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 shrub endemic to New Caledonia, as sister to all other extant flowering plants. Here we present new observations on leaf anatomy and morphology, leaf photosynthetic physiology, and xylem hydraulic function for Amborella in the field. We focus on experimental measurements of the degree of photosynthetic and xylem hydraulic acclimation to sunlight by examining Amborella plants growing in deeply shaded understories (0.7% full sunlight) compared with individuals from exposed montane ridge crests in New Caledonia. Amborella leaves exhibit few anatomical differences between sun and shade. Instead of primarily adjusting leaf anatomical characters 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 a fluorescence 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. In addition, 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 of A. trichopoda. Taken together, A. trichopoda appears to possess limited developmental and physiological flexibility to light flux density. The bearing of these observations, in the context of other early-diverging clades of angiosperms identified by recent phylogenetic studies, on the physiological ecology of the first angiosperms is discussed.