T. G. A. Green

Water status related photosynthesis and carbon isotope discrimination in species of the lichen genusPseudocyphellaria with green or blue-green photobionts and in photosymbiodemes

SummaryGreen lichens have been shown to attain positive net photosynthesis in the presence of water vapour while blue-green lichens require liquid water (Lange et al. 1986). This behaviour is confirmed not only for species with differing photobionts in the genusPseudocyphellaria but for green and blue-green photobionts in a single joined thallus (photosymbiodeme), with a single mycobiont, and also when adjacent as co-primary photobionts. The different response is therefore a property of the photobiont. The results are consistent with published photosynthesis/water content response curves. The minimum thallus water content for positive net photosynthesis appears to be much lower in green lichens (15% to 30%, related to dry weight) compared to blue-greens (85% to 100%). Since both types of lichen rehydrate to about 50% water content by water vapour uptake only green lichens will show positive net photosynthesis. It is proposed that the presence of sugar alcohols in green algae allow them to retain a liquid pool (concentrated solution) in their chloroplasts at low water potentials and even to reform it by water vapour uptake after being dried. The previously shown difference in δ13C values between blue-green and green lichens is also retained in a photosymbiodeme and must be photobiont determined. The wide range of δ13C values in lichens can be explained by a C3 carboxylation system and the various effects of different limiting processes for photosynthetic CO2 fixation. If carboxylation is rate limiting, there will be a strong discrimination of13CO2, at high internal CO2 partial pressure. The resulting very low δ13C values (-31 to-35‰) have been found only in green lichens which are able to photosynthesize at low thallus water content by equilibraiton with water vapour. When the liquid phase diffusion of CO2 becomes more and more rate limiting and the internal CO2 pressure decreases, the13C content of the photosynthates increases and less negative δ13C values results, as are found for blue-green lichens.

Temperate rainforest lichens in New Zealand : high thallus water content can severely limit photosynthetic CO2 exchange

CO2 exchange rate in relation to thallus water content (WC, % of dry weight) was determined for 22 species of lichens, mainly members of the genera Pseudocyphellaria and Sticta, from a temperate rainforest, Urewere National Park, New Zealand. All data were obtained in the field, either using a standard technique in which the lichens were initially wetted (soaked or sprayed, then shaken) and allowed to slowly dry, or from periodic measurements on samples that were continuously exposed in their natural habitat. A wide range of WC was found, with species varying from 357 to 3360% for maximal WC in the field, and from 86 to 1300% for optimal WC for photosynthesis. Maximal WC for lichens, wetted by the standard technique, were almost always much less than the field maxima, due to the presence of water on the thalli. The relationships between CO2 exchange rate and WC could be divided into four response types based on the presence, and degree, of depression of photosynthesis at high WC. Type A lichens showed no depression, and Type B only a little at maximal WC. Type C had a very large depression and, at the highest WC, CO2 release could occur even in the light. Photosynthetic depression commenced soon after optimal WC was reached. Type D lichens showed a similar depression but the response curve had an inflection so that net photosynthesis was low but almost constant, and never negative, at higher WC. There was little apparent relationship between lichen genus or photobiont type and the response type. It was shown that high WC does limit photosynthetic CO2 uptake under natural conditions. Lichens, taken directly from the field and allowed to dry under controlled conditions, had net photosynthesis rates that were initially strongly inhibited but rose to an optimum, before declining at low WC. The limiting effects of high WC were clearly shown when, under similar light conditions, severe photosynthetic depression followed a brief, midday, rain storm. Over the whole measuring period the lichens were rarely at their optimal WC for photosynthesis, being mostly too wet or, occasionally, too dry. Photosynthetic performance by the lichens exposed in the field was similar to that expected from the relationship between the photosynthetic rate and WC established by the standard procedure.

Carbon-dioxide exchange in lichens: determination of transport and carboxylation characteristics

Measurements were made of net rates of CO2 assimilation in lichens at various ambient concentrations of CO2 in air and in helox (79% He, 21% O2). Because of the faster rate of CO2 diffusion in the pores of lichen thalli when filled with helox than when filled with air, a given net rate of assimilation was achieved at a lower ambient concentration of CO2 in helox. The differences were used to estimate resistances to diffusion through the gas-filled pore systems in lichens. The technique was first tested with five lichen species, and then applied in a detailed study with Ramalina maciformis, in which gas-phase resistances were determined in samples at four different states of hydration and with two irradiances. By assuming, on the basis of previous evidence, that the phycobiont in R. maciformis is fully turgid and photosynthetically competent at the smallest hydration imposed (equilibration with vapour at 97% relative humidity), and that, with this state of hydration, diffusion of CO2 to the phycobiont takes place through continuously gas-filled pores, it was possible also to determine both the dependence of net rate of assimilation in the phycobiont on local concentration of CO2 in the algal layer, and, with the wetter samples, the extents to which diffusion of CO2 to the phycobiont was impeded by water films. In equilibrium with air of 97% relative humidity, the thallus water content being 0.5 g per g dry weight, the resistance to CO2 diffusion through the thallus was about twice as large as the resistance to CO2 uptake within the phycobiont. Total resistance to diffusion increased rapidly with increase in hydration. At a water content of 2 g per g it was about 50 times as great as the resistance to uptake within the phycobiont and more than two-thirds of it was attributable to impedance of transfer by water. The influences of water content on rate of assimilation at various irradiances are discussed. The analysis shows that the local CO2 compensation concentration of the phycobiont in R. maciformis is close to zero, indicating that photorespiratory release of CO2 does not take place in the alga, Trebouxia sp., under the conditions of these experiments.

Monitoring photosynthetic activity of crustose lichens using a PAM-2000 fluorescence system

SummaryCO2 exchange and fluorescence yield of the crustose lichen Buellia frigida were measured in situ by means of a CO2 porometer and a PAM-2000, a newly developed portable fluorescence system. The pulse amplitude modulation system of the PAM-2000 allows measurements in the field under ambient light, temperature and moisture conditions without dark adaptation of the sample. CO2 exchange and fluorescence measurements were well correlated when measured under natural conditions in continental Antarctica during a drying cycle of melt-water-soaked lichen thalli. It was shown that the fluorescence parameter ΔF/Fm′ is a measure of the photosynthetic activity of the lichen. It proved possible, using the PAM-2000, to differentiate the physiological performance of the thallus centre and the marginal lobes. The distribution of water in the thallus during a drying cycle was shown to be inhomogeneous. The photosynthetic rates of B. frigida calculated on an area basis are comparatively high and indicate that this lichen is well adapted to its habitat conditions in this part of continental Antarctica.

Chlorophyll a fluorescence and CO2 exchange of Umbilicaria aprina under extreme light stress in the cold

Abstract A lichen growing in a continental Antarctic region with low temperatures and strong irradiance in summer was investigated for evidence of photoinhibition. Field experiments with Umbilicaria aprina from a sheltered site with heavy snowpack showed no effects of photoinhibition when the lichen was exposed to strong sun irradiance for nearly 11 h a day. This was evident from CO2 exchange and simultaneous chlorophyll a fluorescence measurements. CO2 exchange was also not affected if quartz glass allowing greater UV penetration, was used as a lid for the cuvette. The dependency of net photosynthesis on photosynthetic photon flux density suggests that the lichen is photophilous.

An assessment of the relationship between chlorophyll a fluorescence and CO2 gas exchange from field measurements on a moss and lichen

Abstract. The relationship between CO2 exchange and relative electron-transport rate through photosystem II (ETR, measured using chlorophyll a fluorescence) was determined for a moss and a green algal lichen, photobiont probably Trebouxia sp., in the field in Antarctica. Net photosynthesis (NP) and dark respiration (DR) were measured over temperatures from zero to 25 °C and gross photosynthesis (GP) calculated (GP = NP + DR). The strong response of DR to temperature in these organisms resulted in substantial changes in CO2 exchange rates. The moss Bryum argenteum Hedw. showed a strong, linear relationship between GP and ETR. This was an unexpected result since mosses are C3 plants and, in higher plants, this group normally has a curvilinear GP versus ETR relationship. It is suggested that suppression of DR in the light might be involved. The lichen, Umbilicaria aprina Nyl., had nonlinear relationships between ETR and GP that were different at each measurement temperature. In some cases the lowest ETR was at the higher CO2 exchange rates. It is suggested that these relationships are the result of strong quenching mechanisms that are inversely proportional to GP. The results support a growing impression that the relationships between ETR and CO2 exchange are complex in these organisms and different from those found for higher plants.

Terrestrial vegetation at Canada Glacier, Southern Victoria Land, Antarctica

SummaryBryophyte flushes in the vicinity of Canada Glacier in S.S.S.I. No. 12, Taylor Valley, Southern Victoria Land, were investigated in order to describe the vegetation present and to investigate factors affecting vegetation distribution. Biomass values from 950 to 1,250 g m−2 (dry weight) and vegetated areas up to 14,450 m2 indicate this is a significant area of bryophyte growth in Southern Victoria Land. The pattern of plant species in relation to water flow was investigated through detailed mapping. This is the first confirmed identification of Bryum argenteum, and Pottia heimii, and the first report of Bryum pseudotriquetrum from this area. Tissue nitrogen values for plants at this site are lower than other reported data, but it seems unlikely this would be a limiting factor for growth. It is concluded that, in this area, summer water flow in conjunction with microtopography has the greatest influence in determining where mosses, cyanobacteria and algae grow.

Differences in the susceptibility to light stress in two lichens forming a phycosymbiodeme, one partner possessing and one lacking the xanthophyll cycle

SummaryThe effect of high light levels on the two partners of a Pseudocyphellaria phycosymbiodeme (Pseudocyphellaria rufovirescens, with a green phycobiont, and P. murrayi with a blue-green phycobiont), which naturally occurs in deep shade, was examined and found to differ between the partners. Green algae can rapidly accumulate zeaxanthin, which we suggest is involved in photoprotection, through the xanthophyll cycle. Blue-green algae lack this cycle, and P. murrayi did not contain or form any zeaxanthin under our experimental conditions. Upon illumination, the thallus lobes with green algae exhibited strong nonphotochemical fluorescence quenching indicative of the radiationless dissipation of excess excitation energy, whereas thallus lobes with blue-green algae did not possess this capacity. The reduction state of photosystem II was higher by approximately 30% at each PFD beyond the light-limiting range in the blue-green algal partner compared with the green algal partner. Furthermore, a 2-h exposure to high light levels resulted in large reductions in the efficiency of photosynthetic energy conversion which were rapidly reversible in the lichen with green algae, but were long-lasting in the lichen with blue-green algae. Changes in fluorescence characteristics indicated that the cause of the depression in photosynthetic energy conversion was a reversible increase in radiationless dissipation in the green algal partner and “photoinhibitory damage” in the blue-green algal partner. These findings represent further evidence that zeaxanthin is involved in the photoprotective dissipation of excessive excitation energy in photosynthetic membranes. The difference in the capacity for rapid zeaxanthin formation between the two partners of the Pseudocyphellaria phycosymbiodeme may be important in the habitat selection of the two species when living separate from one another.

Pseudocyphellaria dissimilis: a desiccation-sensitive, highly shade-adapted lichen from New Zealand

SummaryPseudocyphellaria dissimilis, a foliose, cyanobacterial lichen, is shown not to fit into the normal ecological concept of lichens. This species is both extremely shade-tolerant and also more intolerant to drying than aquatic lichens previously thought to be the most desiccation-sensitive of lichens. Samples of P. dissimilis from a humid rain-forest site in New Zealand were transported in a moist state to Germany. Photosynthesis response curves were generated. The effect of desiccation was measured by comparing CO2 exchange before and after a standard 20-h drying routine. Lichen thalli could be equilibrated at 15° C to relative humidities (RH) from 5% to almost 100%. Photosynthesis was saturated at a photosynthetically active radiation (PAR) level of 20 μmol m-2 s-1 (350 μbar CO2) and PAR compensation was a very low 1 μmol m-2 s-1. Photosynthesis did not saturate until 1500 μbar CO2. Net photosynthesis was relatively unaffected by temperature between 10° C and 30° C with upper compensation at over 40° C. Temporary depression of photosynthesis occurred after a drying period of 20 h with equilibration at 45–65% relative humidity (RH). Sustained damage occurred at 15–25% RH and many samples died after equilibration at 5–16% RH. Microclimate studies of the lichen habitat below the evergreen, broadleaf forest canopy revealed consistently low PAR (normally below 10–20 μmol m-2 s-1) and high humidities (over 80% RH even during the day time). The species shows many features of an extremely deep shade-adapted plant including low PAR saturation and compensation, low photosynthetic and respiratory rates and low dry weight per unit area.

Episodic forest mortality in the Kaimai Ranges, North Island, New Zealand

Abstract Investigations of the upland vegetation of the Kaimai Ranges have revealed widespread mortality affecting a wide range of species and forest types. The mortality is not caused by browsing by Introduced mammals, although they can be shown to have slowed recent forest recovery. Dendrochronological data define 2 mortality episodes closely linked with severe droughts which occurred in 1914 and 1946. Continued ill-thrift in the surviving vegetation and slow growth in the seral forests is the result of complex causes which include Changes in soil water table, increased exposure of residual trees, low nutrient status of the soils, and attacks by pathogens. On steeper slopes the mortality appears to have contributed to a period of Increased erosion, and for this reason further study of the return frequency of the periods of mortality may enable the frequency of the erosion episodes to be determined. The association of drought with the primary causes of the mortality suggests that it may be an under-rated...