Leaflet photosynthesis rate and carbon metabolite accumulation patterns in nitrogen-limited, vegetative soybean plants

Prolonged inorganic nitrogen (NO₃ ^–+NH₄ ⁺) limitation of non-N₂-fixing soybean plants affected leaflet photosynthesis rates, photosynthate accumulation rates and levels, and anaplerotic carbon metabolite levels. Leaflets of nitrogen-limited (N-Lim), 27–31-day-old plants displayed 15 to 23% lower photosynthesis rates than leaflets of nitrogen-sufficient (N-Suff) plants. In contrast, N-Lim plant leaflets displayed higher sucrose and starch levels and rates of accumulation, as well as higher levels of carbon metabolites associated with sucrose and starch synthesis, e. g., glycerate-3-phosphate and glucose phosphates, than N-Suff plant leaflets. Concurrently, levels of soluble protein, chlorophyll, and anaplerotic metabolites, e.g., malate and phosphoenolpyruvate, were lower in leaflets of N-Lim plants than N-Suff plants, suggesting that the enzymes of the anaplerotic carbon metabolite pathway were lower in activity in N-Lim plant leaflets. Malate net accumulation rates in the earliest part of the illumination period were lower in N-Lim than in N-Suff plant leaflets; however, by the midday period, malate accumulation rate in N-Lim plant leaflets exceeded that in leaflets of N-Suff plants. Further, soluble protein accumulation rates in leaflets of N-Suff and N-Lim plants were similar, and the rate of dark respiration, measured in the early part of the dark period, was higher in N-Lim plant leaflets than in N-Suff plant leaflets. It was concluded that during prolonged N-limitation, foliar metabolite conditions favored the channelling of a large proportion of the carbon assimilate into sucrose and starch, while assimilate flow through the anaplerotic pathway was diminished. However, in some daytime periods, there was a normal level of carbon assimilate channelled through the anaplerotic pathway for ultimate use in amino acid and protein synthesis.Abbreviations ADPG-PPiase ADPglucose pyrophosphorylase - Ce CO₂ in the leaf photosynthesis measuring cuvette - Ci leaf internal CO₂ during photosynthesis measurement - Chl chlorophyll - DHAP dihydroxyacetone phosphate - GAP glyceraldehyde-3-phosphate - G_sw stomatal conductance with units as mmol H₂O m^–2 s^–1 - G1P glucose-1-phosphate - G6P glucose-6-phosphate - F6P fructose-6-phosphate - FBP fructose-1,6-bisphosphate - FBPase-pH 8.1 chloroplastic fructose-1,6-bisP (C-1) phosphatase (pH 8.1) - MAL malate - N inorganic nitrogen, i.e. NO₃ ^–+NH₄ ⁺ (at levels and molar ratios indicated) - PE post-emergence - PEP phosphoenolpyruvate - PEPCase phosphoenolpyruvate carboxylase - PGA 3-phosphoglycerate - PYR pyruvate - PYR kinase pyruvate kinase - Pn net CO₂ photoassimilation in leaves - PPFD photosynthetic photon flux density - PPRC pentose phosphate reductive cycle - RuBP ribulose-1,5-bisphosphate; rubisco-ribulose-1,5-bisphosphate carboxylase/oxygenase - SLW specific leaf mass - SPS sucrose-6-phosphate synthase - TCA cycle tricarboxylic acid cycle; triose-P-DAP+GAP     

Carbohydrate metabolism in one- and two-year-old spruce needles, and stem carbohydrates from three months before until three months after bud break

Starch and sucrose metabolism of one- and two-year-old needles of Norway spruce (Picea abies [L.] Karst., about 30 years old) was investigated from three months before until three months after bud break at a natural site. We distinguish different metabolic states according to the extractable activities of enzymes (α-amylase [EC 3.2.1.1], ADP-glucose pyrophosphorylase [AGP, EC 2.7.7.27], D-enzyme [EC 2.4.1.25], starch phosphorylase [STP. EC 2.4.1.1]), sucrose phosphate synthase [SPS, EC 2.4.1.14], sucrose syntbase [SS, EC 2.4.1.13]. acid invertase [AI, EC 3.2.1.261) and pool sizes of related metabolites (starch, glucose, fructose, sucrose, raffinose, stachyose, fructose 6-phosphate [F6P], glucose 6-phosphate [G6P], fructose 2,6-bisphosphate [F26BP], and inorganic phosphate [P₁]). The period ending with bud break was characterized by high rates of net photosynthesis, a pronounced decrease in the amount of soluble sugars, and a steep rise in starch (from the detection limit to approximately 600 nmol glycosyl units [mg dry weight]_-1). In parallel, the extractable activity of AGP increased, while D-enzyme was on a relative high level when compared with the period after bud break. With respect to sucrose metabolism, F26BP, an inhibitor of sucrose synthesis, decreased from 1 to 0.4 pmol (mg dry weight)_-1. This was complemented by SPS activity, which was due to both increased protein levels shown by immunoblotting and activation under metabolite control (high levels of G6P and a low P_i/G6P ratio). This indicates a high capacity of synthesis of starch and sucrose in the period before bud break. These observations are in accordance with estimates of photosynthetic carbon gain, which indicate that in early spring large amounts of carbon from current photosynthesis are exported out of the needles. In addition, the content of nonstructural carbohydrates (expressed as hexoses) increased in the bark of the stem. This could also be a consequence of an enhanced carbon export from the needles. After the onset of bud break, starch concentration decreased in all tissues under investigation. In contrast, the level of total nonstructural carbohydrates in the outermost sapwood nearly doubled from bud break until the end of sampling. In the needles, net photosynthesis was reduced by about 75% and a decrease in SPS activity and protein level were found together with lower G6P concentration, and an increased P_i/G6P ratio. These results suggest that during that period sucrose synthesis was reduced in the older needles. In addition, under conditions of reduced photosynthesis, carbon demand of current year needles was in part ensured by the mobilization of starch in the older needles. Taken together our data show that before bud break carbon metabolism of mature leaves is related with the sink demands of storage organs. After bud break the accumulated assimilate pools in needles and stem, mainly the bark, are mobilized and support carbon supply to new tissues.     

Growth and maintenance respiration in leaves of northern red oak seedlings and mature trees after 3 years of ozone exposure

A two-component model of growth and maintenance respiration is used to study the response of northern red oak (Quercus rubra L.) seedlings and 32-year-old trees to sub-ambient (10 μmol h; cumulative dose based on 7 h daily mean), ambient (43 μmol h), and twice-ambient (85 μmolh) ozone. The relative growth rates (RGR) of leaves sampled from seedlings and trees were similar across treatments, as were specific leaf respiration rates (SRR). Growth coefficients estimated from the SRR versus RGR relationship averaged 25-3 mol CO2 kg^?1 leaf dry mass produced for seedlings and 21-5 mol kg^?1 for trees. Maintenance coefficients ranged from 0-89 to 1-07 mol CO₂ kg^?1 leaf dry mass d^?1 for seedlings and from 0-64 to 0-84 mol kg-1 d^?1 for trees. Neither coefficient was affected by ozone. Leaves sampled throughout the growing season also showed little response of respiration to ozone. This occurred despite a 30% reduction in net photosynthesis for trees grown at twice-ambient ozone. These results suggest that growth and maintenance respiration in young northern red oak leaves are not affected by ozone and that in older leaves injury can occur without a parallel increase in so-called ‘maintenance’ respiration.     

Photosynthesis and carbon export in white clover plants grown at various levels of phosphorus supply

Net photosynthesis, concurrent carbon export and starch, sucrose and inorganic phosphorus concentrations were measured in leaves of white clover ( Trifolium repens L. cv. Grasslands Huia) grown at four levels of phosphorus supply in the presence or absence of mineral nitrogen. The nitrogen treatments had no effect on growth, photosyntheis or carbon export. At the three higher levels of phosphorus supply, the amount of carbon exported was about 77% of net fixation. Photosynthesis and export per leaf decreased with phosphorus supply, primarily through the effect of phosphorus supply on leaf area. The rate of photosynthesis was reduced only at the lowest level of phosphorus supply.
Inorganic phosphorus rose with phosphorus supply but starch concentration was unaffected. Sucrose was reduced at the lowest level of phosphorus supply but not significantly affected at higher levels. The ratio between starch and sucrose concentration was also unaffected at the higher levels, but was increased at the lowest level of supply. There thus appeared to be direct effects of phosphorus supply on photosynthesis, partitioning of carbon to carbohydrates and, by implication, export, only at the lowest level of phosphorus supply. As leaf area and plant growth were affected over the whole range of phosphorus supply, factors other than photosynthesis per se must have determined the response of growth to phosphorus supply.     

Soluble acid invertase activity in leaves is independent of species differences in leaf carbohydrates, diurnal sugar profiles and paths of phloem loading

Leaf sucrose, starch, hexose and maximum extractable soluble acid invertase activity were compared throughout the day in source leaves of 13 plant species chosen for their putative phloem-loading type (apoplastic or symplastic). Four species which represent the different phloem-loading types (tomato, barley, maize and Fuchsia ) were studied in detail. Using this information we wished to determine whether a positive correlation between foliar carbohydrates and acid invertase activity exists in leaves from different species and, furthermore, whether this relationship is determined by phloem-loading type. Acid invertase activity was relatively constant throughout the day in all species. The extent of sucrose, hexose and starch accumulation and the sucrose: starch ratio measured at a given time were species-dependent. No correlations were found between foliar soluble acid invertase activity and the hexose, sucrose or starch content of the leaves in any of the species, regardless of phloem-loading type. The species examined could be divided into three distinct groups: (1) high sucrose, low invertase; (2) low sucrose, low invertase; and (3) low sucrose, high invertase. The absence of an inverse relationship between leaf sucrose, hexose or starch contents and endogenous soluble acid invertase suggests that this enzyme is not directly involved in carbon partitioning in leaves but serves an auxiliary function.     

Localisation of sucrose-phosphate synthase and starch in leaves of C4 plants

The activity and intercellular distribution of sucrose-phosphate synthase (SPS; EC 2.4.1.14) were determined in fully expanded leaves from a range of C₄ plants. In Zea mays L. and Atriplex spongiosa F. Muell., SPS was located almost exclusively in the mesophyll cells. In other species, SPS was found in both cell types, with the activity in the bundle sheath cells ranging from 5% of the total leaf activity in Echinochloa crus-galli (L.) Beauv. to 35% in Sorghum bicolor Moench. At the end of the light period, starch was found only in the bundle sheath cells in all of the species examined. There appears to be little correlation between C₄-acid decarboxylation type and the location of sucrose and starch synthesis in the leaves of C₄ plants. Received: 18 October 1996 / Accepted: 20 November 1996     

Comparison of the effect of rapidly and gradually developing water-stress on carbohydrate metabolism in spinach leaves

Abstract. The effect of water-stress on photosynthetic carbon metabolism in spinach ( Spinacia oleracea L.) has been studied in experiments in which water-stress was induced rapidly by floating leaf discs on sorbitol solutions or wilting detached leaves, and in experiments in which water-stress was allowed to develop gradually in whole plants as the soil dried out. In both short- and long-term water stress, the rate of photosynthesis in saturating CO₂ did not decrease until leaf water potential decreased below -1.0 MPa. However, at smaller water deficits there was already an inhibition of starch synthesis, while sucrose synthesis remained constant or increased. This change in partitioning was accompanied by an increase in activation of sucrose-phosphate synthase (revealed as an increase in activity assayed in the presence of low hexose-phosphate and inorganic phosphate, while the activity assayed with saturating hexosephosphates remained unaltered). Water-stressed leaves had a two- to three-fold higher sucrose content at the end of the night, and contained less starch than non-stressed leaves. When leaves were held in the dark, sucrose was mobilized initially, while starch was not mobilized until the sucrose had decreased to a low level; in water-stressed leaves, starch mobilization commenced at a two-fold higher sucrose content. It is concluded that water-stressed leaves maintain higher sucrose and lower starch levels than non-stressed leaves. This response is found in rapid and long-term stress, and represents an inherent response to water deficits.     

The response of sensitive and tolerant clones of Populus tremuloides to dynamic ozone exposure under controlled environmental conditions

Both sensitive and tolerant clones of aspen ( Populus tremuloides ) were exposed to ozone using four different exposure regimes under controlled environmental conditions. Based on data on ambient ozone from 10 cities in the USA, three treatments of 4-wk exposure to the same SUM06 (an accumulation of hourly O₃ concentrations greater than 0.06 ml l⁻¹) were constructed. The regimes allowed us to investigate: (a) the importance of long (3 wk, treatment 1) versus short (1 wk, treatment 2) duration of regimes with high peaks; (b) the effect of treatments with variable peak occurrence (treatments 1 and 2) versus uniform peak occurrence (treatment 3) during the exposure period. Nonfumigated control plants were maintained at ozone concentrations <10 nl l⁻¹. Bifacial black necrosis, a typical symptom of ozone injury on aspen leaves, occurred on both clones after 2 wk exposure. Up to 60% of the leaves on the sensitive clone were injured, with an average of 6% of total leaf area injured. In the tolerant clone only 10% of the leaves were injured, with less than 1% of the total leaf area symptomatic. The severity of injury was consistently greatest in treatment 2, followed by treatments 1 and 3, respectively. The interval between peak exposures was less important than the occurrence of peaks versus a stable maximum concentration. Premature leaf abscission occurred in the sensitive clone. Measures of gas exchange demonstrated reduced photosynthesis under ozone fumigation, but exposure regime was not a significant factor. Concentrations of two antioxidants, ascorbic acid and glutathione, were almost always greater in the resistant than in the sensitive clone, but the differences were not statistically significant. The levels of these antioxidants in aspen leaves did not change with ozone fumigation or leaf age.     

Short-term UV-B radiation and ozone exposure effects on aromatic secondary metabolite accumulation and shoot growth of flavonoid-deficient Arabidopsis mutants

The presence of UV-absorptive substances in the epidermal cells of leaves is thought to protect mesophyll tissues from the harmful effects of UV-B radiation. We examined the influence of short-term UV-B exposures on UV-absorptive (330 nm) sinapates and flavonols, and on shoot growth of the Arabidopsis wild type ecotype Landsberg erecta and two mutants. 114 deficient in chalcone synthase, and 115 , deficient in chalcone/flavonone isomerase. Sequential ozone exposures were used to determine the effects of oxidative stress The levels of sinapates and flavonols on a leaf fresh weight basis increased substantially in the wild type and sinapates increased in the 114 mutant in vegetative vegetative/reproductive transitional and reproductive stage plants in response to short-term (48h) UV-B radiation. When UV-B was discontinued the levels generally decreased lo pre-exposure levels after 48 h in vegetative/reproductive but not in reproductive plants. Exposure to ozone before or alter UV-B treatment did not consistently affect the levels of these UV-absorptive compounds. Dry matter accumulation was less affected by UV-B at the vegetative and reproductive stages than at the vegetative/reproductive stage. At the vegetative/reproductive stage, shoot growth of all 3 genotypes was retarded by UV-B. Growth was not retarded by short-term ozone exposure alone but when exposure to ozone followed UV-B exposure, growth was reduced in all genotypes. Leaf cupping appeared on 115 plants exposed to UV-B.     

Crop responses to ozone: uptake, modes of action, carbon assimilation and partitioning

The inhibitory effects of tropospheric O₃ on crop photosynthesis, growth, and yield have been documented in numerous studies over the past 35 years. In large part, the results of this research supported governmental regulations designed to limit tropospheric O₃ levels to concentrations that affected crop production at economically acceptable levels. Recent studies have brought into question the efficacy of these concentration-based O₃ standards compared with flux-based approaches that incorporate O₃ uptake along with environmental and biotic factors that influence plant responses. In addition, recent studies provide insight into the biochemical mechanisms of O₃ injury to plants. Current interpretations suggest that upon entry into the leaf intercellular space O₃ rapidly reacts with components of the leaf apoplast to initiate a complex set of responses involving the formation of toxic metabolites and generation of plant defence responses that constitute variably effective countermeasures. Plant species and cultivars exhibit a range of sensitivity to O₃, evident as heritable characteristics, that must reflect identifiable biochemical and molecular processes that affect sensitivity to O₃ injury, although their exact makeup remains unclear. Ozone clearly impairs photosynthetic processes, which might include the effects on electron transport and guard cell homeostasis as well as the better-documented effects on carbon fixation via decreased Rubisco activity. Translocation of photosynthate could be inhibited by O₃ exposure as well. Further, the influence of tropospheric O₃ needs to be considered when assessing potential effects of rising concentrations of atmospheric CO₂ on crop production. Advances in O₃ flux modelling and improved understanding of biochemical and molecular effects of O₃ on photosynthetic gas exchange and plant defence processes are leading to more complete, integrated assessments of O₃ impacts on crop physiology that continue to support the rationale for maintaining or improving current O₃ air quality standards as well as providing a basis for development of more O₃-tolerant crop lines.     

Phosphate accumulation in leaves of wheat seedlings measured in leaf cell protoplasts isolated after root or foliar treatment with orthophosphate

Leaf cell protoplasts were isolated from wheat seedlings ( Triticum aestivum L. cv. Urquie) after orthophosphate (Pi) treatment of the plant to determine the capacity for intracellular phosphate accumulation. Seedlings were treated with Pi concentrations near the phytotoxic level to maximize the Pi concentration in the leaf prior to protoplast isolation 1 day later. Both foliar and root treatment of seedlings with Pi increased the phosphate content of leaf protoplasts by approximately 20 μmol (mg chlorophyll)⁻¹ over Pi levels in untreated controls. Phosphate-loaded protoplasts from treated seedlings had similar photosynthetic rates and starch content but 50% more soluble reducing sugar than protoplasts from untreated seedlings. Protoplast dark respiration decreased after treatments which increased protoplast potassium content. The results suggest that similar amounts of Pi can be accumulated by leaf cells of wheat after foliar or root application of Pi to the seedling without hindering Pi-sensitive processes such as photosynthesis and starch synthesis.     

Effects of elevated ozone and low light on diurnal and seasonal carbon gain in sugar maple

The long‐term interactive effects of ozone and light on whole‐tree carbon balance of sugar maple (Acer saccharum Marsh.) seedlings were examined, with an emphasis on carbon acquisition, foliar partitioning into starch and soluble sugars, and allocation to growth. Sugar maple seedlings were fumigated with ambient, 1·7 × ambient and 3·0 × ambient ozone in open‐top chambers for 3 years under low and high light (15 and 35% full sunlight, respectively). Three years of ozone fumigation reduced the total biomass of seedlings in the low‐ and high‐light treatments by 64 and 41%, respectively, but had no effect on whole‐plant biomass allocation. Ozone had no effect on net photosynthesis until late in the growing season, with low‐light seedlings generally exhibiting more pronounced reductions in photosynthesis. The late‐season reduction in photosynthesis was not due to impaired stomatal function, but was associated more with accelerated senescence or senescence‐like injury. In contrast, the 3·0 × ambient ozone treatment immediately reduced diurnal starch accumulation in leaves by over 50% and increased partitioning of total non‐structural carbohydrates into soluble sugars, suggesting that injury repair processes may be maintaining photosynthesis in late spring and early summer at the expense of storage carbon. The results in the present study indicate that changes in leaf‐level photosynthesis may not accurately predict the growth response of sugar maple to ozone in different light environments. The larger reduction in seedling growth under low‐light conditions suggests that seedlings in gap or closed‐canopy environments are more susceptible to ozone than those in a clearing. Similarly, understanding the effects of tropospheric ozone on net carbon gain of a mature tree will require scaling of leaf‐level responses to heterogeneous light environments, where some leaves may be more susceptible than others.     

Changes in photosynthetic rates and gene expression of leaves during a source-sink perturbation in sugarcane

BACKGROUND AND AIMS: In crops other than sugarcane there is good evidence that the size and activity of carbon sinks influence source activity via sugar-related regulation of the enzymes of photosynthesis, an effect that is partly mediated through coarse regulation of gene expression. METHODS: In the current study, leaf shading treatments were used to perturb the source-sink balance in 12-month-old Saccharum spp. hybrid 'N19' (N19) by restricting source activity to a single mature leaf. Changes in leaf photosynthetic gas exchange variables and leaf and culm sugar concentrations were subsequently measured over a 14 d period. In addition, the changes in leaf gene response to the source-sink perturbation were measured by reverse northern hybridization analysis of an array of 128 expressed sequence tags (ESTs) related to photosynthetic and carbohydrate metabolism. KEY RESULTS: Sucrose concentrations in immature culm tissue declined significantly over the duration of the shading treatment, while a 57 and 88% increase in the assimilation rate (A) and electron transport rate (ETR), respectively, was observed in the source leaf. Several genes (27) in the leaf displayed a >2-fold change in expression level, including the upregulation of several genes associated with C(4) photosynthesis, mitochondrial metabolism and sugar transport. Changes in gene expression levels of several genes, including Rubisco (EC 4.1.1.39) and hexokinase (HXK; EC 2.7.1.1), correlated with changes in photosynthesis and tissue sugar concentrations that occurred subsequent to the source-sink perturbation. CONCLUSIONS: These results are consistent with the notion that sink demand may limit source activity through a kinase-mediated sugar signalling mechanism that correlates to a decrease in source hexose concentrations, which, in turn, correlate with increased expression of genes involved in photosynthesis and metabolite transport. The signal feedback system reporting sink sufficiency and regulating source activity may be a potentially valuable target for future genetic manipulation to increase sugarcane sucrose yield.     

Effects of sucrose on starch accumulation and rate of photosynthesis in Rosa cultured in vitro

Shootlets of Rosa multiflora L. cv. Montse were cultured in vitro with four different levels of sucrose (0, 1, 3 and 5%). Chloroplasts of shootlets grown in a medium without sucrose contained numerous, large plastoglobuli and were lacking in starch granules. The size and number of starch granules increased with the level of sucrose in the culture medium. Starch content in leaves of shootlets grown with 5% sucrose was higher (ca 1, 3%) than those grown with 3% (ca 0, 45%) and 1% sucrose (ca 0, 27%). Starch might be used by the in vitro shootlets during the acclimation period.Abbreviations BA benzyladenine - Pi orthophosphate - S sucrose - Rubisco ribulose 1,5-bisphosphate carboxylase - TEM transmission electron microscopy     

Carbohydrate partitioning, photosynthesis and growth in Lemna gibba G3. II. Effects of phosphorus limitation

The relationship between CO₂ assimilation rate, growth and partitioning of carbon among starch, sucrose, glucose and fructose were studied in phosphorus (P_i)-limited Lemna gibba L. G3. Two experimental models were used: 1) Cultures were grown at various stable, suboptimal rates regulated by the supply of P_i; 2) cultures growing at optimal rates were transferred to P_i-free medium. The response to a P_i deficiency can be divided into two phases. Phase I is characterized by hyperactivity of the sucrose synthesis pathway, leading to high levels of glucose and fructose. Phase II is characterized by starch accumulation associated with a decrease in the cytoplasmic pools of soluble sugars owing to inhibition of carbon export from the chloroplast. A strong negative correlation was found between the CO₂ assimilation rate and starch levels. No significant correlation was found between assimilation and ATP levels and decrease in relative growth rate did not significantly affect the adenylate energy charge (EC). The regulatory aspects of the partitioning of carbon among soluble sugars and starch as well as the negative correlation between carbohydrate levels and CO₂ assimilation at P_i-limited growth are discussed.     

Carbohydrate partitioning, photosynthesis and growth in Lemna gibba G3.I. Effects of nitrogen Limitation

The growth rate of Lemna giba L. G3 was varied by limiting the supply of nitrogen (N) under otherwise constant condition. Two experimental approaches were used. 1) A series of suboptimally growing cultures were supplid daily with exponentially increasing doses on N. 2) Optimally growing cultures were transferred to a N-fre medium and cultivated in it for 10 days. Lveles of starch, sucrose, glucose, fructose, and systems. At RGR ranging from optimal to 50% of optimal caused decreased levels of soluble sugars, but increased lvels of starch.Starch accumulation showed a strong negative correlation with the CO² assimilation rate, indicating increased triose phosphate/inorganic phosphate (TP/Pⁱ) ratio in the chloroplast causing end product inhibition of photosyntheisis. The data indicat the quantitative rlationship betwen the photosynthetic activity and the carbon utilization rate influnces the activity of the sucrose synthesis pathway and thus the rate of the triose hosphate/Pⁱ exchange at the chloroplast membrane, which in turn regulates the activity of starch synthesis and the Calvin cycle.     

Concentration‐ and flux‐based dose–responses of isoprene emission from poplar leaves and plants exposed to an ozone concentration gradient

Concentration‐ and flux‐based O₃ dose–responses of isoprene emission from single leaves and whole plants were developed. Two poplar clones differing in O₃ sensitivity were exposed to five O₃ levels in open‐top chambers for 97 d: charcoal‐filtered ambient air (CF), non‐filtered ambient air (NF) and NF plus 20 ppb (NF + 20), 40 ppb (NF + 40) and 60 ppb (NF + 60). At both leaf and plant level, isoprene emission was significantly decreased by NF + 40 and NF + 60 for both clones. Although intra‐specific variability was found when the emissions were up‐scaled to the whole plant, both leaf‐ and plant‐level emissions decreased linearly with increasing concentration‐based (AOT40, cumulative exposure to hourly O₃ concentrations >40 ppb) and flux‐based indices (POD_Y, cumulative stomatal uptake of O₃ > Y nmol O₃ m^?2 PLA s^?1). AOT40‐ and POD₇‐based dose–responses performed equally well. The two clones responded differently to AOT40 and similarly to POD_Y (with a slightly higher R² for POD₇) when the emission was expressed as change relative to clean air. We thus recommend POD₇ as a large‐scale risk assessment metric to estimate isoprene emission responses to O₃ in poplar.     

Elevated ozone reduces photosynthetic carbon gain by accelerating leaf senescence of inbred and hybrid maize in a genotype‐specific manner

Exposure to elevated tropospheric ozone concentration ([O₃]) accelerates leaf senescence in many C₃ crops. However, the effects of elevated [O₃] on C₄ crops including maize (Zea mays L.) are poorly understood in terms of physiological mechanism and genetic variation in sensitivity. Using free air gas concentration enrichment, we investigated the photosynthetic response of 18 diverse maize inbred and hybrid lines to season‐long exposure to elevated [O₃] (~100 nl L^?1) in the field. Gas exchange was measured on the leaf subtending the ear throughout the grain filling period. On average over the lifetime of the leaf, elevated [O₃] led to reductions in photosynthetic CO₂ assimilation of both inbred (?22%) and hybrid (?33%) genotypes. There was significant variation among both inbred and hybrid lines in the sensitivity of photosynthesis to elevated [O₃], with some lines showing no change in photosynthesis at elevated [O₃]. Based on analysis of inbred line B73, the reduced CO₂ assimilation at elevated [O₃] was associated with accelerated senescence decreasing photosynthetic capacity and not altered stomatal limitation. These findings across diverse maize genotypes could advance the development of more O₃ tolerant maize and provide experimental data for parameterization and validation of studies modeling how O₃ impacts crop performance.