Differences In Photosynthetic Responses Research Articles

Impact of photoluminescent carbon quantum dots on photosynthesis efficiency of rice and corn crops

Photosynthesis is one of the most fundamental biochemical processes on earth such that it is vital to the existence of most lives on this planet. In fact, unravelling the potentials in enhancing photosynthetic efficiency and electron transfer process, which are thought to improve plant growth is one of the emerging approaches in tackling modern agricultural shortcomings. In light of this, zero-dimensional carbon quantum dots (CQD) have emerged and garnered much interest in recent years which can enhance photosynthesis by modulating the associated electron transfer process. In this work, CQD was extracted from empty fruit bunch (EFB) biochar using a green acid-free microwave method. The resulting CQD was characterized using HRTEM, PL, UV–Vis and XPS. Typical rice (C3) and corn (C4) crops were selected in the present study in order to compare the significant effect of CQD on the two different photosynthetic pathways of crops. CQD was first introduced into crop via foliar spraying application instead of localised placement of CQD before seedling development. The influence of CQD on the photosynthetic efficiency of rice (C3) and corn (C4) leaves was determined by measuring both carbon dioxide conversion and the stomatal conductance of the leaf. As a result, the introduction of CQD greatly enhanced the photosynthesis in CQD-exposed crops. This is the first study focusing on phylogenetically constrained differences in photosynthetic responses between C3 and C4 crops upon CQD exposure, which gives a better insight into the understanding of photosynthesis process and shows considerable promise in nanomaterial research for sustainable agriculture practices.

Impact of K deficiency on leaves and siliques photosynthesis via metabolomics in Brassica napus

Potassium (K) deficiency is known for its impact on photosynthesis via the synthetic constraints of CO2 diffusion and assimilation. However, insights in the different responses of net photosynthetic rate (A) to K depletion inside primary metabolites, produced specifically by each organ, remain to be elucidated. Leaves and siliques of Brassica napus were investigated to clarify the responses of A, the CO2 conductance and that of main metabolism pathways under K deficiency. Leaves were far more sensitive to K deficiency, displaying simultaneous decreases in A and CO2 diffusion reflected in a broader range of metabolites. To compensate the carbon starvation under K deficiency, leaf metabolic profile was adaptively regulated thorough downregulation of the carbon flux into carbohydrates and organic acids and upregulation of nitrogen-rich amino acids. Significant relationships between leaf A and sugars, organic acids and amino acids were observed. In comparison with leaves, the carbon shortage observed in siliques was negligible or even absent, with no obvious change in carbon flux through the glycolysis and tricarboxylic acid cycle pathways. Proline represented the only metabolite that correlated positively with silique A. These results provide new insights into differences in photosynthetic responses to K deficiency between leaf and non-leaf organs based on the potential metabolic processes involved.

Parameters of electrical signals and photosynthetic responses induced by them in pea seedlings depend on the nature of stimulus.

Local damage induces generation and propagation of variation potentials (VPs) that affect physiological processes in plants. The aims of the work presented here were to investigate parameters of VP induced by burning, heating and mechanical injury in pea seedlings, and to undertake a theoretical analysis of the mechanisms underlying the differences in VP parameters and a study of the photosynthetic responses caused by VPs induced by the damaging factors. The velocity of propagation of burn-induced VP decreased with distance from the damaged area whereas the velocities of heating- and injury-induced VPs were constant. The amplitudes of burn- and heating-induced VPs did not depend on distance whereas the amplitude of VP induced by mechanical injury decreased. VP propagation has been simulated on the basis of wound substance spread. The simulation revealed two possible ways of wound substance propagation: turbulent diffusion from the damaged area and secondary active production in intact cells. The photosynthetic response (decrease in the quantum yield of PSII and raising the level of non-photochemical fluorescence quenching (NPQ)) developed in case of VP entering the intact leaf under heating and burn but was not registered after mechanical injury. An increase in NPQ level was biphasic under burn in comparison with a single-phase one under heating, and the NPQ amplitude was slightly higher under burn. We suggest that differences in photosynthetic responses may be determined by the parameters of VPs induced by stimuli of different nature.

Differences in photosynthetic responses of NADP-ME type C4 species to high light

Main conclusionThree species chosen as representatives of NADP-ME C4 subtype exhibit different sensitivity toward photoinhibition, and great photochemical differences were found to exist between the species. These characteristics might be due to the imbalance in the excitation energy between the photosystems present in M and BS cells, and also due to that between species caused by the penetration of light inside the leaves. Such regulation in the distribution of light intensity between M and BS cells shows that co-operation between both the metabolic systems determines effective photosynthesis and reduces the harmful effects of high light on the degradation of PSII through the production of reactive oxygen species (ROS).We have investigated several physiological parameters of NADP-ME-type C4 species (e.g., Zea mays, Echinochloa crus-galli, and Digitariasanguinalis) grown under moderate light intensity (200 µmol photons m−2 s−1) and, subsequently, exposed to excess light intensity (HL, 1600 µmol photons m−2 s−1). Our main interest was to understand why these species, grown under identical conditions, differ in their responses toward high light, and what is the physiological significance of these differences. Among the investigated species, Echinochloa crus-galli is best adapted to HL treatment. High resistance of the photosynthetic apparatus of E. crus-galli to HL was accompanied by an elevated level of phosphorylation of PSII proteins, and higher values of photochemical quenching, ATP/ADP ratio, activity of PSI and PSII complexes, as well as integrity of the thylakoid membranes. It was also shown that the non-radiative dissipation of energy in the studied plants was not dependent on carotenoid contents and, thus, other photoprotective mechanisms might have been engaged under HL stress conditions. The activity of the enzymes superoxide dismutase and ascorbate peroxidase as well as the content of malondialdehyde and H2O2 suggests that antioxidant defense is not responsible for the differences observed in the tolerance of NADP-ME species toward HL stress. We concluded that the chloroplasts of the examined NADP-ME species showed different sensitivity to short-term high light irradiance, suggesting a role of other factors excluding light factors, thus influencing the response of thylakoid proteins. We also observed that HL affects the mesophyll chloroplasts first hand and, subsequently, the bundle sheath chloroplasts.

Photosynthetic Light Responses May Explain Vertical Distribution of Hymenophyllaceae Species in a Temperate Rainforest of Southern Chile.

Some epiphytic Hymenophyllaceae are restricted to lower parts of the host (<60 cm; 10–100 μmol photons m-2 s-1) in a secondary forest of Southern Chile; other species occupy the whole host height (≥10 m; max PPFD >1000 μmol photons m-2 s-1). Our aim was to study the photosynthetic light responses of two Hymenophyllaceae species in relation to their contrasting distribution. We determined light tolerance of Hymenoglossum cruentum and Hymenophyllum dentatum by measuring gas exchange, PSI and PSII light energy partitioning, NPQ components, and pigment contents. H. dentatum showed lower maximum photosynthesis rates (Amax) than H. cruentum, but the former species kept its net rates (An) near Amax across a wide light range. In contrast, in the latter one, An declined at PPFDs >60 μmol photons m-2 s-1. H. cruentum, the shadiest plant, showed higher chlorophyll contents than H. dentatum. Differences in energy partitioning at PSI and PSII were consistent with gas exchange results. H. dentatum exhibited a higher light compensation point of the partitioning of absorbed energy between photochemical Y(PSII) and non-photochemical Y(NPQ) processes. Hence, both species allocated energy mainly toward photochemistry instead of heat dissipation at their light saturation points. Above saturation, H. cruentum had higher heat dissipation than H. dentatum. PSI yield (YPSI) remained higher in H. dentatum than H. cruentum in a wider light range. In both species, the main cause of heat dissipation at PSI was a donor side limitation. An early dynamic photo-inhibition of PSII may have caused an over reduction of the Qa+ pool decreasing the efficiency of electron donation to PSI. In H. dentatum, a slight increase in heat dissipation due to acceptor side limitation of PSI was observed above 300 μmol photons m-2s-1. Differences in photosynthetic responses to light suggest that light tolerance and species plasticity could explain their contrasting vertical distribution.

Adaptation of Trifolium repens × T. uniflorum hybrid clovers to drought stress

Abstract White clover (Trifolium repens L.) is valued for its contribution to pasture quality and utilisation by animals, compatibility with grass, and fixation of nitrogen. However, it is limited by poor adaptation to drought. Hybridisation with Trifolium uniflorum L. may have potential to improve the drought resistance of white clover. An experiment in a rain shelter facility with contrasting moisture treatments, and a field evaluation under dryland conditions, were used to compare the agronomic potential of these interspecific hybrids (ISH) with white clover in moisture limited conditions. In the rain shelter experiment, there were smaller effects of water stress on shoot dry weight (DW), leaf area, internode length and senescence of first backcross generation hybrids compared with white clover and second backcross generation hybrids. Differences in photosynthetic responses were possibly influenced by the effect of root DW allocation on water uptake. In the field evaluation, growth scores of a wider range of hybrid families during summer moisture stress concurred with the results under water stress in the rain shelter. Growth of some ISH families outperformed the best white clover cultivars, particularly in the third and most stressful summer and this result was a key performance indicator of the value of the hybrids for drought prone areas. These findings using early, unselected, hybrid populations indicate the potential for further selection of elite, adapted cultivars from ISH breeding strategies. Keywords: Interspecific clover hybrid, drought resistance, rain shelter, field evaluation

Photosynthetic sensitivity to drought varies among populations of Quercus ilex along a rainfall gradient.

Drought frequency and intensity are expected to increase in the Mediterranean as a consequence of global climate change. To understand how photosynthetic capacity responds to long-term water stress, we measured seasonal patterns of stomatal (SL), mesophyll (MCL) and biochemical limitations (BL) to net photosynthesis (Amax) in three Quercus ilex (L.) populations from sites differing in annual rainfall. In the absence of water stress, stomatal conductance (gs), maximum carboxylation capacity (Vcmax), photosynthetic electron transport rate (Jmax) and Amax were similar among populations. However, as leaf predawn water potential (Ψl,pd) declined, the population from the wettest site showed steeper declines in gs, Vcmax, Jmax and Amax than those from the drier sites. Consequently, SL, MCL and BL increased most steeply in response to decreasing Ψl,pd in the population from the wettest site. The higher sensitivity of Amax to drought was primarily the result of stronger stomatal regulation of water loss. Among-population differences were not observed when gs was used instead of Ψl,pd as a drought stress indicator. Given that higher growth rates, stature and leaf area index were observed at the wettest site, we speculate that hydraulic architecture may explain the greater drought sensitivity of this population. Collectively, these results highlight the importance of considering among-population differences in photosynthetic responses to seasonal drought in large scale process-based models of forest ecosystem function.

Influence of water deficit and genotype on photosynthetic activity, dry mass partitioning and grain yield changes of winter wheat

Pn-PAR response curves, gas exchange, dry mass partitioning and grain yield of four different genotypes of wheat were investigated in a randomized complete block design under different watering regimes (85, 55 and 25% FC). The results indicated that genotype differences existed in the adaptation response to water deficit which included change to growth strategies coupled with photo-physiological strategies. Water treatment was the dominating factor which limited plant growth, productivity and yield. As water deficit increased, stomatal conductance (gs), saturation irradiance (SI) and maximum net photosynthetic rate (Pmax) decreased, while irrigation water use efficiency (IWUE), compensation irradiance (CI) and dark respiration (R(D)) increased in all genotypes, with a more rapid decline in wet climate cultivars. Differences in photosynthetic responses existed in different climate genotypes at 55% FC. Reduction of Pn for the wet climate cultivar was mainly due to gs, while for the dry climate cultivar, Pn coupled with photosynthetic pigment increased. At 25% FC, water deficit decreased Pn attributed to non-stomatal limitations in four genotypes. Dry mass was reduced in droughted plants and the percentage increased in dry mass allocated to roots. Furthermore, the wet climate genotypes had the greatest decrease in Chla/b ratio. These findings demonstrated that the superior drought resistance in dry climate cultivars could be attributed to higher photosynthetic capacity, dry matter allocation and then grain yield.

Photosynthetic Consequences of Late Leaf Spot Differ between Two Peanut Cultivars with Variable Levels of Resistance

ABSTRACTLate leaf spot (LLS) caused by Cercosporidium personatum (Berk. &amp; Curt.) Deighton reduces leaf CO2 assimilation rate (Asat) and accelerates leaf defoliation, which together lead to major reductions in peanut (Arachis hypogaea L.) yield worldwide. This study was conducted to determine whether differences in photosynthetic response to LLS severity exist among peanut cultivars of differing resistance. Field experiments were conducted in 2008 and 2009 to study the effects of LLS on Asat of tagged leaf cohorts, and photosynthetic response of similar age leaves to LLS in peanut cultivars with more (York) and less (Carver) quantitative resistance. A nonlinear model, y = (1 − x)β was used to analyze Asat data, where y is relative Asat, x is measured visual lesion area, and β represents the relationship between virtual and visual lesion area. Progression of LLS severity on leaf cohorts was slower in York than in Carver. However, the reduction in Asat with leaf cohort age was similar across the cultivars. This paradox could be explained by a higher β value in York (4.6) than in Carver (3.6), indicating a greater relative reduction in Asat beyond the necrotic lesion area in York. This greater reduction in Asat in York compared to Carver was most closely related to a reduction in maximum carboxylation velocity. Results indicated that future efforts to improve LLS resistance should include sustaining Asat in response to LLS infection along with slower disease progress.

Elevated CO2 affects photosynthetic responses in canopy pine and subcanopy deciduous trees over 10 years: a synthesis from Duke FACE

AbstractLeaf responses to elevated atmospheric CO2 concentration (Ca) are central to models of forest CO2 exchange with the atmosphere and constrain the magnitude of the future carbon sink. Estimating the magnitude of primary productivity enhancement of forests in elevated Ca requires an understanding of how photosynthesis is regulated by diffusional and biochemical components and up‐scaled to entire canopies. To test the sensitivity of leaf photosynthesis and stomatal conductance to elevated Ca in time and space, we compiled a comprehensive dataset measured over 10 years for a temperate pine forest of Pinus taeda, but also including deciduous species, primarily Liquidambar styraciflua. We combined over one thousand controlled‐response curves of photosynthesis as a function of environmental drivers (light, air Ca and temperature) measured at canopy heights up to 20 m over 11 years (1996–2006) to generate parameterizations for leaf‐scale models for the Duke free‐air CO2 enrichment (FACE) experiment. The enhancement of leaf net photosynthesis (Anet) in P. taeda by elevated Ca of +200 μmol mol−1 was 67% for current‐year needles in the upper crown in summer conditions over 10 years. Photosynthetic enhancement of P. taeda at the leaf‐scale increased by two‐fold from the driest to wettest growing seasons. Current‐year pine foliage Anet was sensitive to temporal variation, whereas previous‐year foliage Anet was less responsive and overall showed less enhancement (+30%). Photosynthetic downregulation in overwintering upper canopy pine needles was small at average leaf N (Narea), but statistically significant. In contrast, co‐dominant and subcanopy L. styraciflua trees showed Anet enhancement of 62% and no Anet–Narea adjustments. Various understory deciduous tree species showed an average Anet enhancement of 42%. Differences in photosynthetic responses between overwintering pine needles and subcanopy deciduous leaves suggest that increased Ca has the potential to enhance the mixed‐species composition of planted pine stands and, by extension, naturally regenerating pine‐dominated stands.

Photosynthetic and growth responses of reciprocal hybrids to variation in water and nitrogen availability

Fitness of plant hybrids often depends upon the environment, but physiological mechanisms underlying the differential responses to habitat are poorly understood. We examined physiological responses of Ipomopsis species and hybrids, including reciprocal F(1)s and F(2)s, to variation in soil moisture and nitrogen. • To examine responses to moisture, we subjected plants to a dry-down experiment. Nitrogen was manipulated in three independent experiments, one in the field and two in common environments. • Plants with I. tenuituba cytoplasmic background had lower optimal soil moisture for photosynthesis, appearing better adapted to dry conditions, than plants with I. aggregata cytoplasm. This result supported a prediction from prior studies. The species and hybrids did not differ greatly in physiological responses to nitrogen. An increase in soil nitrogen increased leaf nitrogen, carbon assimilation, integrated water-use efficiency, and growth, but the increases in growth were not mediated primarily by an increase in photosynthesis. In neither the field, nor in common-garden studies, did physiological responses to soil nitrogen differ detectably across plant types, although only I. aggregata and hybrids increased seed production in the field. • These results demonstrate differences in photosynthetic responses between reciprocal hybrids and suggest that water use is more important than nitrogen in explaining the relative photosynthetic performance of these hybrids compared to their parents.

Physiological and environmental regulation of interannual variability in CO2 exchange on rangelands in the western United States

AbstractFor most ecosystems, net ecosystem exchange of CO2 (NEE) varies within and among years in response to environmental change. We analyzed measurements of CO2 exchange from eight native rangeland ecosystems in the western United States (58 site‐years of data) in order to determine the contributions of photosynthetic and respiratory (physiological) components of CO2 exchange to environmentally caused variation in NEE. Rangelands included Great Plains grasslands, desert shrubland, desert grasslands, and sagebrush steppe. We predicted that (1) week‐to‐week change in NEE and among‐year variation in the response of NEE to temperature, net radiation, and other environmental drivers would be better explained by change in maximum rates of ecosystem photosynthesis (Amax) than by change in apparent light‐use efficiency (α) or ecosystem respiration at 10 °C (R10) and (2) among‐year variation in the responses of NEE, Amax, and α to environmental drivers would be explained by changes in leaf area index (LAI). As predicted, NEE was better correlated with Amax than α or R10 for six of the eight rangelands. Week‐to‐week variation in NEE and physiological parameters correlated mainly with time‐lagged indices of precipitation and water‐related environmental variables, like potential evapotranspiration, for desert sites and with net radiation and temperature for Great Plains grasslands. For most rangelands, the response of NEE to a given change in temperature, net radiation, or evaporative demand differed among years because the response of photosynthetic parameters (Amax, α) to environmental drivers differed among years. Differences in photosynthetic responses were not explained by variation in LAI alone. A better understanding of controls on canopy photosynthesis will be required to predict variation in NEE of rangeland ecosystems.

Photosynthetic and morphological responses of eelgrass ( Zostera marina L.) to a gradient of light conditions

Zostera marina L. (eelgrass) from Great Bay Estuary, New Hampshire and Maine (USA), was transplanted in outdoor mesocosms and subjected to four light treatments (100, 58, 34 and 11% surface irradiance, SI) between May and September 2003 to investigate the relationship between light availability and the growth and survival of eelgrass. Evaluating eelgrass seedlings and adult mature plants demonstrated no differences in photosynthetic response after 22 days of acclimation. During at least the first 19 days of shading, maximum electron transport rate (ETR max) rate of eelgrass did not differ significantly between light treatments. After 40 days, a significant reduction in ETR max and minimum saturating light was observed in plants growing at 34% SI and below. Morphological responses exhibited a linear increasing trend with greater light. 34% SI exhibited drastic reductions (to less than 25% of control) in rhizome growth, shoot density, shoot production, number of nodes per plant and plant weight at the end of the study (81 days). Shoot to root ratio at 34% SI increased by > 50%. Plants shaded to 58% SI showed no significant difference from the control in plant parameters except an increased rate of rhizome elongation. Our results link the lower shoot densities with shading to the slow growth rate of horizontal rhizomes and a total lack of lateral expansion at 11% SI. ETR max declined over time in plants at 11% SI resulting in 81% mortality, no lateral branching and no morphological development, indicating that the minimum light required for long-term eelgrass growth and survival is greater than the previously suggested 11% SI. We demonstrate that eelgrass plants at these latitudes can persist at light levels of 58% SI and above, and are light-limited at 34% SI and below.

Photosynthetic responses of subtidal seagrasses to a daily light cycle in Torres Strait: A comparative study

In this study, we examined the photosynthetic responses of five common seagrass species from a typical mixed meadow in Torres Strait at a depth of 5–7 m using pulse amplitude modulated (PAM) fluorometry. The photosynthetic response of each species was measured every 2 h throughout a single daily light cycle from dawn (6 am) to dusk (6 pm). PAM fluorometry was used to generate rapid light curves from which measures of electron transport rate (ETR max ), photosynthetic efficiency ( α), saturating irradiance ( E k ) and light-adapted quantum yield (Δ F/ F′ m ) were derived for each species. The amount of light absorbed by leaves (absorption factor) was also determined for each species. Similar diurnal patterns were recorded among species with 3–4 fold increases in maximal electron rate from dawn to midday and a maintenance of ETR max in the afternoon that would allow an optimal use of low light by all species. Differences in photosynthetic responses to changes in the daily light regime were also evident with Syringodium isoetifolium showing the highest photosynthetic rates and saturating irradiances suggesting a competitive advantage over other species under conditions of high light. In contrast Halophila ovalis, Halophila decipiens and Halophila spinulosa were characterised by comparatively low photosynthetic rates and minimum light requirements (i.e. low E k ) typical of shade adaptation. The structural makeup of each species may explain the observed differences with large, structurally complex species such as Syringodium isoetifolium and Cymodocea serrulata showing high photosynthetic effciciencies ( α) and therefore high-light-adapted traits (e.g. high ETR max and E k ) compared with the smaller Halophila species positioned lower in the canopy. For the smaller Halophila species these shade-adapted traits are features that optimise their survival during low-light conditions. Knowledge of these characteristics and responses improves our understanding of the underlying causes of changes in seagrass biomass, growth and survival that occur when modifications in light quantity and quality arise from anthropogenic and climatic disturbances that commonly occur in Torres Strait.

Relationship between salt tolerance and photosynthetic machinery performance in citrus

In citrus, salt stress has been related to the build up of chloride ions in plant tissues that affect photosynthesis, growth and yield. We investigated the effects of salt stress on the stability of the photosynthetic machinery with respect to the relative salt tolerance of different citrus genotypes including: Swingle Citrumelo, Carrizo citrange, C35 citrange, Cleopatra mandarin and Forner-Alcaide #5. Under identical salt-stress conditions, Forner-Alcaide #5 and Cleopatra mandarin accumulated less chloride ions in leaves than the other genotypes and showed a better plant performance. Chlorophyll fluorescence parameters indicated severe impairments of photosynthetic activity in salt-sensitive Citrumelo and citranges but Cleopatra and Forner-Alcaide #5 were less affected. In addition, differences in photosynthetic responses between these two moderately tolerant genotypes suggested different strategies to cope with salinity. The high tolerance to salinity shown by Forner-Alcaide #5 can be associated to the ability of keeping an active photosynthetic system at elevated saline conditions whereas the tolerance of Cleopatra was linked to rapid reductions of net photosynthetic rate, stomatal conductance, performance of PSII and photosynthetic efficiency.

Cutting‐edge international tree research in New Phytologist

Cutting‐edge international tree research in <i>New Phytologist</i>

Fluorescence and photosynthetic competency in single eggs and embryos of Ascophyllum nodosum (Phaeophyceae)

K.Y. Kim, H.J. Jeong, H.P. Main and D.J. Garbary. 2006. Fluorescence and photosynthetic competency in single eggs and embryos of Ascophyllum nodosum (Phaeophyceae). Phycologia 45: 331–336. DOI: 10.2216/05-40.1Pulse amplitude modulation (PAM) fluorometry was used in 2004 and 2005 to measure chlorophyll α fluorescence associated with photosystem II in eggs and embryos of Ascophyllum nodosum (Linnaeus) Le Jolis from the Atlantic coast of Nova Scotia. Fucoid eggs and embryos were maintained in the laboratory for 8 days over which fluorescence measurements were taken. Newly released eggs saturated at about 20 μmol photons m−2 s−1 with maximum electron transport rate (rETRmax) declining to <3 after 3 days. Embryos saturated at about 50 μmol photons m−2 s−1 and rETRmax increased to >11 by day 3. Although some egg senescence was apparent by day 3, some eggs continued to generate a strong photosynthetic signal throughout the 8-day experimental period, even though rETRmax was reduced. Differences in photosynthetic responses associated with fertilisation were apparent in Fv/Fm, within 24 h, and in rETRmax after 2 days. Eggs showed relatively stable or declining responses, whereas embryos showed stable or increasing responses. By day 8, quantum yield of embryos was similar to values reported elsewhere for mature fucoids.

Precipitation pulse use by an invasive woody legume: the role of soil texture and pulse size

Plant metabolic activity in arid and semi-arid environments is largely tied to episodic precipitation events or "pulses". The ability of plants to take up and utilize rain pulses during the growing season in these water-limited ecosystems is determined in part by pulse timing, intensity and amount, and by hydrological properties of the soil that translate precipitation into plant-available soil moisture. We assessed the sensitivity of an invasive woody plant, velvet mesquite (Prosopis velutina Woot.), to large (35 mm) and small (10 mm) isotopically labeled irrigation pulses on two contrasting soil textures (sandy-loam vs. loamy-clay) in semi-desert grassland in southeastern Arizona, USA. Predawn leaf water potential (psi(pd)), the isotopic abundance of deuterium in stem water (deltaD), the abundance of 13C in soluble leaf sugar (delta13C), and percent volumetric soil water content (theta(v)) were measured prior to irrigation and repeatedly for 2 weeks following irrigation. Plant water potential and the percent of pulse water present in the stem xylem indicated that although mesquite trees on both coarse- and fine-textured soils quickly responded to the large irrigation pulse, the magnitude and duration of this response substantially differed between soil textures. After reaching a maximum 4 days after the irrigation, the fraction of pulse water in stem xylem decreased more rapidly on the loamy-clay soil than the sandy-loam soil. Similarly, on both soil textures mesquite significantly responded to the 10-mm pulse. However, the magnitude of this response was substantially greater for mesquite on the sandy-loam soil compared to loamy-clay soil. The relationship between psi(pd) and delta13C of leaf-soluble carbohydrates over the pulse period did not differ between plants at the two sites, indicating that differences in photosynthetic response of mesquite trees to the moisture pulses was a function of soil water availability within the rooting zone rather than differences in plant biochemical or physiological constraints. Patterns of resource acquisition by mesquite during the dynamic wetting-drying cycle following rainfall pulses is controlled by a complex interaction between pulse size and soil hydraulic properties. A better understanding of how this interaction affects plant water availability and photosynthetic response is needed to predict how grassland structure and function will respond to climate change.

High‐Temperature Effects on Photosynthetic Processes in Temperate and Tropical Cereals

High temperature limits productivity of crops in many regions of the world, but the physiological basis of tolerance to the stress is not known. Our objective was to determine the relationship between photosynthetic responses to high temperature at the whole‐plant level and sensitivity of light reactions, the most labile systems, to brief heat treatment. Wheat (Triticum aestivum L.), a C3 species adapted to cool environments; rice (Oryza sativa L.), a C3 species adapted to warm environments; and millet [Pennesitum glaucum (L.) R. Br.], a C4 species adapted to hot environments, were compared. Photosynthesis was measured in plants grown at 22, 32, or 42°C, and light reactions were measured in protoplasts, chloroplasts, and thylakoids isolated from seedlings grown at 27° C and treated in vitro at 22, 32, and 42°C. Leaf photosynthesis of millet and rice increased from 22 to 32°C and then decreased to 42°C, whereas in wheat it was highest at 22°C and decreased as temperature increased. Photosynthetic rates of protoplasts and chloroplasts from all species decreased after being treated in vitro over the same temperature range. Photosystem II (PSII) activity declined steadily in protoplasts, chloroplasts, and thylakoids of millet and rice from 22 to 42°C but decreased abruptly in organelles of wheat from 32 to 42°C. The results suggest that differences in photosynthetic responses to high temperature are associated with light reactions, and extreme sensitivity of wheat may be attributable to injury to PSII.

Comparison of the response of red oak ( Quercus rubra) seedlings and mature trees to ozone exposure using simulation modeling

Field studies have determined that the photosynthetic rates of mature northern red oak ( Quercus rubra L.) trees are sensitive to ozone exposure whereas those of seedlings are not. We used a model of tree physiology to determine the consequences of these differences in photosynthetic response to carbon allocation and tissue growth in seedlings and mature trees over a 2-year period. In the seedling simulations at twice ambient ozone, only the total non-structural carbohydrate (TNC) storage pool was affected. The effects in the simulated mature tree were much greater, with large decreases predicted for TNC, fine root, leaf, stem, branch, and coarse root tissues. The model produced many ozone-induced responses in the mature tree that were similar or consistent with observations made in a field study, but the simulations overestimated the effect of twice ambient ozone on root TNC and growth. The discrepancy between field and simulated results suggests that the field study trees exposed to elevated ozone levels may use carbon at a reduced rate, particularly through reduced respiration.