Comparative ecophysiology of CAM and C3 bromeliads. IV. Plant water relations
SMITH JAC., GRIFFITHS H., LÜTTGE U., CROOK CE., GRIFFITHS NM., STIMMEL K.
Abstract An investigation was carried out into the water relations of CAM and C3 bromeliads in their natural habitat during the dry season in Trinidad. Measurements were made of xylem tension with the pressure chamber and of cell‐sap osmotic pressure and titratable acidity on crushed leaf samples. A steady‐state CO2 and H2O‐vapour porometer was also used so that changes in leaf water relations during individual day‐night cycles could be directly related to gas‐exchange patterns in situ. Xylem tension changed in parallel with transpiration rate and in general reached its maximum value in CAM bromeliads at night and in C3 bromeliads during the day. In addition, large nocturnal increases in cell‐sap osmotic pressure and titratable acidity (ΔH+) typically occurred in the CAM bromeliads. The C3‐CAM intermediate Guzmania monostachia showed slight nocturnal acidification, but had higher values of xylem tension during the day. Very high values of AH+ were observed in the CAM species when the tanks of the epiphytic bromeliads contained water: Aechmea nudicaulis showed a mean maximum ΔH+ of 474 mol m−3, the highest value so far observed for CAM plants. On some nights dew formed on the leaf surfaces of the epiphytes, partially curtailing gas exchange and leading to a marked decrease in xylem tension in both C3 and CAM species. Between‐site comparisons were also made for a wide range of habitats from arid coastal scrub to montane rain forest. Compared with values characteristic of other life‐forms, xylem tension and cell‐sap osmotic pressure were low for all bromeliads, and did not differ significantly in co‐occurring CAM and C3 bromeliads. Mean maximum xylem tension (10 species in total) ranged from 0.29 M Pa at the montane sites to 0.67 MPa at the most arid site, and mean minimum osmotic pressure (17 species) from 0.51 to 0.97 MPa. At the arid sites the bromeliads were exclusively CAM species, two of which (Aechmea aquilega and Bromelia plumieri) grew terrestrially in the undergrowth of the coastal scrub. Xylem tension in these species was low enough to indicate that they must be functionally independent of the substratum during the dry season. In the wetter part of Trinidad, no between‐site differences in leaf water relations were found along an altitudinal gradient in the Northern Mountain Range; seasonal differences in this area were also small. Overall, leaf water relations and gas exchange in the bromeliads were strongly affected both by short‐term changes in water availability and by longer‐term climatic differences in the various regions of the island. Copyright © 1986, Wiley Blackwell. All rights reserved