ALBERT

Notes: Plants of Cirsium vulgare (Savi) Ten. were cultivated under five different nitrogen regimes in order to investigate the effects of nitrogen supply on the storage processes in a biennial species during its first year of growth.External N supply increased total biomass production without changing the relationship between ‘productive plant compartments’ (i.e. shoot plus fine roots) and ‘storage plant compartments’ (i.e. structural root dry weight, which is defined as the difference between tap root biomass and the amount of stored carbohydrates and N compounds). The amount of carbohydrates and N compounds stored per unit of structural tap root dry weight was not affected by external N availability during the season, because high rates of N supply increased the concentration of N compounds whilst decreasing the carbohydrate concentration, and low rates of N supply had the opposite effect. Mobilization of N from senescing leaves was not related to the N status of the plants. The relationship between nitrogen compounds stored in the tap root and the maximum amount of nitrogen in leaves was an increasing function with increasing nitrogen supply. We conclude that the allocation between vegetative plant growth and the growth of storage structures over a wide range of N availability seems to follow predictions from optimum allocation theory, whereas N storage responds in a rather plastic way to N availability.

Notes: The cost of nitrogen storage to current growth was examined in relation to N availability in the biennial Cirsium vulgare. Plants were grown outdoors, in sand culture, with continuous diel drip irrigation of fertilization medium containing one of five different N concentrations. Plants grown at the highest N concentration stored twice as much N in their tap roots as did plants grown at the lowest N concentration. In high-N-grown plants, the storage of N reserves occurred during the period of maximum growth, at the same time as tap-root production. At the time of maximum biomass, stored N was also at a maximum. During the period following maximum biomass, no additional storage of N occurred. This pattern was observed despite frequent late-season leaf senescence which resulted in a large pool of potentially mobile N which could have been stored at no cost to growth. In low-N-grown plants, the production of tap-root storage tissue and the filling of that tissue with stored N were staggered. Tap-root production and growth occurred during the period of maximum growth, as in the high-N-grown plants. However, filling of the storage tissue with N occurred late in the growing season, when the pool of mobile N from senescent leaves was large. The utilization of this late-season N source occurred with little or no cost to growth, and this N is labelled, according to previous definitions, as ‘accumulated’. The costs of storing N in plants of the different N treatments were calculated using two models based on different growth constraints. In one model, the cost of N storage was represented as lost growth due to allocation of N to storage, rather than to the photosynthetic shoot (i.e. growth was assumed to be limited by carbon acquisition). In the second model, the storage cost was calculated as lost growth due to allocation of N to storage, rather than to the nitrogen-acquiring fine-root system (i.e. growth was assumed to be limited by nitrogen acquisition). In both models, the total cost of N storage was predicted to decrease as N availability decreased due to smaller storage pool sizes in plants of the low-N treatments. The cost of filling the tap root with stored N as a percentage of the total storage cost was also reduced as N availability decreased due to the occurrence of late-season accumulation. By relying, at least in part, on late-season accumulation, plants grown at the lowest three levels of N availability reduced total storage costs by 15 to 22%. The results demonstrate that plants are capable of adjusting their storage patterns in response to low nitrogen availability such that the costs of storage are reduced.

Notes: A model is presented which solves simultaneously for leaf-scale stomatal conductance, CO2 assimilation and the energy balance as a function of leaf position within canopies of well-watered vegetation. Fluxes and conductances were calculated separately for sunlit and shaded leaves. A linear dependence of photosynthetic capacity on leaf nitrogen content was assumed, while leaf nitrogen content and light intensity were assumed to decrease exponentially within canopies. Separate extinction coefficients were used for diffuse and direct beam radiation. An efficient Gaussian integration technique was used to compute fluxes and mean conductances for the canopy. The multilayer model synthesizes current knowledge of radiation penetration, leaf physiology and the physics of evaporation and provides insights into the response of whole canopies to multiple, interacting factors. The model was also used to explore sources of variation in the slopes of two simple parametric models (nitrogen- and light-use efficiency), and to set bounds on the magnitudes of the parameters.For canopies low in total N, daily assimilation rates are ∼10% lower when leaf N is distributed uniformly than when the same total N is distributed according to the exponentially decreasing profile of absorbed radiation. However, gains are negligible for plants with high N concentrations. Canopy conductance, Gc should be calculated as Gc=Aσ(fslgsl+fshgsh), where Δ is leaf area index, fsi and fsh are the fractions of sunlit and shaded leaves at each level, and gsi and gsh are the corresponding stomatal conductances. Simple addition of conductances without this weighting causes errors in transpiration calculated using the ‘big-leaf’ version of the Penman-Monteith equation. Partitioning of available energy between sensible and latent heat is very responsive to the parameter describing the sensitivity of stomata to the atmospheric humidity deficit. This parameter also affects canopy conductance, but has a relatively small impact on canopy assimilation.Simple parametric models are useful for extrapolating understanding from small to large scales, but the complexity of real ecosystems is thus subsumed in unexplained variations in parameter values. Simulations with the multilayer model show that both nitrogen- and radiation-use efficiencies depend on plant nutritional status and the diffuse component of incident radiation, causing a 2- to 3-fold variation in these efficiencies.

Notes: We have investigated the interactions between resource assimilation and storage in rosette leaves, and their impact on the growth and reproduction of the annual species Arabidopsis thaliana. The resource balance was experimentally perturbed by changing (i) the external nutrition, by varying the nitrogen supply; (ii) the assimilation and reallocation of resources from rosette leaves to reproductive organs, by cutting or covering rosette leaves at the time of early flower bud formation, and (iii) the internal carbon and nitrogen balance of the plants, by using isogenic mutants either lacking starch formation (PGM mutant) or with reduced nitrate uptake (NU mutant).When plants were grown on high nitrogen, they had higher concentrations of carbohydrates and nitrate in their leaves during the rosette phase than during flowering. However, these storage pools did not significantly contribute to the bulk flow of resources to seeds. The pool size of stored resources in rosette leaves at the onset of seed filling was very low compared to the total amount of carbon and nitrogen needed for seed formation. Instead, the rosette leaves had an important function in the continued assimilation of resources during seed ripening, as shown by the low seed yield of plants whose leaves were covered or cut off. When a key resource became limiting, such as nitrogen in the NU mutants and in plants grown on a low nitrogen supply, stored resources in the rosette leaves (e.g. nitrogen) were remobilized, and made a larger contribution to seed biomass. A change in nutrition resulted in a complete reversal of the plant response: plants shifted from high to low nutrition exhibited a seed yield similar to that of plants grown continuously on a low nitrogen supply, and vice versa. This demonstrates that resource assimilation during the reproductive phase determines seed production.The PGM mutant had a reduced growth rate and a smaller biomass during the rosette phase as a result of changes in respiration caused by a high turnover of soluble sugars (Caspar et al. 1986; W. Schulze et al. 1991). During flowering, however, the vegetative growth rate in the PGM mutant increased, and exceeded that of the wild-type. By the end of the flowering stage, the biomass of the PGM mutant did not differ from that of the wild-type. However, in contrast to the wild-type, the PGM mutant maintained a high vegetative growth rate during seed formation, but had a low rate of seed production. These differences in allocation in the PGM mutant result in a significantly lower seed yield in the starchless mutants. This indicates that starch formation is not only an important factor during growth in the rosette phase, but is also important for whole plant allocation during seed formation. The NU mutant resembled the wild-type grown on a low nitrogen supply, except that it unexpectedly showed symptoms of carbohydrate shortage as well as nitrogen deficiency.In all genotypes and treatments, there was a striking correlation between the concentrations of nitrate and organic nitrogen and shoot growth on the one hand, and sucrose concentration and root growth on the other. In addition, nitrate reductase activity (NRA) was correlated with the total carbohydrate concentration: low carbohydrate levels in starchless mutants led to low NRA even at high nitrate supply. Thus the concentrations of stored carbohydrates and nitrate are directly or indirectly involved in regulating allocation.

Notes: Root exudates were sampled from detopped root systems of castor bean (Ricinus communis). Different volume flux rates were imposed by changing the pneumatic pressure around the root system using a Passioura-type pressure chamber. The concentrations of cations, anions, amino acids, organic acids and abscisic acid decreased hyperbolically when flux rates increased from pure root exudation up to values typical for transpiring plants. Concentrations at low and high fluxes differed by up to 40 times (phosphate) and the ratio of substances changed by factors of up to 10. During the subsequent reduction of flux produced by lowering the pneumatic pressure in the root pressure chamber, the concentrations and ratios of substances deviated (at a given flux rate) from those found when flux was increased. The flux dependence of exudate composition cannot therefore be explained by a simple dilution mechanism. Xylem sap samples from intact, transpiring plants were collected using a Passioura-type root pressure chamber. The concentrations of the xylem sap changed diurnally. Substances could be separated into three groups: (1) calcium, magnesium and amino acid concentrations correlated well with the values expected from their concentration-flux relationships, whereas (2) the concentrations of sulphate and phosphate deviated from the expected relationships during the light phase, and (3) nitrate and potassium concentrations in intact plants varied in completely the opposite manner from those in isolated root systems. Abscisic acid concentrations in the root exudate were dependent on the extent of water use and showed strong diurnal variations in the xylem sap of intact plants even in droughtstressed plants. Calculations using root exudates overestimated export from the root system in intact plants, with the largest deviation found for proton flux (a factor of 10). We conclude that root exudate studies cannot be used as the sole basis for estimating fluxes of substances in the xylem of intact plants. Consequences for studying and modelling xylem transport in whole plants are discussed.

Notes: Abstract Experiments were carried out to determine how decreased expression of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) affects photosynthetic metabolism in ambient growth conditions. In a series of tobacco (Nicotiana tabacum L.) plants containing progressively smaller amounts of Rubisco the rate of photosynthesis was measured under conditions similar to those in which the plants had been grown (310 μmol photons · m−2 · s−1, 350 μbar CO2, 22° C). (i) There was only a marginal inhibition (6%) of photosynthesis when Rubisco was decreased to about 60% of the amount in the wildtype. The reduced amount of Rubisco was compensated for by an increase in Rubisco activation (rising from 60 to 100%), with minor contributions from an increase of its substrates (ribulose-1,5-bisphosphate and the internal CO2 concentration) and a decrease of its product (glycerate-3-phosphate). (ii) The decreased amount of Rubisco was accompanied by an increased ATP/ADP ratio that may be causally linked to the increased activation of Rubisco. An increase of highenergy-state chlorophyll fluorescence shows that thylakoid membrane energisation and high-energy-state-dependent energy dissipation at photosystem two had also increased. (iii) A further decrease of Rubisco (in the range of 50–20% of the wildtype level) resulted in a strong and proportional inhibition of CO2 assimilation. This was accompanied by a decrease of fructose-1,6-bisphosphatase activity, coupling-factor 1 (CF1)-ATP-synthase protein, NADP-malate dehydrogenase protein, and chlorophyll. The chlorophyll a/b ratio did not change, and enolase and sucrose-phosphate synthase activity did not decrease. It is argued that other photosynthetic enzymes are also decreased once Rubisco decreases to the point at which it becomes strongly limiting for photosynthesis. (iv) It is proposed that the amount of Rubisco in the wildtype represents a balance between the demands of light, water and nitrogen utilisation. The wildtype overinvests about 15% more protein in Rubisco than is needed to avoid a strict Rubisco limitation of photosynthesis. However, this “excess” Rubisco allows the wildtype to operate with lower thylakoid energisation, and decreased high-energy-state-dependent energy dissipation, hence increasing light-use efficiency by about 6%. It also allows the wildtype to operate with a lower internal CO2 concentration in the leaf and a lower stomatal conductance at a given rate of photosynthesis, so that instantaneous water-use efficiency is marginally (8%) increased.

Notes: Abstract Transgenic tobacco (Nicotiana tabacum L.) plants transformed with ‘antisense’ rbcS to produce a series of plants with a progressive decrease in the amount of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) have been used to investigate the contribution of Rubsico to the control of photosynthesis at different irradiance, CO2 concentrations and vapour-pressure deficits. Assimilation rates, transpiration, the internal CO2 concentration and chlorophyll fluorescence were measured in each plant. (i) The flux-control coefficient of Rubisco was estimated from the slope of the plot of Rubisco content versus assimilation rate. The flux-control coefficient had a value of 0.8 or more in high irradiance, (1050 μmol·m−2·s−1), low-vapour pressure deficit (4 mbar) and ambient CO2 (350 μbar). Control was marginal in enhanced CO2 (450 μbar) or low light (310 μmol·m−2·s−1) and was also decreased at high vapour-pressure deficit (17 mbar). No control was exerted in 5% CO2. (ii) The flux-control coefficients of Rubisco were compared with the fractional demand placed on the calculated available Rubisco capacity. Only a marginal control on photosynthetic flux is exerted by Rubisco until over 50% of the available capacity is being used. Control increases as utilisation rises to 80%, and approaches unity (i.e. strict limitation) when more than 80% of the available capacity is being used. (iii) In low light, plants with reduced Rubisco have very high energy-dependent quenching of chlorophyll fluorescence (qE) and a decreased apparent quantum yield. It is argued that Rubisco still exerts marginal control in these conditions because decreased Rubisco leads to increased thylakoid energisation and high-energy dependent dissipation of light energy, and lower light-harvesting efficiency. (iv) The flux-control coefficient of stomata for photosynthesis was calculated from the flux-control coefficient of Rubisco and the internal CO2 concentration, by applying the connectivity theorem. Control by the stomata varies between zero and about 0.25. It is increased by increased irradiance, decreased CO2 or decreased vapour-pressure deficit. (v) Photosynthetic oscillations in saturating irradiance and CO2 are suppressed in decreased-activity transformants before the steady-state rate of photosynthesis is affected. This provides direct evidence that these oscillations reveal the presence of “excess” Rubisco. (vi) Comparison of the flux-control coefficients of Rubisco with mechanistic models of photosynthesis provides direct support for the reliability of these models in conditions where Rubisco has a flux-control coefficient approach unity (i.e. “limits” photosynthesis), but also indicates that these models are less useful in conditions where control is shared between Rubisco and other components of the photosynthetic apparatus.

Notes: Abstract Throughfall nitrogen of a 15-year-old Picea abies (L.) Karst. (Norway spruce) stand in the Fichtelgebirge, Germany, was labeled with either 15N-ammonium or 15N-nitrate and uptake of these two tracers was followed during two successive growing seasons (1991 and 1992). 15N-labeling (62 mg 15N m-2 under conditions of 1.5 g N m-2 atmospheric nitrogen deposition) did not increase N concentrations in plant tissues. The 15N recovery within the entire stand (including soils) was 94%±6% of the applied 15N-ammonium tracer and 100%±6% of the applied 15N-nitrate tracer during the 1st year of investigation. This decreased to 80%±24% and 83%±20%, respectively, during the 2nd year. After 11 days, the 15N tracer was detectable in 1-year-old spruce needles and leaves of understory species. After 1 month, tracer was detectable in needle litter fall. At the end of the first growing season, more than 50% of the 15N taken up by spruce was assimilated in needles, and more than 20% in twigs. The relative distribution of recovered tracer of both 15N-ammonium and 15N-nitrate was similar within the different foliage age classes (recent to 11-year-old) and other compartments of the trees. 15N enrichment generally decreased with increasing tissue age. Roots accounted for up to 20% of the recovered 15N in spruce; no enrichment could be detected in stem wood. Although 15N-ammonium and 15N-nitrate were applied in the same molar quantities (15NH 4 + : 15NO 3 - =1:1), the tracers were diluted differently in the inorganic soil N pools (15NH 4 + /NH 4 + : 15NO 3 - /NO 3 - =1:9). Therefore the measured 15N amounts retained by the vegetation do not represent the actual fluxes of ammonium and nitrate in the soil solution. Use of the molar ammonium-to-nitrate ratio of 9:1 in the soil water extract to estimate 15N uptake from inorganic N pools resulted in a 2–4 times higher ammonium than nitrate uptake by P. abies.

Notes: Abstract Natural abundances of nitrogen isotopes, δ15N, indicate that, in the same habitat, Alaskan Picea glauca and P. mariana use a different soil nitrogen compartment from the evergreen shrub Vaccinium vitis-idaea or the deciduous grass Calamagrostis canadensis. The very low δ15N values (-7.7 ‰) suggest that (1) Picea mainly uses inorganic nitrogen (probably mainly ammonium) or organic N in fresh litter, (2) Vaccinium (-4.3 ‰) with its ericoid mycorrhizae uses more stable organic matter, and (3) Calamagrostis (+0.9 ‰) exploits deeper soil horizons with higher δ15N values of soil N. We conclude that species limited by the same nutrient may coexist by drawing on different pools of soil N in a nutrient-deficient environment. The differences among life-forms decrease with increasing N availability. The different levels of δ15N are associated with different nitrogen concentrations in leaves, Picea having a lower N concentration (0.62 mmol g−1) than Vaccinium (0.98 mmol g−1) or Calamagrostis (1.33 mmol g−1). An extended vector analysis by Timmer and Armstrong (1987) suggests that N is the most limiting element for Picea in this habitat, causing needle yellowing at N concentrations below 0.5 mmol g−1 or N contents below 2 mmol needle−1. Increasing N supply had an exponential effect on twig and needle growth. Phosphorus, potassium and magnesium are at marginal supply, but no interaction between ammonium supply and needle Mg concentration could be detected. Calcium is in adequate supply on both calcareous and acidic soils. The results are compared with European conditions of excessive N supply from anthropogenic N depositions.