Temporal variability in basal isoprene emission factor

Seasonal variability in basal isoprene emission factor (&mgr;g C g(-1) h(-1) or nmol m(-2) s(-1), leaf temperature at 30 degrees C and photosynthetically active radiation (PAR) at 1000 &mgr;mol m(-2) s(-1)) was studied during the 1998 growing season at Duke Forest in the North Carolina Piedmont. Emissions from eight upper-canopy white oak (Quercus alba L.) leaves were measured periodically from the onset of isoprene emission on Day of Year (DOY) 119 (April 29) to leaf senescence in late October (DOY 299). Emissions from four leaves were measured under basal conditions with a controlled-environment cuvette system equipped with 10-ml gas-tight syringes and a reduction gas detector. Emissions from the other four leaves were measured under ambient conditions with the same system. Emission rates from the four leaves measured under ambient conditions were adjusted to basal conditions based on the PAR and leaf temperature algorithms of Guenther et al. (1993). The seasonal onset of isoprene emission was in agreement with previous studies where cumulative degree days from the date of the last spring frost were used to estimate bud break, leaf expansion, and increase in basal emission factor (EF). Between DOY 141 (May 21) and 240 (August 28), mean meteorological conditions 6 to 18 h prior to the EF measurements (ambient PAR and temperature) explained up to 78% of the variability in mean basal EF between measurement periods. Summertime mean isoprene emission potential was reached on DOY 141 (May 21) and was maintained until DOY 240 (August 28), when isoprene emission began to decline monotonically as leaf senescence approached. The mean value for leaves measured under ambient conditions and adjusted to basal conditions for DOY 141-240 was 75.6 &mgr;g C g(-1) h(-1) (74.2-79.1), whereas the mean value for leaves measured under basal conditions was 72.9 &mgr;g C g(-1) h(-1) (64.7-88.9). Between DOY 141 and 240, daily mean isoprene EFs varied from 54 to 96 &mgr;g C g(-1) h(-1) (27 to 49 nmol m(-2) s(-1)). In agreement with previous work at this and other sites, basal isoprene emission rates of fully exposed leaves at the crown apex of this tree were about 20% higher than those of the selected leaves. The length of the period prior to measurement of isoprene emission, during which meteorology was correlated with basal EF, appeared to be related to the timing and periodicity of meteorological change, and probably explains quantitative differences in the length of this period among studies. The empirical equation that we derived for this effect explained variability in midday EFs at the study site, but its general applicability remains to be tested. Strong diurnal changes in EF (as high as a factor of 2) are implied in this study, and should be examined further.     

Field measurements of isoprene emission from trees in response to temperature and light

The atmospheric hydrocarbon budget is important for predicting ozone episodes and the effects of pollution mitigation strategies. Isoprene emission from plants is an important part of the atmospheric hydrocarbon budget. We measured isoprene emission capacity at the bottom, middle, and top of the canopies of a white oak (Quercus alba L.) tree and a red oak (Quercus rubra L.) tree growing adjacent to a tower in the Duke University Forest. Leaves at the top of the white oak tree canopy had a three- to fivefold greater capacity for emitting isoprene than leaves at the bottom of the tree canopy. Isoprene emission rate increased with increasing temperature up to about 42 degrees C. We conclude that leaves at the top of the white oak tree canopy had higher isoprene emission rates because they were exposed to more sunlight, reduced water availability, and higher temperature than leaves at the bottom of the canopy. Between 35 and 40 degrees C, white oak photosynthesis and stomatal conductance declined, whereas red oak (Quercus rubra) photosynthesis and stomatal conductance increased over this range. Red oak had lower rates of isoprene emission than white oak, perhaps reflecting the higher stomatal conductance that would keep leaves cool. The concentration of isoprene inside the leaf was estimated with a simplified form of the equation used to estimate CO(2) inside leaves.     

Rapid temperature acclimation of leaf respiration rates in Quercus alba and Quercus rubra

We conducted controlled (chamber) and natural (field) environment experiments on the acclimation of respiration in Quercus alba L. and Quercus rubra L. Three-year-old Louisiana, Indiana and Wisconsin populations of Q. alba were placed in growth chambers and exposed to alternating 5-week periods of cool (20 degrees C mean) and warm (26 degrees C mean) temperatures. We measured respiration rates on fully expanded leaves immediately before and approximately every 2 days after a switch in mean temperature. In a second chamber experiment, 3-year-old potted Q. alba seedlings were exposed to alternating warm (26 degrees C mean) and cool (16 degrees C mean) temperatures at 4-day intervals. Leaf dark respiration rates were measured on days 2, 3 and 4 after each change in temperature. In a third, field-based study, we measured leaf respiration rates in the same three sources of Q. alba and in Arkansas, Indiana and Minnesota sources of Q. rubra before and after a natural 16 degrees C change in mean daily ambient temperature. We observed rapid, significant and similar acclimation of leaf respiration rates in all populations of Q. alba and Q. rubra. Cold-origin populations were no more plastic in their acclimation responses than populations from warmer sites. All geographic sources showed lower respiration rates when measured at 24 degrees C after exposure to higher mean temperatures. Respiration rates decreased 13% with a 6 degrees C increase in mean temperature in the first chamber study, and almost 40% with a 10 degrees C increase in temperature in the second chamber study. Acclimation was rapid in all three studies, occurring after 2 days of exposure to changed temperature regimes. Acclimation was reversible when changes in ambient temperature occurred at 4-day intervals. Respiration response functions, ln(R) = ln(beta0) + beta1T, were statistically different among treatments (cool versus warm, first chamber study) and among sources in a pooled comparison. Pair-wise comparisons indicated statistically significant (P<0.05) differences in cool- versus warm-measured temperature/respiration response functions for Indiana and Wisconsin sources of Q. alba. Log-transformed base respiration rates were significantly lower during periods of higher mean temperatures. Indiana Q. alba showed a significantly higher beta1 when plants were grown at 16 degrees C than when grown at 26 degrees C. Acclimation in Q. alba was unaccompanied by changes in leaf nitrogen concentration, but was associated with a change in leaf total nonstructural carbohydrate concentration. Total nonstructural carbohydrate concentration was slightly, but statistically, lower (13.6 versus 12%, P<0.05) after a 10 degrees C increase in temperature.     

Isoprene emission inventory for the BOREAS southern study area

An isoprene emission inventory for a section of boreal forest in central Saskatchewan was developed based on measured emission rates from the two dominant isoprene-emitting species, black spruce (Picea mariana (Mill.) BSP) and aspen (Populus tremuloides Michx.). The micrometeorological gradient technique was used to determine isoprene emission factors for establishing the inventory. Isoprene fluxes were measured during each of the three BOREAS intensive field campaigns (IFCs) during the 1994 growing season. Measured isoprene fluxes varied from 0.04 to 3.3 mg C m(-2) h(-1) over the black spruce canopy, and from 0.05 to 7.3 mg C m(-2) h(-1) above the aspen forest. Midsummer standard isoprene emission fluxes were 1.2 mg C m(-2) h(-1) and 2.3 mg C m(-2) h(-1) (at 20 degrees C and photosynthetically active radiation (PAR) of 1000 &mgr;mol m(-2) s(-1)) for black spruce and aspen, respectively. With light and temperature differences accounted for, there was an apparent seasonal effect on emissions with the highest rates in the summer months. The total amount of isoprene emitted from this section of the boreal forest was estimated to be 8.6 Gg C year(-1), which is about 1% of the net ecosystem carbon exchange for the study area. Aspen was the largest contributor, accounting for approximately 70% of the total. Branch enclosure and relaxed eddy accumulation measurements made at the black spruce site were used to define the uncertainty associated with flux measurements. Emission rates obtained by the gradient, enclosure and relaxed eddy accumulation methods showed good agreement when normalized to standard light and temperature conditions. The coefficient of variance between the three techniques was 12% for summer (IFC-2) measurements. The sensitivity of the annual isoprene emission total to the assignment of mean irradiance and temperature was also examined. If the hourly mean temperatures were increased by 1 degrees C throughout the growing season, annual carbon loss due to isoprene emission would increase by 14% from 8.6 to 9.8 Gg C.     

Environmental controls over isoprene emission in deciduous oak canopies

In summer 1992, isoprene emission was measured on intact leaves and branches of Quercus alba (L.) at two heights in a forest canopy. Isoprene emission capacity (measured at 30 degrees C and a photosynthetic photon flux density of 1000 micro mol m(-2) s(-1)) was significantly higher in sun leaves than in shade leaves when expressed on a leaf area basis (51 versus 31 nmol m(-2) s(-1); P < 0.01). Because leaf mass per unit area (LMA, g m(-2)) was higher in sun leaves than in shade leaves, emissions of sun and shade leaves expressed on a dry mass basis did not differ significantly (99 versus 89 micro g C g(DW) (-1) h(-1); P = 0.05). Similar measurements in 1995 were consistent with the 1992 data, but data from leaves in more shaded locations demonstrated that isoprene emission capacity decreased with decreasing growth irradiance, irrespective of units of expression. Isoprene emission capacity in leaves of Q. coccinea Muenchh. and Q. velutina Lam. also declined steeply with canopy depth. Emission capacity, on a dry mass basis, showed no obvious pattern with canopy position in Q. prinus L. There was no difference in the temperature response of sun versus shade leaves of Q. alba, but shade leaves exhibited a greater quantum efficiency and saturated at lower irradiance than sun leaves. Rates of isoprene emission measured on branches of Q. alba were approximately 60% of those measured on individual leaves, as a result of self-shading within branch enclosures. It is recommended that within-canopy variation in isoprene emission capacity be incorporated into regional emission models.     

Isoprene emission, photosynthesis, and growth in sweetgum (Liquidambar styraciflua) seedlings exposed to short- and long-term drying cycles

Isoprene emissions were studied in one-year old sweetgum (Liquidambar styraciflua L.) seedlings during nine drying-rewatering cycles extending over five months. Each drying cycle lasted to the point of leaf wilting. Growth was essentially stopped in response to the first drying cycle, though seedling survival and capacity to recover turgor on rewatering remained high throughout the entire nine cycles. Photosynthetic rates of leaves were inhibited by the drying treatments. Under severe drought, isoprene emission rates of leaves were also inhibited, though isoprene emission was generally less sensitive to drought than photosynthesis. The lower drought sensitivity of isoprene emission compared with photosynthesis resulted in a higher percentage of fixed carbon lost as isoprene as seedlings became more stressed. During the recovery phase of the drying-rewatering cycles, isoprene emission rates in several seedlings were higher than in well-watered control seedlings. Following the ninth drying-rewatering cycle, sustained daily watering resulted in recovery of isoprene emission rates to control values within four days. Photosynthetic rates only recovered to 50% of control values after seven days. We conclude that the mechanisms regulating photosynthetic rate and isoprene emission rate are differentially influenced by limited water supplies. The results are consistent with past studies that predict a protective role for isoprene emission during stress, particularly protection from excessive leaf temperatures during drought.     

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.     

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.     

Relationship of isopentenyl diphosphate (IDP) isomerase activity to isoprene emission of oak leaves

Oaks emit large amounts of isoprene, a compound that plays an important role in tropospheric chemistry. Isopentenyl diphosphate isomerase (IDI, E.C. 5.3.3.2) catalyzes the isomerization of isopentenyl diphosphate (IDP) to dimethylallyl diphosphate (DMADP), and in isoprene-emitting plants, isoprene synthase (IS) converts the DMADP to isoprene. To study the role of IDI in isoprene biosynthesis of oak leaves, we compared IDI and IS activities in pedunculate oak (Quercus robur L.) and pubescent oak (Quercus pubescens Willd.) with the isoprene emission rates of these species. We developed a non-radioactive enzyme assay to detect IDI activity in crude leaf extracts of Q. robur. The substrate dependency of IDI activity showed biphasic kinetics with Michaelis constants (K(m)(IDP)) of 0.7 +/- 0.2 micro M for a high-affinity phase and 39.5 +/- 6.9 micro M for a low-affinity phase, potentially attributable to different IDI isoforms. Under standard assay conditions, the temperature optimum for IDI activity was about 42 degrees C, but IDI activity was detectable up to 60 degrees C. A sharp pH optimum appeared around pH 7, with 20 mM Mg(2+) also required for IDI activity. Neither IDI activity nor IS activity showed diurnal variation in Q. robur leaves. The sum of IDI activities showed a significant linear correlation with IS activity in both Q. robur and Q. pubescens leaves, and both enzyme activities showed a linear relationship to isoprene emission factors in leaves of these oak species, indicating the possible involvement of IDI in isoprene biosynthesis by oak leaves.     

Coarse and fine root respiration in aspen (Populus tremuloides)

Coarse and fine root respiration rates of aspen (Populus tremuloides Michx.) were measured at 5, 15 and 25 degrees C. Coarse roots ranged from 0.65 to 4.45 cm in diameter, whereas fine roots were less than 5 mm in diameter. To discriminate between maintenance and growth respiration, root respiration rates were measured during aboveground growing periods and dormant periods. An additional measurement of coarse root respiration was made during spring leaf flush, to evaluate the effect of mobilization of resources for leaf expansion on root respiration. Fine roots respired at much higher rates than coarse roots, with a mean rate at 15 degrees C of 1290 micromol CO2 m-3 s-1 during the growing period, and 660 micromol CO2 m-3 s-1 during the dormant period. The temperature response of fine root respiration rate was nonlinear: mean Q10 was 3.90 for measurements made at 5-15 degrees C and 2.19 for measurements made at 15-25 degrees C. Coarse root respiration rates measured at 15 degrees C in late fall (dormant season) were higher (370 micromol CO2 m-3 s-1) than rates from roots collected at leaf flush and early summer (200 micromol CO2 m-3 s-1). The higher respiration rates in late fall, which were accompanied by decreased total nonstructural carbohydrate (TNC) concentrations, suggest that respiration rates in late fall included growth expenditures, reflecting recent radial growth. Neither bud flush nor shoot growth of the trees caused an increase in coarse root respiration or a decrease in TNC concentrations, suggesting a limited role of coarse roots as reserve storage organs for spring shoot growth, and a lack of synchronization between above- and belowground growth. Pooling the data from the coarse and fine roots showed a positive correlation between nitrogen concentration and respiration rate.     

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.     

Reduction of isoprene emissions from live oak (Quercus fusiformis) with oak wilt

Many plants emit isoprene, a hydrocarbon that has important influences on atmospheric chemistry. Pathogens may affect isoprene fluxes, both through damage to plant tissue and by changing the abundance of isoprene-emitting species. Live oaks (Quercus fusiformis (Small) Sarg. and Q. virginiana Mill) are major emitters of isoprene in the southern United States, and oak populations in Texas are being dramatically reduced by oak wilt, a widespread fungal vascular disease. We investigated the effects of oak wilt on isoprene emissions from live oak leaves (Q. fusiformis) in the field, as a first step in exploring the physiological effects of oak wilt on isoprene production and the implications of these effects for larger-scale isoprene fluxes. Isoprene emission rates per unit dry leaf mass were 44% lower for actively symptomatic leaves than for leaves on healthy trees (P = 0.033). Isoprene fluxes were significantly negatively correlated with rankings of disease activity in the host tree (fluxes in leaves on healthy trees > healthy leaves on survivor trees > healthy leaves on the same branch as symptomatic leaves > symptomatic leaves; isoprene per unit dry mass: Spearman's rho = -0.781, P = 0.001; isoprene per unit leaf area: Spearman's rho = -0.652, P = 0.008). Photosynthesis and stomatal conductance were reduced by 57 and 63%, respectively, in symptomatic relative to healthy leaves (P < 0.05); these reductions were proportionally greater than the reductions in isoprene emissions. Low isoprene emission rates in symptomatic leaves are most simply explained by physiological constraints on isoprene production, such as water stress as a result of xylem blockage, rather than direct effects of the oak wilt fungus on isoprene synthesis. The effects of oak wilt on leaf-level isoprene emission rates are probably less important for regional isoprene fluxes than the reduction in oak leaf area across landscapes.     

Nocturnal isoprene emission from mature trees and diurnal acceleration of isoprene oxidation rates near Quercus serrata Thunb. leaves

Quercus serrata Thunb. ex Murray is a widespread deciduous oak in China, the Korean Peninsula, and Japan, and a strong isoprene emitter. Establishing accurate inventories of this species and estimating net carbon budgets, including biogenic volatile organic compounds (BVOC), necessitates detailed evaluation of BVOC emission and oxidation characteristics. Emissions of isoprene, the most abundant BVOC, presumably contribute to atmospheric chemistry through the formation of photochemical oxidants and secondary organic aerosols. We built an isoprene flux monitoring system to simultaneously reveal characteristics of the flux and fate of isoprene at multiple locations in Q. serrata forests. We used proton transfer reaction mass spectrometry (PTR-MS) and an automated closed chamber to measure isoprene emissions from soil and leaves in a warm-temperate Q. serrata forest. We used a relaxed eddy accumulation system with PTR-MS to simultaneously measure the canopy flux. In continuous foliage chamber measurements, we observed daily variations of isoprene emissions and continuous nocturnal emissions from leaves. Nocturnal emissions exceeded 25 % of total daily leaf emissions and were relatively high at sunset and low at sunrise. These results suggest that nocturnal emissions from mature trees may not be negligible. When leaf emissions were high in the daytime, the canopy isoprene flux tended to plateau at an upper limit. Observations of isoprene concentrations and gradients suggest that the plateau was caused by acceleration of isoprene oxidation, and sequential formation of secondary organic aerosols may occur near the leaf just after emission. Elucidation of these linkages may require continuous field measurements with a simultaneous multi-flux monitoring system.     

Frost resistance and ice nucleation in leaves of five woody timberline species measured in situ during shoot expansion

Frost resistance and ice nucleation temperatures of leaves, from bud swelling until after full expansion, were measured in situ for five major woody timberline species with recently developed field freezing equipment. Frost resistance determined in situ on leaves of attached twigs was significantly higher than values determined on detached leaves in laboratory tests (e.g., the temperature at which incipient frost damage was observed (LTi) was 1.2 degrees C higher for detached leaves than for attached leaves of Picea abies (L.) Karst.). Frost resistance of leaves of all species changed significantly during shoot expansion (e.g., changes of 7.2 and 11 degrees C for Rhododendron ferrugineum L. and Larix decidua Mill., respectively). Expanding leaves (between 0 and 60% of full expansion) were the most sensitive to frost, with LTi values ranging from -3.4 degrees C in R. ferrugineum to -6.3 degrees C in L. decidua. Among the studied species, P. abies and R. ferrugineum were the most frost sensitive throughout the shoot elongation period. In situ freezing patterns of leaves of attached twigs also differed from those of leaves of excised twigs. During leaf expansion, two distinct freezing exotherms were always registered in situ. The first freezing event (E1, high-temperature exotherm) was recorded at -1.5 +/- 0.2 degrees C and reflected extracellular ice formation. Exposure of leaves to temperatures at which E1 occurred was, in all cases, noninjurious. The low-temperature exotherm (E2) mostly coincided with frost damage, except for some stages of leaf expansion in R. ferrugineum and P. abies, indicating that in situ freezing exotherms were not accurate estimators of frost damage in these species.     

Impact of ozone on monoterpene emissions and evidence for an isoprene-like antioxidant action of monoterpenes emitted by Quercus ilex leaves

Quercus ilex (L.) leaves emit monoterpenes, particularly alpha-pinene, beta-pinene and sabinene. Apart from the monoterpene pools that are stored in specialized structures and have a clear defensive or attractive role, the function of monoterpenes in Q. ilex leaves is unknown. We tested whether monoterpenes have an antioxidant role, as has previously been found for isoprene in isoprene-emitting leaves. We exposed Q. ilex leaves to either mild and repeated ozone exposure (Experiment I) or to a single acute ozone exposure (Experiment II) at temperatures ranging between 20 and 32 degrees C. Both ozone treatments rapidly stimulated monoterpene synthesis, but had no effect on photosynthesis and caused no visible damage to leaves maintained at 25, 30 or 32 degrees C. Ozone inhibited both photosynthesis and monoterpene synthesis in leaves maintained at 20 degrees C. To characterize the relationship between monoterpenes and ozone-induced damage, we fed detached leaves fosmidomycin, a selective inhibitor of isoprene synthesis. Fosmidomycin caused rapid and complete inhibition of monoterpene emissions in leaves maintained at 30 degrees C, confirming that monoterpenes are synthesized by the same biochemical pathway as isoprene. However, over the experimental period, fosmidomycin did not affect concentrations of compounds that are formed from chloroplastic isoprenoids and that might have conferred antioxidant protection, either directly (carotenoids) or indirectly (chlorophylls, xanthophylls). In leaves whose monoterpene synthesis had been inhibited by fosmidomycin, ozone rapidly and significantly inhibited photosynthesis and increased the production of hydrogen peroxide and malonyldialdehyde. We conclude that monoterpenes produced by Q. ilex leaves share the same biosynthetic pathway and function as isoprene. Furthermore, all volatile isoprenoids may have similar antioxidant properties and may be stimulated by the same stress-inducing conditions.     

Control of growth of juvenile leaves of Eucalyptus globulus: effects of leaf age

Biophysical variables influencing the expansion of plant cells (yield threshold, cell wall extensibility and turgor) were measured in individual Eucalyptus globulus leaves from the time of emergence until cessation of growth. Leaf water relations variables and growth rates were determined as relative humidity was changed on an hourly basis. Yield threshold and cell wall extensibility were estimated from plots of leaf growth rate versus turgor. Cell wall extensibility was also measured by the Instron technique, and yield threshold was determined experimentally both by stress relaxation in a psychrometer chamber and by incubation in a range of polyethylene glycol solutions. Once emerging leaves reached approximately 5 cm(2) in size, increases in leaf area were rapid throughout the expansive phase and varied little between light and dark periods. Both leaf growth rate and turgor were sensitive to changes in humidity, and in the longer term, both yield threshold and cell wall extensibility changed as the leaf aged. Rapidly expanding leaves had a very low yield threshold and high cell wall extensibility, whereas mature leaves had low cell wall extensibility. Yield threshold increased with leaf age.     

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.     

High temperature and drought stress effects on survival of Pinus ponderosa seedlings

We studied the effects of high temperature and drought on the survival, growth and water relations of seedlings of Pinus ponderosa (Dougl.) Lawson, one of few coniferous tree species that can successfully colonize drought-prone sites with high soil surface temperatures. Temperature profiles were measured with 0.07-mm thermocouples in a sparse ponderosa pine forest in northern Idaho. The soil surface and the adjacent 5 mm of air reached maximum temperatures exceeding 75 degrees C, well above the lethal temperature threshold for most plants. Air temperatures 50 mm above the soil surface (seedling needle height) rarely exceeded 45 degrees C. Pinus ponderosa seedlings that survived maintained basal stem temperatures as much as 15 degrees C lower than the surrounding air. The apparent threshold temperature at the seedling stem surface resulting in death was approximately 63 degrees C for less than 1 min. No correlation between seedling mortality and needle temperature was found, although some needles reached temperatures as high as 60 degrees C for periods of 相似文献   

Component and whole-system respiration fluxes in northern deciduous forests

We measured component and whole-system respiration fluxes in northern hardwood (Acer saccharum Marsh., Tilia americana L., Fraxinus pennsylvanica Marsh.) and aspen (Populus tremuloides Michx.) forest stands in Price County, northern Wisconsin from 1999 through 2002. Measurements of soil, leaf and stem respiration, stem biomass, leaf area and biomass, and vertical profiles of leaf area were combined with biometric measurements to create site-specific respiration models and to estimate component and whole-system respiration fluxes. Hourly estimates of component respiration were based on site measurements of air, soil and stem temperature, leaf mass, sapwood volume and species composition. We also measured whole-system respiration from an above-canopy eddy flux tower. Measured soil respiration rates varied significantly among sites, but not consistently among dominant species (P < 0.05 and P > 0.1). Annual soil respiration ranged from 8.09 to 11.94 Mg C ha(-1) year(-1). Soil respiration varied linearly with temperature (P < 0.05), but not with soil water content (P > 0.1). Stem respiration rates per unit volume and per unit area differed significantly among species (P < 0.05). Stem respiration per unit volume of sapwood was highest in F. pennsylvanica (up to 300 micro mol m(3) s(-1)) and lowest in T. americana (22 micro mol m(3) s(-1)) when measured at peak summer temperatures (27 to 29 degrees C). In northern hardwood stands, south-side stem temperatures were higher and more variable than north-side temperatures during leaf-off periods, but were not different statistically during leaf-on periods. Cumulative annual stem respiration varied by year and species (P < 0.05) and averaged 1.59 Mg C ha(-1) year(-1). Leaf respiration rates varied significantly among species (P < 0.05). Respiration rates per unit leaf mass measured at 30 degrees C were highest for P. tremuloides (38.8 nmol g(-1) s(-1)), lowest for Ulmus rubra Muhlenb. (13.1 nmol g(-1) s(-1)) and intermediate and similar (30.2 nmol g(-1) s(-1)) for T. americana, F. pennsylvanica and Q. rubra. During the growing season, component respiration estimates were dominated by soil respiration, followed by leaf and then stem respiration. Summed component respiration averaged 11.86 Mg C ha(-1) year(-1). We found strong covariance between whole-ecosystem and summed component respiration measurements, but absolute rates and annual sums differed greatly.     

Influence of climate and fertilization on net photosynthesis of mature slash pine

Net photosynthesis was measured under field conditions in 23-year-old slash pine (Pinus elliottii Engelm. var. elliottii) trees to determine how it was affected by fertilization and climate. There was only a small decrease in rates of net photosynthesis from late summer through winter demonstrating that appreciable carbon gain occurs throughout the year in slash pine. Although fertilization substantially increased leaf area and aboveground biomass, it only slightly increased the rate of net photosynthesis. Simultaneous measurements of gas exchange in fertilized and unfertilized (control) plots allowed the detection of a small, but statistically significant difference in average net photosynthesis of 0.14 micro mol m(-2) s(-1). Irradiance, and to a lesser extent air temperature, were the environmental factors that exerted the most control on net photosynthesis. The highest rates of net photosynthesis occurred between air temperatures of 25 and 35 degrees C. Because air temperatures were within this range for 46% of all daylight hours during the year, air temperature was not often a significant limitation. Soil and atmospheric water deficits had less effect on photosynthesis than irradiance or air temperature. Although the depth to the water table changed during the year from 10 to 160 cm, predawn and midday xylem pressure potentials only changed slightly throughout the year. Predawn values ranged from -0.63 to -0.88 MPa in the control plot and from -0.51 to -0.87 MPa in the fertilized plot and were not correlated with water table depth. There was no correlation between xylem pressure potentials and net photosynthesis, presumably because water uptake was adequate. Although vapor pressure deficits reached 3.5 kPa during the summer, they had little effect on net photosynthesis. Over a vapor pressure deficit range from 1.0 to 3.0 kPa, net photosynthesis only decreased 21%. No differences in responses to these environmental factors could be attributed to fertilization.