Comparative ecophysiology of CAM and C 3 bromeliads. V. Gas exchange and leaf structure of the C 3 bromeliad Pitcairnia integrifolia

Pitcairnia integrifolia is endemic to northern Trinidad and the Paria peninsula of Venezuela and is the only member of the bromeliad‐subfamily Pitcairnioideae in Trinidad. It is terrestrial with roots fully functional in water and solute uptake, grows on exposed steep rocky cliffs and can occur just above the spill zone of sea waves under continuous sea spray. Thus, it can be subject both to water stress, particularly during the dry season, and to salt stress. Gas exchange of P. integrifolia leaves was measured on a clear day in Trinidad. Uptake of CO 2 and leaf conductance to diffusion of water vapour had two peaks during the light period, a larger one in the early morning and a smaller one in the late afternoon, which were separated by an extended midday depression of gas exchange. CO 2 partial pressure in the leaf air‐spaces increased during the midday depression. Leaf temperatures reached a maximum of 51.6°C and leaf‐air water‐vapour‐concentration differences were also very high during the midday depression, when quantum fluxes were up to 2 mmol m −2 s −1 and higher. The midday depression is considered as a functional adaptation to temporary water stress. Although P. integrifolia is subject to sea‐spray, an internal osmotic pressure of only 0.91 MPa indicated that NaCl is not accumulated. Leaf epidermis cells are thick‐walled and have a prominent cuticle. The abaxial leaf surface with the trichomes is not wettable, and the trichomes apparently do not function in water and solute uptake. They cover the stomata densely and may create a favourable microenvironment around them. They also have a high reflectance. This does not prevent overheating of the leaves, but does reduce the photosynthetically active radiation penetrating to the mesophyll. The trichomes may thus contribute to the prevention of photoinhibition at high incident quantum flux.