Concentration of CO2, above and below the soil surface and δ13C values of plant tissues, soil litter and organic matter were measured in a caatinga forest of the upper Rio Negro basin in southern Venezuela. CO2, concentrations near the forest floor were consistently higher than in the atmosphere. CO2, gradient in the soil was very steep probably because of the poor aeration in this flood-prone forest. δ13C values of plant tissues showed a clear pattern with lower values in the ground herbaceous plants and under-canopy trees. Tree seedlings showed δ13C values similar to the upper-canopy trees indicating their dependence on reserves carried in the seed from the mother tree. Decomposing litter and soil organic matter also showed δ13C values similar to the upper-canopy trees. It is suggested that lower δ13C values of the shade flora result primarily from the assimilation of CO2, depleted in δ13C originating from soil respiration. Probable effects of low light intensity and physiological factors are discussed.

Evidence for the operation of CAM in the deciduous climber, Cissus trifoliata L., was obtained in field and laboratory studies. Under natural conditions, diurnal oscillations of titratable acidity and colorimetric measurements of night CO fixation, determined for a period of two and a half years, suggested that acid accumulation was related to plant water status, assessed through the daily courses of stomatal resistance and xylem water potential during dry and rainy seasons. These findings were confirmed by gas exchange studies under controlled conditions which showed that the plant fixed all its CO during the day when it was well irrigated; as water stress increased, dark CO uptake gradually replaced fixation during the day until the plant only performed dark fixation. In severe water stress, even the rate of the latter process decreased until leaves fell.

In this work orange pollen grains (Helianthus annuus) were immersed in a supersaturated ionic solution (similar to human extracellular fluid) during different times at 37 °C, atmospheric pressure and pH of 7.4. By Fourier transform infrared spectroscopy (FT-IR) we confirmed the presence of the functional groups carbonyl, amide I, amide II, carboxyl and phosphate in both samples. Nevertheless, yellow pollen owns a greater amount of phosphate groups than orange pollen. By scanning electron microscopy (SEM), differences in the morphology, shape and size of both samples were not observed. In orange pollen grains, amorphous agglomerates can be observed for periods smaller to 14 days and spherical structures formed by nanocrystals of laminar form were observed on their surface at 17 days. By energy dispersive spectroscopy (EDS) confirms the presence of calcium and phosphorous in the agglomerates with different Ca/P ratio and by X-ray diffraction (XRD) the hydroxyapatite (HAp) phase was verified.