Ecophysiology of xerophytic and halophytic vegetation of a coastal alluvial plain in northern Venezuela

summary A comparison of the performance of two epiphytes with crassulacean acid metabolism (CAM) was made during the rainy season and dry season at the Ciénega el Ostional, Chichiriviche in northern Venezuela. The epiphytic bromeliad, Tillandsia flexuosa has water‐retaining tanks and leaf trichomes, and propagates mainly vegetatively to produce large populations in the shrubby island vegetation. The epiphytic orchid, Schomburgkia humboldtiana formed smaller populations, and had large succulent leaves with uniform chlorenchyma and no distinct water‐storage parenchyma, unlike T. flexuosa. Both epiphytes were myrmecophilous. Leaf succulence (kg m−2) declined by ∼ 10% in the dry season for both plants. Both epiphytes showed reduced CO2 uptake during Phase I (dark period) and (dawn‐dusk) titratable acidity (ΔH+) in the dry season. Water‐use efficiency (WUE) was higher for S. humboldtiana (16.0 × 10−3 mol CO2 per mol H2O compared with 5.0 × 10 −3 for T. flexuosa) although WUE remained constant during rainy and dry season for each species. Sixty to seventy per cent of the dawn dusk titratable acidity was derived internally from respiratory CO2 (recycling) for both species, and in absolute terms, recycling decreased in the dry season, in contrast to the expected progression under drought stress. Recycling is an important facet of carbon balance for both species in both rainy and dry seasons. Leaf Na+ concentration was higher than values quoted for terrestrial salt stressed CAM plants. Fructose and glucose declined in leaf bases of T. flexuosa during the dark period, but not in the more distal regions of the leaf. S. humboldtiana showed a decrease in sucrose at night, and mannitol was also an important constituent of the leaves. Xylem sap tension maxima increased from 0.38 ± 0.09 MPa (rainy season) to only 0.55 ± 0.06 MPa (dry season) for T. flexuosa, these values being much lower than those found for terrestrial shrubs and herbs at the same site, the two epiphytes use CAM in conjunction with differing morphological adaptations to maintain growth throughout the year at the Ciénega el Ostional, but it would seem that T. flexuosa has better physiological characteristics for maintenance of carbon acquisition during the dry season.