Acclimation of Leaf Anatomy, Photosynthetic Light Use, and Xylem, Hydraulics to Light in Amborella Trichopoda (Amborellaceae)

Recent phylogenetic analyses place Amborella trichopoda, a semiclimbing premontane rain forest shrubendemic to New Caledonia, as sister to all other extant flowering plants. Here we present new observationson leaf anatomy and morphology, leaf photosynthetic physiology, and xylem hydraulic function for Amborellain the field. We focus on experimental measurements of the degree of photosynthetic and xylem hydraulicacclimation to sunlight by examining Amborella plants growing in deeply shaded understories (0.7% fullsunlight) compared with individuals from exposed montane ridge crests in New Caledonia. Amborella leavesexhibit few anatomical differences between sun and shade. Instead of primarily adjusting leaf anatomicalcharacters to irradiance, A. trichopoda leaves exposed periodically to full sunlight modify their orientation,becoming folded up along the midvein, presumably to avoid increases in light interception. Chlorophyll afluorescence measurements demonstrate that the photosynthetic apparatus of A. trichopoda is shade adapted,based on a low capacity for excess light energy dissipation and low photosynthetic electron transport rate. Inaddition, these characteristics exhibit limited upregulation in response to increased irradiance. In parallel,there are small or no differences in leaf area-specific hydraulic conductivity between sun and shade shoots ofA. trichopoda. Taken together, A. trichopoda appears to possess limited developmental and physiologicalflexibility to light flux density. The bearing of these observations, in the context of other early-diverging cladesof angiosperms identified by recent phylogenetic studies, on the physiological ecology of the first angiospermsis discussed.