Water Relations of Tropical Epiphytes

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Abstract
Relative water content R, water potential ψ and leaf diffusive conductance C5 were monitored while five tropical epiphytes were subjected to an extended period of drought. The species studied were: ferns Pyrrosia adnascens (Forst.) Ching and Pyrrosia angustata (Sw.) Ching, family Polypodiaceae; orchids: Eria velutina Lindl., Dendrobium tortile Lindl. and Dendrobium crumenatum Sw. The ferns reached zero turgor rapidly and after only small declines in relative water content (R) and ψ. Beyond this point stomatal movement seemed strongly suppressed, but the leaves continued to lose water vapour until very low values of R were reached. Nevertheless, on re-watering water potential (ψ), R and diffusive conductance (C5) returned to pre-stress levels within 3 d. The orchids showed a more gradual decline in R and ψ, stomatal activity was not so strongly suppressed, and night opening of stomata was observed under stress. The relationship between ψ and R was found for each species, curves being fitted to the data points by non-linear regression. From these analyses it was concluded that these species had very dilute cell sap, and consequently the change in ψ for a given decline in R was smaller than for most other species recorded in the literature.