Nitrogen nutrition of C3 plants at elevated atmospheric CO2 concentrations

The atmospheric CO₂ concentration has risen from the preindustrial level of approximately 290 μl l⁻¹ to more than 350 μl l⁻¹ in 1993. The current rate of rise is such that concentrations of 420 μl l⁻¹ are expected in the next 20 years. For C₃ plants, higher CO₂ levels favour the photosynthetic carbon reduction cycle over the photorespiratory cycle, resulting in higher rates of carbohydrate production and plant productivity. The change in balance between the two photosynthetic cycles appears to alter nitrogen and carbon metabolism in the leaf, possibly causing decreases in nitrogen concentrations in the leaf. This may result from increases in the concentration of storage carbohydrates of high molecular weight (soluble or insoluble) and/or changes in distribution of protein or other nitrogen containing compounds. Uptake of nitrogen may also be reduced at high CO₂ due to lower transpiration rates. Decreases in foliar nitrogen levels have important implications for production of crops such as wheat, because fertilizer management is often based on leaf chemical analysis, using standards estimated when the CO₂ levels were considerably lower. These standards will need to be re-evaluated as the CO₂ concentration continues to rise. Lower levels of leaf nitrogen will also have implications for the quality of wheat grain produced, because it is likely that less nitrogen would be retranslocated during grain filling.     

Effects of elevated CO2 on grain yield and quality of wheat: results from a 3-year free-air CO2 enrichment experiment

Spring wheat ( Triticum aestivum L. cv. TRISO) was grown for three consecutive seasons in a free-air carbon dioxide (CO₂) enrichment (FACE) field experiment in order to examine the effects on crop yield and grain quality. CO₂ enrichment promoted aboveground biomass (+11.8%) and grain yield (+10.4%). However, adverse effects were predominantly observed on wholegrain quality characteristics. Although the thousand-grain weight remained unchanged, size distribution was significantly shifted towards smaller grains, which may directly relate to lower market value. Total grain protein concentration decreased significantly by 7.4% under elevated CO₂, and protein and amino acid composition were altered. Corresponding to the decline in grain protein concentration, CO₂ enrichment resulted in an overall decrease in amino acid concentrations, with greater reductions in non-essential than essential amino acids. Minerals such as potassium, molybdenum and lead increased, while manganese, iron, cadmium and silicon decreased, suggesting that adjustments of agricultural practices may be required to retain current grain quality standards. The concentration of fructose and fructan, as well as amounts per area of total and individual non-structural carbohydrates, except for starch, significantly increased in the grain. The same holds true for the amount of lipids. With regard to mixing and rheological properties of the flour, a significant increase in gluten resistance under elevated CO₂ was observed. CO₂ enrichment obviously affected grain quality characteristics that are important for consumer nutrition and health, and for industrial processing and marketing, which have to date received little attention.     

The interaction of high temperature and elevated CO2 on photosynthetic acclimation of single leaves of rice in situ

Rice ( Oryza sativa L. cv. IR72) was grown at three different CO₂ concentrations (ambient, ambient + 200 μmol mol⁻¹, ambient + 300 μmol mol⁻¹) at two different growth temperatures (ambient, ambient + 4°C) from sowing to maturity to determine longterm photosynthetic acclimation to elevated CO₂ with and without increasing temperature. Single leaves of rice showed a cooperative enhancement of photosynthetic rate with elevated CO₂ and temperature during tillering, relative to the elevated CO₂ condition alone. However, after flowering, the degree of photosynthetic stimulation by elevated CO₂ was reduced for the ambient + 4°C treatment. This increasing insensitivity to CO₂ appeared to be accompanied by a reduction in ribulose-1.5-bisphosphate carboxylase/oxygenase (Rubisco) activity and/or concentration as evidenced by the reduction in the assimilation (A) to internal CO₂ (C₁) response curve. The reproductive response (e.g. percent filled grains, panicle weight) was reduced at the higher growth temperature and presumably reflects a greater increase in floral sterility. Results indicate that while CO₂ and temperature could act synergistically at the biochemical level, the direct effect of temperature on floral development with a subsequent reduction in carbon utilization may change sink strength so as to limit photosynthetic stimulation by elevated CO₂ concentration.     

Seasonal changes in the effects of free-air CO2 enrichment (FACE) on growth, morphology and physiology of rice root at three levels of nitrogen fertilization

Over time, the relative effects of elevated [CO₂] on the aboveground photosynthesis, growth and development of rice (Oryza sativa L.) are likely to be changed with increasing duration of CO₂ exposure, but the resultant effects on rice belowground responses remain to be evaluated. To investigate the impacts of elevated [CO₂] on seasonal changes in root growth, morphology and physiology of rice, a free‐air CO₂ enrichment (FACE) experiment was performed at Wuxi, Jiangsu, China, in 2002–2003. A japonica cultivar with large panicle was exposed to two [CO₂] (ambient [CO₂], 370 μmol mol⁻¹; elevated [CO₂], 570 μmol mol⁻¹) at three levels of nitrogen (N): low (LN, 15 g N m⁻²), medium (MN, 25 g N m⁻²) and high N (HN, 35 g N m⁻²). Elevated [CO₂] increased cumulative root volume, root dry weight, adventitious root length and adventitious root number at all developmental stages by 25–71%, which was mainly associated with increased root growth rate during early growth period (EGP) and lower rate of root senescence during late growth period (LGP), while a slight inhibition of root growth rate occurred during middle growth period (MGP). For individual adventitious roots, elevated [CO₂] increased average length, volume, diameter and dry weight early in the season, but the effects gradually disappeared in subsequent stages. Total surface area and active adsorption area per unit root dry weight reached their maxima 10 days earlier in FACE vs. ambient plants, but both of them together with root oxidation ability per unit root dry weight declined with elevated [CO₂] during MGP and LGP, the decline being larger during MGP than LGP. The CO₂‐induced decreases in specific root activities during MGP and LGP were associated with a larger amount of root accumulation during EGP and lower N concentration and higher C/N ratio in roots during MGP and LGP in FACE vs. ambient plants. The results suggest that most of the CO₂‐induced increases in shoot growth of rice are similarly associated with increased root growth.     

Elevated CO2 and water deficit effects on photosynthesis, ribulose bisphosphate carboxylase‐oxygenase, and carbohydrate metabolism in rice

Rice (Oryza sativa[L.] cv. IR-72) was grown for a season in sunlit, controlled-environment chambers at 350 or 700 µmol CO₂ mol^?1 under continuously flooded (unstressed) or drought-imposed periods at panicle initiation (stressed). The midday canopy photosynthetic rates (P_n), measured at the CO₂ concentration ([CO₂]) used for growth, were enhanced by high [CO₂] but reduced by drought. High [CO₂] increased P_n by 18 to 34% for the unstressed plants, and 6 to 12% for the stressed plants. In the unstressed plants, CO₂ enrichment increased water-use efficiency (WUE) by 26%, and reduced evapotranspiration (ET) by 8 to 14%. Both high [CO₂] and severe drought decreased the activity and content of ribulose bisphosphate carboxylase-oxygenase (Rubisco). High-CO₂-unstressed plants had 6 to 22% smaller content and 5 to 25%, lower activity of Rubisco than ambient-CO₂-unstressed plants. Under severe drought, reductions of Rubisco were 53 and 27% in activity and 40 and 12% in content, respectively, for ambient- and high-CO₂ treatments. The apparent catalytic turnover rate (K_cat) of midday fully activated Rubisco was not altered by high [CO₂], but severe drought reduced K_cat by 17 to 23%. Chloroplasts of the high-CO₂ leaves contained more, and larger starch grains than those of the ambient CO₂ leaves. High [CO₂] did not affect the leaf sucrose content of unstressed plants. In contrast, severe drought reduced the leaf starch and increased the sucrose content in both CO₂ treatments. The activity of leaf sucrose phosphate synthase of unstressed plants was not affected by high [CO₂], whereas that of ambient-CO₂-grown plants was reduced 45% by severe drought. Reduction in ET and enhancements in both P_n and WUE for rice grown under high [CO₂] helped to delay the adverse effects of severe drought and allowed the stressed plants to assimilate CO₂ for an extra day. Thus, rice grown in the next century may utilize less water, use water more efficiently, and be able to tolerate drought better under some situations.     

Seasonal CO2 exchange patterns of developing peach (Prunus persica) fruits in response to temperature, light and CO2 concentration

CO₂ exchange rates per unit dry weight, measured in the field on attached fruits of the late-maturing Cal Red peach cultivar, at 1200 μmol photons m^?2S^?1 and in dark, and photosynthetic rates, calculated by the difference between the rates of CO₂ evolution in light and dark, declined over the growing season. Calculated photosynthetic rates per fruit increased over the season with increasing fruit dry matter, but declined in maturing fruits apparently coinciding with the loss of chlorophyll. Slight net fruit photosynthetic rates ranging from 0. 087 ± 0. 06 to 0. 003 ± 0. 05 nmol CO₂ (g dry weight)^?1 S^?1 were measured in midseason under optimal temperature (15 and 20°C) and light (1200 μmol photons m^?2 S^?1) conditions. Calculated fruit photosynthetic rates per unit dry weight increased with increasing temperatures and photon flux densities during fruit development. Dark respiration rates per unit dry weight doubled within a temperature interval of 10°C; the mean seasonal O₁₀ value was 2. 03 between 20 and 30°C. The highest photosynthetic rates were measured at 35°C throughout the growing season. Since dark respiration rates increased at high temperatures to a greater extent than CO₂ exchange rates in light, fruit photosynthesis was apparently stimulated by high internal CO₂ concentrations via CO₂ refixation. At 15°C, fruit photosynthetic rates tended to be saturated at about 600 μmol photons m^?2 S^?1. Young peach fruits responded to increasing ambient CO₂ concentrations with decreasing net CO₂ exchange rates in light, but more mature fruits did not respond to increases in ambient CO₂. Fruit CO₂ exchange rates in the dark remained fairly constant, apparently uninfluenced by ambient CO₂ concentrations during the entire growing season. Calculated fruit photosynthetic rates clearly revealed the difference in CO₂ response of young and mature peach fruits. Photosynthetic rates of younger peach fruits apparently approached saturation at 370 μl CO₂1^?2. In CO₂ free air, fruit photosynthesis was dependent on CO₂ refixation since CO₂ uptake by the fruits from the external atmosphere was not possible. The difference in photosynthetic rates between fruits in CO₂-free air and 370 μl CO₂ 1^?1 indicated that young peach fruits were apparently able to take up CO₂ from the external atmosphere. CO₂ uptake by peach fruits contributed between 28 and 16% to the fruit photosynthetic rate early in the season, whereas photosynthesis in maturing fruits was supplied entirely by CO₂ refixation.     

Acquisition and within-plant allocation of 13C and 15N in CO2-enriched Quercus robur plants

We assessed the effects of doubling atmospheric CO₂ concentration, [CO₂], on C and N allocation within pedunculate oak plants (Quercus robur L.) grown in containers under optimal water supply. A short-term dual ¹³CO₂ and ¹⁵NO₃^? labelling experiment was carried out when the plants had formed their third growing flush. The 22-week exposure to 700 μl l^?1 [CO₂] stimulated plant growth and biomass accumulation (+53% as compared with the 350 μl l^?1 [CO₂] treatment) but decreased the root/shoot biomass ratio (-23%) and specific leaf area (-18%). Moreover, there was an increase in net CO₂ assimilation rate (+37% on a leaf dry weight basis; +71% on a leaf area basis), and a decrease in both above- and below-ground CO₂ respiration rates (-32 and -26%, respectively, on a dry mass basis) under elevated [CO₂]. ¹³C acquisition, expressed on a plant mass basis or on a plant leaf area basis, was also markedly stimulated under elevated [CO₂] both after the 12-h ¹³CO₂ pulse phase and after the 60-h chase phase. Plant N content was increased under elevated CO₂ (+36%), but not enough to compensate for the increase in plant C content (+53%). Thus, the plant C/N ratio was increased (+13%) and plant N concentration was decreased (-11%). There was no effect of elevated [CO₂] on fine root-specific ¹⁵N uptake (amount of recently assimilated ¹⁵N per unit fine root dry mass), suggesting that modifications of plant N pools were merely linked to root size and not to root function. N concentration was decreased in the leaves of the first and second growing flushes and in the coarse roots, whereas it was unaffected by [CO₂] in the stem and in the actively growing organs (fine roots and leaves of the third growth flush). Furthermore, leaf N content per unit area was unaffected by [CO₂]. These results are consistent with the short-term optimization of N distribution within the plants with respect to growth and photosynthesis. Such an optimization might be achieved at the expense of the N pools in storage compartments (coarse roots, leaves of the first and second growth flushes). After the 60-h ¹³C chase phase, leaves of the first and second growth flushes were almost completely depleted in recent ¹³C under ambient [CO₂], whereas these leaves retained important amounts of recently assimilated ¹³C (carbohydrate reserves?) under elevated [CO₂].     

Assimilate relations in source and sink leaves during acclimation to a CO2-enriched atmosphere

Evidence from previous studies suggested that adjustments in assimilate formation and partitioning in leaves might occur over time when plants are exposed to enriched atmospheric CO₂. We examined assimilate relations of source (primary unifoliolate) and developing sink (second mainstem trifoliolate) leaves of soybean [ Glycine max (L.) Merr. cv. Lee] plants for 12 days after transfer from a control (350 μl l⁻¹) to a high (700 μ l⁻¹) CO₂ environment. Similar responses were evident in the two leaf types. Net CO₂ exchange rate (CER) immediately increased and remained elevated in high CO₂. Initially, the additional assimilate at high CO₂ levels in the light and was utilized in the subsequent dark period. After approximately 7 days, assimilate export in the light began to increase and by 12 days reached rates 3 to 5 times that of the control. In the developing sink leaf, high rates of export in the light occurred as the leaf approached full expansion. The results indicate that a specific acclimation process occurs in source leaves which increases the capacity for assimilate export in the light phase of the diurnal cycle as plants adjust to enriched CO₂ and a more rapid growth rate.     

Seasonal changes in the effects of elevated CO2 on rice at three levels of nitrogen supply: a free air CO2 enrichment (FACE) experiment

Over time, the stimulative effect of elevated CO₂ on the photosynthesis of rice crops is likely to be reduced with increasing duration of CO₂ exposure, but the resultant effects on crop productivity remain unclear. To investigate seasonal changes in the effect of elevated CO₂ on the growth of rice (Oryza sativa L.) crops, a free air CO₂ enrichment (FACE) experiment was conducted at Shizukuishi, Iwate, Japan in 1998–2000. The target CO₂ concentration of the FACE plots was 200 µmol mol^?1 above that of ambient. Three levels of nitrogen (N) were supplied: low (LN, 4 g N m^?2), medium [MN, 8 (1998) and 9 (1999, 2000) g N m^?2] and high N (HN, 12 and 15 g N m^?2). For MN and HN but not for LN, elevated CO₂ increased tiller number at panicle initiation (PI) but this positive response decreased with crop development. As a result, the response of green leaf area index (GLAI) to elevated CO₂ greatly varied with development, showing positive responses during vegetative stages and negative responses after PI. Elevated CO₂ decreased leaf N concentration over the season, except during early stage of development. For MN crops, total biomass increased with elevated CO₂, but the response declined linearly with development, with average increases of 32, 28, 21, 15 and 12% at tillering, PI, anthesis, mid‐ripening and grain maturity, respectively. This decline is likely to be due to decreases in the positive effects of elevated CO₂ on canopy photosynthesis because of reductions in both GLAI and leaf N. Up to PI, LN‐crops tended to have a lower response to elevated CO₂ than MN‐ and HN‐crops, though by final harvest the total biomass response was similar for all N levels. For MN‐ and HN‐crops, the positive response of grain yield (ca. 15%) to elevated CO₂ was slightly greater than the response of final total biomass while for LN‐crops it was less. We conclude that most of the seasonal changes in crop response to elevated CO₂ are directly or indirectly associated with N uptake.     

Growth and onto-morphogenesis of soybean (Glycine max Merril) in an open, naturally CO2-enriched environment

Springs emitting carbon dioxide are frequent in Central Italy and provide a way of testing the response of plants to CO₂ enrichment under natural conditions. Results of a CO₂ enrichment experiment on soybean at a CO₂ spring (Solfatara) are presented. The experimental site is characterized by significant anomalies in atmospheric CO₂ concentration produced by a large number of vents emitting almost pure CO₂ (93%) plus small amounts of hydrogen sulphide, methane, nitrogen and oxygen. Within the gas vent area, plants were grown at three sub-areas whose mean CO₂ concentrations during daytime were 350,652 and 2370 μmol mol^-1, respectively. Weekly harvests were made to measure biomass growth, leaf area and ontogenetic development. Biomass growth rate and seed yield were enhanced by elevated CO₂. In particular, onto-morphogenetic development was affected by elevated CO₂ with high levels of CO₂ increasing the total number of main stem leaf nodes and the area of the main stem trifoliolate leaves. Biochemical analysis of plant tissue suggested that there was no effect of the small amounts of H₂S on the response to CO₂ enrichment. Non-protein sulphydryl compounds did not accumulate in leaf tissues and the overall capacity of leaf extracts to oxidize exogenously added NADH was not decreased. The limitations and advantages of experimenting with crop plants at elevated CO₂ in the open and in the proximity of carbon dioxide springs are discussed.