Influence of nitrogen supply on the photoprotective response of Neoregelia cruenta under high and low light intensity

We investigated preference for nitrogen source and the influence of ammonium nitrate on leaf pigment content, crassulacean acid metabolism (CAM) activity, and the efficiency of PSII in Neoregelia cruenta (R.Graham) L.B. Smith, a CAM bromeliad of major ecological importance to restinga (coastal sand ridge plains) environments. Plants showed a preference for ammonium over nitrate in a 24-h experiment where plants were exposed to 15 NH 4 NO 3 or NH 4 15 NO 3 . Mature individuals of N. cruenta were exposed to 95 and 20% full sunlight, and treated with 5 mM NH 4 NO 3 or tap water only. After 4 months under the different treatments we found that nitrogen uptake and carotenoid content were independent of light exposure. Total chlorophyll decreased under nitrogen limitation and high light. Net titratable acid accumulation was not influenced by light or nitrogen regimes. Plants under low light showed consistently high photochemical efficiency of PSII (F v /F m ) throughout the day. In contrast, plants under high light and nitrogen limitation showed a significant decline in F v /F m around midday, which recovered by the end of the light period. This decline in F v /F m was attributed to increased non-photochemical quenching. Our findings that plants under high light and with high nitrogen behave similarly to shade plants were unexpected. They suggest that the high light, high nitrogen leaves used a greater portion of the light absorbed in PSII antennae for photochemistry than the high light, low nitrogen plants. High nitrogen content in the leaves of N. cruenta appears to protect this CAM bromeliad against photoinhibition.