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.

Xerosicyos danguyi H. Humb. (Cucurbitaceae) is a Crassulacean acid metabolism (CAM) species native to Madagascar. Previously, it was shown that when grown under good water conditions, it is a typical CAM plant, but when water stressed, it shifts to a dampened form of CAM, termed CAM-idling, in which stomata are closed day and night but with a continued, low diurnal organic acid fluctuation. We have now studied the kinetics of some metabolic features of the shift from CAM to CAM-idling under severe water stress and the recovery upon rewatering. When water is withheld, there is a steady decrease in relative water content (RWC), reaching about 50%, at which point the water potential decreases precipitously from about -2 or -3 bars to -12 bars.Abscisic acid (ABA) increases sharply at about 75% RWC. Stomata close, which limits COz uptake, and there is a dampened diurnal organic acid fluctuation typical of CAM-idling. Throughout an extended stress period to 50% RWC, there is no change in chlorophyll, protein, and ribulose bisphosphate carboxylase activity compared with the well-watered plants. Despite the fact that the tissue was already in CAM, the stress is accompanied by an increase in phosphoenolpyruvate carboxylase (PEPc) mRNA, extractable PEPc activity, and PEPc protein (such that the specific activity remained approximately constant) and a decrease in the apparent KJPEP). It is not known if the changes in KJPEP) in response to drought are related to or are separate from the increases in PEPc protein and mRNA. The changes in Km(PEP) could be in response to the decreased endogenous levels of organic acids, but evidently are not an assay artifact. The increases in PEPc protein and mRNA appear to be related to the water-stress treatment and may result from the increased concentration of ABA or the decreased levels of endogenous organic acids. When rewatered, the metabolism quickly returns to the well-watered control typical of CAM.

Clusia rosea Jacq. is a hemiepiphyte having Crassulacean Acid Metabolism (CAM). In its natural habitat Clusia begins its life cycle as an epiphyte and eventually becomes a rooted tree. These two stages of the life cycle of Clusia represent markedly different water regimes. Our CO exchange, stomatal conductance, titratable acidity, and stable carbon isotope ratio measurements indicate that Clusia has a flexible photosynthetic mode, where CO is fixed mostly via CAM during its epiphytic stage, when water availability is low, and via both CAM and C during its rooted stage.

Hemiepiphytic species in the genera Clusia and Ficus were investigated to study their mode of photosynthetic metabolism when growing under natural conditions. Despite growing sympatrically in many areas and having the same growth habit, some Clusia species show Crassulacean acid metabolism (CAM) whereas all species of Ficus investigated are C. This conclusion is based on diurnal CO fixation patterns, diurnal stomatal conductances, diurnal titratable acidity fluctuations, and δC isotope ratios. Clusia minor, growing in the savannas adjacent to Barinas, Venezuela, shows all aspects of Crassulacean acid metabolism (CAM) on the basis of nocturnal gas exchange, stomatal conductance, total titratable acidity, and carbon isotope composition when measured during the dry season (February 1986). During the wet season (June 1986), the plants shifted to C-type gas exchange with all CO uptake occurring during the daylight hours. The carbon isotope composition of new growth was-28 to-29‰ typical of C plants.

Strains of Haplopappus gracilis (Nutt.) Gray cells resistant to 6-azauracil have been isolated from cultures of diploid cells. These strains are also resistant to 8-azaguanine, as is their parent. The variants are 100- to 125-fold more resistant to 6-azauracil than their parent, and they exhibit different spectra of cross resistance to other pyrimidine analogues. The phenotype of each variant is stable in the absence of selection. The majority of cells in cultures of the variants are diploid; all others examined were tetraploid. Initial rates of uptake of uracil are not reduced in the variants. Fluorouracil, to which two variants are resistant, is taken up by one of them as well as by the parent. Responses of the other two to fluorouracil are not correlated with decreased ability to accumulate this analogue.