confer simultaneous adaptation to drought and shade as they require low light and water. These traits include thick phylloclades with low rates of maximum assimilation, low respiration, low light compensation points, low cuticular conductance, low hydraulic conductance, and low modulus of elasticity associated with internal water storage tissue. Despite very low stomatal conductance, very low carbon isotope ratios implied that extremely low photosynthetic rates are typical in these species. Overall, these trait values were comparable to shade tolerant broadleaf woody evergreens grown in low light. Ruscus appears to be highly specialized both physiologically and morphologically to occupy shaded-drought regions across a wide geographical range, including extremely low resource understory sites.