Rate Of Apparent Photosynthesis Research Articles

SEASONAL VARIATIONS OF PHOTOSYNTHETIC PIGMENTS, TOTAL C, N, AND P CONTENT, AND PHOTOSYNTHESIS IN PHYLLARIOPSIS PURPURASCENS: (PHAEOPHYTA) FROM THE STRAIT OF GIBRALTAR

ABSTRACT Photosynthetic pigments, C, N, and P tissue composition, and photosynthetic rate were measured from April to October in the brown alga Phyllariopsis purpurascens (C. Agardh) Henry et South (Laminariales, Phaeophyta) growing at a 30‐m depth in the Strait of Gibraltar. Ir‐radiance reaching the population ranged from 13.5 to 27.5 mol.m‐2.mo‐1. The available light for this species, expressed as a percentage of the irradiance above the water, was 1.8%. Dissolved inorganic nitrogen forms, NO3‐and NH4+, were constant from April to October, whereas phosphate was depleted in August. Chlorophyll a decreased from 520.0 ± 165.0 to 199.6 ± 159.9 μg.g‐1 dry weight; in contrast, chlorophyll c and carotenoids did not change until September but increased threefold in October. C:N and N:P ratios changed in the same way and in the same range. They were constant until July but increased from 15–17 up to 42 (C:N) and from 14 to 40 (N:P) in October, suggesting a severe P limitation of growth of this species. The dark respiration rate and the light compensation point were constant from April to October (0.5 ± 0.1 μmol O2. m‐2.s‐1 and 6.5 ± 0.2 μmol.m‐2. s‐1, respectively), whereas the maximum rate of apparent photosynthesis, light onset saturation parameter, and half saturation constant for light were maximum in April to May (3.7 μmol O2. m‐2.s‐1and 40 and 41.5 μmol.m‐2. s‐1, respectively) and October (3.6 μmol O2. m‐2.s‐1 and 50 and 53.7 μmol.m‐2. s‐1, respectively). They were minimum in August (1.2 μmol O2.m‐2.s‐1 and 11.3 and 12 μmol.m‐2.s‐1, respectively). These minimum figures yielded a negative carbon budget in August and 0 in September, whereas it was positive the rest of the year. Photosynthetic efficiency, estimated by the ratio between maximum apparent photosynthesis and light half saturation constant, showed a strong agreement with productivity measured by means of an independent method. These results indicate that lamina expansion in this species is controlled by photosynthetic efficiency.

Rates of apparent photosynthesis, respiration and dry matter accumulation in maize canopies

The rates of canopy apparent photosynthesis (PC) and canopy respiration (Rc) were studied during vegetation season in two erectophile and two planophile hybrids of maize (Zea mays L.) grown at two canopy densities [7.5 plants m-2 (HD) and 4.5 plants m-2 (LD)]. Large differences in PC, Rc, RC/PC, leaf area index (LAI), dry matter accumulation and grain yield were found among hybrids and plant densities. Variations in PC and RC were associated mainly with changes in LAI. There was also found change in PC per unit LAI with time. The average RC/PC was 28.9 % for all treatments throughout the vegetation season. PC and RC per unit dry matter were higher in LD than in HD and decreased throughout the measurement period. The HD stand had higher PC and yield in hybrids with erectophile foliage, whereas LD stand had higher PC after male tetrad and got higher yield in hybrids with planophile foliage. Only RC in hybrids of the two foliage types was higher under HD than under LD throughout the vegetation period.

Intensifying Plant Density Response of Corn with Artificial Shade

AbstractCompetition among corn (Zea mays L.) plants for interception of photosynthetically active radiation (PAR) at high plant density can result in a reduction in kernel number per ear and complete ear barrenness. The response of field corn to long‐term shading as density increased was investigated. A single‐ear hybrid was grown at the Agricultural Experiment Station Farm, Univ. of Massachusetts, at three densities of 3, 7.5, and 12 plants m−2. Shading was provided by using black polypropylene fabric with 50% light penetration installed 44 d after emergence. The rate of apparent photosynthesis in ear leaves was reduced significantly by both increased plant density and shading. The reduction was attributed to reduced PAR in higher densities and shaded plots and to the decreased chlorophyll concentration measured in leaves of plants at high density in both ambient light and shaded plots. Tassel emergence was slightly delayed in high density and shaded plots. However, the time of silking was delayed significantly because of both high density and shading. Seven days after anthesis, 10% of plants in high densities and 50% of plants in shaded plots had no silk. The response of grain yield per unit area in ambient light to increasing density was parabolic. No increase in yield was obtained with density increase in the shaded plots. Shade reduced the number of productive ears per plant, and the number of kernels per row resulting in a 23 and 66% yield reduction in low and high plant densities respectively, when compared to ambient light. Weight per kernel was reduced 28% with increased plant density. Shading did not further decrease kernel weight. The number of kernel rows per ear was least sensitive to both high plant density and shading.

Recovery of normal photosynthesis and respiration in common wheat with Agropyron cytoplasms by telocentric Agropyron chromosomes

Alloplasmic common wheat (Triticum aestivum L. cultivais 'Penjamo 62' and 'Siete Cerros 66') with cytoplasms of wheatgrass (Agropyron trichophorum and Ag. glaucum) showed two aberrant phenotypes, i.e., gross reduction in plant vigor and male sterility. Plant vigor and male fertility were restored by cytoplasm-specific telocentric chromosomes (telosomes). Studies on carbon assimilation and consumption and on oxygen evolution and uptake showed that maximum rates of apparent photosynthesis were significantly lower in the alloplasmic lines than in their corresponding euplasmic lines and that the telosomes restored a normal level of photosynthesis. The decreased apparent photosynthetic rates in the alloplasmic lines were shown to be not due to decreased rates of true photosynthesis but to increased rates of dark respiration in the green leaves. In contrast, dark respiration in the roots was significantly low in the alloplasmic lines. The alloplasmic lines also showed decreased rates of respiratory consumption of new photosynthates. These results suggest that growth depression and male sterility in the alloplasmic lines are related to aberrant mitochondrial function, which is compensated for by cytoplasm-specific telosomes.

Fluoride‐induced enhancement and inhibition of photosynthesis in four taxa of Pinus

summary Pinus elliottii Englem. var. elliottii (PEE) and three varities of Pinus caribaea Morelet [var. caribaea Barr, et Golf. (PCC), var. bahamensis Barr. et Golf. (PCB) and var. hondurensis Bafr. et Golf (PCH)] were grown for a total of 125 days under gaseous hydrogen fluoride at 0.0, 1.2, 1.8.and 4.3 μg F m−3. Apparent photosynthesis was measured on 10–12 plants in each taxon and treatment during the last 10 days of the experiment, using needles free from visible injury that had matured before (old) or during the experiment (young). Rates of apparent photosynthesis were significantly greater in young needles of PCB and PCH exposed to 1.2 than to 4.3 μg F m−3, and in old needles of PCH exposed to 1.2 than to 0.0, 1.8 and 4.3 μg F m−3. Chlorophyll concentrations in old needles were consistently lower than those in young needles, but varied less with changes in ambient fluoride concentration, differences in photosynthetic rates of PCB, PCC and PCH corresponded generally with differences in concentrations of chlorophylls a, b and (a+b). Ambient fluoride concentrations less than about 1 μg F m3 appear unlikely to have deleterious short‐term effects on chlorophyll concentrations and photosynthesis in these taxa.

Spatial and Temporal Distribution of Photosynthesis and Transpiration by Single Leaves in a Rainforest Tree, Argyrodendron peralatum

The light environments and rates of photosynthesis by leaves of Argyrodendron peralatum (F. M. Bailey) I.H. Boas were examined at eight positions within a natural rainforest canopy. Daily integrals of irradiance at leaf surfaces expressed as a percentage of that reaching a horizontal sensor in the open decreased from about 50% in the uppermost leaves to about 20% at a point 2 m below the top of the crown and to 10-15% in the lower crown and understory. Leaves in the upper crown had maximum instantaneous rates of apparent photosynthesis of 11.5 �mol CO2 m-2 s-1, whilst leaves in the lower crown of the same tree, and in understory saplings had maximum photosynthesis rates of 8.2 �mol m-2 s-1. These rates were not significantly different, and are within the range observed in a number of rainforest species characteristic of later successional stages. Under conditions of adequate water supply, photosynthetic rate was light-saturated by 500 �mol quanta m-2 s-1, irrespective of level in the canopy.

Photosynthesis in an Australian rainforest tree, Argyrodendron peralatum, during the rapid development and relief of water deficits in the dry season.

Rates of apparent photosynthesis were measured in situ at five positions between the upper crown and a lower branch of a 34 m tall Argyrodendron peralatum (F.M. Bailey) H.L. Edlin ex I.H. Boas tree, and on an understorey sapling of the same species growing in a northern Australian rainforest. At the end of the dry season, rapid reductions in photosynthetic rates occurred in the upper crown within three days after a rain event, but changes in the lower crown and the sapling were less marked. Complete recovery of photosynthesis followed a second rain event. At high photon flux densities, stomatal conductance to water vapour decreased in a curvilinear fashion as the vapour pressure difference between leaf and air increased. Apparent photosynthesis was linearly related to stomatal conductance on the first clear day after each rain event, but there was no relationship between these parameters at the end of a brief natural drying cycle. Under conditions of adequate water supply, stomatal conductances of both upper crown and understorey leaves increased linearly with increasing photon flux density up to about 300 μmol m-2 s-1. During water deficits, stomatal conductances in leaves from the understorey increased much more rapidly at very low photon flux densities than did conductances in leaves from the upper canopy.

Physiological and morphological responses of Pinus strobus L. and Pinus sylvestris L. seedlings subjected to low-level continuous gamma irradiation at a radioactive waste disposal area

About 100 one-year old Pinus strobus and Pinus sylvestris seedlings were placed at the Welcome Residue Site (WRS), a radioactive waste disposal area located near Port Hope, Ontario, and thereafter continuously exposed to an average gamma dose rate of 10.15 mR/hr. An additional 100 seedlings were placed at a nearby control site where the background dose rate was about 0.03 mR/hr. Seedlings from both locations were sampled on three occasions for the analysis of various parameters. Data collected at the end of the growing season show that, although the low-level continuous irradiation treatment had not affected the chlorophyll content of the new needles of both species, it had suppressed their normal rates of apparent photosynthesis by about 16–19% and respiration rates by about 14–23%, and had reduced 80% ethanol soluble sugar content by about 14–25%. This treatment also suppressed stem elongation which led to considerable crowding of new needles and stimulated the outgrowth of lateral branches. These results suggest that both the morphological responses exhibited by the irradiated seedlings and the changes observed in the physico-chemical parameters of their needles were intermediated by the effect of continuous irradiation on the level of tree auxin, IAA.

カブの生育並びに光合成に及ぼす紫外線の影響

Two experimental plots were prepared to investigate the effect of near-ultraviolet (UV) radiation on the growth and photosynthesis of turnip (Brassica cam pestris L.). They were treated by covering with UV-transmitting vinyl film (UVT) or UV-eliminating vinyl film (UVE) (Fig.1).The results were summarized as follows.1. UV energy in UVE plot was 3 to 4% of that in UVT plot (Table 1). The temperature of UVE plot was similar to that of UVT plot throughout the growth period (Fig.2). The daily mean temperature throughout the growth period was about 19°C.2. UVE plot was superior to UVT plot in all the characters investigated on growth (Table 2). Particularly plant length and fresh weight were significant at 5% level throughout the growth period. The growth pattern of each character was almost common to each other between the two experimental plots, except that the relative growth rate during exponential growth period of UVE plot was higher than that of UVT plot (Fig.3-1-Fig.6).3. Apparent photosynthesis rate per unit of leaf area in UVE plot was higher than that in UVT plot at 24 days after sowing, but at 42, 48 and 70 days of after sowing the result was reversed (Fig.8). From this experiment, the effect UV on photosynthesis was not clearly observed.

Leaflet expansion rates for fifteen soybean cultivars

Abstract In determining soybean (Glycine max (L.) Merr.) yield, the rate of leaf area expansion may be more important than the rate of apparent photosynthesis per unit leaf area. Fifteen soybean cultivars from two Maturity Groups were surveyed for the rate at which terminal leaflets of the sixth, eighth, and tenth trifoliate leaf expanded under field conditions. Leaflets lenght (LL) and width (LW) were measured every day from when the leaflets were 10–15 mm long until full expansion, a period of 11 or 12 days. Leaflet lenght expansion (LLE) and leaflet width expansion (LWE) rates were calculated from these data. After full expansion, leaflet area (LA) and specific leaf weight (SLW) were determined. Maturity Group III cultivars differed for: LA, LW, and LWE of the sixth, eighth, and tenth leaves; SLW of the sixth and eighth leaves; LLE of the eighth and tenth leaves; and LL of the tenth leaf. Maturity Group IV cultivars differed for SLW, LA, LL, LW, LLE, and LWE for all three leaves. The tenth leaf of most cultivars was smaller and expanded at a slower rate than the sixth leaf. Highly significant correlations between LA and leaflet expansion rates were found (r = 0.55 to 0.90). For the eighth and tenth leaves of Maturity Group IV, high values for SLW were associated with slow rates of leaflet expansion. Specific leaf weight of Maturity Group III was not correlated with any other leaflet characteristic. Because single leaflet expansion and total leaf area expansion rates are negatively related to SLW or apparent photosynthesis, it may be difficult to identify cultivars that combine a fast rate of leaf expansion with a fast rate of apparent photosynthesis.

Manganese Deficiency and Toxicity Effects on Photosynthesis, Chlorophyll, and Transpiration in Wheat1

Wheat (Triticum aestivum L.) production areas are expanding to soils low in available Mn in southern Georgia and to soils high in available Mn in northern Georgia. Useful data on Mn deficient and toxic levels in leaf tissue for top growth and physiological processes in wheat are needed to evaluate growth processes limiting wheat production. The objectives of this study were to correlate Mn deficiency and toxicity levels with top dry weight production, photosynthesis, chlorophyll concentration, and transpiration. Wheat plants were grown in 10 experimental concentrations ranging from 0.09 μM to 9.1 mM in nutrient solution. Photosynthesis and transpriration measurements were made on blade 1 of plants grown in the greanhouse. Total chlorophyll and Mn concentrations were determined on the experimental tissue after freeze‐drying. Foliar dry weights on a per pot (15 plants) basis were also determined. Severe Mn deficiency depressed top dry weight, photosynthesis, and total chlorophyll concentration, but transpiration was unaffected. Mild Mn toxicity reduced top dry weight but not photosynthesis, total chlorophyll concentration, or transpiration. Critical Mn deficiency levels for reducing top dry weight and apparent photosynthesis rates were similar, namely 0.23 and 0.30 mmol kg−1, repectively. Growth reduction from Mn deficiency is possibly due to reduced photosynthesis. Minimum Mn requirements for chlorophyll (0.14 mmol kg−1) is about one‐half of that for top production. The critical Mn deficiency level for transpiration could not be determined. The critical Mn toxicity level for top dry weight occurred at 7 mmol kg−1. Concentrations of 19, 19, and 24 mmol kg−1 were required for the depression of photosynthesis, total chlorophyll concentration, and transpiration, respectively. Maximum growth and yield are expected when the range of Mn concentration in blade 1 is 0.23 to 7 mmol kg−1. The critical levels are prelimilinary values to be used in evaluating Mn status of field‐grown wheat for possible deficiency and toxicity.

Heterosis of Apparent Photosynthesis Rate in Arachis hypogaea L.1

Abstract Complete diallel crosses were made among three parental peanut (Arachis hypogaea L.) genotypes to gain a better understanding of photosynthetic inheritance. Highly controlled environmental conditions were used throughout the study to determine apparent photosynthetic rates. Results indicated significant differences among parents and F1 hybrids. Reciprocal differences were also observed between certain cross combinations. Dominance and overdominance effects were found under these test regimes which suggest a heterotic response for this important characteristic.

Canopy Structure and Photosynthesis of Alfalfa Genotypes Differing in Nodule Effectiveness1

Photosynthesis is linked to N2 fixation and to other plant metabolic processes, but little is known of how N2‐fixation capability affects the development of the leaf canopy and leaf photosynthetic rates. Field experiments were undertaken to determine the effect of differences in N2‐fixation capability mediated by plant genes on canopy structure and apparent photosynthesis of alfalfa (Medicago saliva L.). An alfalfa genotype lacking the capability of establishing an effective symbiosis with Rhizobium was compared to normal alfalfa. Entries were grown at two rates of supplemental N (0 and 100 kg ha−1 as NH4NO5) to provide genetic‐ and culturally‐controlled differences in N2 fixation. Hostmediated genetic differences in N2‐fixation capability (ineffective vs. effective nodules) elicited pronounced changes in rates of apparent photosynthesis, canopy structure, and specific, leaf weight. Compared with alfalfa having effective nodules (Eff), alfalfa with ineffective nodules (In) averaged 17% greater specific leaf weight, but 73% less leaf area over both N regimes and three growth‐harvest cycles. For unshaded leaves at the top of the canopies, rate of apparent photosynthesis per leaf area of In was similar to that of Eff on vegetative plants 2 weeks before harvest and in canopies at 5% flower. Leaf canopies of In contained a more vertically uniform leaf area than Eff canopies unless they were supplied with 100 kg ha−1 combined N. In vegetative In plants 2 weeks before harvest, supplemental N at a rate of 100 kg ha−1 enhanced leaf area by 50%, but tended to reduce rate of apparent photosynthesis per leaf area. In Eff plants at this same growth stage, supplemental N also reduced apparent photosynthesis, but leaf area was not affected. In canopies at 5% flower, supplemental N did not change rate of apparent photosynthesis of either In or Eff. These results show that N stress induced by host‐mediated ineffective nodulation greatly reduced leaf area and photosynthetic rate per ground area, but that effective nodules were not a prerequisite for maintaining normal photosynthetic rates per leaf area even under severely limiting soil N supply.

Effects of Air Temperature on the Growth of Mitsumata (Edgeworthia papyrifera Sieb. et Zucc.)

The influences of air temperature on the growth of mitsumata under pot conditions, were examined from the view point of dry matter production. The results were summarized as follows: 1. The apparent photosynthetic rate was light-saturated at about 20 klx, though slight depression was observed above 40 klx, when the plants were grown under 50% shading (Fig. 1). The optimum temperature of apparent photosynthesis was found at about 26°C. The respiratory rate increased from 20 to 40°C with 1.98 of Q10 value (Fig. 2). 2. The Relative Growth Rate (RGR) increased with increasing air temperature from 15 to 25°C. The Net Assimilation Rate (NAR) showed the same behaviour to RGR. The Leaf Area Ratio (LAR) decreased with increasing air temperature (Fig. 3). The rate of apparent photosynthesis grown under 25, 20 and 15°C for about 90 days, was the highest at 24, 21 and about 15°C, respectibly (Fig. 4). 3. The length of the new shoot and the thickness of the stem decreased markedly at 30°C, and the ratio of the stem thickness to the shoot length was the highest at 20°C (Fig. 5 and Fig. 6).

Effect of Pretreatment Temperature on Response of Photosynthesis Rate in Maize to Current Temperature.

Maize plants (hybrid XL45) were exposed to three temperature treatments of different duration: a 'growth' temperature (extending from sowing to the day before photosynthetic measurements were made), an 'acclimation' temperature (exposure to a given temperature for 24 h) and a 'measurement' temperature (the temperature during the photosynthetic measurement, 30-45 min duration). The measurement temperature had the greatest single effect on photosynthetic rate, which increased 2-3-fold between 16 and 35�C. The highest rate of apparent photosynthesis was measured in plants grown at 25�C, acclimated at 35�C and measured at 35�C (2.1 mg m-� s-�). The relative contribution of stomatal and residual resistances to the total resistance to CO2 uptake changed little with temperature treatment, but the absolute magnitude of these resistances was strongly affected by temperature. Stomatal resistances ranged from 200 to 700 s m-�. The stomatal resistance dominated, accounting for 65-80% of the total resistance to apparent photosynthesis. Chlorophyll concentrations changed in response to both growth and acclimation temperatures. Those grown at 16�C had the lowest concentration, those grown at 35�C the highest. The chlorophyll concentration changed over a 24-h acclimation period in both expanding and fully expanded leaves, increasing when plants were transferred to higher temperature and decreasing when the transfer was to lower temperature.

The Functional Morphology of Turf-Forming Seaweeds: Persistence in Stressful Marine Habitats

Many seaweeds that occur in physically stressful habitats or habitats subject to mod- erate herbivory grow as colonial turfs rather than as spatially separated individuals. The turf growth form is energetically expensive (the net production per gram ash free dry mass of turfs being 33-61% lower than that of individuals), but turfs suffer less physiological damage during desiccating low tides and lose less biomass to herbivores. The upper portions of turf-forming species show significantly greater rates of apparent photosynthesis and dark respiration than do the lower portions. This spatial partitioning of photosynthetic activity decreases the energetic cost of the turf arrangement and may allow basal portions to function as persistent resting stages during periods of adverse conditions when uprights cannot be maintained. Turf-forming species are specialized for areas that are subject to moderate grazing pressure and physical stresses. They are dependent upon these factors to prevent their competitive exclusion by more productive, but less resistant, seaweeds. Damage to apical por- tions causes increased branching that results in a more tightly compacted turf. Algae that regenerate in this way can adjust their growth form in accordance with varying levels of disturbance encountered in different habitats and thus incur the minimal cost consistent with survival in that area. For seaweeds occurring in stressful habitats, selection has favored characteristics that increase persistence in space and time even though these involve considerable losses in competitive ability and productivity.

RELATIONSHIPS AMONG PHOTOSYNTHETIC RATE, BEAN YIELD AND OTHER CHARACTERS IN FIELD-GROWN CULTIVARS OF SOYBEAN

In field experiments with young spaced plants of 48 soybean (Glycine max (L.) Merr.) cultivars, derived lines tended to have higher rates of apparent photosynthesis (PA) than their parent lines over 3 yr of tests using the 14CO2 method. For 2 yr in which chlorophyll and specific leaf weight (SLW) were also measured, a significant number of derived lines had higher PA, chlorophyll and SLW than their parent lines. With 12 cultivars of similar maturity, grown as row-crop tests for 4 yr, PA and leaf percent N were correlated with each other during August (pod-filling period) but not during July (flowering period). August PA and leaf percent N were correlated with bean yield, but July values were not. Chlorophyll and SLW were correlated with each other in July and in August; each was correlated with PA in July but not in August. Although July measurements of leaf characters were not correlated with bean yield, July and August values were correlated for PA and for chlorophyll. The correlations observed among the characters in different sampling periods are discussed in relation to possibilities for selection.

Dependence of rates of apparent photosynthesis on tissue phosphorus concentrations in Myriophyllum spicatum L.

The relationship between internal phosphorus concentration and apparent photosynthesis was examined in 10-cm shoot tips of the submersed macrophyte Myriophyllum spicatum L. A pseudo-Michaelis-Menten type relationship was observed in both laboratory and field-grown plants. Curves fit to the data showed phosphorus compensation points (tissue phosphorus concentrations at which the predicted rates of apparent photosynthesis = 0) of 0.024 and 0.023% for laboratory and field-grown plants respectively. Critical concentrations (tissue phosphorus concentration resulting in a predicted rate of apparent photosynthesis equal to 90% of a “mean saturated” photosynthetic rate) were determined to be 0.29 and 0.32%, respectively. Literature values for tissue phosphorus concentrations frequently fall in the range between Gerloff's critical concentration of 0.07% and the 0.3% critical concentration derived in the current study, indicating that data originally interpreted as showing phosphorus sufficiency may indeed indicate suboptimal levels of phosphorus.

Effect of Irradiance on Development of Apparent Nitrogen Fixation and Photosynthesis in Soybean

Soybeans grown with 2 millimolar NO(3) (-), which optimized apparent N(2) fixation by Rhizobium symbionts, showed significantly different rates of apparent photosynthesis and C(2)H(2) reduction during seedling development at two irradiances. Those physiological processes were lower for several weeks in plants grown at 1,500 microeinsteins per meter(2) per second than in those exposed to 700 microeinsteins per meter(2) per second. The irradiance-induced retardation was evident in short-term rates of apparent photosynthesis and N(2) fixation, as well as in measures of dry matter and total N accumulation. In spite of their previously inhibited development, plants grown at 1,500 microeinsteins per meter(2) per second were indistinguishable by day 28 from those exposed to 700 microeinsteins per meter(2) per second in terms of whole-shoot CO(2)-exchange rate; by day 35 they were identical in terms of whole-plant C(2)H(2)-reduction rate. On day 38 there was no significant difference in dry weight or N content between treatments. Shifting plants between irradiance treatments on day 21 showed that the higher irradiance also had a short-term inhibitory effect on C(2)H(2) reduction. The fact that 16 millimolar NO(3) (-) prevented the continuous exposure to 1,500 microeinsteins per meter(2) per second from inhibiting apparent photosynthesis suggested that seedlings grown on 2 millimolar NO(3) (-) with Rhizobium were N-limited. Although rates of apparent photosynthesis were similar by day 28, the additional week required to produce equal rates of apparent N(2) fixation between irradiance treatments showed that physiological adaptations of shoots, as well as photosynthesis per se, can affect root nodule activity.

Carbon Exchange Rates of Shoots Required to Utilize Available Acetylene Reduction Capacity in Soybean and Alfalfa Root Nodules

The CO(2)-exchange rate required to make full use of available N(2)-fixation capacity, measured as acetylene reduction, was determined in soybean and alfalfa. Carbohydrates of root systems were depleted during a 40-hour dark treatment; then plants were exposed to a 24-hour light period during which different CO(2)-exchange rates were maintained with various CO(2) concentrations. In three- and four-week-old soybeans and four-week-old alfalfa plants, acetylene-reduction capacity was used fully with CO(2)-exchange rates as low as 10 milligrams CO(2) per plant per hour. In six-week-old alfalfa plants, however, acetylene reduction rates increased linearly, and apparent N(2)-fixation capacity was not used fully when CO(2)-exchange rates were higher than 40 milligrams CO(2) per plant per hour. Under the conditions established, the energy cost of N(2) fixation, measured as Delta(respiration of roots + nodules)/Deltaacetylene reduction over dark-treatment values, was 0.453 milligrams CO(2) per micromole C(2)H(4) for all rates of acetylene reduction and for both ages of soybean and alfalfa plants. Thus, root-plus-nodule respiration was not promoted by higher rates of apparent photosynthesis after C(2)H(2)-reduction capacity became saturated, and all available capacity for apparent N(2) fixation had the same energy requirement.