Light and leaf hydraulics: The case for two tropical epiphytes

Background/Question/Methods

The light environment of leaves can influence their hydraulic capacity. Epiphytic plants in the tropical rainforest, such as tank bromeliads, are exposed to a wide range of sunlight, varying with several factors including position on the host tree. We investigated two congeneric tank bromeliads, a C3 species, Guzmania lingulata, and a C3-CAM intermediate, G. monostachia, in order to compare the effects of different light environments on two closely related and morphologically similar species. We hypothesized that the leaf hydraulic conductance of G. monostachia would be the higher of the two, because it tends to occur higher in the canopy than its congener. We measured instantaneous and integrated light availability along two host trees and measured leaf hydraulic conductance and related morphological and anatomical variables for plants from adjacent recently fallen trees. Leaf hydraulic conductance (K_leaf) was measured using the evaporative flux method, leaf xylem conductance (K_x) was estimated by using tracheid diameters and numbers in the Hagen-Poiseuille equation, and leaf conductance outside the xylem (K_ox) was derived from a model using K_leaf, K_x, and leaf dimensions as inputs. Stomatal conductance and other key leaf traits were also measured to put possible differences  in K_leaf in context.

Results/Conclusions

For G. monostachia, K_leaf was nearly 3 times higher for plants from the high light group (> 30 m) than for the low light group (< 17 m; P = 0.013), yet only 60% higher for high light G. lingulata  (> 4 m) than for the low light group (< 2 m; P = 0.251). Between the two species, K_leaf was similar for low light, 1.5 – 1.6 mmol m^-2 s^-1 MPa^-1), whereas for the high light groups, K_leaf for G. monostachia was about 60% greater than for G. lingulata (P = 0.053). For both species, K_x (xylem) was higher for the high light groups, but only significantly so for G. monostachia. Leaf hydraulic conductance outside the xylem (K_ox) also increased in response to high light, by about 40% for G. lingulata and by about 3 times for G. monostachia. In almost all cases, K_x and K_ox were similarly limiting for the two species, suggesting that xylem and mesophyll traits were coordinated developmental responses to different water demands in the different light environments. The greater responsiveness of leaf hydraulic traits to light in G. monostachia was associated with greater differences in stomatal conductance and with its occurrence higher in the canopy.