Combined effects of varying water content and CO2 concentration on photosynthesis in Spagnum fuscum

The photosynthesis of Spagnum fuscum (Schimp.) Klinggr. at different water contents and CO₂ concentrations was measured in the laboratory. The optimal water content for photosynthesis near the current atmospheric CO₂ concentration is 600–800% (percentage of dry weight). The decrease in photosynthesis is very steep towards lower water contents and less steep towards higher water contents. The optimal water content range moves higher and becomes wider with increasing CO₂ concentration. At 3000 ppm there is no longer any decrease in photosynthesis with increasing water content. The water content of S. fuscum has a considerable effect on the response of photosynthesis to CO₂ concentration. In a moss saturated with water, photosynthesis increases gradually until 8000 ppm CO₂, but this saturation concentration becomes lower with decreasing water content, being c. 1500 ppm at a water content of 700–800%. An increase in CO₂ concentration over 300 ppm will raise photosynthesis very little in dry moss with a water content of only 300–400%.     

Moss functioning in different taiga ecosystems in interior Alaska

Summary Carbon dioxide exchange rates in excised 2-year-old shoot sections of five common moss species were measured by infrared gas analysis in mosses collected from different stands of mature vegetation near Fairbanks, Alaska. The maximum rates of net photosynthesis ranged from 2.65 mg CO₂ g^-1h^-1 in Polytrichum commune Hedw. to 0.25 in Spagnum nemoreum Scop. Intermediate values were found in Sphagnum subsecundum Nees., Hylocomium splendens (Hedw.) B.S.G., and Pleurozium schreberi (Brid.) Mitt. Dark respiration rates at 15°C ranged from 0.24 mg CO₂ g^-1h^-1 in S. subsecundum to 0.57 mg CO₂ g^-1h^-1 in H. splendens. The dark respiration rates were found to increase in periods of growth or restoration of tissue (i.e., after desiccation). There was a strong decrease in the rates of net photosynthesis during the winter and after long periods of desiccation.Due to increasing amounts of young, photosynthetically active tissue there was a gradual increase in the rates of net photosynthesis during the season to maximum values in late August. As an apparent result of constant respiration rates and increasing gross photosynthetic rates, the optimum temperature for photosynthesis at light saturation and field capacity increased during the season in all species except Polytrichum, with a corresponding drop in the compensation light intensities. Sphagnum subsecundum seemed to be the most light-dependent species.Leaf water content was found to be an important limiting factor for photosynthesis in the field. A comparison between sites showed that the maximum rates of net photosynthesis increased with increasing nutrient content in the soil but at the permafrostfree sites photosynthesis was inhibited by frequent moisture stress.     

A COMPARISON OF AIR AND WATER AS ENVIRONMENTS FOR PHOTOSYNTHESIS BY THE INTERTIDAL ALGA FUCUS SPIRALIS (PHAEOPHYTA)1

The response of photosynthesis and respiration of the intertidal brown alga Fucus spiralis L. to light and temperature at ambient and elevated concentrations of inorganic carbon was investigated. The light-saturated rate of photosynthesis was greater in air at 15° C and 20° C, but greater in water at 10° C. Light compensation point and I_k was about 50% lower under submerged relative to emerged conditions, whereas the initial slope of photosynthesis versus irradiance was higher, except at 20° C. Under both submerged and emerged conditions light-saturated photosynthesis was limited to a similar degree (78%, and 65%, respectively) by the availability of inorganic carbon at naturally occuring concentrations. In air, slight desiccation at tissue water contents of about 96% to 92% caused a stimulation in the rate of net photosynthesis to 110–148% of fully hydrated fronds. At lower water contents the rate of net photosynthesis declined linearly with decreasing water content and became zero at a water content of about 15%. Dark respiration declined linearly with tissue water content and remained positive to a water content of 8%. Upon reimmersion the fronds showed a complete recovery within 35 min following desiccation to a water content of 20–30%. Thus F. spiralis seems to be very tolerant to desiccation. Since F. spiralis photosynthesizes effectively in air, even at a higher rate than in water as long as it has not lost a large proportion of its water in desiccation, the alternating exposure to air may be beneficial by increasing the daily carbon gain compared to a fully submerged situation.     

COMPARATIVE PATTERNS OF NET PHOTOSYNTHESIS IN AN ASSEMBLAGE OF MOSSES WITH CONTRASTING MICRODISTRIBUTIONS

Effects of plant water content, previous desiccation, photon flux density, and temperature on rate of net photosynthesis were measured in the common species in the assemblage of desiccationtolerant mosses that dominates the flora on granitic boulders at a semiarid site in the inland chaparral of San Diego County, California. Species differed significantly in their responses to water content, desiccation, and light. Species previously found to occur in microsites of lower water availability attained maximum net photosynthesis at lower water contents and showed greater ability to recover from prolonged desiccation. Species found in microsites of lower light availability had higher rates of net photosynthesis at low photon flux density. Results suggest that differences in potential rates of net photosynthesis partly determine relative ability of these species to colonize xeric or dark microsites, but that other factors also determine microdistribution.     

Comparative desiccation tolerance of two Sphagnum mosses

Summary Sphagnum fallax (Klinggr.) Klinggr., a moss growing in hollows close to the water table, is more desiccation tolerant than S. nemoreum Scop., a hummock former distributed high above the hollows. Sphagnum fallax recovered to a greater proportion of its predesiccation photosynthetic rate after one and five days of tissue dryness. Further, a greater percentage of S. fallax plants survived five and ten day periods at low tissue water contents. Longer desiccated periods and lower water contents during these periods decreased both photosynthetic recovery and survival.Water contents measured in Bloomingdale Bog (Adirondack Mountains, NY, USA) showed that S. fallax probably dries more frequently and for longer periods than S. nemoreum. These results support previous findings that the greater ability of S. nemoreum to remain moist in the field is the most important character in its success as a hummock former. Greater tolerance of desiccation helps S. fallax to compensate for its greater tendency to become dry, and is a key physiological feature enabling it to dominate hollows.     

Analysis of the decrease in photosynthesis on desiccation of mosses from xeric and hydric environments

Three moss species [ Tortula ruraliformis (Besch.) Grout. Bryum pseudotriquetrum (Hedw.) Schaegr and Dicranella palustris (Dicks.) Crund. ex. E. F. Warb. ( D. squarrosa (Starke) Schp.] collected from a range of habitats differing in water availability were desiccated in controlled conditions. All species became photosynthetically inactive when dried below a water content of 100–200% dry weight. Only Tortula ruraliformis , a moss from arid sand dunes. was able to recover fully to pre-desiccated rates of photosynthetic electron transport during subsequent rehydration. The rate of recovery was influenced by irradiance during desiccation. Mosses from hydric habitats showed some resumption of photosynthetic electron transport (following rehydration) if dried in the dark. but did not do so if dried even in low light. In these circumstances the mosses showed evidence of lasting photoinhibition of photosynthesis after rehydration. The desiccation-tolerant T. ruraliformis became significantly photoinhibited only when continually exposed to high irradiance (1200 μmol m⁻² s⁻¹) in the hydrated state. If allowed to desiccate whilst exposed to high irradiance this species showed less evidence of photoinhibition after rehydration, and was not at all affected by desiccation in low irradiance. Photon flux absorption in dry moss was 50–60% less than that in hydrated moss as a result of leaf curling. However, the reduction in absorption of photosynthetically active radiation cannot account for the total loss of photosynthetic oxygen evolution and variable chlorophyll fluorescence observed in the desiccated mosses.     

The recovery of photosynthesis in tomato subsequent to chilling exposure

The overall success of a plant in coping with low temperature sensitivity of photosynthesis is dependent not only on the maximum extent of inhibition suffered for a given time of low temperature exposure but also on the persistence of the inhibition after normal growth temperatures are restored. Thus the capacity of recovery and the speed with which a plant can recover from the effects of chilling exposure are important parameters in determining how devastating the chilling event will be on season-long growth and yields. We have studied the recovery of CO₂-saturated photosynthesis from the injury caused by exposing intact tomato plants (Lycopersicon esculentum Mill. cv. Floramerica) or detached tomato leaves to a temperature of 1°C in the dark for varying periods of time. We found that net photosynthesis was fully recovered within 12 h after returning the plants to 25°C in the dark, even after chilling exposures as long as 45 h. This was true for intact plants as well as for detached leaves that were supplied with water. When chilling took place in the light (4°C, 1000 E · m^-2 · s^-1, PAR) inhibition of photosynthesis was more severe and appeared more quickly and the recovery was slower and incomplete. A 12 h chilling exposure in the light resulted in injury to net photosynthesis that was not fully recovered even after 50 h. Chilling damage to photosynthesis developing in the light was distinguished from chilling in the dark by the decreased photosynthetic quantum yield. Not only did high intensity illumination enhance chilling damage of photosynthesis but bright light subsequent to the chilling exposure also delayed the recovery of photosynthesis. At none of the three ambient CO₂ concentrations investigated (300, 1500 and 5000 1.1^-1) did the recovery of photosynthesis depend on stomatal conductance.     

Photosynthesis during desiccation in an intertidal alga and a land plant

This study was undertaken to determine how photosynthesis tolerates desiccation in an intertidal alga Fucus vesiculosus L. and a terrestrial sunflower Helianthus annuus L. Photosynthetic O2 evolution generally was inhibited at low water potentials (psiw) but more in sunflower leaves than in Fucus fronds at the same psiw. As psiw decreased, less carbon accumulated in an organic carbon store in Fucus. The inhibition of photosynthesis appeared to be mostly biochemical because it could not be prevented by supplying additional CO2 or by supplying CO2 from the internal organic carbon store. The inhibition of photosynthesis and carbon storage occurred after turgor disappeared and thus when solute concentrations were increasing in the cells. Solute concentrations were much higher in Fucus than in sunflower. After desiccation to the air-dry state (psiw below - 10 MPa), photosynthesis could not recover in sunflower but it recovered rapidly when Fucus was exposed to seawater. The lack of recovery in sunflower was associated with inability to recover turgor probably because of breaks in cell membranes. The ability to recover in Fucus was gradually lost during 1.5 d of desiccation at 45% relative humidity. At lower humidities, recovery was lost sooner as small amounts of water were removed. We conclude that photosynthesis tolerated desiccation more in Fucus than in sunflower because of differences in the molecular environment around the photosynthetic enzymes. Important aspects of this environment were features that prevented membrane breakage but promoted the retention of small amounts of water that were critical for viability.     

Changes in chlorophyll a fluorescence, photosynthetic CO2 assimilation and xanthophyll cycle interconversions during dehydration in desiccation-tolerant and intolerant liverworts

The interactions among water content, chlorophyll a fluorescence emission, xanthophyll interconversions and net photosynthesis were analyzed during dehydration in desiccation-tolerant Frullania dilatata (L.) Dum. and desiccation-intolerant Pellia endiviifolia (Dicks) Dum. Water loss led to a progressive suppression of photosynthetic carbon assimilation in both species. Their chlorophyll fluorescence characteristics at low water content were: low photosynthetic quantum conversion efficiency, high excitation pressure on photosystem II and strong non-photochemical quenching. However, dissipation activity was lower in P. endiviifolia and was not accompanied by a rise in the concentration of de-epoxidised xanthophylls as F. dilatata. The photosynthetic apparatus of F. dilatata remained fully and speedily recuperable after desiccation in as indicated by the restoration of chlorophyll fluorescence parameters to pre-desiccation values upon rehydration. A lack of recovery upon remoistening of P. endiviifolia indicated permanent and irreversible damage to photosystem II. The results suggest that F. dilatata possesses a desiccation-induced zeaxanthin-mediated photoprotective mechanism which might aid photosynthesis recovery when favourable conditions are restored by alleviating photoinhibitory damage during desiccation. This avoidance mechanism might have evolved as an adaptative response to repeated cycles of desiccation and rehydration that represent a real threat to photosynthetic viability. Received: 12 January 1998 / Accepted: 14 July 1998     

PHOTOSYNTHESIS OF THE RED ALGA GASTROCLONIUM COULTERI (RHODOPHYTA) IN RESPONSE TO CHANGES IN TEMPERATURE,LIGHT INTENSITY,AND DESICCATION1

Previously reported transplantation experiments in the field showed that Gastroclonium coulteri (Harvey) Kylin could survive above its normal intertidal range (0.0–0.5 m above MLLW), except during periods of daytime low tides in spring. Net photosynthetic rate measurements in the laboratory were performed to determine which physical factors might determine the upper boundary for this species in the intertidal zone. Maximum net photosynthesis occurred between 15 and 20° C, but remained positive between 4 and 35° C. The air temperature extremes observed in the field were 2° C (only seen once) and 26° C. Net photosynthesis increased as expected with light intensity to the highest value obtainable in the laboratory, 1400 μEin m^?2 s^?1. Plants collected from the field under higher light intensity (up to 2000 μEin m^?2 s^?2) also showed high rates of photosynthesis. Neither the temperature nor light levels observed in the field were directly damaging to photosynthesis. Desiccation, however, resulted in a sharp decrease in both photosynthesis and respiration. G. coulteri fully recovered from successive daily treatments of about 35% desiccation, but not from successive treatments of 50% desiccation. One exposure to 70% desiccation allowed no recovery of photosynthetic capacity.     

COMPARATIVE PHOTOSYNTHESIS OF TWO SPECIES OF INTERTIDAL EPIPHYTIC MACROALGAE ON MANGROVE ROOTS DURING SUBMERSION AND EMERSION

Photosynthesis and dark respiration rates were measured in water and in air, and the capacity to recover photosynthetic activity from emersion stress was examined for two species of intertidal, epiphytic macroalgae—Bostrychia calliptera (Montagne) Montagne and Caloglossa leprieurii (Montagne) J. Agardh—collected on prop roots of the red mangrove Rhizophora mangle L. in Buenaventura Bay, Pacific coast of Colombia. In both species, net photosynthetic rates were significantly higher under submersed conditions. Maximum photosynthetic rates (P_max) in water and in air were highest in B. calliptera, 126 ± 4 versus 52 ± 9 μmol O₂·mg chl a⁻¹·h⁻¹, respectively. In C. leprieurii, P_max of submerged plants in water and in air were 98 ± 9 versus 30 ± 11 μmol O₂·mg chla⁻¹·h⁻¹. The photoinhibition model of Platt et al. (1980) was used to fit the experimental data in both water and air for both species. Photoinhibition occurred at irradiance as low as 200 μmol·m⁻²·s⁻¹. The photosynthesis–light response curves demonstrated an adaptation to shaded habitats for both species, as light compensation points in water and air for both species were below 17 ± 5 μmol·m⁻²·s⁻¹. The rate of dehydration was significantly lower in thalli of B. calliptera compared to C. leprieurii. An increase of photosynthetic activity in B. calliptera was evident between 5% and 15% water loss, but rates decreased thereafter with declining water content. In C. leprieurii, desiccation negatively influenced photosynthetic rates that significantly decreased linearly with declining water content. In B. calliptera, net photosynthesis reached zero only at a water content between 29% and 35%, whereas in C. leprieurii no net photosynthesis occurred in plants containing less than about 50% of their relative water content. Resubmerged plants ofB. calliptera exhibited 100% photosynthetic recovery after 45 min, whereas C. leprieurii recovered 100% at about 120 min. On the basis of the comparison of rates of light-saturated net photosynthesis for B. calliptera in air versus in water, aerial photosynthetic activity ranged from 35% to 42% of that in water, whereas the emersed photosynthetic capacity of C. leprieurii ranged from 24% to 29% of that in water. Using tidal predictions and the emersed photosynthetic rates, a carbon balance model was constructed for both species over a single daylight period. The calculations indicated that emersed photosynthesis increased average daily carbon production of B. calliptera by 17% and C. leprieuri by 12%. The physiological responses to desiccation stress and the photosynthetic recovery capacities between species correlated with, and may determine, their vertical distribution in the mangrove habitats of Buenaventura Bay.     

In situ studies of water relations and CO2 exchange of the tropical macrolichen, Sticta tomentosa

Diel (24-h) time courses of CO₂ exchange, water relations, and microclimate of the foliose lichen, Sticta tomentosa (Swartz) Ach., and responses to experimentally manipulated conditions were measured at a forest edge in a lower montane rainforest in Panama.
Similar to earlier observations on two other rain forest lichens, daily desiccation suppressed net photosynthesis (NP) during the period when irradiation was highest. Not surprisingly, the light response curves of NP showed saturation at rather low light levels. Rehydration was associated with an initial resaturation burst of short duration, which could be demonstrated both under natural conditions and experimentally. This additional loss of CO₂ seems too low to be ecologically relevant. Moreover, high thallus hydration was also detrimental to NP: at maximum water content net CO₂ uptake was depressed by >50%. Although NP was well adapted to the prevailing high temperatures, the latter also stimulated dark respiration. On average, almost 60% of the diurnal carbon gain was lost during the night.
In spite of these limitations, the integrated 24-h C gain was quite high, on average 0·5% of the thallus C content. Whilst these figures were determined for horizontally exposed samples, we also assessed the role of different exposures on photosynthetic performance. Diel C gain was highest under conditions of semi-shade (westerly exposure), which allowed long periods of activity, whilst much higher irradiance at other exposures could not be utilized for photosynthetic production: easterly exposed thalli dried out even faster than horizontally exposed samples.     

哀牢山湿性常绿阔叶林树干4种附生苔藓植物的水分变化对光合、荧光参数的影响

以云南哀牢山湿性常绿阔叶林中的附生苔藓植物为对象,研究林内4种树干附生苔藓植物阿萨羽苔(Plagiochila assamica Steph.)、西南树平藓(Homaliodendron montagneanum(Müll.Hal.)M.Fleisch.)、刀叶树平藓(H.scalpellifolium(Mitt.)M.Fleisch.)、大羽藓(Thuidium cymbifolium(Dozy et Molk.)Dozy et Molk.)的吸水力、失水特征和脱水耐性,稳定碳同位素(δ~(13)C)值,以及水分变化对苔藓净光合速率和荧光参数的影响。结果显示:(1)4种附生苔藓植物均具有较强的吸水能力和较低持水力,脱水耐性较强,脱水后均可迅速复水;(2)4种附生苔藓植物的δ~(13)C值较低,其水分利用效率小于其他维管束植物;除大羽藓外,其余3种附生苔藓雨季的δ~(13)C值与水分利用效率均高于干季;(3)4种附生苔藓植物的净光合速率(Pn)随着含水量的降低而降低,在含水量小于60%~80%时,其最大光化学效率(Fv/Fm)急剧下降,反映出这些附生苔藓植物的光合、荧光特性对水分变化非常敏感。     

Early morning photosynthesis of the moss Tortula ruralis following summer dew fall in a Hungarian temperate dry sandy grassland

Air temperature and humidity, moss surface temperature, moss water content, and photosynthetically active radiation were measured through a clear dry night and early morning in July 1998; CO₂ gas exchange of the moss was measured by infra-red gas analysis. The measurements showed progressive absorption of water by the moss through much of the night. The moss reached sufficient water content for about 1.5 h of positive net CO₂ uptake immediately after dawn. The cumulative net carbon balance on this occasion was negative, but mornings with heavier dew could give a positive daily carbon balance, and short, early morning periods of photosynthesis during prolonged dry weather may mitigate long-term desiccation damage and allow for regular molecular repair.     

Recovery of photosynthesis in sunflower after a period of low leaf water potential

Photosynthesis was studied in sunflower plants subjected to 1 to 2 days of desiccation and then permitted to recover. The leaf water potential to which leaves returned after rewatering was dependent on the severity of desiccation and the evaporative conditions. Under moderately evaporative conditions, leaf water potential returned to predesiccation levels after 3 to 5 hours when desiccation was slight. Leaf water potentials remained below predesiccation levels for several days after rewatering when leaf water potentials decreased to −13 to −19 bars during desiccation. Leaf water potential showed no sign of recovery when leaf water potentials decreased to −20 bars or below during desiccation. The lack of full recovery of leaf water potential was attributable to increased resistance to water transport in the roots and stem. The resistance ultimately became large enough to result in death of the leaves because net water loss continued even after the soil had been rewatered.     

In vitro desiccation tolerance of the epiphytic Ghost Orchid,Dendrophylax lindenii (Lindl.) Benth x. Rolfe

The Ghost Orchid, Dendrophylax lindenii (Lindl.) Benth x. Rolfe, is a rare and endangered epiphytic orchid native to south Florida and Cuba. The orchid is considered difficult to propagate under greenhouse conditions, requiring high humidity and low air movement. In contrast, the orchid’s native habitat seasonally dries out with decreased precipitation and humidity. This suggests some level of desiccation tolerance. Ghost Orchid plants were assessed for potential desiccation tolerance and ability to recover from desiccation stress under in vitro conditions. In vitro-derived plants were placed into sterile baby food jars and transferred to chambers maintained at 10% relative humidity, which is extremely low compared to relative humidity levels (40–100%) recorded under natural field conditions. Plants were removed every week for 4 wk and recovered on P723 medium supplemented with banana powder for 4 wk. Data were collected at the initiation of the experiment, after the desiccation periods, and after 4 wk in vitro recovery. Ghost Orchid plants demonstrated extremely high desiccation tolerance. Even after 4 wk desiccation, plant survival was observed at 79.2% after recovery. Desiccated plants exhibited significant decreases in tissue water potential (− 18.44 MPa), fresh weight (65.5% loss), and water content (14.2%); however, high plant survival was still observed under these conditions similar to poikilohydric plants. Overall, the Ghost Orchid demonstrated high desiccation tolerance, which should be considered for future greenhouse culture and for its application in the direct field establishment of in vitro-derived plants without greenhouse acclimatization.

     

Protective mechanism of desiccation tolerance in <Emphasis Type="Italic">Reaumuria soongorica</Emphasis>: Leaf abscission and sucrose accumulation in the stem

Reaumuria soongorica (Pall.) Maxim., a perennial semi-shrub, is widely found in semi-arid areas in northwestern China and can survive severe desiccation of its vegetative organs. In order to study the protective mechanism of desiccation tolerance in R. soongorica, diurnal patterns of net photosynthetic rate (Pn), water use efficiency (WUE) and chlorophyll fluorescence parameters of Photosystem II (PSII), and sugar content in the source leaf and stem were investigated in 6-year-old plants during progressive soil drought imposed by the cessation of watering. The results showed that R. soongorica was characterized by very low leaf water potential, high WUE, photosynthesis and high accumulation of sucrose in the stem and leaf abscission under desiccation. The maximum Pn increased at first and then declined during drought, but intrinsic WUE increased remarkably in the morning with increasing drought stress. The maximal photochemical efficiency of PSII (Fv/Fm) and the quantum efficiency of noncyclic electric transport of PSII(Φ_PSII) decreased significantly under water stress and exhibited an obvious phenomenon of photoinhibition at noon. Drought stressed plants maintained a higher capacity of dissipation of the excitation energy (measured as NPQ) with the increasing intensity of stress. Conditions of progressive drought promoted sucrose and starch accumulation in the stems but not in the leaves. However, when leaf water potential was less than −21.3 MPa, the plant leaves died and then abscised. But the stem photosynthesis remained and, afterward the plants entered the dormant state. Upon rewatering, the shoots reactivated and the plants developed new leaves. Therefore, R. soongorica has the ability to reduce water loss through leaf abscission and maintain the vigor of the stem cells to survive desiccation.     

Desiccation-time limits of photosynthetic recovery in Equisetum hyemale (Equisetaceae) spores

The chlorophyllous spores of Equisetum survive desiccation, yet cannot tolerate this quiescent state for more than ~2 wk. The hypothesis that spore viability of Equisetum hyemale L. is limited by inhibition of photosynthetic recovery was tested using chlorophyll a fluorescence and oxygen-exchange analyses. Experimental spores were desiccated at 2% relative humidity and 25C for time periods of 24 h, 1 wk, and 2 wk, and then rehydrated at 200 mmol photons/m2s (PAR) and 25C for up to 24 h. Spores desiccated for 24 h recovered photosynthetic competence very rapidly during rehydration, reaching the O2 compensation point in 6.3 ~ 0.3 (mean +/- SE) min. Recovery of photosynthetic performance of spores desiccated for 1 wk was slower, as judged by significantly slower increases of (1) photochemical efficiency of photosystem (PS) II, (2) PS II quinoneB-reducing center concentration, (3) quinoneB concentration, (4) water-oxidation activity, (5) rate of light-induced O2 evolution, and (6) apparent quantum yield of net O2 exchange. Photosystem-II and whole-spore photosynthetic competence of 2-wk desiccated spores was increasingly impaired, and did not recover during rehydration. Origin fluorescence yield and dark respiration were not affected by desiccation time following rehydration. The results suggest that the extremely short viability of disseminated spores of Equisetum hyemale is due to the inability to recover losses of water oxidation and photosystem II-core function following 2 wk of desiccation.     

Potential NH4+ and NO3− uptake in seven Sphagnum species

The rate of nitrogen uptake by seven Sphagnum species, which from a gradient from hummock to hollow and from ombrotrophic to minerotrophic conditions, was measured as the decrease in the concentrations of NH₄⁺ and NO₃⁻ from solutions in which capitula were grown under laboratory conditions.
The highest uptake rate was by individuals of each species with large capitula and a high number of ion exchange sites, i.e. lawn species ( S. pulchrum , S. fallax , S. papillosum and S. magellanicum ). On a dry-mass basis, the most effective species were the hummock species ( S. fuscum and S. rubellum ), even though these species have a low dry mass. Hummock species, which occur in high densities and have high potential N-uptake rates on a dry-mass basis, were the most effective species in retaining available nitrogen.