Does growth temperature affect the temperature responses of photosynthesis and internal conductance to CO2? A test with Eucalyptus regnans

Internal conductance to CO(2) transfer from intercellular spaces to chloroplasts (g(i)) poses a major limitation to photosynthesis, but only three studies have investigated the temperature dependance of g(i). The aim of this study was to determine whether acclimation to 15 versus 30 degrees C affects the temperature response of photosynthesis and g(i) in seedlings of the evergreen tree species Eucalyptus regnans F. Muell. Six-month-old seedlings were acclimated to 15 or 30 degrees C for 6 weeks before g(i) was estimated by simultaneous measurements of gas exchange and chlorophyll fluorescence (variable J method). There was little evidence for acclimation of photosynthesis to growth temperature. In seedlings acclimated to either 15 or 30 degrees C, the maximum rate of net photosynthesis peaked at around 30 or 35 degrees C. Such lack of temperature acclimation may be related to the constant day and night temperature acclimation regime, which differed from most other studies in which night temperatures were lower than day temperatures. Internal conductance averaged 0.25 mol m(-2) s(-1) at 25 degrees C and increased threefold from 10 to 35 degrees C. There was some evidence that g(i) was greater in seedlings acclimated to 15 than to 30 degrees C, which resulted in seedlings acclimated to 15 degrees C having, if anything, a smaller relative limitation due to g(i) than seedlings acclimated to 30 degrees C. Stomatal limitations were also smaller in seedlings acclimated to 15 degrees C than in seedlings acclimated to 30 degrees C. Based on chloroplast CO(2) concentration, neither maximum rates of carboxylation nor RuBP-limited rate of electron transport peaked between 10 and 35 degrees C. Both were described well by an Arrhenius function and had similar activation energies (57-70 kJ mol(-1)). These findings confirm previous studies showing g(i) to be positively related to measurement temperature.     

Acclimation of loblolly pine (Pinus taeda) seedlings to high temperatures

To determine the extent to which loblolly pine seedlings (Pinus taeda L.) acclimate to high temperatures, seedlings from three provenances-southeastern Texas (mean annual temperature 20.3 degrees C), southwestern Arkansas (mean annual temperature 16.2 degrees C) and Chesapeake, Maryland (mean annual temperature 12.8 degrees C)-were grown at constant temperatures of 25, 30, 35 or 40 degrees C in growth chambers. After two months, only 14% of the seedlings in the 40 degrees C treatment survived, so the treatment was dropped from the experiment. Provenance and family differences were not significant for most measured variables. Total biomass was similar in the 25 and 30 degrees C treatments, and less in the 35 degrees C treatment. Foliage biomass was higher, and root biomass lower, in the 30 degrees C treatment compared with the 25 degrees C treatment. Net photosynthesis and dark respiration of all seedlings were measured at 25, 30 and 35 degrees C. Both net photosynthesis and dark respiration exhibited acclimation to the temperature at which the seedlings were grown. For each temperature treatment, the highest rate of net photosynthesis was measured at the growth temperature. Dark respiration rates increased with increasing measurement temperature, but the basal rate of respiration, measured at 25 degrees C, decreased from 0.617 &mgr;mol m(-2) s(-1) in the 25 degrees C treatment to 0.348 &mgr;mol m(-2) s(-1) in the 35 degrees C treatment, resulting in less carbon loss in the higher temperature treatments than if the seedlings had not acclimated to the growth conditions. Temperature acclimation, particularly of dark respiration, may explain why total biomass of seedlings grown at 30 degrees C was similar to that of seedlings grown at 25 degrees C.     

Effects of atmospheric humidity and acclimation temperature on the temperature response of photosynthesis in young Larix decidua Mill

Larch (Larix decidua Mill.) seedlings of a low altitude (600 m) Austrian provenance were raised outdoors and acclimated in chambers for 14 to 24 days during August and September at either 8 degrees C and an atmospheric saturation vapor pressure deficit (DeltaW) of 2.5 Pa kPa(-1), or 24 degrees C and a DeltaW of 6.2 Pa kPa(-1). Subsequently, their rates of photosynthesis, dark respiration and transpiration were measured at temperatures between 5 and 30 degrees C with DeltaW either maintained below 10 Pa kPa(-1) or allowed to increase with temperature up to 38 Pa kPa(-1). Below 15 degrees C the photosynthetic rate of cold-acclimated plants was higher, but above 15 degrees C it was lower, than that of warm-acclimated plants. Temperature acclimation caused a greater shift in the temperature optimum for photosynthesis when DeltaW was kept small than when it was allowed to increase with temperature. When DeltaW was kept small, leaf conductance of cold-acclimated plants, unlike that of warm-acclimated plants, did not increase with temperature above 15 degrees C. When DeltaW increased with temperature, leaf conductance of cold-acclimated plants decreased more rapidly with temperature than that of warm-acclimated plants. Low temperature acclimation increased the rate of photosynthesis below 15 degrees C without affecting leaf conductance, which indicates that there was an adaptation in leaf internal processes. Further evidence of a metabolic adaptation to acclimation temperature is that dark respiration of cold-acclimated plants was twice that of warm-acclimated plants at all temperatures.     

Acclimation of photosynthesis and respiration to simulated climatic warming in northern and southern populations of Acer saccharum: laboratory and field evidence

Physiological acclimation and genotypic adaptation to prevailing temperatures may influence forest responses to future climatic warming. We examined photosynthetic and respiratory responses of sugar maple (Acer saccharum Marsh.) from two portions of the species' range for evidence of both phenomena in a laboratory study with seedlings. A field study was also conducted to assess the impacts of temperature acclimation on saplings subjected to an imposed temperature manipulation (4 degrees C above ambient temperature). The two seedling populations exhibited more evidence of physiological acclimation to warming than of ecotypic adaptation, although respiration was less sensitive to short-term warming in the southern population than in the northern population. In both seedling populations, thermal compensation increased photosynthesis by 14% and decreased respiration by 10% in the warm-acclimated groups. Saplings growing in open-top field chambers at ambient temperature and 4 degrees C above ambient temperature showed evidence of temperature acclimation, but photosynthesis did not increase in response to the 4 degrees C warming. On the contrary, photosynthetic rates measured at the prevailing chamber temperature throughout three growing seasons were similar, or lower (12% lower on average) in saplings maintained at 4 degrees C above ambient temperature compared with saplings maintained at ambient temperature. However, the long-term photosynthetic temperature optimum for saplings in the field experiment was higher than it was for seedlings in either the 27 or the 31 degrees C growth chamber. Respiratory acclimation was also evident in the saplings in the field chambers. Saplings had similar rates of respiration in both temperature treatments, and respiration showed little dependence on prevailing temperature during the growing season. We conclude that photosynthesis and respiration in sugar maple have the potential for physiological acclimation to temperature, but exhibit a low degree of genetic adaptation. Some of the potential for acclimation to a 4 degrees C increase above a background of naturally fluctuating temperatures may be offset by differences in water relations, and, in the long term, may be obscured by the inherent variability in rates under field conditions. Nevertheless, physiologically based models should incorporate seasonal acclimation to temperature and permit ecotypic differences to influence model outcomes for those species with high genetic differentiation between regions.     

Morphological and physiological acclimation of Quercus coccifera L. seedlings to water availability and growing medium

One of the main constraints of reforestation in the Mediterranean region is low summer water availability during the first years after out planting. Plant water availability depends on the precipitation regime, but also on the physical properties of the soil. Higher survival rates result when seedlings are soil acclimated. Our main goal was to describe the morpho-physiological responses of 6-, 10- and 18-month-old Quercus coccifera seedlings growing in a natural soil (terra rossa) or a standard nursery growing medium, and to assess in the nursery if seedlings growing in natural soil were more resistant to deficit irrigation. The high growth rate achieved after 10?C18 months by terra rossa-grown seedlings in contrast with those grown in the nursery substrate suggests that the former were acclimated to the soil. Higher photosynthetic rate (A), transpiration (E) and stomatal conductance (g_s) were observed in terra rossa seedlings, mainly during the first months. The higher carbon availability may account for the higher root nitrogen concentration in terra rossa-grown seedlings, which could favor their later field growth. Low-watered seedlings showed a certain degree of hardening, since after 18 months, they showed higher A, E, g_s and lower photoinhibition than well-watered seedlings, likely attributable to the sharp leaf-to-root biomass ratio reduction. Carbon isotope discrimination (??) values were similar to those of well-watered plants and indicated a non-stomatal component as the main factor controlling photosynthesis in these leaves. Eighteen-month-old low irrigated seedlings had the highest mortality. Overall, results suggest that nursery terra rossa-acclimated Q. coccifera seedlings with improved physiological status and hydraulic soil-root continuity would have a higher survival rate in the field.     

Critical night length for bud set and its variation in two photoperiodic ecotypes of Betula pendula

We studied the variation in critical night length for bud set in two photoperiodic ecotypes (two latitudinally distant stands) of silver birch (Betula pendula Roth) in three phytotron experiments. Seeds from 21 open-pollinated mother trees in a southern (Tuusula, 60 degrees N) and a northern (Kittil?, 67 degrees N) Finnish stand were germinated and grown for 4 weeks in a 24-h photoperiod in a greenhouse and then moved to different night length treatments at 18 degrees C for 4 to 6 weeks. Night lengths from 5 to 8.5 h were used for southern origin seedlings and from 1 to 4.5 h for northern origin seedlings. At the end of the treatments, apical bud set was observed and the percentage of seedlings with bud set calculated for each treatment and tree progeny. The critical night lengths (CNL) for 50% bud set were determined separately for seedlings from each mother tree by regression analysis. In both ecotypes, the mean percentage of seedlings with bud set was lowest for the shortest night lengths and increased rapidly as night lengths increased. Mean CNL with its 95% confidence interval for the southern and northern ecotypes was 6.3 +/- 0.2 and 3.1 +/- 0.3 h, respectively. The CNL of the two ecotypes differed significantly in three experiments. Within-ecotype variance of the CNL was significantly higher in the northern ecotype (0.484) than in the southern ecotype (0.150). Significant differences in CNL were detected between individual mother trees of the southern ecotype, but not between mother trees of the northern ecotype. The ranking of individual mother trees, based on CNL, differed in the three experiments.     

Thermal optima of photosynthetic functions and thermostability of photochemistry in cork oak seedlings

Temperature effects on photosynthesis were studied in seedlings of evergreen Mediterranean cork oak (Quercus suber L.). Responses to changes in temperature and the temperature optima of maximal carboxylation rate (V(cmax)) and maximal light-driven electron flux (J(max)) were estimated from gas exchange measurements and a leaf-level photosynthesis model. The estimated temperature optima were approximately 34 and 33 degrees C for V(cmax) and J(max), respectively, which fall within the lower range of temperature optima previously observed in deciduous tree species. The thermostability of the photosynthetic apparatus was estimated according to the temperature at which basal chlorophyll a fluorescence begins to increase (T(c)). The T(c) was highly variable, increasing from 42 to 51 degrees C when ambient temperature rose from 10 to 40 degrees C, and increasing from 44 to 54 degrees C with decreasing soil water availability while net CO(2) assimilation rate dropped to almost zero. When a heat shock was imposed, an additional small increase in T(c) was observed in drought-stressed and control seedlings. Maximal T(c) values following heat shock were about 56 degrees C, which, to our knowledge, are the highest values that have been observed in tree species. In conclusion, the intrinsic temperature responses of cork oak did not differ from those of other species (similar T(c) under ambient temperature and water availability, and relatively low thermal optima for photosynthetic capacity in seedlings grown at cool temperatures). However, the large ability of cork oak to acclimate to drought and elevated temperature may be an important factor in the tolerance of this evergreen Mediterranean species to summer drought and high temperatures.     

Inhibition and acclimation of C(3) photosynthesis to moderate heat: a perspective from thermally contrasting genotypes of Acer rubrum (red maple)

Effects of moderate heat on growth and photosynthesis were investigated in two clonal genotypes of Acer rubrum L., originally collected from the thermally contrasting habitats of Florida and Minnesota, USA, and known in the horticultural trade for sensitivity and insensitivity to heat, respectively. Under both common garden and warm greenhouse conditions (day/night temperature of 33/25 degrees C), the Florida genotype exhibited more growth than the Minnesota genotype. To determine the physiological parameters associated with this response, plants were acclimated to ambient (27/25 degrees C) or moderately elevated (33/25 degrees C) temperatures for 21 days before measurement of net photosynthesis at temperatures ranging from 25 to 48 degrees C. In vivo measurements of gas exchange and chlorophyll a fluorescence of ambient-acclimated plants revealed that, compared with the Minnesota genotype, the Florida genotype maintained a higher photosynthetic rate, higher stomatal conductance, more open PSII reaction centers, a greater PSII quantum yield and a lower quantum requirement for photosystem II (phi(PSII)) per mole of CO(2) fixed (phi(CO(2) )) throughout the measurement temperature range. When both genotypes were acclimated at 33/25 degrees C and measured at 33 degrees C, analysis of the response of net photosynthesis to calculated intercellular CO(2) concentration indicated that the maximal rate of Rubisco carboxylation (V(cmax)) decreased more in the Minnesota genotype than in the Florida genotype in response to elevated temperature. Additionally, phi(PSII)/phi(CO(2) ) at 33 degrees C was markedly higher for Minnesota plants under photorespiratory conditions, but similar to Florida plants under non-photorespiratory conditions. The results indicate that the higher net photosynthetic rate at 33/25 degrees C of the Florida genotype compared with the Minnesota genotype could be a result of several mechanisms, including the maintenance of a higher V(cmax )and a more efficient quantum requirement of PSII per mole of CO(2) fixed, which is likely the result of lower photorespiration.     

Temperature control of the development of frost hardiness in two populations of Leptospermum scoparium

Seedlings of Leptospermum scoparium J.R. et G. Forst (manuka) originating from seed from a low altitude coastal site (Auckland) and from a high altitude inland site (Desert Road) were grown for 96 days in four controlled environments to compare the relationship between growth temperature and frost hardening. Day/night temperature treatments were 12/6, 12/3, 12/0 and 12/-3 degrees C. Frost hardiness was determined at 14-day intervals by exposing whole seedlings to temperatures ranging from -2 to -8 degrees C. Frost damage differed significantly between the two populations: Desert Road seedlings were less affected than Auckland seedlings. At all growth temperatures, the time courses of frost hardiness of both populations followed curvilinear relationships reaching a maximum hardiness at about Day 50, after which the seedlings spontaneously dehardened. The rate of frost hardening increased linearly with decreasing temperature from 6 to 0 degrees C, but thereafter, no further increase occurred with decreasing temperature to -3 degrees C. The frost hardening process was more sensitive to temperature in the Desert Road seedlings than in the Auckland seedlings, and this difference may account for the intraspecific variation in frost hardening capacity of this species. Comparisons with Pinus radiata D. Don and Lolium perenne L. indicated that interspecific variation in frost hardening capacity can also be accounted for by differences in the sensitivity of the hardening process to temperature.     

Effectiveness of freezer storage in fulfilling the chilling requirement of fall-lifted ponderosa pine seedlings

The degree to which freezer storage fulfilled the chilling requirement of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) seedlings of two sources was determined by monitoring their development after potting or planting. The seedlings were lifted in September, October, November, or March and subjected to storage before outplanting. The fulfillment of chilling was assessed by measuring days to budbreak, cumulative percentage of seedlings flushing, foliated shoot length, and rate of bud abortion. The effect of freezer storage depended on stage of seedling development at lifting, length of storage, and seed source. Storage did not totally replace winter conditions, especially for seedlings lifted in September and October. Those from a high-elevation seed source flushed sooner than those from a low-elevation source. Delayed budbreak after planting of early lifted seedlings from the high-elevation source disappeared in the second year, but those from the low-elevation source continued to show effects. Seedlings lifted and stored in November had patterns of budbreak that were similar to those of seedlings that had overwintered in beds.     

Chlorophyll fluorescence and CO(2) assimilation of black spruce seedlings following frost in different temperature and light conditions

Effects of artificial frosts on light-saturated photosynthesis (A(max)) and ground, maximal and variable fluorescence variables (F(o), F(m), and F(v) and F(v)/F(m)) were monitored on 1-year-old foliage of black spruce seedlings (Picea mariana (Mill.) BSP) grown at high (25 degrees C), moderate (15 degrees C) and low (5 degrees C) temperatures and moderate (240 &mgr;mol m(-2) s(-1)) and low (80 &mgr;mol m(-2) s(-1)) irradiances. Photoinhibition of 1-year-old foliage was greater in seedlings grown in moderate light than in seedlings grown in low light. Photoinhibition increased with decreasing growth chamber temperature at both irradiances. Most changes in F(v)/F(m) were caused by changes in F(v). Exposure to -4 degrees C decreased both F(v)/F(m) and A(max) compared with control values. The effect of the -4 degrees C frost treatment was greater in seedlings grown in low light than in seedlings grown in moderate light, probably because seedlings grown in moderate light were already partially photoinhibited before the frost treatment. Following -4 degrees C treatment, neither F(v)/F(m) nor A(max) recovered in seedlings grown in low light. Light-saturated photosynthesis decreased with decreasing growth chamber temperature. Light-saturated photosynthesis was more sensitive to the -3 and -4 degrees C frost treatments in seedlings grown at 25 degrees C than in seedlings grown at 15 and 5 degrees C. The A(max) of seedlings grown at 15 degrees C was sensitive only to the -4 degrees C frost treatment, whereas A(max) of seedlings grown at 5 degrees C was not sensitive to any of the frost treatments. Recovery of A(max) following frost took longer in seedlings grown at high temperatures than in seedlings grown at low temperatures. For seedlings grown at the same temperature but under different irradiances, both A(max) and F(v)/F(m) reflected damage to the photosynthetic system following a moderate frost. However, for seedlings grown at the same irradiance but different temperatures, A(max) provided a more sensitive indicator of frost damage to the photosynthetic system than F(v)/F(m) ratio.     

Root water flow and growth of aspen (Populus tremuloides) at low root temperatures

Effects of root zone temperature on growth, shoot water relations, and root water flow were studied in 1-year-old aspen (Populus tremuloides Michx.) seedlings. Seedlings were grown in solution culture and exposed to day/night air temperatures of 22/16 degrees C and solution culture temperatures of 5, 10, or 20 degrees C for 28 days after bud flush. Compared with root growth at 20 degrees C, root growth was completely inhibited at 5 degrees C and inhibited by 97% at 10 degrees C. The 5 and 10 degrees C treatments severely reduced shoot growth, leaf size, and total leaf area. Root water flow was inhibited by the 5 and 10 degrees C treatments. However, when seedlings were grown for 28 days at 5 degrees C and root water flow was measured at 20 degrees C, there was an increase in flow rate. This increase in root water flow was similar in magnitude to the decrease in root water flow observed when seedlings were grown for 28 days at 20 degrees C and root water flow was measured at 5 degrees C. Reduced root water flow of seedlings grown at 5 and 10 degrees C resulted in decreased stomatal conductance, net assimilation, and shoot water potentials. Root water flow was positively correlated with leaf size, total leaf area, shoot length, and new root growth. Transferring seedlings from 5 to 20 degrees C for 24 h significantly increased root water flow, shoot water potential, and net photosynthesis, whereas transferring seedlings from 10 to 20 degrees C resulted in only a slightly increased shoot water potential. Transferring seedlings from 20 to 5 degrees C greatly reduced root water flow, stomatal conductance, and net photosynthesis, whereas shoot water potential decreased only slightly.     

Photosynthetic and transpirational responses of red spruce understory trees to light and temperature

Understory red spruce (Picea rubens Sarg.) trees, between 20 and 50 cm in height and 12 years or more in age, were collected from mid- and high-elevation stands in north-central Vermont and placed in a closed-cuvette system to measure photosynthetic and transpirational responses to photosynthetic photon flux density (PPFD) and temperature. Photosynthesis, dark respiration, transpiration and water-use efficiency of trees from both stands responded to changes in PPFD and temperature in similar ways. Trees from both stands exhibited maximum rates of net photosynthesis at temperatures between 15 and 20 degrees C, and exposure to higher temperatures resulted in reduced rates of photosynthesis and increased rates of respiration. Net photosynthetic rates generally increased with increasing light intensity but began to level off at 250 micro mol m(-2) s(-1). Water-use efficiency was maximal when temperature and PPFD were at 15 degrees C and above 400 micro mol m(-2) s(-1), respectively.     

Contrasting effects of elevated carbon dioxide concentration and temperature on Rubisco activity,chlorophyll fluorescence,needle ultrastructure and secondary metabolites in conifer seedlings

Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) seedlings were grown for 50 days in growth chambers in an ambient or twice ambient carbon dioxide concentration ([CO2]) at a day/night temperature of 19/12 degrees C or 23/16 degrees C. Although elevated [CO2] (EC) had only slight effects on the growth parameters measured, elevated temperature (ET) increased above ground dry mass of both species. Among treatments, biomass accumulation of both species was greatest in the combined EC + ET treatment. The EC treatment induced thylakoid swelling and increased numbers of plastoglobuli observed in Scots pine needles. Although EC had little effect on Rubisco protein or N concentration of needles, ET had a large effect on N-containing compounds and enhanced N allocation from 1-year-old needles. Terpenoids were more responsive to EC and ET than total phenolics. Generally, terpene concentrations were reduced by EC and increased by ET. Increased terpenoid concentrations in response to ET might be associated with thermotolerance of photosynthesis. In Norway spruce, EC decreased total phenolic concentrations in needles, probably as a result of increased growth. We conclude that, in seedlings of these boreal species, the effects of elevated [CO2] on the studied parameters were small compared with the effects of elevated temperature.     

Relationships between gas exchange adaptation of Sitka x interior spruce genotypes and ribosomal DNA markers

Adaptive physiological changes were investigated in seven populations of Sitka (Picea sitchensis (Bong.) Carr.) x interior spruce (P. glauca (Moench) Voss x P. engelmannii Parry ex Engelm.) spanning the Nass-Skeena transition zone in British Columbia, Canada. Each population was represented by an Si rDNA index that was calculated from the relative optical densities on a gel autoradiogram of five ribosomal DNA bands characteristic of Sitka spruce and interior spruce. This index estimates the proportion of the genome contributed by interior spruce. Physiological adaptations were assessed by gas exchange parameters measured under both well-watered and drought conditions. Under well-watered conditions, Sitka spruce populations had higher maximal photosynthesis at saturating light and ambient CO(2), higher quantum yield at the light compensation point, and higher dark respiration than interior spruce populations. Sitka spruce populations also reached maximal photosynthesis at lower photosynthetically active radiation and higher CO(2) concentrations, and had higher stomatal densities that resulted in lower stomatal limitations to photosynthesis than interior spruce populations. In contrast, interior spruce populations exhibited greater drought tolerance than Sitka spruce populations. Their gas exchange rates declined at a slower rate in response to drought. They maintained higher gas exchange rates in response to moderate to severe drought (predawn plant water potentials = -1.5 MPa), and their photosynthetic rates recovered faster when they were rewatered after exposure to drought. Comparison of the seven populations indicated that physiological parameters were significantly related to the Si rDNA index. An increase in Si rDNA index was associated with proportional changes in physiological measurements, suggesting that genetic interchange among species with contrasting ecological adaptations can enhance the environmental adaptation of natural populations.     

Seasonal photosynthetic responses to light and temperature in white spruce (Picea glauca) seedlings planted under an aspen (Populus tremuloides) canopy and in the open

Photosynthetic light and temperature response curves were measured seasonally in seedlings of white spruce (Picea glauca (Moench.) Voss) grown for two years in the understory of aspen (Populus tremuloides Michx.) or in the open in central Alberta. Light-saturated rate of net photosynthesis, the optimum temperature for net photosynthesis, transpiration rate, photochemical efficiency, and stomatal and mesophyll conductances increased from spring to summer and declined thereafter, whereas dark respiration rate and compensation and saturation points were highest in spring. Depression of photosynthetic parameters was greater in open-grown seedlings than in understory seedlings during the periods in spring and autumn when night frosts were common. Net photosynthetic rates were similar in understory and open-grown seedlings in summer, but they were significantly lower in open-grown seedlings in spring and autumn. Significantly lower transpiration rates and stomatal conductances in open-grown seedlings than in understory seedlings were also observed at 15 and 25 degrees C in the autumn. Shoot and needle growth were less in open-grown seedlings than in understory seedlings. In summer, when irradiances were low in the aspen understory, understory white spruce seedlings maintained a positive carbon balance by decreasing their compensation and saturation points and increasing their photochemical efficiency compared to spring and autumn.     

Fall lifting and long-term freezer storage of ponderosa pine seedlings: effects on post-storage leaf water potential, stomatal conductance, and root growth potential

Post-storage water relations, stomatal conductance, and root growth potential were examined in ponderosa pine (Pinus ponderosa Dougl. ex Laws.) seedlings from high- and low-elevation seed sources that had been lifted either in October or November and freezer stored, or in March, and then grown hydroponically in a greenhouse for 31 days. Seedlings lifted in October had poor root initiation (< 17 new roots per seedling), low predawn leaf water potentials (< -1.5 MPa), and low stomatal conductance (7.10 mmol m(-2) s(-1)) compared with seedlings lifted in November or March. There was little difference in post-storage water relations and stomatal conductance between seedlings lifted in November and those lifted in March. Throughout the 31-day test, seedlings from the high-elevation seed source produced 3-9 times more new roots, had higher predawn leaf water potentials (-0.6 to -0.7 MPa versus -1.1 to -1.6 MPa), and 1.3-5 times greater stomatal conductance than seedlings from the low-elevation seed source. For all seedlings on Day 31, the number of new roots was significantly related to predawn leaf water potential (r(2) = 0.65) and stomatal conductance (r(2) = 0.82). Similarly, the dry weight of new roots per seedling on Day 31 accounted for a significant amount of the variation in predawn leaf water potential (r(2) = 0.81) and stomatal conductance (r(2) = 0.49).     

Seedlings of five boreal tree species differ in acclimation of net photosynthesis to elevated CO(2) and temperature

Biochemical models of photosynthesis suggest that rising temperatures will increase rates of net carbon dioxide assimilation and enhance plant responses to increasing atmospheric concentrations of CO(2). We tested this hypothesis by evaluating acclimation and ontogenetic drift in net photosynthesis in seedlings of five boreal tree species grown at 370 and 580 &mgr;mol mol(-1) CO(2) in combination with day/night temperatures of 18/12, 21/15, 24/18, 27/21, and 30/24 degrees C. Leaf-area-based rates of net photosynthesis increased between 13 and 36% among species in plants grown and measured in elevated CO(2) compared to ambient CO(2). These CO(2)-induced increases in net photosynthesis were greater for slower-growing Picea mariana (Mill.) B.S.P., Pinus banksiana Lamb., and Larix laricina (Du Roi) K. Koch than for faster-growing Populus tremuloides Michx. and Betula papyrifera Marsh., paralleling longer-term growth differences between CO(2) treatments. Measures at common CO(2) concentrations revealed that net photosynthesis was down-regulated in plants grown at elevated CO(2). In situ leaf gas exchange rates varied minimally across temperature treatments and, contrary to predictions, increasing growth temperatures did not enhance the response of net photosynthesis to elevated CO(2) in four of the five species. Overall, the species exhibited declines in specific leaf area and leaf nitrogen concentration, and increases in total nonstructural carbohydrates in response to CO(2) enrichment. Consequently, the elevated CO(2) treatment enhanced rates of net photosynthesis much more when expressed on a leaf area basis (25%) than when expressed on a leaf mass basis (10%). In all species, rates of leaf net CO(2) exchange exhibited modest declines with increasing plant size through ontogeny. Among the conifers, enhancements of photosynthetic rates in elevated CO(2) were sustained through time across a wide range of plant sizes. In contrast, for Populus tremuloides and B. papyrifera, mass-based photosynthetic rates did not differ between CO(2) treatments. Overall, net photosynthetic rates were highly correlated with relative growth rate as it varied among species and treatment combinations through time. We conclude that interspecific variation may be a more important determinant of photosynthetic response to CO(2) than temperature.     

Effects of soil temperature on shoot and root growth and nutrient uptake of 5-year-old Norway spruce seedlings

Soil temperature is a main factor limiting root growth in the boreal forest. To simulate the possible soil-warming effect of future climate change, 5-year-old Norway spruce (Picea abies (L.) Karst.) seedlings were subjected to three simulated growing seasons in controlled environment rooms. The seedlings were acclimated to a soil temperature of 16 degrees C during the first (GS I) and third growing seasons (GS III), but were assigned to random soil-temperature treatments of 9, 13, 18 and 21 degrees C during the second growing season (GS II). In GS II, shoot diameter growth was lowest in the 21 degrees C treatment and root growth was lowest in the 9 degrees C treatment. In GS III, shoot height and root length growth improved in seedlings that had been kept at 9 degrees C during GS II, indicating compensatory growth in response to increased soil temperature. The temporary decrease in soil temperature had no long-lasting significant effect on seedling biomass or total nutrient uptake. At the end of GS III, fine roots of seedlings exposed to a soil temperature of 21 degrees C in GS II were distributed more evenly between the organic and mineral soil layers than roots of seedlings in the other treatments. During GS II and GS III, root growth started earlier than shoot growth, decreased during the rapid shoot elongation phase and increased again as shoot growth decreased.     

Midday depression of net photosynthesis in the tropical rainforest tree Eperua grandiflora: contributions of stomatal and internal conductances,respiration and Rubisco functioning

High midday temperatures can depress net photosynthesis. We investigated possible mechanisms underlying this phenomenon in leaves of Eperua grandiflora (Aubl.) Benth. saplings. This tropical tree establishes in small gaps in the rainforest canopy where direct sunlight can raise midday temperatures markedly. We simulated this microclimate in a growth chamber by varying air temperature between 28 and 38 degrees C at constant vapor pressure. A decrease in stomatal conductance in response to an increase in leaf-to-air vapor pressure difference (deltaW) caused by an increase in leaf temperature (Tleaf) was the principal reason for the decrease in net photosynthesis between 28 and 33 degrees C. Net photosynthesis decreased further between 33 and 38 degrees C. Direct effects on mesophyll functioning and indirect effects through deltaW were of similar magnitude in this temperature range. Mitochondrial respiration during photosynthesis was insensitive to Tleaf over the investigated temperature range; it thus did not contribute to midday depression of net photosynthesis. Internal conductance for CO2 diffusion in the leaf, estimated by combined gas exchange and chlorophyll fluorescence measurements, decreased slightly with increasing Tleaf. However, the decrease in photosynthetic rate with increasing Tleaf was larger and thus the difference in CO2 partial pressure between the substomatal cavity and chloroplast was smaller, leading to the conclusion that this factor was not causally involved in midday depression. Carboxylation capacity inferred from the CO2 response of photosynthesis increased between 28 and 33 degrees C, but remained unchanged between 33 and 38 degrees C. Increased oxygenation of ribulose-1,5-bisphosphate relative to its carboxylation and the concomitant increase in photorespiration with increasing Tleaf were thus not compensated by an increase in carboxylation capacity over the higher temperature range. This was the principal reason for the negative effect of high midday temperatures on mesophyll functioning.