Leaf Gas Exchange Rates Research Articles

Differential ecophysiological response of deciduous shrubs and a graminoid to long-term experimental snow reductions and additions in moist acidic tundra, Northern Alaska

Changes in winter precipitation that include both decreases and increases in winter snow are underway across the Arctic. In this study, we used a 14-year experiment that has increased and decreased winter snow in the moist acidic tussock tundra of northern Alaska to understand impacts of variation in winter snow depth on summer leaf-level ecophysiology of two deciduous shrubs and a graminoid species, including: instantaneous rates of leaf gas exchange, and δ(13)C, δ(15)N, and nitrogen (N) concentrations of Betula nana, Salix pulchra, and Eriophorum vaginatum. Leaf-level measurements were complemented by measurements of canopy leaf area index (LAI) and depth of thaw. Reductions in snow lowered summer leaf photosynthesis, conductance, and transpiration rates by up to 40% compared to ambient and deep snow conditions for Eriophorum vaginatum, and reduced Salix pulchra conductance and transpiration by up to 49%. In contrast, Betula nana exhibited no changes in leaf gas exchange in response to lower or deeper snow. Canopy LAI increased with added snow, while reduced winter snow resulted in lower growing season soil temperatures and reduced thaw depths. Our findings indicate that the spatial and temporal variability of future snow depth will have individualistic consequences for leaf-level C fixation and water flux by tundra species, and that these responses will be manifested over the longer term by changes in canopy traits, depth of thaw, soil C and N processes, and trace gas (CO2 and H2O) exchanges between the tundra and the atmosphere.

Instantaneously measured traits may detect non-plastic ecophysiological performances in response to drought, explaining distributions of Styrax species in the Cerrado

We analyzed the differences between irrigated and non-irrigated plants of three congeneric Styrax species that present distinct distribution patterns in the physiognomies of the Cerrado vegetation in Brazil. Styrax ferrugineus showed a stomatal conductance (g s) unresponsive to soil water deficit in potted plants. This may explain the high gas exchange and photochemical efficiency found in this species, which is well adapted to the Cerrado sensu stricto (s. str.), a savanna-type vegetation. S. camporum, which is widely distributed in the Cerrado sensu lato (s. l.) areas, was the only species that exhibited increased intrinsic water use efficiency on the days of maximum water deficit. This result distinguishes S. camporum from S. pohlii, which is a forest species, since the g s of both species decreased during the days of maximum water stress. In contrast to other studies, we propose that instantaneously measured traits, such as leaf gas exchange rates and chlorophyll fluorescence, may be used to detect non-plastic performances in response to environmental stress, helping explain distinct geographical distributions of congeneric species in the Cerrado vegetation.

Relationships between stomatal regulation, water-use, and water-use efficiency of two coexisting key Mediterranean tree species

This study aims to test the hypothesis that as leaf water potential decreases, stomatal conductance (gs) and total water use decrease faster in trees tending toward isohydric behavior than in coexisting anisohydric trees.We measured leaf gas exchange rates in two key Mediterranean species: Pinus halepensis (isohydric) and Quercus calliprinos (anisohydric) growing together in two different sites during seven field campaigns over 14months. Intrinsic water-use efficiency (WUEi) was calculated from gas exchange ratios, and independently from carbon isotopic composition, δ13C, of annual tree-ring sub-sections in four representative growth years.As expected, gs was greatly restricted already at VPD<3kPa in pine trees whereas in oak trees gs was dynamically adjusted even at VPD>5kPa. Consequently, mean transpiration rates were 0.2–2.2 and 0.5–3.9mmolm2s−1 in coexisting pines and oaks, respectively. Mean δ13C values were 1.5‰ higher in tree-rings of the pine compared to the oak trees, consistent with the differences in WUEi between 75 and 64μmol CO2mol−1 H2O in pines and oaks, respectively, based on the short-term gas exchange measurements.A preliminary attempt to upscale the results to typical forest stands of the two species, on annual time-scales, demonstrated that the differences in stomatal regulation and water-use could imply ∼30% higher water-use (or ∼70% lower water yield) in oak stand compared to pine stand, related to its tendency toward anisohydric behavior. This sets the limit for typical 300treesha−1 oak and pine stands at the 460 and 360mm iso-precipitation lines, respectively, consistent with their current distribution along the precipitation gradient in our region. The results can help predict or manage changes in species composition in the face of increasing water limitations in Mediterranean regions.

Leaf gas-exchange characteristics of four Japanese and four Dutch tomato cultivars grown in a greenhouse

Modern Dutch tomato cultivars reportedly yield 1.2–1.6 times as much fresh fruit as Japanese cultivars. To analyze a factor potentially contributing to the yield difference, leaf gas-exchange characteristics were determined for four Japanese (‘Asabiyori 10’, ‘Momotaro York’, ‘Reiyo’ and ‘Renaissance’) and four Dutch (‘Dundee’, ‘Levanzo’, ‘Tomimaru Mucho’ and ‘Vinchy’) tomato cultivars and evaluated according to a C3 photosynthesis model. Plants were grown hydroponically with a high-wire system in a glass greenhouse, and leaf gas-exchange rates were measured at different photosynthetic photon flux and CO2 partial pressures with 130- to 140-d-old plants. Overall, Dutch cultivars generally had higher photosynthetic capacity than Japanese cultivars both under the light-limited and -saturated conditions, and both under the CO2-limited and -saturated conditions; thus, both capacities of ribulose-1,5-bisphosphate (RuBP) carboxylation and regeneration were relatively higher in Dutch cultivars. However, not all Dutch cultivars were superior than Japanese cultivars in terms of photosynthesis: although Dutch cultivars ‘Levanzo’ and ‘Tomimaru Mucho’ tended to have higher leaf photosynthetic capacity than Japanese cultivars ‘Asabiyori 10’ and ‘Renaissance’, Japanese cultivars ‘Momotaro York’ and ‘Reiyo’ and Dutch cultivars ‘Dundee’ and ‘Vinchy’ possessed comparable photosynthetic capacity. Thus, there is variation of photosynthetic capacity even within Dutch cultivars and within Japanese cultivars.

Regulation and acclimation of leaf gas exchange in a piñon–juniper woodland exposed to three different precipitation regimes

Leaf gas-exchange regulation plays a central role in the ability of trees to survive drought, but forecasting the future response of gas exchange to prolonged drought is hampered by our lack of knowledge regarding potential acclimation. To investigate whether leaf gas-exchange rates and sensitivity to drought acclimate to precipitation regimes, we measured the seasonal variations of leaf gas exchange in a mature piñon-juniper Pinus edulis-Juniperus monosperma woodland after 3 years of precipitation manipulation. We compared trees receiving ambient precipitation with those in an irrigated treatment (+30% of ambient precipitation) and a partial rainfall exclusion (-45%). Treatments significantly affected leaf water potential, stomatal conductance and photosynthesis for both isohydric piñon and anisohydric juniper. Leaf gas exchange acclimated to the precipitation regimes in both species. Maximum gas-exchange rates under well-watered conditions, leaf-specific hydraulic conductance and leaf water potential at zero photosynthetic assimilation all decreased with decreasing precipitation. Despite their distinct drought resistance and stomatal regulation strategies, both species experienced hydraulic limitation on leaf gas exchange when precipitation decreased, leading to an intraspecific trade-off between maximum photosynthetic assimilation and resistance of photosynthesis to drought. This response will be most detrimental to the carbon balance of piñon under predicted increases in aridity in the southwestern USA.

Evaluation of Some Wheat Landraces for Drought Tolerance based on Physiological Traits

Drought is a seriously wide-spread problem influencing wheat (Triticum aestivum) production and quality. Developing of abiotic tolerant cultivars is hampered by the lack of wheat genetic resources.The main objective of this research was to evaluate drought tolerance of some wheat landraces that were naturally acclimatized to stress environmental conditions in Egypt.Eight wheat landracescollected from diverse areas of Egypt as well as two wheat cultivars, i.e.YecoraKojo (exotic drought tolerant cultivar) and Gemmeiza 9 (Egyptian high yielding cultivar) were evaluated under four different water regimes, i.e., 75%, 50% and 25% of well-watered regime (100%) after planting irrigationduring 2010/2011 and 2011/2012 seasons. Field work carried out at Gemmeiza Agricultural Research Station. Measurements included leaf gas exchange rate and some agronomic traits. The results showed significant differencesamong genotypes in gas exchange rate, i.e. Net photosynthesis (PN), transpiration rate (E) and stomatal conductance (gs), yield and its components and drought tolerance indices,i.e. water use efficiency (WUE),drought susceptibility index (DSI)and grain yield response index (GYRI),indicating the presence of wide genetic variability among them. The reduction in soil water contents led to reduction of the gas exchange rate, yield and its components. Landraces 1and 8 had higher gas exchange rate, yield and its components under different irrigation regimes as compared to Gemmeiza 9.Also drought tolerance indices indicated that both landraces 1 and 8 are promising as drought tolerantand high yielding genotypes.

Isotopes reveal contrasting water use strategies among coexisting plant species in a Mediterranean ecosystem

Variation in the stable carbon and oxygen isotope composition (δ13C, Δ18O) of co-occurring plant species may reflect the functional diversity of water use strategies present in natural plant communities. We investigated the patterns of water use among 10 coexisting plant species representing diverse taxonomic groups and life forms in semiarid southeast Spain by measuring their leaf δ13C and Δ18O, the oxygen isotope ratio of stem water and leaf gas exchange rates. Across species, Δ18O was tightly negatively correlated with stomatal conductance (gs), whereas δ13C was positively correlated with intrinsic water use efficiency (WUEi). Broad interspecific variation in Δ18O, δ13C and WUEi was largely determined by differences in gs, as indicated by a strong positive correlation between leaf δ13C and Δ18O across species The 10 co-occurring species segregated along a continuous ecophysiological gradient defined by their leaf δ13C and Δ18O, thus revealing a wide spectrum of stomatal regulation intensity and contrasting water use strategies ranging from 'profligate/opportunistic' (high gs, low WUEi) to 'conservative' (low gs, high WUEi). Coexisting species maintained their relative isotopic rankings in 2 yr with contrasting rainfall, suggesting the existence of species-specific 'isotopic niches' that reflect ecophysiological niche segregation in dryland plant communities.

Determining the Optimum Plant Temperature of Cotton Physiology and Yield to Improve Plant‐Based Irrigation Scheduling

ABSTRACTA plant‐based thermal optimum approach to irrigation scheduling provides potential benefits in that water applications are scheduled on the basis of plant response to water status. Such irrigation systems require a defined thermal optimum for the crop and while such optimum values have been identified for cotton (Gossypium hirsutum L.) cultivars in the United States, there is no information of this type for cultivars common in Australian production. This paper outlines a threefold approach to determining the optimum temperature (Topt) of the commercial Australian cotton cultivar Sicot 70BRF in an Australian production system. It combines the use of a laboratory‐based fluorescence assay, field‐based net C assimilation rate (A) and stomatal conductance to water vapor (gs), and canopy temperature (Tc)–yield relations. The fluorescence assay showed a Topt between 28 and 30°C while leaf gas exchange rates peaked at a leaf temperature (Tl) of 29°C. The Tc–yield relations peaked at 26°C, with yield reductions observed when Tc &gt; 28°C. We conclude the Topt of the Australian upland cotton cultivar Sicot 70BRF to be 28 ± 2°C. This Topt will provide valuable information for use in thermal optimum irrigation scheduling systems.

Ecophysiological comparison of 50-year-old longleaf pine, slash pine and loblolly pine

Longleaf pine (Pinus palustris Mill.), a species that once dominated the southeastern USA, is considered to be more drought tolerant than the principle plantation species in the South, loblolly pine (Pinus taeda L.) and slash pine (Pinus elliottii Engelm.), and so is predicted to better cope with increases in drought frequency associated with climate change. To determine if longleaf pine displays a more conservative water use strategy than the other two southern pine species, we examined diurnal patterns in leaf light-saturated photosynthesis, stomatal conductance, water use efficiency and leaf water potential (ΨL) over one growing season in a 50-year-old replicated field experiment. Short-term photosynthetic response to temperature was examined in August. No consistent differences among species in leaf gas exchange rates were observed, but ΨL was higher in longleaf pine compared to loblolly and slash pine across the growing season. Foliar δ13C measured at the end of the growing season was higher in longleaf pine than in loblolly pine but not slash pine. No temperature optimum of photosynthesis was observed in any species and photosynthesis did not respond to changing temperature. Based on leaf physiological traits, these results do not support the contention that longleaf pine has a more conservative leaf water use strategy than the other two pine species. The results do suggest that differences in hydraulic architecture or hydraulic efficiency may account for higher ΨL and perhaps greater drought tolerance in longleaf pine.

Photosynthetic gas exchange characteristics in &lt;i&gt;Jatropha curcas&lt;/i&gt; L.

The use of Jatropha (Jatropha curcas L.) as a source of biofuel has been well-documented. However, the physiological characteristic and growth analysis studies of Jatropha have received considerably lesser attention. In the present study, to confirm the physiological characteristics of Jatropha, we measured the leaf gas exchange characteristics in response to various environmental conditions. Seedlings were grown in 1/5,000 a pots for 2-3 months under greenhouse conditions. Leaf gas exchange rates were measured in a handmade assimilation chamber (26x30x9 cm), in which a fully expanded whole leaf could be set. Based on the leaf gas exchange characteristics, Jatropha was considered to be a C-3 photosynthesis plant. The photosynthetic rate ranged between 10 and 25 mu mol m(-2) s(-1) and light saturation generally occurred at 500-1,000 mu mol m(-2) s(-1) photon flux densities (PFD), depending on the growth conditions and leaf positions. Leaf conductance and transpiration rates were saturated at 400-1,000 mu mol m(-2) s(-1) PFD, also depending on the growth conditions and leaf positions. Maximum rates of transpiration and leaf conductance were 2-6 mmol m(-2) s(-1) and 200-1,200 mmol m(-2) s(-1), respectively, which are very similar to those in C-3 rice plants. Optimum temperature for photosynthesis was approximately at 25-30 degrees C and the maximum rate was 20 mu mol m(-2) s(-1). Application of varying vapor pressure difference (from 1 to 3 kPa) did not affect the photosynthetic rate. Photorespiration in Jatropha was 28.5%, which is within the range of typical C-3 plants. Based on the photosynthetic parameters presented in this study, field performance of Jatropha under severe environmental condition is discussed.

Effects of salt and water stress on plant biomass and photosynthetic characteristics of Tamarisk (Tamarix chinensis Lour.) seedlings

Tamarix chinensis Lour. (Tamarisk) is regarded as a potential shrub that improves the marginal lands, and is conversely considered as an invasive species in many areas. This study conducted controlled experiments to investigate the biomass accumulation, allocation and photosynthetic responses o f T. chinensis seedlings to different salt and water conditions. The results showed that T. chinensis seedlings died when soil salt content attained 16 to 20 g kg -1 ; total biomass, root biomass, shoot biomass and leaf biomass decreased by 63.3, 56.3, 70.6 and 62.4% respectively, from 0 to 16 g kg -1 (added NaCl content), and decrease by 37.9, 25.3, 48.4 and 37.9%, respectively, from 60 to 20% field capacity (FC, soil moisture). Water deficit led to earlier peaks of net photosynthetic rate (P N ) during the day. Relative rate of electron transport (ETR) decreased, but optimal quantum yield of photosystem II (F v /F m ) showed no significant difference (P 0.05) on biomass allocation of T. chinensis seedlings, while water deficit significantly favoured root biomass accumulation. T. chinensis seedlings were able to maintain high leaf gas exchange rates under 20% FC, but water use efficiency (WUE) decreased and sufficient water supply may increase the ability of T. chinensis to survive in higher soil salinity conditions. Keywords: Salt cedar, biomass allocation, gas-exchange, fluorescence, photosystem II.

The Infiuences of Ozone and Soil Water Stress, Singly and in Combination, on Leaf Gas Exchange Rates, Leaf Ultrastructural Characteristics and Annual Ring Width of Fagus crenata Seedlings

The Infiuences of Ozone and Soil Water Stress, Singly and in Combination, on Leaf Gas Exchange Rates, Leaf Ultrastructural Characteristics and Annual Ring Width of Fagus crenata Seedlings

Leaf δ18O of remaining trees is affected by thinning intensity in a semiarid pine forest

Silvicultural thinning usually improves the water status of remaining trees in water-limited forests. We evaluated the usefulness of a dual stable isotope approach (δ¹³C, δ¹⁸O) for comparing the physiological performance of remaining trees between forest stands subjected to two different thinning intensities (moderate versus heavy) in a 60-year-old Pinus halepensis Mill. plantation in semiarid southeastern Spain. We measured bulk leaf δ¹³C and δ¹⁸O, foliar elemental concentrations, stem water content, stem water δ¹⁸O (δ¹⁸O(stem water)), tree ring widths and leaf gas exchange rates to assess the influence of forest stand density on tree performance. Remaining trees in low-density stands (heavily thinned) showed lower leaf δ¹⁸O, and higher stomatal conductance (g(s)), photosynthetic rate and radial growth than those in moderate-density stands (moderately thinned). By contrast, leaf δ¹³C, intrinsic water-use efficiency, foliar elemental concentrations and δ¹⁸O(stem water) were unaffected by stand density. Lower foliar δ¹⁸O in heavily thinned stands reflected higher g(s) of remaining trees due to decreased inter-tree competition for water, whereas higher photosynthetic rate was largely attributable to reduced stomatal limitation to CO₂ uptake. The dual isotope approach provided insight into the early (12 months) effects of stand density manipulation on the physiological performance of remaining trees.

Simulation of evapotranspiration and carbon dioxide flux in the wheat‐maize rotation croplands of the North China Plain using the Simple Biosphere Model

AbstractThe North China Plain, which is critical for food production in China, is encountering serious water shortage due to heavy agricultural water requirement. The accurate modelling of carbon dioxide flux and evapotranspiration (ET) in croplands is thus essential for yield prediction and water resources management. The land surface model is powerful in simulating energy and carbon dioxide fluxes between land and atmosphere. Some key processes in the Simple Biosphere Model (Version 2, SiB2) were parameterized based on the observations. The simulated fluxes were tested against the eddy covariance flux measurements over two typical winter wheat/maize double cropping fields. A simple diagnostic parameterisation of soil respiration, not included in SiB2, was added and calibrated using the observations to model the carbon budget. The Ball‐Berry stomatal conductance model was calibrated using observed leaf gas exchange rate, showing that the original SiB2 could significantly underpredict the ET in the wheat field. SiB2 significantly underpredicted soil resistance at the Weishan site, leading to overpredict the ET. Overall, there was a close agreement between the simulated and observed latent heat fluxes and net CO2 exchange using the re‐parameterized SiB2. These findings are important when the model is used for the regional simulation in the North China Plain. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Ontogeny and leaf gas exchange mediate the carbon isotopic signature of herbaceous plants

Values (Δ(i)) predicted by a simplified photosynthetic discrimination model, based only on diffusion through air followed by carboxylation, are often used to infer ecological conditions from the ¹³C signature of plant organs (δ¹³C(p)). Recent studies showed that additional isotope discrimination (d that includes mesophyll conductance, photorespiration and day respiration, and post-carboxylation discrimination) can strongly affect δ¹³C(p); however, little is known about its variability during plant ontogeny for different species. Effect of ontogeny on leaf gas exchange rates, Δ(i) , observed discrimination (Δ(p)) and d in leaf, phloem and root of seven herbaceous species at three ontogenetic stages were investigated under controlled conditions. Functional group identity and ontogeny significantly affected Δ(i) and Δ(p). However, predicted Δ(i) did not match Δ(p). d, strongly affected by functional group identity and ontogeny, varied by up to 14 ‰. d scaled tightly with stomatal conductance, suggesting complex controls including changes in mesophyll conductance. The magnitude of the changes in δ¹³C(p) due to ontogeny was similar to that due to environmental factors reported in other studies. d and ontogeny should, therefore, be considered in ecosystem studies, integrated in ecosystem models using δ¹³C(p) and limit the applicability of δ¹³C(leaf) as a proxy for water-use efficiency in herbaceous plants.

Steady and dynamic photosynthetic responses of seedlings from contrasting successional groups under low-light growth conditions

To test the hypothesis that leaf-level photosynthetic-related traits might confer late successionals a competitive advantage over early successionals in low-light growth conditions, steady photosynthetic assimilation- and dynamic photosynthetic induction-related traits were examined in low-light-grown seedlings with contrasting successional status. Compared with the early successionals, late successionals as a group significantly exhibited lower leaf gas exchange rates. While late successionals required a longer time to respond to simulated sunflecks, they had lower rates of induction losses after sunflecks. Such photosynthetic induction traits allowed late successionals to more effectively utilize subsequent sunflecks. It was observed that plants with lower gas exchange rates responded more slowly to simulated sunfelcks, but they had lower rates of induction losses after sunflecks. In addition, the rate of response to sunflecks was positively correlated with the rate of induction loss after sunflecks across the successional status of species. A principal components analysis (PCA) demonstrated that early and late successionals were separated along the first axis of the PCA, and that early successionals were grouped on the right and were associated with higher gas exchange rates, fast responses to sunflecks, and rapid rates of induction loss after sunflecks; late successionals held an opposite pattern. Overall, our results suggest that smaller respiratory carbon losses and lower metabolic costs give late successionals a competitive advantage in low-light growth conditions, that late successionals have an advantage over early successionals in utilizing sunflecks, and thus that the successional status of species are mainly associated with the leaf-level photosynthetic-related traits.

Effects of ozone on growth, yield and leaf gas exchange rates of four Bangladeshi cultivars of rice ( Oryza sativa L.)

To assess the effects of tropospheric O 3 on rice cultivated in Bangladesh, four Bangladeshi cultivars (BR11, BR14, BR28 and BR29) of rice ( Oryza sativa L.) were exposed daily to charcoal-filtered air or O 3 at 60 and 100 nl l −1 (10:00–17:00) from 1 July to 28 November 2008. The whole-plant dry mass and grain yield per plant of the four cultivars were significantly reduced by the exposure to O 3. The exposure to O 3 significantly reduced net photosynthetic rate of the 12th and flag leaves of the four cultivars. The sensitivity to O 3 of growth, yield and leaf gas exchange rates was not significantly different among the four cultivars. The present study suggests that the sensitivity to O 3 of yield of the four Bangladeshi rice cultivars is greater than that of American rice cultivars and is similar to that of Japanese rice cultivars and that O 3 may detrimentally affect rice production in Bangladesh.

Response to dehydration and irrigation in invasive and native saplings: osmotic adjustment versus leaf shedding

To clarify the mechanism underlying successful invasion by tree species into xeric sites on Japan's Bonin Islands, we compared the water use of an alien species, Psidium cattleianum, which is rapidly expanding on ridge sites with shallow soil, with that of a native species, Trema orientalis. We hypothesized that there is a trade-off between leaf shedding with low cavitation resistance (frequent xylem cavitation plus refilling ability) and leaf osmotic adjustment with high cavitation resistance (cessation of xylem cavitation plus canopy leaf retention), indicating contrasting strategies for drought tolerance and water use in semi-arid regions. We examined leaf turnover, leaf gas exchange, leaf water potential and water distribution in stem xylem conduits using cryo-scanning electron microscopy for the saplings of both species under three cycles of artificial drought and sudden pulse irrigation. Invasive P. cattleianum saplings were highly resistant to cavitation in stem xylem conduits, retained their leaves and exhibited effective leaf osmotic adjustment under the drought treatment. In contrast, native T. orientalis saplings exhibited xylem cavitation, conspicuous leaf shedding and less effective leaf osmotic adjustment under the drought treatment. Leaf gas exchange rate recovered more rapidly in P. cattleianum saplings than in T. orientalis saplings immediately following pulse irrigation after a period without irrigation, especially in the first drought cycle. Embolized conduits in T. orientalis were refilled by pulse irrigation, and leaf gas exchange rate recovered following refilling. The two tree species showed contrasting strategies for drought tolerance and water use along a trade-off axis. Cavitation avoidance and effective leaf osmotic adjustment in P. cattleianum saplings under drought conditions partially support their survival at the xeric ridge sites on the Bonin Islands. Our results help to explain the success of P. cattleianum in its invasion of a sub-arid environment.

Effects of ozone on growth, yield and leaf gas exchange rates of two Bangladeshi cultivars of wheat ( Triticum aestivum L.)

To clarify the effects of O 3 on crop plants cultivated in Bangladesh, two Bangladeshi wheat cultivars (Sufi and Bijoy) were grown in plastic boxes filled with Andisol and exposed daily to charcoal-filtered air or O 3 at 60 and 100 nl l −1 (10:00-17:00) from 13 March to 4 June 2008. The whole-plant dry mass and grain yield per plant of the two cultivars at the final harvest were significantly reduced by the exposure to O 3. Although there was no significant effect of O 3 on stomatal diffusive conductance to H 2O of flag leaf, net photosynthetic rate of the leaf was significantly reduced by the exposure to O 3. The sensitivity of growth, yield, yield components and leaf gas exchange rates to O 3 was not significantly different between the two cultivars. The results obtained in the present study suggest that ambient levels of O 3 may detrimentally affect wheat production in Bangladesh.

The role of photosynthesis in the recruitment of juvenile Quercus gambelii into mature Q. gambelii communities

Abstract Previous work with Quercus gambelii (Gambel's oak), a widespread oak species from the mountains of the western United States and northern Mexico, showed high juvenile density (< 3 cm in DBH [diameter at breast height] or less than 150 cm in height), but few or no small (3–5 cm in DBH) tree sized plants, suggesting a recruitment bottleneck. We postulate that there could be several factors preventing the recruitment of juveniles into the adult population, including reduced light levels, herbivory, competition for soil resources, or a combination of these factors. In this study, we evaluated the response of leaves of Q. gambelii to various light levels. Surface light levels and leaf gas exchange rates were measured for sun (open) and shade (understory) Q. gambelii seedlings in Q. gambelii communities in the Lincoln National Forest, New Mexico, USA. Mean daily photosynthetic flux densities (PFD) in the understory during the monsoon period (wet season) were 40 ± 46 µmol m−2 sec−1, or 6% of the open co...