In situ Comparison of Daily Photosynthetic Activity Patterns of Saxicolous Lichens and Mosses in Sierra de Guadarrama, Central Spain

Abstract

Diurnal time courses of in situ photosynthetic activity of lichens and mosses growing on a granitic boulder in Sierra de Guadarrama, central Spain, were measured during five days in October, 1993. A portable chlorophyll fluorometer was used for assessing photosynthetic activity of four lichen and two bryophyte species together with photosynthetic photon flux density (PPFD) incident at the thallus surface and thallus temperature. The quantum efficiency of photosystem II (PSII; AF/Fm'), and the apparent electron transport rate through PSII (AF/Fm' X PPFD) were calculated. The weather changed from wet to dry conditions during the period of measurements. Exposureand species-related differences in the photosynthetic performance were observed. Both moss species, Hedwigia ciliata and Grimmia pulvinata, showed no sign of thallus drying and the photochemical efficiency of both species was mainly controlled by the diel patterns of PPFD. In contrast, water loss was the major limiting factor for metabolic activity in the lichen species. Exposureand species-related changes in thallus color indicating water loss occurred as early as on 18 October in Umbilicaria spodochroa and with a 24-hr-delay in Lasallia hispanica and Umbilicaria grisea (19 October). In Lasallia pustulata thallus color changes were visible only on 20 October when dry weather induced severe dehydration in all lichen species. Within the same species, different microhabitat resulted in a different physiological performance depending on water balances as well as on PPFD conditions. The data demonstrate the potential of field measurements of chlorophyll a fluorescence as a non-intrusive tool for the analysis of the in situ photosynthetic performance of poikilohydric organisms without altering rates of water loss and uptake, which are always crucial in the photosynthetic performance of poikilohydric organisms under conditions. In contrast to attached leaves of higher plants, which are easily accessible (Schulze et al. 1982), the 3-dimensional thallus structure of lichens and bryophytes and their often close attachment to their substrate cause serious technical problems for in situ measurements of photosynthetic activity of these poikilohydric organisms. The development of the CO2-porometer (Lange et al. 1984, 1985), which minimizes the time periods of the exposure of the sample in the measuring cuvette, resulted in a substantial increase of knowledge about the diurnal photosynthetic activity patterns of cryptogams under natural or semi-natural conditions (Bruns-Strenge & Lange 1991; Hahn et al. 1989, 1993; Kappen & Breuer 1991; Kappen et al. 1996; Lange et al. 1991, 1996; Sancho et al. 1997a,b; Schroeter et al. 1991a,b). The crucial problem, however, remained unresolved: the lichen or moss sample still had to be removed from its habitat at regular intervals even if only for short periods to measure the gas exchange. Modifications to the technique allowed the in situ measurement of CO2 gas exchange of crustose lichens without removing them from their substratum (Kappen et al. 1990), but the system was very laborious and was little used. Recently, Lange and coworkers (Lange & Green 1996; Lange et al. 1997) developed a fixed version of the CO2-porometer which by automatic enclosure of an epilithic lichen thallus allows unattended, intermittent measurements of the CO2 gas exchange under conditions. However, even this system has limitations, in particular, the occasional enclosure of the sample in a cuvette will still lead to changes in water exchange because of unavoidable alterations to the radiation environment and through deflection of rain and fog and the sample is still required to be removed from its original location with consequent disruption from water supply from the substrate. Progress in the measurement of chlorophyll a 0007-2745/99/623-633$1.25/0 This content downloaded from 157.55.39.148 on Tue, 12 Jul 2016 05:31:28 UTC All use subject to http://about.jstor.org/terms 624 THE BRYOLOGIST [VOL. 102 fluorescence has now made available a different technique for the assessment of photosynthetic activity. Primarily used as a tool for the quantification of stress effects on photosynthesis (Krause & Weis 1991), highly sensitive chlorophyll fluorometers, e.g., PAM-technique (Pulse-Amplitude-Modulation), now allow rapid, non-destructive measurements of the quantum efficiency of photosystem II (PS II) under ambient light conditions without special pre-adaptation of the sample (Bolhdr-Nordenkampf et al. 1989; Schreiber et al. 1986, 1994). Portable PAM-systems are now available that allow measurements of in situ photosynthetic performance making them especially suitable for studies on samples, such as lichens and mosses, which are difficult to handle in gas exchange measurements (Schroeter et al. 1997b), and for automatic measurements over long periods even in extreme environments like the Antarctic (Leisner et al. 1996; Schroeter 1991; Schroeter et al. 1991b, 1997a). Although PS II activity measured by fluorescence may allow calculation, with certain assumptions, of CO2 fixation for higher plants (Edwards & Baker 1993; Genty et al. 1989) this is far from certain for lichens and mosses and, in these groups, it can really only be used as an indicator of activity rather than as an absolute indicator of carbon gain (Green et al. 1998; Schroeter et al. 1995; Sundberg et al. 1997). In the study presented here we have used the chlorophyll a fluorescence yield as a tool to assess the photosynthetic performance of different cryptogams, mosses and lichens, growing together on one large boulder in the Sierra de Guadarrama in central Spain. The primary objective was to use the nonintrusive methodology to demonstrate the complex interactions between, position, microclimate, and plant form that may not be revealed when the samples are removed for measurement. SITE DESCRIPTION, VEGETATION, PLANT MATERIAL,