EFFECTS OF NITRATE CONCENTRATION ON THE GROWTH AND PHYSIOLOGY OF LAMINARIA SACCHARINA (PHAEOPHYTA) IN CULTURE1,2

Laminaria saccharina Lamour. sporophytes were grown in enriched and synthetic media through a range of nitrate concentrations, There was an approximately linear relationship between growth and nutrient concentration up to 10 μ substrate concentration. The half-saturation constant (K_{2) was ca. 1.4 μ NO3-. The internal levels of NO3- increased at substrate concentrations above 10 μM b>3- and reached levels several thousand times higher than the surrounding medium. Thus there is evidence for luxury consumption of NOsb>3}^-. The chlorophyll content and photosynthetic capacities of plants also increased with increasing external NO₃^- The ecological implications of this work are considered.     

Estimation of growth yield and maintenance coefficient of plant cell suspensions

Methodology is presented for the determination of growth yield (Y(g)) and maintenance coefficient (m) for carbon utilization of plant cells grown in suspension culture. Estimation of Y(g) and m requires measurements of specific growth rate (micro) and specific rate of substrate uptake (q) at different growth limiting substrate concentrations. Batch culture of tobacco cells did not permit evaluation of Y(g) and m because micro is constant and maximal during most of the growth cycle. In batch culture, the period of declining specific growth rate is extremely brief because of the rapid transition from logarithmic growth to stationary phase. This occurs because the K(m) for growth is relatively small compared to the initial sucrose concentration. Thus, when the substrate level reaches the K(m), the large mass of cells rapidly depletes the remaining substrate. In contrast, semicontinuous culture facilitates the determination of Y(g) and m because various steady-state growth rates can be achieved. Mathematical expressions were developed to determine the effective values of micro and q over the semicontinuous replacement interval. The validity of this approach was verified by conducting simulations using experimentally determined parameters.     

Discrimination between12C and13C by marine plants

Summary The natural abundance¹³C/¹²C ratios (as δ¹³C) of organic matter of marine macroalgae from Fife and Angus (East Scotland) were measured for comparison with the species' ability to use CO₂ and HCO ₃ ^- for photosynthesis, as deduced from previously published pH-drift measurements. There was a clear difference in δ¹³C values for species able or unable to use HCO ₃ ^- . Six species of Chlorophyta, 12 species of Phaeophyta and 8 species of Rhodophyta that the pH-drift data suggested could use HCO ₃ ^- had δ¹³C values in the range -8.81‰ to -22.55‰. A further 6 species of Rhodophyta which the pH-drift data suggested could only use CO₂ had δ¹³C values in the range -29.90‰ to-34.51‰. One of these six species (Lomentaria articulata) is intertidal; the other five are subtidal and so have no access to atmospheric CO₂ to complicate the analysis. For these species, calculations based on the measured δ¹³C of the algae, the δ¹³C of CO₂ in seawater, and the known¹³C/¹²C discrimination of CO₂ diffusion and RUBISCO carboxylation suggest that only 15–21% of the limitation to photosynthesisin situ results from CO₂ diffusion from the bulk medium to the plastids; the remaining 79–85% is associated with carboxylation reactions (and, via feedback effects, down-stream processes). This analysis has been extended for one of these five species,Delesseria sanguinea, by incorporating data onin situ specific growth rates, respiratory rates measured in the laboratory, and applying Fick's law of diffusion to calculate a boundary layer thickness of 17–24 μm. This value is reasonable for aDelesseria sanguinea frondin situ. For HCO ₃ ^- -using marine macroalgae the range of δ¹³C values measured can be accommodated by a CO₂ efflux from algal cells which range from 0.306 of the gross HCO ₃ ^- influx forEnteromorpha intestinalis (δ¹³C=-8.81‰) in a rockpool to 0.787 forChondrus crispus (δ¹³C=-22.55‰). The relatively high computed CO₂ efflux for those HCO ₃ ^- -users with the more negative δ¹³C values implies a relatively high photon cost of C assimilation; the observed photon costs can be accommodated by assuming coupled, energy-independent inorganic carbon influx and efflux. The observed δ¹³C values are also interpreted in terms of water movement regimes and obtaining CO₂ from the atmosphere. Published δ¹³C values for freshwater macrophytes were compared with the ability of the species to use CO₂ and HCO ₃ ^- and again there was an apparent separation in δ¹³C values for these two groups. δ¹³C values obtained for marine macroalgae for which no pH-drift data are available permit predictions, as yet untested, as to whether they use predominantly CO₂ or HCO ₃ ^-     

Intraspecific variation in the response of rice (Oryza sativa) to increased CO2– photosynthetic, biomass and reproductive characteristics

Two rice ( Oryza sativa L.) cultivars of contrasting morphologies, IR-36 and Fujiyama-5, were exposed to ambient (360 μl l⁻¹) and ambient plus 300 μl l⁻¹ CO₂ from time of emergence until ca 50% grain fill at the Duke University Phytotron, Durham, North Carolina. Exposure to increased CO₂ resulted in about a 50% increase in the photosynthetic rate for both cultivars and photosynthetic enhancement was still evident after 3 months of exposure to a high CO₂ environment. The photosynthetic response at 5% CO₂ and the response of CO₂ assimilation (A) to internal CO₂ (C_i) suggest a reallocation of biochemical resources from RuBP carboxylation to RuBP regeneration. Increases in total plant biomass at elevated CO₂ were approximately the same in both cultivars, although differences in allocation patterns were noted in root/shoot ratio. Differences in reproductive characteristics were also observed between cultivars at an elevated CO₂ environment with a significant increase in harvest index for IR-36 but not for Fujiyama-5. Changes in carbon allocation in reproduction between these two cultivars suggest that lines of rice could be identified that would maximize reproductive output in a future high CO₂ environment.     

Carbon isotope discrimination at vegetative stage as an indicator of yield and water use efficiency of spring wheat (Triticum turgidum L. var. durum)

A field experiment was conducted to investigate if carbon isotope (¹³C) discrimination () measured at the vegetative stage of spring wheat (Triticum turgidum L. var. durum) is related with the yield and water use efficiency (WUE) at ripening. A line source sprinkler irrigation system exposed the wheat genotypes to different watering regimes, from rainfed to full irrigation and thereby increased the range in yield and WUE attainable in the four genotypes studied. The results indicated that values measured at the late stem elongation stage 60 days after planting (DAP), showed strong positive correlation with total dry matter yield (r=0.732^***), and a highly significant negative correlation with WUE (r=–0.755^***) measured at ripening 105 DAP. The data suggest that the imprints of measured at vegetative growth stage persists throughout the entire growth period, until maturity. Subject to confirmation from additional studies in other crops and locations, early measurements of may prove a useful tool for rapid and early screening of cultivars, for high yield and high WUE.     

Root turnover as determinant of the cycling of C,N, and P in a dry heathland ecosystem

Root production and turnover were studied using sequential core sampling and observations in permanent minirhizotrons in the field in three dry heathland stands dominated by the evergreen dwarfshrub Calluna vulgaris and the grasses Deschampsia flexuosa and Molinia caerulea, respectively. Root biomass production, estimated by core sampling, amounted to 160 (Calluna), 180 (Deschampsia) and 1380 (Molinia) g m^-2 yr^-1, respectively. Root biomass turnover rate in Calluna (0.64 yr^-1) was lower compared with the grasses (Deschampsia: 0.96 yr^-1; Molinia 1.68yr^-1)). Root length turnover rate was 0.75–0.77 yr^-1 (Deschampsia) and 1.17–1.49 yr^-1 (Molinia), respectively. No resorption of N and P from senescing roots was observed in either species. Input of organic N into the soil due to root turnover, estimated using the core sampling data, amounted to 1.8 g N m^-2 yr^-1(^Calluna), 1.7 g N m^-2 yr^-1 (Deschampsia) and 19.7 g N m^-2 yr^-1 (Molinia), respectively. The organic P input was 0.05, 0.07 and 0.55 g P M^-2 yr^-1, respectively. Using the minirhizotron turnover estimates these values were20–22% (Deschampsia) and 11–30% (Molinia) lower.When the biomass turnover data were used, it appeared that in the Molinia stand root turnover contributed 67% to total litter production, 87% to total litter nitrogen loss and 84% to total litter phosphorus loss. For Calluna and Deschampsia these percentages were about three and two times lower, respectively.This study shows that (1) Root turnover is a key factor in ecosystem C, N, and P cycling; and that (2) The relative importance of root turnover differs between species.     

Dynamics of carbon assimilation by St Lawrence estuary phytoplankton at the end of summer

This study presents data of in situ measurements of inorganic carbon assimilation by phytoplankton communities of the St Lawrence estuary during the end of summer 1982. We used carboxylase activity measurements (ribulose-1,5-bisphosphate carboxylase, carboxylases) and the ¹³C/¹²C ratio of phytoplankton organic carbon, expressed as ¹³C, to study patterns of assimilation. Upper estuary phytoplankton communities showed a smaller turn-over rate in carbon assimilation than lower estuary phytoplankton communities. Carbon assimilation was limited by light intensity in the upper estuary and by CO₂ availability in the lower estuary. In the St Lawrence estuary, stable carbon isotope ratios of phytoplankton organic carbon seemed to be controlled by inorganic carbon availability rather than by phytoplankton metabolism.     

EFFECTS OF VARIATIONS IN LIGHT INTENSITY ON THE EFFICIENCY OF GROWTH OF SKELETONEMA COSTATUM (BACILLARIOPHYCEAE) IN A CYCLOSTAT1

The relative importance of respiration and organic carbon release to the efficiency of carbon specific growth of Skeletonema costatum (Grev.) Clave was evaluated over a light range from 1500–15 μE · m^?2· s^?1. Net growth efficiency ranged from 0.45–0.69 with a maximum at 130 μE · m^?2· s^?1. Respiration was 93% or more of the variations in growth efficiency. Organic carbon release ranged from 0–7% of gross production and increased with light intensity. Carbon specific particulate production was a hyperbolic function of incident light intensity and was related exponentially to particulate carbon production per unit chlorophyll a. Full sunlight conditions, 1500 μE · m^?2· s^?1, did not induce photoinhibition of gross production. Variations in the efficiency of growth of S. costatum were minimized over a wide range of light intensities mainly because of variations in cellular pigments which permitted the efficient utilization of available light energy, and a reduction in the losses of carbon which increases the growth rate, possibly as a consequence of the recycling of respired carbon within the cell.     

Carbon and nitrogen metabolism in legume root nodules

The literature concerning the metabolism of carbon compounds during the reduction, assimilation and translocation of nitrogen in root nodules of leguminous plants is reviewed. The reduction of dinitrogen requires an energy source (ATP) and a reluctant which are both supplied by respiratory catabolism of carbohydrates produced by the host plant. Photosynthates are also required to generate the carbon skeletons for amino acid or urcide synthesis during the assimilation of ammonia produced by the bacteria within the nodule tissue. Competition for photosynthates occurs between the bacteroids, nodule tissue and the various vegetative and reproductive sinks in the host plant. The nature of carbon compounds involved in these processes, their routes of metabolism, the mechanisms of control and the partitioning of metabolises between the various sites of utilization are only poorly understood. It is apparent that dinitrogen is reduced to ammonia in the bacteroids. Both fast- and slow-growing strains of Rhizobium possess the Entner-Doudoroff pathway of glucose catabolism, and some, if not all, enzymes of the Emden-Meyerhof pathway. Some bacterial cultures also metabolize carbon through the ketogluconate pathway but only the fast-growing strains of cultured rhizobia possess the key enzyme of the pentose phosphate pathway (6-phosphogluconate dehydrogenase). The host cells are thought to contain the complete Emden-Meyerhof pathway and tricarboxylic acid cycle, which provides the carbon skeletons for assimilation of the ammonia, formed by the bacteroids, into α-amino acids. A pathway of anapleurotic carbon conservation, operative in the host cells, synthesizes oxaloacetic acid through β-carboxylation of phosphoenol pyruvate. This process could be important in the recapture and assimilation of respired CO₂ in the rhizosphere. The main route of assimilation of ammonia produced by the bacteroids would appear to be via the glutamine synthetase-glutamate synthase pathway in the host cells. However, glutamate dehydrogenase may also be involved in ammonia assimilation. These enzymes also occur in in vitro cultures of Rhizobium and in bacteroids where they presumably participate in the synthesis of amino acids for growth of the bacteria or bacteroids. Nitrogen assimilated into glutamine or glutamate is exported from the nodules in a variety of forms, which include asparagine, glutamine, aspartate, homoserine and allantoates, in proportions which depend on the legume species. Studies on regulation of the overall process have focussed on expression of bacteroid genes and on the control of enzyme activity, at the level of nitrogenase and enzymes of nitrogen assimilation in particular. However, due to the wide range of experimental techniques, environmental conditions and plant species which have been used, no clear conclusions can yet be drawn. The pathways of carbon flow in nitrogen metabolism, particularly in relation to the synthesis of ureides and the regulation of carbon metabolism, remain key areas for future research in symbiotic nitrogen fixation.