Optimum Temperature For Photosynthesis Research Articles

Time course of photosynthetic temperature acclimation in Carex eleocharis Bailey

Abstract Photosynthetic temperature acclimation in Carex eleocharis has been demonstrated in a previous study in which warm grown (35/15°C) plants were shown to have photosynthetic temperature optima approximately 14°C higher than cool grown (20/15°C) plants (Monson, Littlejohn & Williams, 1983). The current study examined the time course of this acclimation by determining photo‐synthetic temperature optima as a function of time, of cool grown plants moved to warm growing conditions. Leaves which had developed under cool conditions were capable of an upward adjustment of 6–8°C of their optimum photosynthetic temperature within a time span of 6–14 d. For greatest photosynthetic temperature acclimation it was necessary for leaves to form and develop entirely under warm conditions. These leaves exhibited a 14–15°C upward adjustment of their optimum temperature for photosynthesis within 20–31 d since moving plants from cool to warm growing conditions. Thus, the time course of this acclimation is of short enough duration to be significant during the growing season and presumably contributes toward the ability of this species to maintain active growth during the cool and warm portions of the growing season. It is also noted that the plant with its capacity to form new leaves, has a much wider acclimation capacity than any single leaf.

Photosynthesis and water relations of the mistletoe, Phoradendron villosum, and its host, the California valley oak, Quercus lobata.

Water vapor and CO2 exchange characteristics were studied in Phoradendron villosum, a hemiparasitic mistletoe and its host, Quercus lobata. The hemiparasite had stomatal conductances equal to or higher than the host but a much lower capacity to fix carbon. Respiration was high in the mistletoe relative to maximum photosynthesis because of the high leaf specific weight (weight per unit area) in this species. The temperature optima for photosynthesis were similar in both species although photosynthesis in Phoradendron declined more steeply below 20 degrees C. Consequences of the high conductance and low rate of photosynthesis in the mistletoe include a low water-use efficiency and a relatively high internal concentration of CO2.

The Effect of Temperature Preconditioning on the Temperature Sensitivity of Net CO2 Flux in Geographically Diverse Populations of the Moss Polytrichum Commune

Gametophytes of five geographically diverse populations of the moss Polytrichum commune ranging from Barrow, Alaska (71°N), to Gainesville, Florida (29°N), were grown under constant temperatures of 5° and 20°C with 16 h of light each day. The net CO2 exchange of the shoot tissue produced under these conditions was subsequently studied in relation to temperature. The following patterns were noted. (1) The maximum net photosynthetic rate decreased in P. commune populations from sites along a gradient of increasing latitude, except that highest rates were found in the temperature St. Hilaire population; the difference in photosynthetic rates between the extreme values was about sevenfold, (2) Cool growth conditions resulted in low maximum net CO2 uptake rates. (3) The growth temperature affected the maximum net photosynthetic rate of low—latitude populations more than high—latitude populations. (4) The optimum temperatures for photosynthesis were equal to or higher than the growth temperatures. (5) The temperature at the center of the estimated optimal range for net CO2 uptake was correlated with both preconditioning temperature and the latitude of origin. (6) The upper temperature compensation points, which were unusually high for bryophytes, were correlated positively with the preconditioning temperature and negatively with the latitude of origin. (7) Northern populations had dark—respiration rates which were equal to or lower than those of southern populations. (8) After low—temperature acclimation, the dark respiration increased only in the southernmost population.

Ecophysiology of lichens in the dry valleys of Southern Victoria Land, Antarctica

CO2 exchange was measured in three cryptoendolithic lichen samples from the ice-free mountains of Southern Victoria Land. Optimum temperature for net photosynthesis at different light intensities ranged from below 2°C to about 7°C; the upper compensation point ranged from 5°C to 15.7°C, and the lower compensation point could be estimated in two samples as being between-6°C and-8°C. Dark respiration rates were higher than those of net photosynthesis. The results indicate that these cryptoendolithic lichens are not more adapted to low temperatures than are crustose lichens from coastal Antarctica. Calculations show that optimum temperatures for photosynthesis may not be reached in the natural environment. Photosynthetic rates were low, but they increased when samples were split into smaller pieces, permitting gas exchange through broken rock surfaces. In nature, when gas exchange is possible only through the intact rock crust, these apparent rates are probably even lower.

Yield, Water Relations, Gas Exchange, and Surface Reflectances of Near‐Isogenic Wheat Lines Differing in Glaucousness1

Near‐isogenic lines that contrast in the presence or absence of glaucousness were developed in durum wheat (Triticum turgidum L. durum group) and common wheat (T. aestivum L.) and grown in plots in three field environments varying in soil water availability. The glaucous selections yielded significantly more grain and dry matter than non‐glaucous selections in the two higher yielding environments but not in the very dry environment where average yield was 66 gm‐2. No differences within or between lines were found in soil water extraction patterns. Furthermore, leaf water potentials determined at predawn and midday exhibited no consistent differences between glaucous and non‐glaucous selections. Lines selected for glaucousness varied in wax content of the different plant parts. Although differing in glaucousness, the glaucous selections did not necessarily exhibit greater amounts of epicuticular wax. Field gas exchange measurements for flag leaves and sheaths also indicated no beneficial or detrimental effects of glaucousness at saturating light and optimum temperature for photosynthesis. Surface reflectances in the 400 to 700 nm wavelengths were the same for the adaxial surface of the flag leaf, but were 8 to 15% higher in the glaucous than nonglaucous selections of one line for the ear, sheath, and abaxial flag leaf surface. Reflectances increased linearly with amount of epicuticular wax and were greater in the driest environment. The greater yield of the glaucous over the non‐glaucous lines is discussed in relation to differences in reflectance and the possible influence of glaucousness on tissue temperature, leaf area duration, and other factors.

Determinants of leaf temperature in California Mimulus species at different altitudes.

Leaf energy balance and gas-exchange characteristics were studied in Mimulus cardinalis at 400 m and Mimulus lewisii at 2,700 m in the Sierra Nevada of central California. In contrast to previous observations, leaf temperatures were not near 30° C at air temperatures from 20 to 40° C but were coupled quite closely to air temperature. Stomatal conductance in both species decreased in response to increases in the water vapor concentration gradient, a response opposite that required to establish 30°C leaf temperatures over a wide range of air temperatures. The temperature optima for photosynthesis were broad in both species but 5° C higher for M. cardinalis than for M. lewisii. The direct or indirect effects of altitude did not contribute significantly to the maintenance of constant leaf temperatures. For both species, maintaining constant leaf temperatures appears to be less important than avoiding inhibitory water stress or diffusion limitation of photosynthesis.

High Temperature Effects on Leaf Resistance, Leaf Water Potential, and Photosynthesis of Non-bearing Prune Trees1

Abstract The effects of ambient temperatures between 15° and 43°C were determined on net photosynthesis of ‘French’ prune (Prunus domestica L. cv. Agen) trees maintained under a non-limiting soil water supply. The temperature optimum for photosynthesis was about 30° and net CO2 assimilation decreased rapidly above 35° even when water vapor pressure differences (VPD) were only 5 to 10 mb. Leaf resistance (r1) remained very low (2 to 3 sec cm-1) although leaf temperature reached 47° and the leaf water potential (ψ) decreased to -25 bars. Thus, non-stomatal photosynthetic inhibition was responsible for the 80% decline in net CO2 assimilation. Canopy wetting prevented the decline in net CO2 assimilation by reducing leaf temperature 8° (vs. control) and maintaining the ψ 14 bars higher than the non-misted control.

Moss functioning in different taiga ecosystems in interior Alaska : I. Seasonal, phenotypic, and drought effects on photosynthesis and response patterns.

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.

Influence of Sunlight on Photosynthesis, Water Relations, and Leaf Structure in the Understory Species Arnica Cordifolia

Intraspecific variation in photosynthesis, transpiration, and leaf structure according to natural exposure to sunlight was evaluated for the understory species Arnica cordifolia (Compositae) within the Rocky Mountains of southeastern Wyoming. Plants were arbitrarily divided into sun plants (receiving >12.6 MJ°m—2°d—1) and shade plants (receiving <4.2 MJ°m—2°d—1). Significant differences in sunlight patterns and windspeeds at locations with sun or shade plants led to variations in leaf temperature, stomatal resistance, xylem water potentials, and photosynthesis. Very small differences in soil water potentials and air temperatures occurred between sun and shade plant locations. Leaf size, thickness, and trichome density along with chlorophyll contents also varied between leaves from sun and shade plants. Sunlit periods for sun plants contained nearly twice as much energy and quantum flux and lasted nearly twice as long as sunlit periods for shade plants. Windspeeds and soil temperatures were also considerably greater at sun plant locations. Stomatal opening occurred for both sun and shade plants during sunlit periods which led to large increases in pohotosynthesis and transpiration. Aerodynamic leaf resistances accounted for >50% of the total leaf resistance to water loss for plants in full sun, while the mesophyll resistance accounted for >80% of the total leaf resistance to CO2 uptake for both sunlit and shaded plants. Sun plants had higher photosynthetic rates, light saturation values, and temperature optima for photosynthesis. Leaves of sun plants also had large stomata, almost twice as many trichomes, and were °1.3 times as thick as leaves from shade plants. Chlorophyll contents were 36% greater in shade plants when expressed as a mass per mass basis, but 47% less if expressed as mass per leaf area. The results are discussed in terms of adaptive mechanisms for the evolution of sun— or shade—tolerant species in understory habitats.

The influence of water stress on the photosynthetic performance and stomatal behaviour of tree seedlings subjected to variation in temperature and irradiance.

Seedlings of Betula pendula Roth. and Gmelina arborea L. were subjected to variation in temperature and irradiance. The influence of a mild water-stressing treatment on the photosynthetic performance and stomatal behaviour of these plants was assessed. For both species, the shape of the relationships between irradiance and photosynthesis and temperature and photosynthesis resembled those reported for other species. The effect of water stress was to reduce the rate of photosynthesis, particularly at high temperatures. This was largely a function of a reduction in mesophyll conductance under these conditions. The optimum temperature for stomatal opening was significantly lower than the optimum temperature for photosynthesis, which was in turn lowered by the water stress treatment. The stomata of birch seedlings showed maximum opening at an intermediate temperature while the stomata of Gmelina generally exhibited a closing movement when leaf temperatures increased from 15° C. Mesophyll conductances of both species increased with increasing temperature.The physiological basis for the variation in photosynthetic performance and stomatal behaviour and the ecological significance of this variation are discussed.

COMPARATIVE ASPECTS OF PHOTOSYNTHESIS, PHOTORESPIRATION AND TRANSPIRATION IN FOUR SPECIES OF THE CYPERACEAE FROM THE RELICT FLORA OF TEESDALE, NORTHERN ENGLAND

SummaryPopulations of two continental‐northern species, Carex ericetorum Poll, and Eriophonim latifolium Hoppe and two arctic‐alpine species, Kobresia simpliduscula (Wahlenb.) Mackenzie and Carex capillaris L., from the flora of Teesdale were grown under controlled conditions and their rates of photosynthesis, photorespiration and transpiration were measured in an open gas‐analysis system over the leaf temperature range 5 to 30 °C over a range of irradiances. The maximum rates and the temperature optima for photosynthesis were similar for all the species. The rates of photosynthesis at low temperature and the light saturation characteristics of C. ericetorum and E. latifolium were significantly different from those of K. simpliciuscula and C. capillaris. The differences are discussed in relation to the habitats in which these species usually occur.

Coexistence and the comparative light relations of the submersed macrophytes Myriophyllum spicatum L. and Vallisneria americana Michx.

The Eurasian watermilfoil (Myriophyllum spicatum L.) has partially replaced wild celery (Vallisneria americana Michx.) as a community dominant in the littoral zones of lakes of Madison, Wisconsin. The two species have very different growth forms, with that of M. spicatum corresponding more closely to the optimal growth form simulated by the macrophyte production model WEED. The objective of this research was to investigate the mechanisms by which Vallisneria could compensate for its nonoptimal growth form and coexist with Myriophyllum.A quantification of midsummer growth form for the two species at a rooting depth of 80-90 cm showed that M. spicatum had 68% of its shoot biomass within 30 cm of the surface, whereas V. americana had 62% of its leaf biomass within 30 cm of the bottom. Vallisneria had a light extinction coefficient ranging from 0.013 to 0.019 m2·g-1, much higher than the value (ca. 0.006 m2·g-1) for M. spicatum. This indicates less effective penetration of light to lower leaves of V. americana. Half-saturation constants describing the light-dependence of carbon uptake in "shade" and "sun" tissues ranged from 60-197 microeinsteins·m-2·s-1 for V. americana, and 164-365 μeinsteins·m-2·s-1 for M. spicatum. The optimum temperature for photosynthesis was 33.6°C for M. spicatum and 32.6°C for V. americana, but Myriophyllum was nearly twice as effective at carbon uptake at 10°C. Integration of all of the above features with WEED showed that, for midsummer conditions, V. americana more than compensated for apparently disadvantageous morphological features by its greater physiological adaptability to low light regimes. Coupled with the temperature-dependence of photosynthesis, it appears that V. americana is favored by midsummer conditions, whereas M. spicatum is at an advantage at other times.

Effect of Temperature on Blue-Green Algae (Cyanobacteria) in Lake Mendota

The temperature optimum for photosynthesis of natural populations of blue-green algae (cyanobacteria) from Lake Mendota was determined during the period of June to November 1976. In the spring, when temperatures ranged from 0 to 20 degrees C, there were insignificant amounts of blue-green algae in the lake (less than 1% of the biomass). During the summer and fall, when the dominant phytoplankton was blue-green algae, the optimum temperature for photosynthesis was usually between 20 and 30 degrees C, whereas the environmental temperatures during this period ranged from 24 degrees C in August to 12 degrees C in November. In general, the optimum temperature for photosynthesis was higher than the environmental temperature. More importantly, significant photosynthesis also occurred at low temperature in these samples, which suggests that the low temperature alone is not responsible for the absence of blue-green algae in Lake Mendota during the spring. Temperature optima for growth and photosynthesis of laboratory cultures of the three dominant blue-green algae in Lake Mendota were determined. The responses of the two parameters to changes in temperature were similar; thus, photosynthesis appears to be a valid index of growth. However, there was little photosynthesis by laboratory cultures at low temperatures, in contrast to the natural samples. Evidence for an interaction between temperature and low light intensities in their effect on photosynthesis of natural samples is presented.

Leaf hairs: Effects on physiological activity and adaptive value to a desert shrub.

The effects of leaf hairs on photosynthesis, transpiration, and leaf energy balance were measured on the desert shrub Encelia farinosa in order to determine the adaptive significance of the hairs. The pubescence reduces leaf absorptance resulting in a reduced heat load, and as a consequence lower leaf temperatures and lower transpiration rates. In its native habitat where air temperatures often exceed 40° C, the optimum temperature for photosynthesis in E. farinosa occurs at 25° C, and at leaf temperatures above 35° C net photosynthesis declines precipitously. An advantage of leaf pubescence is that it allows a leaf temperature much lower than air temperature. As a result, leaf temperatures are near the temperature optimum for photosynthesis and high, potentially lethal leaf temperatures are avoided. However, there is a disadvantage associated with leaf pubescence. By reflecting quanta that might otherwise be used in photosynthesis, the presence of leaf hairs reduces the rate of photosynthesis. A tradeoff model was used to assess the overall advantage of possessing leaf hairs. In terms of the carbon gaining capacity of the leaf, the model predicted that for different environmental conditions different levels of leaf pubescence were optimal. In other words, under aird conditions and/or high air temperatures, leaves of E. farinosa would have a higher rate of photosynthesis by being pubescent than by not being pubescent. The predictions from this model agreed closely with observed patterns of leaf pubescence in the field.

ANATOMICAL, PHYSIOLOGICAL, AND BIOCHEMICAL EVIDENCE FOR THE C3 PATHWAY IN VERBASCUM THAPSUS L.

SUMMARYAnatomical, physiological and biochemical characteristics of Verbascum thapsus demonstrate that this species is a C3 carbon fixation type rather than C4 as suggested by Parkhurst (1976). Leaf anatomy, net photosynthesis rate, carbon dioxide compensation point, photorespiration rate, temperature optimum for photosynthesis and ratio of ribulose‐1,5‐diphosphate carboxylase to phosphoenolpyruvate carboxylase activity for V. thapsus are all typical of C3 rather than C4 species. The data presented illustrate the danger of assuming a species is either C3or C4 based on a single characteristic. Parkhurst (1976) predicted that optimum photosynthesis for V. thapsus would occur at high temperature and would increase faster than transpiration with increased temperature. Our data are contrary to these predictions.

Influences of Seasonal Changes in Leaf Morphology on Water‐Use Efficiency For Three Desert Broadleaf Shrubs

The influences of seasonal variations in leaf length, pubescence, and the ratio of internal to external leaf area (Ames/A) on transpiration and photosynthesis were evaluated for 3 desert broad—leafs (Encelia farinosa [Grey], Hyptis emoryi [Tour.], and Mirabilis tenuiloba [Wats.]). Field data were incorporated into energy balance and gas exchange equations to determine water—use efficiencies (mass CO2 fixed/mass H2O transpired) at various times throughout a year. These results were compared with simulated values for leaves without the observed seasonal changes in morphology. For Encelia, leaf lengths were over 3—fold greater during the winter and spring than during the drier summer and fall (4.0 cm vs. 1.2cm). Increases in pubescence during dry periods led to substantial decreases in the leaf absorptance to solar irradiation (from 0.61 to 0.43). Similar alterations occurred for Hyptis, but with less month—to—month variation, while Mirabilis had comparatively small changes in these leaf parameters. For Mirabilis Ames/A was about 75% greater (maximum values of °70 vs. 40 for Encelia and Hyptis) and varied less (9% variation) during the year compared with Encelia and Hyptis (46% and 24% variation, respectively). Minimum leaf resistances to H2O vapor diffusion (Rwv) were similar for Encelia and Hyptis throughout most of the year, while Mirabilis had much higher values. The optimal temperature for photosynthesis was several degrees higher for Mirabilis (34°C vs. 30°C for Encelia and Hyptis). The calculated cellular resistance for CO2 diffusion (RcellCO2) was lowest for Mirabilis (147 s/cm) and almost identical for Encelia and Hyptis (282 s/cm). Encelia, and to a lesser degree Hyptis, had maximum photosynthetic rates at a lower water—use efficiency when water was available and greatest water—use efficiency at reduced photosynthetic levels during drier periods of the year. During a shorter growth period in the wet portion of the year, Mirabilis had a consistently high water—use efficiency (0.16) and a moderate photosynthetic rate, reflecting its much higher Ames/A and Rwv combined with a lower RcellCO2. Simulations for leaves without the observed changes in morphology that occurred during the drier months showed that the mean water—use efficiency for the year would have been °48% less for Encelia, 41% less for Hyptis, and only 3% less for Mirabilis. A decrease in leaf length, a decrease in absorptance, and an increase in Ames/A contributed about equally to the estimated seasonal increases in water—use efficiency for Encelia and Hyptis. Simulated transpiration and photosynthesis for leaves with morphology characteristic of drier periods resulted in water—use efficiencies similar to those for natural conditions, but annual photosynthesis was reduced °24% for Encelia, 17% for Hyptis, and <2% for Mirabilis.

Effects of Temperature on Photosynthesis by Maize and Wheat

Maize and wheat plants were grown in controlled environments with day temperatures of 13, 18, 23, or 28 °C. Leaves from maize grown at 23 °C photosynthesized faster than leaves from maize grown at 13 or 18 °C and, except when measured at 28 °C, faster than leaves from maize grownat28 °C; leaves of maize grown at 13 °C were yellow and photosynthesized at insignificant rates. Leaves from wheat grown at 18 ° or 13 °C had faster rates of photosynthesis than leaves from wheat grown at 23 or 28 °C. The best rates for maize were faster than the best rates for wheat when the measurements were made at 23 or 28 °C but at 13 or 18 °C the best rates for maize were not significantly better than the best rates for wheat. Leaves of maize that developed in the environment with 23 °C as the day temperature did not rapidly lose their green colour when transferred to the environment with the day temperature of 13 °C and the rate of photo synthesis of these leaves did not decline rapidly. However, new leaves expanding in the cooler conditions were yellow and not effective in photosynthesis. At 13 or 18 °C maize, a C4 plant, which photorespires slowly, did not photosynthesize more effectively than wheat, which photo respires rapidly. The maize did not produce its most effective leaves at 13 or 18 °C and its optimum temperature for photosynthesis was 23°C or higher. It may therefore be considered ill-adapted to the temperate climate.

ADAPTIVE PHOTOSYNTHESIS RESPONSES TO TEMPERATURE EXTREMES BY THE THERMOPHILIC CYANOPHYTESYNECHOCOCCUS LIVIDUS1

SUMMARYThe physiological and biochemical changes associated with and resulting in adaptation to both sub‐ and supra‐optimal temperatures are presented for the thermophilic cyanophyteSynechococcus lividusCopeland. The optimum temperature for growth was 45 C. An increase in the optimum temperature of photosynthesis from 50 to 55 C was shown for cells grown at the supra‐optimum temperature of 57 C; whereas, cells grown at the sub‐optimal temperature of 35 C exhibited a decrease in the optimal temperature from 50 to 45 C for14CO2uptake. These changes in optimal temperatures are interpreted as adaptive. Associated with the 5 C increase in optimal temperature for photosynthesis was an increase in chlorophyll a, plastoquinone A, and activity of ribulose‐1,5‐diphosphate carboxylase (RuDP carboxylase). However, the increase in the temperature optimum for 57 C grown cells was associated with a reduced O2yield correlated with a reduced ferricyanide photoreduction capacity. RuDP carboxylase activity decreased rapidly above 55 C. Therefore reduced rates above 55 C resulted from damage to ferricyanide reducing systems and reduced RuDP‐carboxylase activity, whereas low photosynthesis rates at sub‐optimal temperatures were probably due to rate limiting effect of low temperatures on RuDP carboxylase activity with no evidence of damage to ferricyanide photoreducing systems.

Physiological aspects of the ecology of Dicranum fuscescens in the subarctic. I. Acclimation and acclimation potential of CO2 exchange in relation to habitat, light, and temperature

Laboratory measurements of net carbon dioxide exchange in relation to light and temperature were made on Dicranum fuscescens Turn, at Schefferville, Quebec (latitude 55° N), during the summer of 1974. Net CO2 exchange was measured using an open-flow infrared gas analysis system. Moss samples were collected from two field sites (a lowland lichen woodland and highland semitundra region) immediately before the experiments. Temperature optima for photosynthesis in plants from both sites showed acclimation to higher temperatures in the middle of the season. Measured maximum rates of photosynthesis, attained in early July, equalled 2.1 mg CO2∙g dry weight−1∙h−1 in plants from the highland site and 0.74 mg CO2∙g dry weight−1∙h−1 in those from the lowland lichen woodland. Dark respiration rates showed no seasonal temperature acclimation. Radiation levels required for saturation of photosynthesis at optimum temperatures showed an increase from early season through midseason in samples from both populations. A reverse trend towards lower light requirements for saturation was detectable in the late season. Field-collected plants were exposed to different temperature regimes for [Formula: see text] months in growth chambers. During this period, temperature acclimation of photosynthesis to ambient temperature conditions elicited a rapid shift in optimum temperatures for photosynthesis over periods as short as 48 h. All results are discussed in relation to measured environmental parameters in the two study sites throughout the 1974 growing season.

Physiological ecology of Juniperus virginiana in oldfields.

Juniperus virginiana plants grow faster than other associated tree species in abandoned fields. During the summer the needles of the species do not light saturate even at 1,750 μE m-2 s-1, reach optimum photosynthesis at ∼20°C, and maintain maximum photosynthesis at-8 to-12 bar twig water potential. In the field, the plants experience pronounced daily changes in water potential. The magnitude of the changes becomes more pronounced later in the summer. Leaves of the mature plants have highest rate of photosynthesis, young trees intermediate, and seedlings lowest. In winter there is a slight shift in optimum temperature for photosynthesis and the plants photosynthesize at 0°C. The rates of photosynthesis are lower in winter than in summer. On sunny days with calm winds, mature individuals and seedlings maintain significantly higher temperatures than air temperature while intermediate plants do not. The latter exhibit a lower photosynthetic rate than both mature plants and seedlings. The trends of photosynthesis, in the 3 size classes, both in winter and summer, correspond to the chlorophyll content of their leaves. It is concluded that J. virginiana grows well in open field habitats because it is a sun-adapted, drought resistant species with a long growing season which includes winter. The species is excluded from mature forests because it is shade-intolerant.