Desiccation Stress and Fluorescence Imaging of the Peltigera Canina

Abstract

Liquid water can restore the photosynthesis of most desiccated lichens within minutes. In contrast to lichens with green algal photobionts, cyanobacterial photobionts cannot uptake enough water from water vapor, i.e. they need liquid water to regain their photosynthetic activity (1). When exposed to wet air for several days, not even variable chlorophyll fluorescence can be detected in cyano-lichens (2). Obviously, their photosystem II reaction centres remain inactive. Moreover, the phycobilisomes remain functionally disconnected from photosystem II during this limited water uptake (3). In phycobilin (PB) fluorescence images (green excitation light), the fluorescence intensity becomes very low during desiccation (4), but until now there have been no data about the PB fluorescence intensity in the air-wetted state. Therefore we were very interested to know whether or not the PB fluorescence reappears in this phase. For this purpose we recorded PB fluorescence images in this and related states and compared the relative fluorescence intensity with chlorophyll fluorescence data obtained from an apparatus (MiniPAM) equipped with a small light guide (1.5 mm diameter). On the basis of 2-dimensional data, we discuss the surprising correlation between PB fluorescence intensity and the reappearance of variable chlorophyll fluorescence in some thallus regions. In addition, the role of the cortex + tomentum transparency and of the extent of pre-desiccation is assessed.