THE EFFECT OF ATMOSPHERIC DESICCATION AND OSMOTIC WATER-STRESS ON PHOTOSYNTHESIS AND DARK RESPIRATION OF LICHENS

Four species of epiphytic lichens from California with varying responses to salinity were studied to determine the response of CO2 exchange to decreasing water potential (Ψ). Changes in Ψ were induced either by allowing dry thalli to come into equilibrium with atmospheres of defined water vapour pressure, or by osmotic stress through incubation of the thalli in sea salt or sorbitol solutions. In two species the gradual decline of CO2 uptake rates with decreasing Ψ was independent of the method which was used to establish ft (and therefore relatively independent of the bulk water content of the thalli); measurable CO2 fixation occurred in Dendrographa minor Darb. and Ramalina menziesii Tayl. to values of Ψ as low as –38 and – 22 MPa, respectively. Photosynthesis of Evernia prunastri (L.) Ach. responded similarly to low Ψ which was induced either through atmospheric desiccation or osmotic dehydration (sorbitol). However, E. prunastri was much more sensitive to low Ψ when the thallus was treated with NaCl solutions. Photosynthesis in Pseudocyphellaria anthraspis Magn., the only species investigated with a blue‐green (cyanobacterial) photobiont, was strictly dependent on wetting of the thallus with water. Net photosynthesis was detectable to – 3−5 MPa in sorbitol and salt solutions, but in moist air of a similar Ψ P. anthraspis showed no CO2 uptake. In all four species, dark respiration was markedly less sensitive to osmotic dehydration in sorbitol or salt solutions than at the same Ψ under atmospheric desiccation. In E. prunastri, salt stress combined with high light resulted in a much more pronounced decrease in the rate of photosynthesis than either salt stress in low light, or high light alone. These species provide a model system to differentiate the effect of atmospheric desiccation and osmotic stress on lichen metabolism, and to study the interaction of drought per se with other stress factors such as salt and high light. Copyright © 1990, Wiley Blackwell. All rights reserved