Growth irradiance effects on photosynthesis and growth in two co-occurring shade-tolerant neotropical perennials of contrasting photosynthetic pathways.

Dieffenbachia longispatha (C3) and Aechmea magdalenae (Crassulacean acid metabolism, CAM) are syntopic, neotropical forest perennials in central Panama that are restricted to shaded habitats. This is of particular interest for A. magdalenae because, like other understory CAM bromeliad species, it appears functionally and structurally to be better suited to life in full sun. Growth irradiance (GI) effects on photosynthesis and growth in both species were explored in the context of sun/shade trade-off concepts largely derived from studies of C3 plants. Potted plants were grown outdoors in 1, 55, and 100% full sun for 5 mo under well-watered conditions. While both species grew faster in high compared to low light, maximum relative growth rates (RGR) in full sun were still extremely slow with A. magdalenae showing a RGR approximately half that of D. longispatha. Photosynthetic capacity increased with GI in D. longispatha but not in A. magdalenae. Aechmea magdalenae responded to GI with shifts in the activity of the different CAM phases. Both species were photoinhibited in full sun, but more so in A. magdalenae. Despite possessing many traits considered adaptive in high light, these results suggest that A. magdalenae is unlikely to attain sufficient growth rates to thrive in productive, high-light habitats.