Epiphytes in the Neotropical rainforest, in addition to experiencing a variable water supply in the canopy, may also be facing longer rainless periods and even drought due to global warming. Tank bromeliads such as Guzmania monostachia, lacking absorbing roots, impound a water supply at the base of their leaves, yet the “tanks” periodically run dry and can be empty for a week or more. We investigated how the leaves of G. monostachia varied in hydraulic conductance (Kleaf) and related physical characteristics during naturally occurring and artificially imposed dry periods followed by rain or re-watering. We predicted that 7-14 days without water would markedly reduce leaf hydraulic conductance by causing embolism in the xylem and several changes in leaf tissues outside the xylem, including decreased aquaporin expression, changes in the amount of aerenchyma, and overall leaf shrinkage. We also predicted that tight stomatal control and low cuticular conductance would help prevent Kleaf from large decreases, and that conductance would return to pre-drying levels due to reversed embolism, restored aquaporin expression, and tissue rehydration in response to re-watering.
For plants of Guzmania monostachia in the field in Costa Rica after 7 rainless days, Kleaf was 50% of its hydrated value, and it increased to that value 4 days after tanks were refilled. For plants with no tank water for 14 d in a glasshouse, Kleaf decreased by 42%, and returned to its initial value within 4 d of tank refilling (one-way RM ANOVA; P < 0.01). Xylem staining indicated that 35% of veins of dry leaves were non-conducting, presumably due to embolism. However, the use of a "leaky cable" model to predict conductances inside and outside the xylem indicated that such a reduction in xylem conductance would account for only a 4-9% decrease in Kleaf. Changes in tissues outside the xylem thus were considered more important, and indeed, aquaporin expression varied in response to drying and even more to rewetting. With respect to anatomical changes, aerenchyma in the leaves decreased during drying, and leaves shrank in area but not significantly in thickness. For plans subjected to drying in the field and in the greenhouse, leaf water potential never decreased below -0.85 MPa, and leaves never appeared to lose turgor. Low values for stomatal and cuticular conductance for leaves during drying, as well as reduced mesophyll conductance to water, may help account for hydraulic resilience in this species.