Past studies have reported two unusual aspects of the water relations of the atmospheric CAM epiphyte Tillandsia usneoides L. (Bromeliaceae): a drought stimulation of nocturnal C0 2 uptake, and nocturnal absorption of water vapor. Contrary to past reports, a 10-d drought did not stimulate nocturnal C0 2 uptake in this species. On the other hand, previous reports of nocturnal water vapor absorption were confirmed in situ throughout a year, although tissue hydration from this source was insufficient to offset daytime water loss. Deposition of dew on the plants was never observed in the field. It is hypothesized that the unusual nature of the water relations of T. usneoides is attributable to the interactions between two "pools" of water and the external atmosphere. The dense indumentum of trichomes obscuring the surface of this epiphyte comprises one pool and is most likely responsible for rapid hydration early in the night and dehydration early in the day. In addition, stomata control water loss from the living mesophyll cells, the second pool, for the remainder of the night. The high rates of water loss observed throughout the day when stomata are closed probably result from leakage through the trichomes.

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SUMMARYMeasurements were performed on leaves of Clusia rosea Jacq. trees in the moist central mountains (330 to 365 m above sea level) and at the dry south coast of St John Island (US Virgin Islands, Lesser Antilles). Seedlings of C. rosea were also studied in the central hills.During the study period (March 1989) all trees showed crassulacean acid metabolism (CAM), in which net COû ptake extended for a remarkably long time in the morning (phase II of CAM: until about 11 to 12 h) and contributed about \ of total net CO.j-uptake. During the night (phase I of CAM) malic acid and citric acid were accumulated concurrently at a molar ratio of malic:citric acid of about 1-6.Internal recycling of respiratory CO.^ was 20% of total COj fixed during the night. Water-use-efficiency (mol COj taken up:mol H^O transpired) was 0-014 to 0-022. The pH of leaf-cell sap at the end of the dark period was 2-85. This would still allow an H+-ATPase at the tonoplast to transport 2H* into the vacuole per ATP hydrolysed when operating near thermodynamic equilibrium.Free sugars, glucose and fructose, and starch were used as precursors for the CO^-acceptor phosphoenolpyruvate during the dark period; contributions of the two hexoses were about equal and together four-times that of starch.Xylem tensions showed increases of up to 8 bar during day-time. Leaf-sap osmotic pressures did not change significantly; the trend was a small decline during day-time.Among the seedlings, three different modes of photosynthesis were encountered, namely C^-photosynthesis in terrestrial and in epiphytic seedlings, continuous stomatal opening and CO.j-uptake day and night in epiphytic seedlings, and CAM in seedlings growing in the tanks of Aechinea lingulata (L.) Baker.

summary Clusia rosea Jacq. is abundant in the moist parts of the Caribbean island of St John (US Virgin Islands, Lesser Antilles) but relatively rare along the dry south coast. Three types of seedlings were encountered, terrestrial seedlings, seedlings growing as humus‐epiphytes on other trees, and seedlings growing inside the tanks of the bromeliad Aechmea lingulata (L.) Baker. Free‐living trees grow from terrestrial seedlings or from epiphytic seedlings strangling and shading their host trees. Leaf‐Na+ levels were always low (1–4 mequiv I−1 tissue water); trees close to the shore were not affected by salinity. In leaves of mature C. rosea trees, levels of Ca2+, Mg2+ and K+ were about 60–90, 40–50, 45–55 mequiv I−1 tissue water, respectively. Epiphytic seedlings tended to contain lower levels of these inorganic cations than seedlings growing terrestrially or in the tanks of Ae. lingulata. Epiphytic seedlings contained significantly less nitrogen than terrestrial seedlings. In the leaves of mature trees N‐levels were independent of altitude and location on the island, but shaded leaves had significantly higher N‐levels than exposed leaves. Light compensation point of photosynthesis in epiphytic seedlings performing C3‐photosynthesis was 17–5 (μmol photons m−2 s−1), photosynthesis was saturated at about 300μmol photons m−2 s−1 showing a maximum rate of CO2‐uptake of 2–3 μmol m−2 s−1.