Higher Net Assimilation Rate Research Articles

Differences in Dry Matter Production and Ecophysiological Characteristics between Soybean Cultivars, Enrei and Tachinagaha.

The soybean cultivar, Tachinagaha, could attain a higher grain yield than Enrei because of higher dry matter production after flowering. The crop growth rate during ripening was clearly high in Tachinagaha because of high net assimilation rates compared with Enrei. No difference in the light extinction coefficient of the whole canopy was observed between the two cultivars, but it was far smaller in the upper layer of the canopy in Tachinagaha than in Enrei, especially at midday. The leaf photosynthetic rates in Tachinagaha were high compared with Enrei. The degradation of leaf color and the decrease in the number of living leaves with ripening slowed in Tachinagaha compared with Enrei. These results indicated that the higher leaf photosynthetic rate because of slow senescence might contribute to the higher NAR in Tachinagaha as well as improved light intercepting characteristics of the upper layer in the canopy.

Growth of Near-isogenic Wheat Lines Differing in Development—Spaced Plants

Abstract The relationship between phenological development in wheat (Triticum aestivumL.) and growth was studied to determine if the switch from a vegetative to a reproductive apex increases plant growth rate. Plant partitioning and relative growth rates during vegetative and pre-flowering reproductive periods were determined in two sets of near-isogenic lines differing in phenological development. Spaced plants were grown in two photoperiods (15 and 10 h) to increase the range of development rates. Lines within each isogenic set and photoperiod treatment did not differ in whole plant growth rate despite large differences in developmental rate. In addition, the partitioning of biomass between roots and shoots was also similar. The transition of the apex from vegetative to reproductive mostly affected the partitioning of shoot biomass into leaf (blades) and stems (rest of the shoot). A longer time to reach floral initiation was associated with the production of more, and larger, leaves as well as more tillers. This resulted in large differences in leaf area between isolines. However, at the whole plant level, all lines accumulated biomass at the same rate with time. The early flowering lines compensated for their reduced leaf area by having a higher net assimilation rate and were thereby able to maintain the same relative growth rate as their later flowering counterparts.

Growth of Broadcasted Rice and Compensatory Effect on Yield at Different Seedling Densities.

苗立ち密度が散播水稲個体の生育と収量に及ぼす影響を明らかにすることにより, 収量補償作用が発現する要因を明確にするため, 偏穂数型品種ヤマヒカリの湛水土壌中散播栽培において, 32, 64, 96, 160本/m2の苗立ち密度を設けて生長解析と収量調査を行った.生育初期の相対分げつ速度(RTR)と相対生長率(RGR)は高密度側(96本区, 160本区)で大きく, 生育中期以降は低密度ほど大きく維持された.RGRの密度間差は, 生育初期は高密度ほど葉面積比(LAR)が大きかったこと, 生育中期以降は低密度ほど純同化率(NAR)が高く維持されたためであった.比葉面積(SLA)は生育期間を通じて密度間差が認められ, 高密度で大きく, 低密度側で小さい傾向にあった.従って, 生育初期のLARの差はSLAの違いに基づくものであり, 生育中期以降のNARの差に対してもSLAの違いが影響を及ぼしているものと考えられた.各収量構成要素には密度間差が認められたが, 苗立ち密度に対する各々の変動は異なっており, 単位面積当たり籾収量に有意差はみられず, 収量の補償作用が確認された.その内容としては, 低密度側(32本区, 64本区)では生育後期までNARが高く, RTRが高く維持され, 個体当たり穂数は低密度ほど多かったことが収量補償に向けた第1番目の変動といえた.しかし, 苗立ち密度の差が大きかったため, 個体当たり穂数の変動で補償しきれなかった分を1穂籾数の変動が補償したものと考えられた.また, 播種後58日以降, NAR並びにRGRと1穂籾数との間には正の関係が窺われた.

Effects of Pre-Flowering Soil Moisture Deficits on Dry Matter Production and Ecophysiological Characteristics in Soybean Plants under Well Irrigated Conditions during Grain Filling

SummarySummer field crop plants in Japan would develop large shoots but poor root systems during the rainy season called “Baiu”. This might have adverse effects on dry matter production in summer thereafter even when they grow under sufficient soil moisture conditions. The effects of pre-flowering soil moisture conditions on dry matter production and ecophysiological characteristics were investigated.Soybean plants were grown under sufficient (W plot) and deficient (D plot) soil moisture before flowering in the field. Under sufficient soil moisture conditions after flowering, the plants in the D plot produced higher dry matter due to higher net assimilation rate (NAR) and higher grain yield due to higher pod-flower set ratio and a heavier seed than in the W plot. The higher NAR in the D plot was attributed to (1) a lower resistance to water transport in plants, which is necessary to maintain a high leaf water potential and high photosynthetic rate during the daytime and (2) delayed senescence. The plants in the D plot had a well developed root system, and had roots with high physiological activity represented by a large amount of exudation from the basal cut end of the stem. The development of physiological activity of the root system maybe reflected in higher capacity of root functions, the higher pod-flower set ratio and the delay in the senescence. Improved cultivation practices such as drainage during the rainy season and breeding of the plants with well-developed root system during the rainy season may be necessary to increase the yield of summer crops in Japan. Irrigation during the summer may not be so effective for the plants with a poorly developed root system.

Photosynthetic strategies of summergreen and evergreen understory herbs of the boreal mixedwood forest.

Seasonal differences in photosynthesis and stomatal conductance of four herbaceous perennials from beneath a deciduous canopy was assessed at two light levels (60 and 400 μmol m-2 s-1 photosynthetic photon flux density, PPFD) and two leaf temperatures (7 and 20°C). Leaves of an evergreen, Pyrola asarifolia Michx., a wintergreen, Cornus canadensis L., and two summergreen species, Rubus pubescens Raf. and Aralia nudicaulis L., were collected at four times during the growing season. In addition, midsummer light response curves were obtained for one summergreen (A. nudicaulis) and one evergreen species (P. asarifolia) at both 7 and 20°C. Gas exchange measurements were made in the laboratory under controlled environmental conditions. For leaves collected in April, when insolation was high due to the leafless overstory, only P. asarifolia had green leaves, and there was no effect of temperature or light on this species' photosynthesis. P. asarifolia's net assimilation rate (NA) in April was about 30% of it's maximum in late summer. In early summer (June), A. nudicaulis and R. pubescens had higher NA at the higher temperature; at this time, these summergreen species also reached their maximum NA. Midsummer photosynthetic light response curves showed that the light-saturation point was higher and more responsive to leaf temperature in the summergreen A. nudicaulis than in the evergreen P. asarifolia. The summergreen species appear to have a photosystem which performs at high rates during early- and mid-summer, as well as a taller stature which allows them to intercept more light. The photosynthetic system of the ever/wintergreen species is adapted to the low ground-level light conditions in the summer and there does not appear to be an adjustment to take further advantage of the higher light in the spring and fall period. The adaptation of the evergreen and wintergreen understory species is tolerance to low temperatures, enabling them to photosynthesize into the fall till the first continuous frosts occur in the understory and also permitting the evergreen species to begin photosynthesis early in the spring.

Carbon and nitrogen allocation in ectomycorrhizal and non-mycorrhizal Pinus sylvestris L. seedlings.

We studied carbon and nitrogen allocation in mycorrhizal and non-mycorrhizal Scots pine (Pinus sylvestris L.) seedlings grown in a semi-hydroponic system with nitrogen as the growth limiting factor. Three ectomycorrhizal fungi were compared: one pioneer species (Thelephora terrestris Ehrh.: Fr.) and two late-stage fungi (Suillus bovinus (L.: Fr.) O. Kuntze, and Scleroderma citrinum Pers.). By giving all plants in each treatment the same amount of readily available nitrogen, we ensured that the external mycelium could not increase the total nitrogen content of the plants, thereby guaranteeing that any change in carbon or nitrogen partitioning was a direct effect of the mycorrhizal infection itself. Carbon and nitrogen partitioning were measured at an early and a late stage of mycorrhizal development, and at a low and a high N addition rate. Although mycorrhizal seedlings had a higher net assimilation rate and a higher shoot/root ratio than non-mycorrhizal seedlings, they had a lower rate of shoot growth. The high carbon demand of the mycobionts was consistent with the large biomass of external mycelia and the increased belowground respiration of the mycorrhizal plants. The carbon cost to the host was similar for pioneer and late-stage fungi. Above- and belowground partitioning of nitrogen was also affected by mycorrhizal infection. The external mycelia of Scleroderma citrinum retained 32% of the nitrogen supplied to the plants, thus significantly reducing nitrogen assimilation by the host plants and consequently reducing their growth rate. By contrast, the external mycelia of T. terrestris and Suillus bovinus retained less nitrogen than the mycelia of Scleroderma citrinum, hence we attributed the decreased growth rates of their host plants to a carbon drain rather than a nitrogen deficiency.

Effects of cold storage and water stress on water relations and gas exchange of white spruce (Picea glauca) seedlings

To determine the effects of lifting time and storage on water-stress resistance of nursery-grown white spruce (Picea glauca (Moench) Voss) seedlings, we compared gas exchange, water relations and mortality of 3-year-old seedlings lifted in October 1991 and stored at -2 degrees C for 3 months with seedlings lifted in January 1992. The seedlings were placed in nutrient solution and subjected to -1.1 or -2.7 MPa water stress induced by polyethylene glycol 3350 for 9 days. Water stress, but not lifting time, had a significant effect on seedling net assimilation, symplastic volume and turgor loss point. In a second experiment, seedlings lifted in October 1991 were stored at -2 degrees C for 7 months and compared with seedlings lifted in May 1992. The seedlings were planted in pots, and their gas exchange and water relation parameters measured in response to gradual water stress. The results suggest that prolonged cold storage retards photosynthetic recovery of seedlings after planting. Higher rates of net assimilation in seedlings lifted in May were not directly related to their water status. Nonstomatal limitations were the primary factor influencing photosynthetic rate. We conclude that the inferior ability of cold-stored seedlings to tolerate water stress was due to poor osmotic adjustment and a lag in recovery of photosynthesis.

Physiological responses of banana (MusaAAA; Cavendish sub-group) in the subtropics. III. Gas exchange, growth analysis and source-sink interaction over a complete crop cycle

SummaryGas-exchange readings as well as dry-matter production and distribution measurements were taken for determining optimum source-sink interactions of ‘Williams’ tissue culture banana plants over a whole crop cycle. Net assimilation rate (NAR), crop growth rate (CGR) and relative growth rate (RGR) were calculated. The results were supported by carbohydrate analysis and the monitoring of 14C-assimilates translocated throughout the plant. High NAR, CGR, RGR and photosynthesis rates (A) were measured during summer and low rates during winter. Growth parameters and CO2 assimilation increased into the second-summer growing season, but quickly declined during the last three months prior to harvest. This was attributed to an aging and rapidly depleting leaf area after flowering. Changes in the sink strength of the leaves and pseudostem during vegetative growth were shown, as with the rhizome during flowering, and the bunch before harvest. The most important sinks for 14C-assimilates during vegetative growth ...

Phosphorus supply affects the photosynthetic capacity of loblolly pine grown in elevated carbon dioxide.

Effects of phosphorus supply and mycorrhizal status on the response of photosynthetic capacity to elevated CO(2) were investigated in loblolly pine (Pinus taeda L.) seedlings. Seedlings were grown in greenhouses maintained at either 35.5 or 71.0 Pa CO(2) in a full factorial experiment with or without mycorrhizal inoculum (Pisolithus tinctorius (Pers.) Coker & Couch) and with an adequate or a limiting supply of phosphorus. Assimilation versus internal CO(2) partial pressure (C(i)) curves were used to estimate maximum Rubisco activity (V(c,max)), electron transport mediated ribulose 1,5-bisphosphate regeneration capacity (J(max)), phosphate regeneration capacity (PiRC) and daytime respiration rates (R(d)). Nonmycorrhizal seedlings grown with limiting phosphorus had significantly reduced V(c,max) and PiRC compared to seedlings in other treatments. Elevated CO(2) increased photosynthetic capacity in nonmycorrhizal seedlings in the low phosphorus treatment by increasing PiRC, whereas it induced phosphorus limitation in mycorrhizal seedlings in the low phosphorus treatment and did not affect the photosynthetic capacity of seedlings in the high phosphorus treatment. Despite the variety of effects on photosynthetic capacity, seedlings in the elevated CO(2) treatments had higher net assimilation rates than seedlings in the ambient CO(2) treatments. We conclude that phosphorus supply affects photosynthetic capacity during long-term exposure to elevated CO(2) through effects on Rubisco activity and ribulose 1,5-bisphosphate regeneration rates.

Effects of Pre-Flowering Soil Moisture Deficits on Dry Matter Production and Ecophysiological Characteristics in Soybean Plants under Drought Conditions during Grain Filling.

Summer crop plants grown in rain-fed fields suffer from drought during a hot and relatively dry summer after the rainy season called "Baiu" in Japan. Since crop plants might develop vigorous shoots with poorly developed root systems during the rainy season, they might suffer from water deficits in the summer, even when soil moisture depletes on the surface of the soil. In order to confirm this, the effects of pre-flowering soil moisture deficits on dry matter production and ecophysiological characteristics thereafter were investigated in soybean plants. Under deficient soil moisture conditions after flowering, higher dry matter production and higher grain yield were attained in the plants grown under deficient soil moisture before flowering (D-plot) than in the plants grown under sufficient soil moisture (W-plot). Large net assimilation rate (NAR) was responsible for high dry matter production in the plants of the D-plot. High NAR in the plants of the D-plot resulted from (1) a root system that was well developed in deep soil layers, (2) their capacity to absorb much soil water, especially in deep soil layers, (3) maintenance of a high leaf water potential and, therefore, a high photosynthetic rate during daytime and (4) a delay in the decrease in photosynthetic rate due to senescence. It was concluded that improved cultivation for developing root systems, such as drainage in the rainy season, as well as irrigation in midsummer, will be important in the cultivation of field summer crop plants in Japan.

Response of small birch plants (Betula pendula Roth.) to elevated CO2 and nitrogen supply

ABSTRACTSmall birch plants were grown for up to 80 d in a climate chamber at varied relative addition rates of nitrogen in culture solution, and at ambient (350 μmol mol‐1) or elevated (700 μmol mol‐1) concentrations of CO2. The relative addition rate of nitrogen controlled relative growth rate accurately and independently of CO2 concentration at sub‐optimum levels. During free access to nutrients, relative growth rate was higher at elevated CO2. Higher values of relative growth rate and net assimilation rate were associated with higher values of plant N‐concentration. At all N‐supply rates, elevated CO2 resulted in higher values of net assimilation rate, whereas leaf weight ratio was independent of CO2. Specific leaf area (and leaf area ratio) was less at higher CO2 and at lower rates of N‐supply. Lower values of specific leaf area were partly because of starch accumulation. Nitrogen productivity (growth rate per unit plant nitrogen) was higher at elevated CO2. At sub‐optimal N‐supply, the higher net assimilation rate at elevated CO2 was offset by a lower leaf area ratio. Carbon dioxide did not affect root/shoot ratio, but a higher fraction of plant dry weight was found in roots at lower N‐supply. In the treatment with lowest N‐supply, five times as much root length was produced per amount of plant nitrogen in comparison with optimum plants. The specific fine root length at all N‐supplies was greater at elevated CO2. These responses of the root system to lower N‐supply and elevated CO2 may have a considerable bearing on the acquisition of nutrients in depleted soils at elevated CO2. The advantage of maintaining steady‐state nutrition in small plants while investigating the effects of elevated CO2 on growth is emphasized.

The interactive effects of light, temperature and CO2/O2 ratios on photosynthesis of Coix lachryma-jobi L.

A portable infra-red gas analyser was used to investigate the interactive effects of light, temperature, and CO 2 /O 2 ratios under controlled environmental conditions in an attempt to model gas exchange characteristics of Coix lachryma - jobi L. Plotting light response curves as a function of temperature (20, 25, 30 and 35°C) revealed no sign of light saturation even at a photosynthetic photon flux density (PPFD) close to 2000 μmol m -2 s -1 . High net assimilation rates ( A ) of approximately 24 μmol CO 2 m -2 s -1 were realized at 30 – 35°C. Assimilation ( A ) versus internal CO 2 partial pressure ( C i ) curves showed a steep rise in A with increase in C i but saturated at approximately 150 μl l -1 and the effect was similar in either the absence or presence of O 2 , under all temperature regimes. C . lachryma - jobi exhibited low CO 2 compensation points ( Γ *) between 0 and 10 μl l -1 at either 0 or 21% O 2 . The slopes of double reciprocal plots of 1/ A versus 1/ C i , were nearly identical and crossed the y-intercept at almost identical points under all O 2 concentrations. This data indicated that there was no apparent O 2 inhibition and that the apparent inhibitor constant ( K ) for O 2 at the site of carboxylation did not change with an increase in [O 2 ] from 0 to 21%. These observations were further confirmed by results obtained from the analysis of apparent carboxylation efficiency where no inhibition of A with increase of [O 2 ] occurred. These characteristics are thus consistent with typical features of C 4 photosynthesis.

Effects of Day and Night Temperature on Goatsrue (Galega officinalis) and Alfalfa (Medicago sativa) Growth

Goatsrue, a perennial legume, is an exotic noxious weed currently found in the United States only in Cache County, Utah. It infests irrigated pastures, alfalfa fields, and noncropland areas. In order to compare their responses to temperature, goatsrue and alfalfa were grown in artificially illuminated controlled-environment chambers in 16 day/night temperature regimes ranging from 15/4 to 36/25 C. Growth analysis was used to evaluate effects of temperature on dry matter accumulation, leaf area production, and biomass allocation. Both species grew best at day/night temperatures of 22/25, 29/ 18, and 29/25 C. Leaf appearance rates were linearly related to mean daily temperature. Goatsrue produced fewer but larger leaves and a greater total leaf area than alfalfa. Biomass partitioning to leaves was greater in goatsrue, whereas partitioning to stems was greater in alfalfa. Response of vegetative dry matter production to temperature closely paralleled response of leaf area duration in both species. Alfalfa generally had a higher net assimilation rate, but the greater leaf area duration of goatsrue resulted in greater dry matter accumulation in this species after 50 d of growth. Overall responses to temperature were similar in the two species. Thus it seems likely that goatsrue could become a much more widely distributed weed in alfalfa.

Water use, water-use efficiency and growth analysis of selected woody ornamental species under a non-limiting water regime

Selected woody ornamental species were grown in a glasshouse under a non-limiting irrigation regime. Relative growth rate (RGR), net assimilation rate (NAR), leaf area ratio (LAR), whole plant transpiration (T), evapotranspiration (ET) and water-use efficiency (WUE) were determined during a 73 day period. Covering the container medium with polyethylene reduced evaporative losses from 13% to 88%, depending on plant species, without affecting growth. The greatest reduction in evaporation occurred in plants with open canopies and low T rates. The shrub/tree species Lagerstroemia indica had the highest NAR and RGR, while the prostrate (plagiotropic) ground covers Trachelospermum asiaticum and Jasminum floridum and the orthotropic shrub/tree species Ligustrum japonicum had the lowest NAR and RGR. Leaf area ratio was greatest in Ligustrum and Pittosporum tobira. Water consumption, leaf area and root:shoot ratio were greatest in Lagerstroemia. Sophora secundiflora, which is adapted to more xeric conditions, had the highest T and lowest WUE. The highest WUE was found in Lagerstroemia, Raphiolepis and Pittosporum.

Morphogenetic Changes Induced by a Low Red: Far-Red Ratio and their Growth Consequences in Water Hyacinth (Eichhornia crassipes)

Plants of water hyacinth (Eichhornia crassipes) were grown under two red/far-red ratios (Z) to investigate the effects on morphology and growth of the light quality component of canopy shade. Experiments were conducted in diffuse sunlight in the presence or absence of far-red radiation. Under low Z conditions, fewer new ramets were produced and their stolons were shorter. The number of leaves per ramet was unchanged, but petiole length and blade area were increased. This change in biomass partitioning resulted in a lower investment in the main resource-acquiring organs (leaf blades and roots). The lower allocation of biomass to the leaf blades was compensated for by a higher net assimilation rate, such that relative growth rate remained unchanged

Growth characteristics and seasonal allocation patterns of biomass and nutrients in Carex species growing in floating fens

Carex acutiformis, C. rostrata and C. diandra occur as dominant species in different floating fens in the ‘Vechtplassen’ area in the Netherlands and have, in this order, a lower annual biomass production concurrent with the deceasing nutrient availability in the fens. The species allocated proportionally equal amounts of biomass to the leaf blades, but Carex diandra invested more in the leaf sheaths and in the roots than the other two species and also had the greatest root length proportional to leaf area. The leaf area ratio was greatest in Carex acutiformis, but C. diandra had the highest net assimilation rate and the highest relative growth rate. These characteristics are presumably connected with the vegetation structure of the fens which becomes, in the order mentioned, more open with increasing irradiance of the whole plant.

Net assimilation rate and shoot area development in birch (Betula pendula Roth.) at different steady-state values of nutrition and photon flux density

Small birch plants (Betula pendula Roth.) were grown in a climate chamber at different, exponentially increasing rates of nitrogen supply and at different photon flux densities. This resulted in treatments with relative growth rate equal to the relative rate of increase in nitrogen supply and with different equilibrium values of plant nitrogen concentration. Nitrogen productivity (rate of dry matter increase per plant nitrogen) was largely independent of nitrogen supply and was greater at higher photon flux density. Leaf weight ratio, average specific leaf area (and thus leaf area ratio) were all greater at better nitrogen supply and at lower values of photon flux density. The dependencies were such that the ratio of total projected leaf area to plant nitrogen at a given photon flux density was similar at all rates of nitrogen supply. The ratio was greater at lower values of photon flux density. At a given value of photon flux density, net assimilation rate and net photosynthetic rate per shoot area (measured at the growth climate) were only slightly greater at better rates of nitrogen supply. Values were greater at higher photon flux densities. Acclimation of the total leaf area to plant nitrogen ratio and of net assimilation rate was such that nitrogen productivity was largely saturated with respect to photon flux density at values greater than 230 μmol m-2 s-1. At higher photon flux densities, any potential gain in nitrogen productivity associated with higher net assimilation rates was apparently offset by lower ratios of total leaf area to plant nitrogen.

Shading, Growth, and Dry-matter Partitioning of Cocoyam [Xanthosoma sagittifolium (L.) Schott

Cocoyam was grown in 100%, 50%, or 30% daylight to determine the effect of light intensity on growth characteristics at various stages of plant development. Beginning ≈ 2 months after planting, growth was monitored at three or four monthly intervals. Plants grown in shade had more petiole and leaf lamina growth and extension, as well as increased top: corm plus cormel ratio (dry-weight basis), than plants grown in 100% daylight. Shade-grown plants had a higher leaf area index and specific leaf area than sun-grown plants. Sun-grown plants had a higher net assimilation rate and specific leaf density than shade-grown plants. Linear equations were developed to predict lamina area through measurements of leaf lamina width and length, petiole length, and lamina dry weight.

A replacement series evaluation of competition between velvetleaf (Abutilon theophrasti) and soybean (Glycine max)

Summary:We examined relative competition abilities of velvetleaf (Abutilon theophrasti) and soybean (Glycine max) grown in a replacement series near Ames, Iowa, U.S.A. in 1986 and 1987. Relative above ground dry weight of soybean in mixtures was higher than expected from mono culture values early in the growing season but lower than expected later in the season, whereas velvetleaf had higher than expected values later in the season. Velvetleaf depressed seed yield of soybean in all mixtures; relative seed yield of velvetleaf was greater in all mixtures than in monoculture. Relative growth rate (RGR), leaf area ratio (LAR) and net assimilation rate (NAR) of soybean did not differ significantly among mixtures and generally decreased over the growing season. Velvetleaf exhibited no consistent differences among mixtures in RGR, LAR, and NAR. Net assimilation rate of velvetleaf was constant over most of the growing season. Relative growth rate and LAR of velvetleaf were highest early in the season and then progressively declined. Velvetleaf had higher NAR and higher RGR than soybean early in the season; LAR of velvetleaf was lower than LAR of soybean over most of the season. Velvetleaf and soybean did not significantly interfere with each other early in the growing season, but velvetleaf gained resources at the expense of soybean during the middle and later parts of the season.

Effects of CO2 Enrichment and Nitrogen Stress on Growth, and Partitioning of Dry Matter and Nitrogen in Wheat and Maize

Atmospheric CO2 levels are increasing, but little is known about how this will affect tissue concentrations and the partitioning of agriculturally important nutrients such as nitrogen (N) within crop plants. To investigate this, a glasshouse experiment was conducted in which wheat, a C3 species, and maize, a C4 species, were grown for 8 weeks at high CO2 (1500 cm3 m-3) on N supplies ranging from deficient (0.5 mol m-3) to more than adequate for maximum growth (25 mol m-3). Wheat responded to both CO2 enrichment and N supply; maize responded only to N supply. CO2-enriched wheat produced about twice the dry matter of control plants at all levels of N supply. Tiller and ear numbers were increased by CO2 enrichment irrespective of N supply. Enriched wheat plants had a lower Leaf Area Ratio but higher Net Assimilation Rate and Relative Growth Rate than control plants. There was no effect of CO2 enrichment on specific leaf weight. The enriched plants had lower shoot to root dry matter ratios than the controls at 6 mol m-3 N and higher. Shoot to root dry matter ratios of both wheat and maize increased with increasing N supply. CO2-enriched wheat plants accumulated more N than the controls but the proportional increase in N content was not as great as that in dry matter, with the result that concentrations of total-N and nitrate-N were lower in all organs of enriched plants, including ears. Nitrate reductase activity was lower in enriched than in control wheat plants. N-use efficiency by wheat was increased by CO2 enrichment. From a practical point of view, the study indicates that critical total-N and NO3-N concentrations used to diagnose the N status of wheat will need to be reassessed as global CO2 levels increase. Elevated CO2 may also reduce the protein content of grain and thus the baking quality of hard wheats.