Ecophysiology of xerophytic and halophytic vegetation of a coastal alluvial plain in northern Venezuela: III. Bromelia humilis Jacq., a terrestrial CAM bromeliad.

The terrestrial CAM plant Bromelia humilis was examined in the salinas of the Ciénega el Ostional, on the north coast of Venezuela, in the rainy and dry seasons. Three colour forms were distinguished; yellow (in full sun), green exposed (also in sun) and green shaded (beneath woodland). Plant size decreased with increasing irradiance. An examination was made of the three phenotypes in terms of CO exchange (J ), dawn-dusk changes in titratable acidity (ΔH ) and malate and citrate levels, osmotic pressure, xylem tension, sugar and amino acids levels, nitrogen and ion concentrations and ambient temperature fluorescence. All phenotypes exhibited lowered J and ΔH in the dry as compared to the rainy season. Citrate, as well as malate, showed dawn-dusk fluctuations. Soluble sugars were the major source of carbon skeletons for nocturnal organic acid production. The dawn-dusk changes in osmotic pressure were negligible. Yellow plants performed poorly in contrast to shaded plants in both seasons. The former showed higher dawn-dusk changes of citrate levels and contained much less nitrogen than shaded plants. Nocturnal recycling of respiratory CO was more important in yellow plants and, in the dry season, reached 87%. These differences were reflected in the overall productivity, shaded plants showing increases in size whereas yellow plants utilized energy mainly for leaf replacement. Water availability and nitrogen supply appear to be the overriding factors determining higher productivity and CO assimilation in partially shaded plants as compared with plants in full sun.