Light-saturated Net Photosynthesis Rate Research Articles

Alternating processes of dry and wet nitrogen deposition have different effects on the function of canopy leaves: Implications for leaf photosynthesis.

Canopy leaves are sinks of dry and wet nitrogen (N) deposition, most studies have not considered the response of canopy leaves to the alternating processes of dry and wet N deposition. We manipulated a close top chamber experiment to observe the effects of simulated N deposition with the same total deposition flux but different dry to wet ratios on leaf structure and physiology by spraying NH4Cl solution or supplying gaseous NH3 over the canopy of seedlings of three species (Betula platyphylla, Fraxinus mandshurica, Pinus koraiensis) placed in the chamber. After 32 days of N deposition and relative to the control, the leaf morphology and mesophyll tissue structure of the three species had no significant changes under all N deposition treatments. With the increase in the ratio of dry to wet N deposition, the N concentration, N metabolizing enzyme activity and soluble protein concentration in leaves of all three species increased continuously, but for the leaf light-saturated net photosynthesis rate, B. platyphylla showed a continuous increase, F. mandshurica showed a continuous decrease, and P. koraiensis showed no significant change. We found that F. mandshurica was the only species whose foliar chlorophyll and potassium concentration decreased with the increase in the ratio of dry to wet N deposition and its leaf light-saturated net photosynthesis rate was positively correlated with foliar chlorophyll and potassium concentration, respectively. Our results indicate that dry deposition is relatively more important on leaf physiological functions in alternating deposition. B. platyphylla and P. koraiensis may better acclimate to canopy NH3/NH4 + deposition than F. mandshurica. Most importantly, the results indicate that a single simulated dry and wet deposition would overestimate and underestimate the response of leaf function to atmospheric N deposition, respectively. Alternating processes of dry and wet deposition should be considered for more realistic assessments of the effects of atmospheric N deposition in forests.

Source and sink activity of Holcus lanatus in response to absolute and relative supply of nitrogen and phosphorus.

Mineral nutrients influence photosynthesis and tissue formation; a shift from nitrogen (N)-limited to phosphorus (P)-limited growth induced by high N deposition may change plant growth in terms of physiology and morphology. This experiment showed that absolute and relative N and P supply affected net photosynthesis (source activity) and biomass formation (sink activity), and the relationship between source and sink activities of Holcus lanatus L. under various nutrient treatments. H. lanatus was grown at three N:P ratios (5, 15, 45) with two absolute supply levels of N and P. Between N:P 5 at low level and N:P 45 at high level, and between N:P 45 at low level and N:P 5 at high level, there was a nine-fold difference in N and P supply. Maximum light-saturated net photosynthesis rate (Amax), specific leaf area (SLA), leaf area, and shoot and root biomass were determined during and after the growth process. Amax was minimal at N:P 5 and increased only with increasing absolute N supply. Neither SLA nor leaf area were affected by N:P; increasing absolute P supply significantly increased leaf area. Shoot and root biomass were minimal at N:P 45 and increased dramatically with increasing absolute P supply. Plant biomass was not correlated with Amax. Our results highlight that H. lanatus growth is predominantly controlled by P supply and to a lesser extent by N, whereas net photosynthesis exerted no apparent control on growth under these sink-limited growth conditions. Our findings contribute to understanding of plant growth under sink-limited conditions.

Physiological responses and ginsenoside production of Panax ginseng seedlings grown under various ratios of red to blue light-emitting diodes

Ginseng, a semi-shade perennial plant, is greatly affected by light. However, light quality has only been studied in a few papers involving growth tests under monochromatic light or based on hairy root cultures. A plant factory with light-emitting diodes (LEDs) may provide high precision and standardization of ginseng seedlings for transplanting, and plant responses to light quality should be investigated for designing the optimal lighting conditions for this environment. In this study, various ratios of red light (R):blue light (B) were set from 100:0 to 0:100% at the same photosynthetic photon flux density of 75 μmol m−2 s−1. As R increased, the shoot length became longer from R75B25, resulting in the single treatment of R being 1.68 times that of B. Compared to monochromatic R or B treatments, the overall growth of ginseng seedlings in R50B50 treatment increased. In the mesophyll structure, mixed light as R50B50 or while LEDs increased starch grains, and only R treatment led to dense chloroplasts in palisade and spongy parenchyma cells. Increasing R ratios had negative effects on CO2 assimilation rate (AN), light-saturated net photosynthesis rate (Asat), and chlorophyll parameters. The higher the R ratio, the higher the ginsenoside content in leaves, while roots were less affected by spectral changes. Monochromatic R induced malformation and senescence of ginseng leaves, while the addition of 25% B was sufficient to prevent the abnormal development of leaves and dysfunctional photosynthetic operation of ginseng seedlings. The results suggest that combinations of R and B should be considered when designing artificial lighting systems for a closed-type plant factory since R affects the morphological characteristics and ginsenoside content of ginseng seedlings.

Ozone-induced changes in physiological and biochemical traits in Elaeocarpus sylvestris and Michelia chapensis in South China

The influences of elevated ozone (O3) concentrations on biomass, leaf gas exchange, fluorescence parameters, and leaf physiological and biochemical traits were examined in Elaeocarpus sylvestris and Michelia chapensis seedlings under four O3 conditions for a growing season in open-top chambers (OTCs). The four O3 concentrations were charcoal-filtered air (CF) (20 ppb), 1 × O3 air (40 ppb), 2 × O3 air (80 ppb), and 4 × O3 air (160 ppb), respectively. The significant decrease in the root/shoot (R/S) ratios of both species indicated that under O3 stress root biomass was more negatively affected than shoot biomass for both tree seedlings. Along with the loss of chlorophyll and carotenoid contents, decrease in the superoxide dismutase (SOD), and enhanced level of lipid peroxidant, the light-saturated net photosynthesis rate (Pnmax) of E. sylvestris, the effective quantum yield of PSII photochemistry (Y(II)) and the electron transport rate (ETR) of both species decreased, suggesting that impaired photosynthesis occurred. The negative effect of O3 on physiological and biochemical parameters was greater for M. chapensis than for E. sylvestris. As the O3 concentration increased, the leaf mass per area (LMA) of E. sylvestris decreased, while that of M. chapensis increased. Therefore, the increased SOD activity as a hormetic-compensatory response, the increases in LMA and longer vegetative period contributed to the good adaptability and high tolerance of M. chapensis.

Phenotypic plasticity in response to light in the coffee tree

Phenotypic plasticity to light availability was examined at the leaf level in field-grown coffee trees ( Coffea arabica). This species has been traditionally considered as shade-demanding, although it performs well without shade and even out-yields shaded coffee. Specifically, we focused our attention on the morpho-anatomical plasticity, the balance between light capture and excess light energy dissipation, as well as on physiological traits associated with carbon gain. A wide natural light gradient, i.e., a diurnal intercepted photon irradiance differing by a factor of 25 between the deepest shade leaves and the more exposed leaves in the canopy, was explored. Responses of most traits to light were non-linear, revealing the classic leaf sun vs. leaf shade dichotomy (e.g., compared with sun leaves, shade leaves had a lower stomatal density, a thinner palisade mesophyll, a higher specific leaf area, an improved light capture, a lower respiration rate, a lower light compensating point and a limited capacity for photoprotection). The light-saturated rates of net photosynthesis were higher in sunlit than in shade leaves, although sun leaves were not efficient enough to use the extra light supply. However, sun leaves showed well-developed photoprotection mechanisms in comparison to shade leaves, which proved sufficient for avoiding photoinhibition. Specifically, a higher non-photochemical quenching coefficient was found in parallel to increases in: (i) zeaxanthin pools, (ii) de-epoxidation state of the xanthophyll cycle, and (iii) activities of some antioxidant enzymes. Intracanopy plasticity depended on the suite of traits considered, and was high for some physiological traits associated with photoprotection and maintenance of a positive carbon balance under low light, but low for most morpho-anatomical features. Our data largely explain the successful cultivation of the coffee tree in both exposed and shade environments, although with a poor resource-use efficiency in high light.

Gas exchange and photosynthetic performance of the tropical tree Acacia nigrescens when grown in different CO2 concentrations

The photosynthetic responses of the tropical tree species Acacia nigrescens Oliv. grown at different atmospheric CO(2) concentrations-from sub-ambient to super-ambient-have been studied. Light-saturated rates of net photosynthesis (A (sat)) in A. nigrescens, measured after 120 days exposure, increased significantly from sub-ambient (196 microL L(-1)) to current ambient (386 microL L(-1)) CO(2) growth conditions but did not increase any further as [CO(2)] became super-ambient (597 microL L(-1)). Examination of photosynthetic CO(2) response curves, leaf nitrogen content, and leaf thickness showed that this acclimation was most likely caused by reduction in Rubisco activity and a shift towards ribulose-1,5-bisphosphate regeneration-limited photosynthesis, but not a consequence of changes in mesophyll conductance. Also, measurements of the maximum efficiency of PSII and the carotenoid to chlorophyll ratio of leaves indicated that it was unlikely that the pattern of A (sat) seen was a consequence of growth [CO(2)] induced stress. Many of the photosynthetic responses examined were not linear with respect to the concentration of CO(2) but could be explained by current models of photosynthesis.

Seasonal changes in canopy photosynthesis and foliage respiration in a Rhizophora stylosa stand at the northern limit of its natural distribution

Gross photosynthesis and respiration rates of leaves at different canopy heights in a Rhizophora stylosa Griff. stand were measured monthly over 1 year at Manko Wetland, Okinawa Island, Japan, which is the northern limit of its distribution. The light-saturated net photosynthesis rate for the leaves at the top of the canopy showed a maximum value of 17 μmol CO2 m−2 s−1 in warm season and a minimum value of 6 μmol CO2 m−2 s−1 in cold season. The light-saturated gross photosynthesis and dark respiration rates of the leaves existing at the top of the canopy were 2−7 times and 3–16 times, respectively, those of leaves at the bottom of the canopy throughout the year. The light compensation point of leaves showed maximum and minimum peaks in warm season and cold season, respectively. The annual canopy gross photosynthesis, foliage respiration, and surplus production were estimated as 117, 49, and 68 t CO2 ha−1 year−1, respectively. The energy efficiency of the annual canopy gross photosynthesis was 2.5%. The gross primary production GPP fell near the regression curve of GPP on the product of leaf area index and warmth index, the regression curve which was established for forests in the Western Pacific with humid climates.

Photosynthetic plasticity of Nothofagus pumilio seedlings to light intensity and soil moisture

Nothofagus seedlings often survive and grow slowly for a long time in the shaded understory. This creates a seedling bank with a potential advantage in reestablishing canopy disturbances. To manage primary forests more effectively, it is important to understand the basis of plant regeneration ecophysiology, and their plasticity to changes in environmental factors. The objective was to evaluate the photosynthetic plasticity of Nothofagus pumilio seedlings to light intensity and soil moisture gradients; and to relate them with silvicultural prescriptions. Six treatments with three light intensities (4, 26 and 64% of the natural incident irradiance) and two soil moistures levels (40–60 and 80–100% soil capacity) were assayed under greenhouse controlled conditions. CO 2 gas exchanges were measured every month on seedlings growing in each condition. In the shaded treatments seedlings grow below their optimum photosynthetic potential (leaf light-saturated net photosynthesis rate of 5.1 μmol CO 2 m −2 s −1) compared with the lighted treatments by improving their photosynthetic performance (8.3–8.4 μmol CO 2 m −2 s −1). Seedling growing under low soil moisture conditions had higher leaf light-saturated net photosynthesis rate than plants grown under 80–100% soil water capacity (7.8 and 6.6 μmol CO 2 m −2 s −1, respectively). When light (up to 150–200 μmol m −2 s −1) and soil moisture (40–60% soil capacity) levels were favourable, seedling plants could exhibit their maximum photosynthetic capacity. If one of these factors became limiting, the plants reduced their photosynthetic rate, e.g. N. pumilio seedlings with enough light and high levels of soil moisture, probably decreased their growth and fine roots activity. For this, application of silviculture systems must take into account the changes in both factors (light and soil moisture) for maximize the growth potential in the natural regeneration. These findings must be combined with morphological variables at a whole-plant, shoot, crown and leaf levels to determine the optimum growth conditions.

Physiological basis of seasonal trend in leaf photosynthesis of five evergreen broad-leaved species in a temperate deciduous forest

The physiological basis of photosynthesis during winter was investigated in saplings of five evergreen broad-leaved species (Camellia japonica L., Cleyera japonica Thunb., Photinia glabra (Thunb.) Maxim., Castanopsis cuspidata (Thunb.) Schottky and Quercus glauca Thunb.) co-occurring under deciduous canopy trees in a temperate forest. We focused on temperature dependence of photosynthetic rate and capacity as important physiological parameters that determine light-saturated rates of net photosynthesis at low temperatures during winter. Under controlled temperature conditions, maximum rates of ribulose bisphosphate carboxylation and electron transport (Vcmax) and Jmax, respectively) increased exponentially with increasing leaf temperature. The temperature dependence of photosynthetic rate did not differ among species. In the field, photosynthetic capacity, determined as Vcmax and Jmax at a common temperature of 25 degrees C (Vcmax(25) and Jmax(25)), increased until autumn and then decreased in species-specific patterns. Values of Vcmax(25) and Jmax(25) differed among species during winter. There was a positive correlation of Vcmax(25) with area-based nitrogen concentration among leaves during winter in Camellia and Photinia. Interspecific differences in Vcmax(25) were responsible for interspecific differences in light-saturated rates of net photosynthesis during winter.

Photosynthesis and physiological traits of evergreen broadleafed saplings during winter under different light environments in a temperate forest

Photosynthetic traits of the evergreen broadleafed species Camellia japonica L. and Quercus glauca Thunb. were continuously investigated during autumn and winter using saplings that grew in different light environments (gap, deciduous canopy understory, and evergreen canopy understory) in a temperate forest. Light-saturated rates of net photosynthesis in midwinter and spring were lower than those in autumn. Photosynthetic capacity, scaled to a common leaf temperature of 25 °C, increased or remained stable after autumn and then decreased in spring in most leaves. Photosynthetic traits per unit leaf area were different among leaves in different light environments of both Camellia and Quercus during most periods. However, photosynthetic traits per unit leaf mass did not differ among leaves in different light environments, suggesting that differences in photosynthetic traits were mainly due to different leaf mass per area among leaves. Photosynthetic rates under light availability typical in the environment were lower in winter than in autumn in leaves in the sun in a gap but were not different in leaves in the shade under evergreen canopy trees. Thus, the importance of winter carbon gain for annual carbon gain is small in leaves in a gap but is large in leaves under evergreen canopy trees.

Differential growth and photosynthetic response of selected cultivars of English ivy to irradiance

SummaryEffects of irradiance on growth and net photosynthesis were studied in four cultivars of Hedera helix L. grown in controlled environments at a mean daily temperature of 24°C. Shoot dry weights of the variegated cultivars Ingelise and Mini Adam were greatest at around 2.9 MJ m–2 d–1. For the two green cultivars, with increasing daily light integral from 1.2 to 7.2 MJ m–2 d–1, shoot dry weight increased in ‘Dark Pittsburg’, but decreased in ‘Evergreen’. Shoot or internode lengths, and leaf area decreased linearly while leaf thickness increased as light integral increased from 1.7 to 7.2 MJ m–2 d–1 in cultivars Ingelise, Dark Pittsburg and Evergreen. Leaf variegation increased in ‘Ingelise’ under 4.3 or 7.2 MJ m–2 d–1, while leaf variegation in ‘Mini Adam’ was affected little by irradiance. Plants of ‘Dark Pittsburg’ had both higher light-saturated net photosynthesis rate and light saturation point than other cultivars. The variegated cultivars Ingelise and Mini Adam had higher light compensation points than the green cultivars Dark Pittsburg and Evergreen.

Impact studies on Nordic forests: effects of elevated CO2 and fertilization on gas exchange

The effects of rising atmospheric carbon dioxide concentration, [CO2], and fertilization on gas exchange of four field-grown tree species were examined using the branch bag technique (Picea abies (L.) Karst., Pinus sylvestris L., Fagus sylvatica L.) or whole tree chambers (Populus trichocarpa Torr. & Gray). Results are presented on changes in light-saturated rates of net photosynthesis (Asat), carboxylation efficiency (α), stomatal conductance (gs), and stomatal limitation of photosynthesis (Ls) after 2–4 years of CO2 exposure. Fertilization alone did not significantly change Asat, α, gs, or Ls for any of the species, but α and Asat were linearly related to foliage nitrogen content when compared across all treatments. No significant CO2 effects were detected for α, gs, or Asat when compared at the same intercellular [CO2], i.e., no downregulation of Asat was apparent. "Long-term" CO2 enrichment increased Asat significantly by 49, 53, 86, and 114% in Populus trichocarpa, Picea abies, F. sylvatica, and Pinus sylvestris, respectively. In all the species the relative CO2 effect on Asat increased linearly with temperature. Thus, application of a simple linear relationship could improve predictions of future tree growth responses to increasing CO2 and temperature in cool climates.

Slow Leaf Development of Evergreen Broad-leaved Tree Species in Japanese Warm Temperate Forests

Rates of light-saturated net photosynthesis (PNmax) and dark respiration (Rd) on a leaf area basis, leaf dry mass per area (LMA), leaf nitrogen content on a leaf area basis (LNa) and instantaneous nitrogen use efficiency (NUE=PNmax/LNa) were followed during leaf development in six evergreen broad-leaved tree species typical of warm-temperate forests in Japan. These species wereCastanopsis sieboldii, Quercus myrsinaefolia, Quercus glauca, Machilus thunbergii, Cinnamomum japonicumandNeolitsea sericea.When expansion of leaf area was complete, PNmax was about one third of its peak value and increased for another 15 to 44 d. Rd at full leaf expansion was about 1.5 to 3.5-times greater than steady-state rates. These facts suggest that leaf development was still underway at the time of full leaf area expansion. Low PNmax at full leaf expansion was caused both by low leaf nitrogen content and low NUE. PNmax increased with the increase in LMA during leaf development in all six species; data from the literature for other species with different life forms also indicated a similar tendency. The steady-state LMA varied markedly among species. Because leaves with larger steady-state LMAs need more resources for their construction, they will also need longer periods for maturation. We hypothesized that the period required for the attainment of peak PNmax, the ‘leaf maturation period’, depends on the steady-state LMA. Plotting data from the present study together with those from literature for other plants across several life forms showed a strong positive relationship between leaf maturation period and steady-state LMA, supporting the hypothesis.

Limitation of carbon assimilation of intertidal Zostera noltii and Z. marina by desiccation at low tide

Carbon dioxide assimilation of emersed Zostera noltii leaves was measured by infrared gas analysis to determine its dependence on leaf water content and the duration of air exposure in the tidal flats of the German Wadden Sea (south east North Sea). After 5 h of exposure to air during low tide, leaves of Z. noltii had lost up to 50% of their water content. The degree of leaf water loss rapidly increased with the distance from the water line and, thus, was a function of the duration of exposure. A decrease in leaf water content by about 30% (from 340–390 to 190–250% DW) resulted in a reversible reduction in light-saturated net photosynthesis rate ( A max) of Z. noltii by more than 50% (24–31 nmol CO 2 g DW −1 s −1 vs. 6–14 nmol g DW −1 s −1). Similarly, laboratory experiments with artificially dried leaves showed a decrease of A max by about 40% for a water content reduction from 400 to 270% DW. Drying experiments with leaves of the co-occurring species Z. marina showed a rapid reduction of A max from 30 nmol CO 2 g DW −1 s −1 (at 500% leaf water content) to zero or even negative CO 2 fluxes (at 180% water content). These results indicate that photosynthesis is more sensitive to desiccation in Z. marina than in Z. noltii under a given leaf water content. The two intertidal Zostera species use low tide periods for rapid assimilation of atmospheric CO 2 as long as their leaf water status is favourable, but desiccation can significantly reduce carbon gain at low tides. We conclude that the presence of seagrasses in the tidal flats of the Wadden Sea is primarily restricted by desiccation at the upper limit.

Effects of carbon dioxide, fertilization, and irrigation on photosynthetic capacity of loblolly pine trees.

Branches of nine-year-old loblolly pine trees grown in a 2 x 2 factorial combination of fertilization and irrigation were exposed for 11 months to ambient, ambient + 175, or ambient + 350 micro mol mol(-1) CO(2). Rates of light-saturated net photosynthesis (A(max)), maximum stomatal conductance to water vapor (g(max)), and foliar nitrogen concentration (% dry mass) were assessed monthly from April 1993 until September 1993 on 1992 foliage (one-year-old) and from July 1993 to March 1994 on 1993 foliage (current-year). Rates of A(max) of foliage in the ambient + 175 CO(2) treatment and ambient + 350 were 32-47 and 83-91% greater, respectively, than that of foliage in the ambient CO(2) treatment. There was a statistically significant interaction between CO(2) treatment and fertilization or irrigation treatment on A(max) on only one measurement date for each age class of foliage. Light-saturated stomatal conductance to water vapor (g(max)) was significantly affected by CO(2) treatment on only four measurement dates. Light-saturated g(max) in winter was only 42% of summer g(max) even though soil water during winter was near field capacity and evaporative demand was low. Fertilization increased foliar N concentration by 30% over the study period when averaged across CO(2) treatments. During the study period, the ambient + 350 CO(2) treatment decreased average foliar N concentration of one-year-old foliage in the control, irrigated, fertilized and irrigated + fertilized plots by 5, 6.4, 9.6 and 11%, respectively, compared with one-year-old foliage in the corresponding ambient CO(2) treatments. The percent increase in A(max) due to CO(2) enrichment was similar in all irrigation and fertilization treatments and the effect persisted throughout the 11-month study period for both one-year-old and current-year foliage.

Comparative study of leaf carbon gain in saplings of Thujopsis dolabrata var. hondai and Quercus mongolica var. grosseserrata in a cool-temperate deciduous forest

Seasonal courses of leaf CO2 gas exchange in a growing season were examined in saplings ofThujopsis dolabrata var.hondai andQuercus mongolica var.grosseserrata in a cool temperate deciduous forest. Between the two tree species there were no large differences in the light compensation point of leaf photosynthesis, except for the season of new leaf expansion. However, light-saturated rates of net photosynthesis were obviously high inT. dolabrata var.hondai. EvergreenT. dolabrata var.hondai saplings had large photosynthetic production in two seasons, before the emergence of new foliage and after foliage fall of the overstory deciduous trees, because of the significantly high solar radiant energy penetrating under the forest canopy during the seasons. Saplings of deciduousQ. mongolica var.grosseserrata were heavily shaded throughout the growing season by foliage of the overstory trees, which resulted in a low daily surplus production. The annual surplus production of leaves in the growing season was estimated to be 2300 mmol CO2 m−2 inT. dolabrata var.hondai and −100 mmol CO2 m−2, slightly negative, inQ. mongolica var.grosseserrata. These results supported the high survivability ofT. dolabrata var.hondai saplings and the high mortality ofQ. mongolica var.grosseserrata in the deciduous forest.

CO 2 gas exchange of two intertidal seagrass species, Zostera marina L. and Zostera noltii Hornem., during emersion

CO 2 gas exchange of emersed Zostera marina L. and Zostera noltii Hornem. plants was measured with a portable infra-red gas analysis system in the intertidal flats of Sylt (Southern North Sea). Light-saturated net photosynthesis rates during emersion were found to be in the same magnitude as values reported in the literature for both species under submersed conditions at various locations. Net photosynthesis of exposed shoots of Z. noltii showed a very low light-compensation point, a high light-saturation value, and a light-saturated assimilation rate nearly twice as high as for co-occuring Z. marina plants. Artificial drying of the leaves of Z. marina in the air stream of the gas exchange cuvette led to a continuous reduction in the light-saturated net photosynthesis rate that was partly reversible after re-wetting of the leaves. More research is needed to qualify whether intensive photosynthetic activity is a common phenomenon in emersed intertidal seagrass populations and how significant this process is for the carbon balance of these plants.

Irradiance and temperature effects on photosynthesis of tussock tundra Sphagnum mosses from the foothills of the Philip Smith Mountains, Alaska.

Photosynthetic characteristics of three species of Sphagnum common in the foothills of the Brooks Range on the North Slope of Alaska were investigated. Generally, light-saturated rates of net photosynthesis decreased in the order S. squarrosum, S. angustifolium, and S. warnstorfii when plants were grown under common growth chamber conditions. For field-grown S. angustifolium, average light compensation point at 10°C was 37 μmol m-2s-1 photosynthetic photon flux density (PPFD), and light saturation occurred between 250 and 500 μmol m-2 s-1. At 20°C, compensation point increased to 127 μmol m-2s-1 and the PPFD required for light saturation increased to approximately 500 μmol m-2s-1, while maximum rates of CO2 uptake increased only slightly. Light response curves of chamber-grown plants exhibited substantially lower compensation points and higher light-saturated rates of CO2 assimilation than field-grown material, due perhaps to a higher percentage of green, photosynthetically competent tissue. All three species exhibited broad responses to temperature, with optima near 20°C, and maintained at least 75% of maximum assimilation between approx. 13° and 30°C. Rates at 5°C were approx. 50% of maximum. Studies of the microclimate of Sphagnum at the field research site suggest that CO2 uptake should occur at near light-saturated rates during the day in open tussock tundra but that PPFD may often be limiting under Salix and Betula canopies in a water track drainage. Simulations using a simple model provided a seasonal estimate of 0.78 g dry weight (DW) of S. angustifolium produced from each initial g of photosynthetic tissue under willow canopies, assuming no water limitations. Although the simulation model suggests that production would be 66% higher in open tussock tundra, S. angustifolium is rarely found in this potentially more stressful habitat. To explain the relative abundance of Sphagnum in shaded water track areas as compared to open tussock tundra, we postulate that the vascular plant canopies provide protection from adverse effects of high temperatures, excess irradiance and reduced water availability. Under conditions of normal water availability, removal of the vascular plant cover did not affect the tissue water content of S. squarrosum, but resulted in a strong decrease in photosynthetic capacity, accompanied by chlorophyll bleaching. These results suggest that photoinhibition may limit production under certain conditions.

The temperature dependance of photoinhibition in the tropical basidiomycete lichen Cora pavonia E. Fries.

The response of net photosynthesis (NP) and dark respiration to periods of high insolation exposure was examined in the tropical basidiomycete lichen Cora pavonia. Photoinhibition of NP proved quite dependant on temperature. Rates of light saturated NP were severely impaired immediately after pretreatment high light exposure at temperatures of 10, 20 and 40°C, while similar exposure at 30°C resulted in only minimal photoinhibition. Apparent quantum yield proved an even more sensitive indicator of photoinhibition, reduced in all temperature treatments, although inhibition was again greatest at low and high temperatures. Concurrent exposure to reduced O2 tensions during high light exposure mitigated some of the deleterious effects of high light exposure at 10 and 20°C, suggesting an interaction of O2 with the inactivation of photosynthetic function. This represents the first reported instance of light dependant chilling stress in lichens, and may be an important limitation on the distribution of this and other tropical lichen species. This narrow range of temperatures within which thalli of C. pavonia can withstand periods of high insolation exposure coincides with that faced by hydrated thalli during rare periods of high insolation exposure within the cloud/shroud zone on La Soufrière, and points to the necessity of considering periods of atypical or unusual climatic events when interpreting patterns of net photosynthetic response, both in tropical and in north temperate lichen species.

Seasonal changes in net photosynthesis rates and photosynthetic capacity in leaves of Cistus salvifolius, a European mediterranean semi-deciduous shrub.

During five different periods between Nov. 1982 and Aug. 1983, the diurnal patterns exhibited in photosynthetic CO2 uptake and stomatal conductance were observed under natural conditions on twigs of Cistus salvifolius, a Mediterranean semi-deciduous shrub which retains a significant proportion of its leaves through the summer drought. During the same periods, net photosynthesis at saturating CO2 partial pressure was measured on the same twigs as a function of irradiance at different temperatures. From these data, photosynthetic capacity, defined here as the CO2- and light-saturated net photosynthesis rate, was obtained as a function of leaf temperature. C. salvifolius is a winter growing species, shoot growth being initiated in Nov. and continuing through May. Photosynthetic capacity was quite high in Nov., March and June, exceeding 40 μmol m-2 s-1 at optimum temperature. In Dec., photosynthetic capacity was somewhat reduced, perhaps due to low night-time temperatures (<5°C) during the measurement period. In Aug., capacity in oversummering shoots at optimum temperature fell to less than 8 μmol m-2 s-1, due to water trees and perhaps leaf aging. Seasonal changes in maximal photosynthetic rates under ambient conditions were similar, and like those found in co-occurring evergreen sclerophylls. Like the evergreens, Cistus demonstrated considerable stomatal control of transpirational water loss, particularly in oversummering leaves. During each measurement period except Aug. when capacity was quite low, the maximum rates of net photosynthesis measured under ambient conditions were less than half the measured photosynthetic capacities at comparable temperatures, suggesting an apparent excess nitrogen investment in the photosynthetic apparatus.