Nutrition and growth of birch and grey alder seedlings in low conductivity solutions and at varied relative rates of nutrient addition

Birch (Betula verrucosa Ehrh.) and grey alder (Alnus incana Moench) seedlings were grown with varied relative addition rates of all nutrients, up to optimum for vegetative growth. The root medium was basically distilled water to which the nutrients, contained in stock solutions in fixed proportions, were added every second hour and in exponentially increased amounts for consumption during the subsequent period. The nutrient weight proportions previously found to be required in birch (100 N:65 K:13 P) were used in all treatments. However, the nutrient proportions required in grey alder were found to be somewhat different (100 N:50 K:18 P). The use of the required proportions in the additions was important for maintenance of maximum growth, efficient nutrient utilization, and low concentrations in the root medium. Luxury consumption and inefficiency occurred at high concentrations. The results show that the nutrient requirements are sufficiently defined, for different relative growth rates, by the nutrient proportions and the relative addition rate. No clear relationships were found between conductivity or concentration in the root medium and the addition rate, net uptake rate, nutrient status, or relative growth rate. The results are in good agreement with data from low concentration and depletion experiments reported in the literature, showing that non-limited uptake rates occur down to very low concentrations. Thus, there is strong evidence that concentration has been incorrectly used when applied as the treatment variable for plant nutrition in plant science and cultivation practice. The dominant factors in sub-optimum and optimum nutrition are the amounts of nutrients available per unit of time, the growth rate, and the nutrient proportions. At low concentration levels, physical factors such as stirring and flow rate of nutrient solution and boundary layer effects are decisive for the rates with which the nutrients become available to the roots. Therefore, at low levels, concentration alone cannot be used as the factor determining nutrient uptake rate. At high levels, concentration is effective as a supra-optimum factor and increased internal percentage contents cause decreased uptake efficiency, thus counter-acting the concentration effect. Nitrogen effects dominated the stress indications when the internal nitrogen percentage content decreased from optimum to the level of the treatments in the beginning of the experiments. Leaf deficiency symptoms disappeared and the root/shoot ratio change ceased when nitrogen status stabilized. Strong linear regressions were found between any two of the variables: relative addition rate of nutrients, relative growth rate, and nutrient status.     

Nutrition and growth of coniferous seedlings at varied relative nitrogen addition rate

The growth of two provenances of Pinus sylvestris L. were compared with two provenances of Picea abies (L.) Karst. and with Pinus contorta Dougl. when grown in solution cultures with low nutrient concentrations. Nitrogen was added at different exponentially increasing rates, and the other nutrients were added at a rate high enough to ensure free access of them to the seedlings. During an initial period of the culture (a lag phase), when the internal nutrient status was changing from optimum to the level of the treatment, deficiency symptoms appeared. The needles yellowed and the root/shoot ratio increased. The initial phase was followed by a period of exponential growth and steady-state nutrition. The needles turned green again, and the root/shoot ratio stabilized at a level characteristic of the treatment. These patterns were the same as previously reported for other tree species. The relative growth rate during exponential growth was numerically closely equal to the relative nitrogen addition rate. The maximum relative growth rates were about 6 to 7.5% dry weight increase day^-1. This is a much lower maximum than for broad-leaved species (about 20 to 30% day^-1) under similar growth conditions. The internal nitrogen concentrations of the seedlings and the relative growth rates were stable during the exponential period. Close linear relationships were found between these parameters and the relative addition rate up to maximum growth. During steady state the relative growth rates of the different plant parts were equal. However, there were large differences between genotypes in absolute root growth rate at the same seedling size because of differences in root/shoot ratio. Lodgepole pine had the highest root growth rate, whereas that of Norway spruce, especially the southern provenance, was remarkably low. Yet, Norway spruce had a high ability to utilize available nutrients. In treatments with free nutrient access, growth allocation to the shoot had a high priority in all genotypes, but there was still a marked tendency for luxury uptake of nutrients. Nitrogen productivity (growth rate per unit of nitrogen) was lower than in broadleaved species and highest in lodgepole pine. The relevance of the dynamic factors, i.e. maximum relative growth rate, nutrient uptake rate, nitrogen productivity, growth allocation and root growth rate, are discussed with regard to conifer characteristics and selection value.     

Biomass and nutrient dynamics of Chinese fir seedlings under conventional and exponential fertilization regimes

Containerized Chinese fir (Cunninghamia lanceolata (Lamb) Hook) were reared from seed at four fertilizer levels (0, 15, 45, 75 mg N seedling-1 season-1) and two topdressing schedules (conventional or exponential) for a 22-week greenhouse rotation to assess growth, nutrition and nutrient loading capacity of seedlings. Extra P supplemented high fertilization (or nutrient loading) treatments to test for induced deficiency of this element. The schedule and rate of fertilization significantly affected growth and nutrient dynamics of the seedlings. Steady-state nutrition and superior growth performance were achieved by seedlings fertilized exponentially at the operational dose (15 mg N), yielding 23, 72 and 52% more in respective biomass, N uptake and P uptake than seedlings fertilized conventionally at the equivalent dose. The improved growth and fertilizer efficiency were attributed to close synchronization of exponential nutrient supply with exponential growth and nutrient demand of plants. High dose exponential fertilization (45 and 75 mg N) induced steady state-nutrition late in the season, increasing seedling N and P uptake by 72–83% and 50–96% compared to low dose exponential fertilization, demonstrating effective nutrient loading of plants without changing biomass. The extra P stimulated P uptake without altering growth or N uptake, thus P was probably not limiting during the greenhouse culture despite high N additions.     

Growth, nutrition, and nitrogen fixation in grey alder at varied rate of nitrogen addition

Seedlings of grey alder (Alnus incana Moench), nodulated or unnodulated, were investigated at varied relative addition rate of nitrogen. Nitrogen fixation alone, without addition of mineral nitrogen, resulted in an almost optimum nitrogen status but only about half the maximum relative growth rate, probably mainly because of energy costs of nodulation and fixation. The growth deficit due to nodulation was much more than can be explained by the theoretical energy requirement for the amount of nitrogen fixed. Thus, the nitrogen fixation process was not very efficiently used. The nitrogen fixation rate was strongly stimulated by increasing nitrogen addition rate up to high levels. The fixation rate decreased rapidly close to optimum (maximum relative growth rate) and was negligible at maximum growth. A feed-back of mineral nitrogen on photosynthesis increased fixation rate with time, and the relative importance of fixation over mineral nitrogen nutrition increased. However, nitrogen fixation, also at maximum rate, supplied only a small proportion of the nitrogen amount required for maximum growth. The optimum nutrient solutions contained comparatively high nitrogen concentrations to secure free access to nitrogen. The nodules were damaged by this treatment, and it is concluded that the nitrogen additions must be adjusted to the current consumption of the plants to avoid an increased external nitrogen concentration. Strong linear regressions were found between relative growth rate, nitrogen status expressed as percentage content of fresh weight, and relative growth rate in unnodulated seedlings. There was a greater variability in nodulated seedlings than in unnodulated ones, because of the nitrogen fixation. The reactions of unnodulated grey alder were largely the same as previously reported for birch seedlings, but the maximum growth capacity was lower in grey alder. During an initial period of change in the internal nitrogen status, deficiency symptoms appeared, especially in unnodulated seedlings. As in birch, the leaves turned green again at stable nitrogen status, independent of level. The results are in sharp contrast to data from the literature where the external nitrogen concentration was used as the driving variable for the internal nitrogen status. The measured fixation rates for grey alder are much higher than those previously reported. Still, the maximum fixation rate observed is small compared to the total nitrogen uptake rate required for maximum growth, in contrast to reported relationships. These comparisons indicate that increased external nitrogen concentration obscures the real relations between mineral and fixed nitrogen, on one hand because of rapid inhibition of nitrogen fixation and, on the other hand, because of failure to obtain stable optimum nutrition and maximum growth by means of this treatment variable.     

Nitrogen limitation of phytoplankton in a Spanish karst lake with a deep chlorophyll maximum: a nutrient enrichment bioassay approach

An in vitro nutrient addition bioassay was performed to testthe relative inorganic nitrogen (N) and phosphorus (P) limitationof phytoplankton in a Spanish karst lake (El Tejo) during thelast part of the stratification period, when nutrient limitationis most pronounced. Nutrient deficiency was tested in samplesfrom three different layers of the lake: the epilimnion, metalimnionand oxic hypolimnion. Nitrogen additions, either without orcombined with P, increased phytoplankton growth in all threestrata, compared with controls or P treatments. This showedthat N was the nutrient limiting phytoplankton growth in latesummer–early fall. Since both hypolimnetic diffusion andgroundwater fluxes of N-rich waters into the lake are much reducedduring summer, N becomes the limiting nutrient as stratificationadvances. We suggest that in this Mediterranean area with lowatmospheric deposition of anthropogenic N and in lakes relativelyfree of surface run-off, nutrient supply by atmospheric depositionmight be a key factor in controlling nutrient deficiency forphytoplankton growth.     

A method for establishing ectomycorrhiza on conifer seedlings in steady-state conditions of nutrition

Scots pine ( Pinus sylvestris L.), and Norway spruce ( Picea abies (L.) Karst.) seedlings, inoculated with Suillus bovinits (L. ex Fr.) O. Kuntze and Paxillus involutus (Batsch) Fr., were grown on sloping plastic plates in growth units. A circulating nutrient solution flowed continuously over the plates. Nutrients in balanced proportions previously found to be appropriate for conifers, were added at a specified relative addition rate, in exponentially increasing amounts. The conductivity of the solution was kept low and stable (<50 μS cm⁻¹). No carbohydrates were added.
Seedling nitrogen concentration and relative growth rate attained steady states in both inoculated and uninoculated treatments. The fungi infected the short roots within a few days and formed typical mantles and Hartig nets. This occurred in all treatments, including conditions of free access to nutrients, in which the nutrient concentration of the seedlings was optimal. The growth rate of the extramatrical mycelium was very high.     

A Definition of Optimum Nutrient Requirements in Birch Seedlings. II

The aim of the experiments described was to specify further the nutrient requirements of birch seedlings. A growth method is described in which the nutrients consumed at maximum growth of birch seedlings can be replaced, without change of the nutrient solution, by means of additions made as pH and conductivity titrations so that three conditions are satisfied: 1) All necessary mineral nutrients are present in the plant in optimum proportions. 2) The nitrogen sources NH⁴+ and NO₃- are present in the nutrient solution in an optimum ratio. 3) Total concentration in the solution is optimum. The three conditions may be regarded as criteria in a definition of nutrient requirements. The possible use of criterion 1 as an expression of the optimum nutrient status is also discussed. A considerable increase in light intensity had no significant effect on the optimum nutrient proportions or the optimum total concentration. A tendency to lower ammonium uptake in relation to nitrate was induced when both nitrogen sources were present. Simple stock nutrient solution systems are listed and discussed.     

Nutrient requirements and stress response of Populus simonii and Paulownia tomentosa

The aim of this investigation was to estimate the optimum nutrient requirements and responses to low relative nutrient addition rates of seedlings of two important broadleaf tree species in China, Populus simonii Carr. and Paulownia tomentosa (Thunb.) Steud. In preliminary experiments the optimum nutrient proportions were estimated under high concentration conditions. The nutrients consumed were replaced by means of daily additions determined by pH and conductivity titrations without changing the nutrient solutions. A relatively high K level was needed in relation to nitrogen; higher than in birch or grey alder seedlings. To obtain a high relative growth rate, suitable proportions by weight were 100 N:70 K:14 P:7 Ca:7 Mg for the Populus seedlings and 100 N:75 K:20 P:8 Ca:9 Mg for the Paulownia seedlings.
In studies of nutrient stress responses the relative nutrient addition rate was used as the treatment variable under low conductivity conditions. The responses and relationships were similar to those described for birch, grey alder and Salix . The relative addition rate, and there was also a strong linear regression between relative growth rate and nitrogen status. Relative growth rates were high and the maximum weight increase was about 19% day⁻¹ in Populus and over 25% day⁻¹ in Paulownia . The nitrogen productivity of Paulownia was very high, 0.26 g dry weight (g N)⁻¹ h⁻¹, and for Populus it was 0.16 g dry weight (g N)⁻¹ h⁻¹.     

Nutrition and growth of birch seedlings at varied relative phosphorus addition rates

Birch ( Betula pendula Roth.) was investigated under steady state nutrition and growth at different relative addition rates of phosphorus (R_p). Phosphorus deficiency symptoms appeared on the leaves when the internal phosphorus concentration decreased, but disappeared again under steady state nutrition, independent of the stress level. The increased root/shoot ratio and the exploratory type of root systems developed during the adjustment stage remained under steady state conditions. At nonoptimum and close to optimum relative addition rates, independent of the rate, the phosphorus concentration of the culture solution did not exceed 2 μmol dm⁻³ and was generally < 1 μmol dm⁻³ immediately after phosphorus additions. The phosphorus concentration just before additions was generally < 0.5 μmol dm⁻³. The nutrition/growth relationships were similar to those for nitrogen, with relative growth rate (R_g) closely related to the R_p applied and with a strong linear relationship between internal phosphorus concentration and R_g. Regression was much steeper than that for nitrogen. The slope of the optimum nutrition was attained at a lower phosphorus weight proportion to nitrogen (8–10 P: 100 N) than previously estimated (= 13 P: 100 N), but a higher relative phosphorus requirement was observed under stress conditions. Birch seedlings had a strong tendency to consume phosphorus in excess of immediate requirements with a small effect on growth above optimum. This resulted in rapidly decreasing phosphorus productivity (P_p, growth rate per unit of phosphorus) with increasing internal phosphorus concentrations above optimum.     

Effects of Nitrogen Exponential Fertilization on Growth and Nutrient Concentration of <i>Hydrangea macrophylla</i> Seedlings

Slow growth rate restricts the development and growth of seedlings due to nutrients deficiency or nutrient imbalance. Exponential fertilization can enhance the internal nutrient reserves in seedlings at the nursery-stage and strengthen their resistance to adverse conditions. In this study, nitrogen requirements for producing Hydrangea macrophylla ‘Hanatemari’ that robust seedlings, nutrient dynamics, biomass and growth, was examined utilizing exponential fertilization. The potted seedlings were fertilized with urea under exponential regime at rates of 0.5, 1.5 and 2.0 g nitrogen/plant (EF1, EF2, and EF3), respectively. In addition, an unfertilized group treated with equal volume of deionized water was used as control. The results showed that seedlings under 1.5 g N/plant (EF2) had the highest plant growth index and total biomass. The nutrient concentrations of different organs varied in different fertilization treatments. Based on the results of current study, it is concluded that 1.5 g N/plant (EF2) is suitable exponential fertilization treatment for the culture of hydrangea seedlings. Our treatments results showed that 2.0 g N/plant is not suitable for seedling culturing, because of serious nutrient toxicity. These findings will help to improve seedling quality and strengthen the production of H. macrophylla for plantation.     

Environmental factors influencing the rate of hydrocarbon oxidation in temperate lakes.

Rates of hydrocarbon biodegradation were estimated by following oxygen uptake during mineral oil oxidation or oxidation of [1-14C]hexadecane to 14CO2, when these substrates were added to natural water samples from Wisconsin lakes. A lag phase preceded hydrocarbon oxidation, the length of which depended on population density or on factors influencing growth rate and on the presence of nonhydrocarbon organic compounds. Hydrocarbon oxidation was coincident with growth and presumably represented the development of indigenous hydrocarbon-degrading microorganisms in response to hydrocarbon additions. In detailed studies in Lake Mendota, it was found that, despite the continued presence of hydrocarbon-degrading microorganisms in water samples, seasonal variations in the rates of mineral oil and hexadecane oxidation occurred which correlated with seasonal changes in temperature and dissolved inorganic nitrogen and phosphorus. The temperature optimum for oil biodegradation remained at 20 to 25 C throughout the year, so that temperature was the main limiting factor during winter, spring, and fall. During summer, when temperatures were optimal, nutrient deficiencies limited oil biodegradation, and higher rates could be obtained by addition of nitrogen and phosphorus. The rates of hydrocarbon biodegradation were thus high only for about 1 month of the ice-free period, when temperature and nutrient supply were optimal. Nutrient limitation of oil biodegradation was also demonstrated in 25 nutrient-poor lakes of northern Wisconsin, although in almost every case oil-degrading bacteria were detected. Knowledge of temperature and nutrient limitations thus will help in predicting the fate of hydrocarbon pollutants in freshwater.     

Environmental factors influencing the rate of hydrocarbon oxidation in temperate lakes.

Rates of hydrocarbon biodegradation were estimated by following oxygen uptake during mineral oil oxidation or oxidation of [1-14C]hexadecane to 14CO2, when these substrates were added to natural water samples from Wisconsin lakes. A lag phase preceded hydrocarbon oxidation, the length of which depended on population density or on factors influencing growth rate and on the presence of nonhydrocarbon organic compounds. Hydrocarbon oxidation was coincident with growth and presumably represented the development of indigenous hydrocarbon-degrading microorganisms in response to hydrocarbon additions. In detailed studies in Lake Mendota, it was found that, despite the continued presence of hydrocarbon-degrading microorganisms in water samples, seasonal variations in the rates of mineral oil and hexadecane oxidation occurred which correlated with seasonal changes in temperature and dissolved inorganic nitrogen and phosphorus. The temperature optimum for oil biodegradation remained at 20 to 25 C throughout the year, so that temperature was the main limiting factor during winter, spring, and fall. During summer, when temperatures were optimal, nutrient deficiencies limited oil biodegradation, and higher rates could be obtained by addition of nitrogen and phosphorus. The rates of hydrocarbon biodegradation were thus high only for about 1 month of the ice-free period, when temperature and nutrient supply were optimal. Nutrient limitation of oil biodegradation was also demonstrated in 25 nutrient-poor lakes of northern Wisconsin, although in almost every case oil-degrading bacteria were detected. Knowledge of temperature and nutrient limitations thus will help in predicting the fate of hydrocarbon pollutants in freshwater.     

Maintaining exponential growth, solution conductivity, and solution pH in low-ionic-strength solution culture using a computer-controlled nutrient delivery system

Studies of plant nutrient requirements in solution culture have often used nutrient concentrations many-fold higher than levels found in fertile soils, creating an artificial rooting environment that can alter patterns of nutrient acquisition. The relative addition rate (RAR) technique addresses this problem by providing nutrients in exponentially increasing quantities to plant roots in solution culture. A computer-controlled RAR nutrient delivery system has been developed to reduce workload and to facilitate more frequent nutrient additions (4x daily) than is possible with manual additions. In initial experiments, a minimum background solution containing 500 microM nitrogen and all other essential nutrients in optimal proportions was required for the healthy growth of Triticum aestivum. This requirement was reduced to 50 microM nitrogen when calcium in the background solutions was increased to 400 microM. Varying the abundance of ammonium and nitrate in both background and delivery solutions provided a means of controlling plant-induced pH changes in growth solutions. In optimized solutions, plant relative growth rates (RGR) in the order of 0.2 g g(-1) plant d(-1) were maintained over a 22 d experimental period. Variation in RARs provided a means of growing plants with varying RGRs under relatively constant conditions of solution electrical conductivity and pH.     

Effect of lateral far-red light supplementation on the growth and morphology of birch seedlings and its interaction with mineral nutrition

The effects of lateral far-red light (FR) and nutrient supply on the growth and nitrogen accumulation of silver birch (Betula pendula) seedlings were studied with the objective of testing the following hypotheses: (1) silver birch seedlings grow taller in response to increased FR irradiance received from the side; (2) this response is modulated by the nutritional status of the seedlings; and (3) an increase in lateral FR irradiance, and concomitant decrease in red to far-red photon ratio, affects the carbon and nitrogen economies of the seedlings. Two factorial experiments, each with two levels of mineral nutrient availability and two light treatments (background 'white' light with and without additional lateral FR simulating light reflected by neighbours in a sparse canopy) were done with small seedlings. The two experiments differed in PAR irradiance. The results of these experiments were that (1) stem elongation rate was increased by lateral FR addition, (2) there was no interaction between this effect and the nutritional status of the seedlings, and (3) neither whole plant accumulation of nitrogen nor dry weight increment was affected by lateral FR under either mineral nutrient supply regime.     

Uses of alkaline phosphatase activity in evaluating phytoplankton community phosphorus deficiency

The phosphorus (P) deficiency status of phytoplankton communities was measured using the physiological indicator, alkaline phosphatase activity (APA) and nutrient-addition growth bioassays in field sampled from four northeastern Minnesota lakes and the far western arm of Lake Superior. Phosphorus additions generally reduced APA, while other treatments increased activity. Samples receiving nitrogen (N) and P increased APA after a long lag period. P-addition bioassays of Lake Superior were consistent with phytoplankton P limitation and variations in APA indicated potential seasonal and spatial changes in P deficiency status. The results suggest that APA reliably reflected the phytoplankton P status, but may not provide sufficient information when N or NP limitation is present.     

Seedling growth of four fire-following Mediterranean plant species deprived of single mineral nutrients

1. Anthyllis vulneraria, Cistus creticus, Hippocrepis unisiliquosa and Pinus brutia are frequent post-fire colonizers whose seedlings are subjected to the mineral conditions imposed by ash in the early stages of growth. We test the hypothesis that the effective internal supply of mineral elements in the seeds of these species may complement the external availability of specific nutrients.
2. Newly germinated seedlings were grown in nutrient solutions, each deficient in one of the following elements: nitrogen, phosphorus or potassium. Control treatments consisted of full nutrients and distilled water. Seedling growth was monitored over 12weeks. The final dry masses of the seedlings were taken as a measure of the availability of each element from the seeds' own stored reserves.
3. For the two legume species nitrogen was the most limiting element followed by phosphorus. Potassium deficiency had no effect on the final legume dry biomass. For Cistus, the order was nitrogen followed by potassium, while phosphorus deprivation had no effect on growth during the 12weeks of the experiment. Deprivation of single minerals had no significant effect on seedling growth of Pinus over the same period. The two legumes and Cistus when denied nitrogen had relatively greater relative root ratios than control plants.
4. The relative seedling requirements of each species for early external supplies of the various nutrients for establishment in the field are discussed with reference to the adaptation of the four species to post-fire regeneration.     

Growth and nutrition of birch seedlings in relation to potassium supply rate

Summary Growth of hydroponically cultivated birch seedlings (Betula pendula Roth.) at sub- and supra-optimum potassium supply rates was investigated. Potassium was supplied either as a relative addition rate (r _k = 5, 10, 15 and 20% increase day^-1) or as fixed concentrations (0.2, 3, 6, 12 and 15 mM) in the culture solution. After an acclimation period the growth rate of the seedlings in the suboptimum treatments reached values close to the treatment variable, the relative rate of K-addition. Deficiency symptoms, in the form of chlorosis and necroses along the leaf margins, developed initially in all suboptimum treatments, but very few new symptoms appeared once the seedlings had reached the phase of steady-state nutrition and growth. At supra-optimum K-supply levels, i.e. at 0.2–15 mM K in the culture solution, no symptoms of deficiency or toxicity developed, and the relative growth rate of the seedlings remained maximum. The relative growth rate of the seedlings was linearly related to the plant K-status for K contents ranging from 0.2 to 1.0% of dry weight (DW). At higher internal K-concentrations, 1.0–3.0% DW, no further increase in relative growth rate was achieved. A shortage of K resulted in a decrease in the net assimilation rate. This effect was counterbalanced by the absence of shift in he leaf weight ratio as well as by the production of relatively thin leaves. The fraction of dry matter allocated to roots decreased in K-limited plants, as did the leaf contents of soluble carbohydrates and starch.     

Growth and nitrogen utilization in seedlings of mountain birch (Betula pubescens ssp. tortuosa ) as affected by ultraviolet radiation (UV-A and UV-B) under laboratory and outdoor conditions

 Growth patterns and nitrogen economy were studied in pot-grown seedlings of mountain birch subjected to different ultraviolet radiation under both laboratory and outdoor conditions at Abisko in northern Sweden. In the laboratory, nutrient supply, temperature, humidity, ultraviolet radiation-A (UV-A, 320–400 nm) and B (UV-B, 280–320 nm) were controlled, while photosynthetically active radiation (PAR, 400–700 nm) and photoperiod varied naturally. Under outdoor conditions nutrient supply was controlled, and the irradiation treatments were ambient and above-ambient UV-B using additional fluorescent lamps. Mountain birch nitrogen economy was affected by increased ultraviolet radiation, as reflected by a changed relationship between plant growth and plant nitrogen both in the laboratory and outdoors. In the laboratory enhanced UV-A decreased leaf area per unit plant biomass (leaf area ratio) but increased biomass productivity, both per unit leaf area (leaf area productivity) and per unit leaf nitrogen (leaf nitrogen productivity). Low levels of UV-B affected growth patterns and nitrogen economy in a similar way to enhanced UV-A. High levels of UV-B clearly decreased relative growth rate and nitrogen productivity, as leaf area ratio, leaf area productivity and leaf nitrogen productivity were all decreased. Under outdoor conditions above-ambient levels of UV-B did not alter growth or biomass allocation traits of the seedlings, whilst nitrogen productivity was increased. Mountain birch seedlings originating from different mother trees varied significantly in their responses to different ultraviolet radiation. Received: 10 April 1997 / Accepted: 19 September 1997     

Effects of aluminium on growth and root reactions of phosphorus stressed Betula pendula seedlings

The impact of two constant non-toxic levels of Al stress (0.2 and 0.4 mM) on growth and ³²P uptake capacity on sub-optimal (P-limited) Betula pendula seedlings grown in sand culture was examined.Seedling growth was under optimum controlled conditions in a growth chamber where nutrient additions were made at a predetermined relative addition rate (R_A) of 10% day^-1. Three treatment groups of seedlings 0, 0.2 and 0.4 mM Al were harvested at 15, 29 and 42 days. The excised roots were exposed to a ³²P-labelled solution for 15 minutes to measure their capacity for P uptake. Growth was determined by weighing the roots, stems and leaves of the seedlings.Growth data showed that relative growth rate (R_G) should equal the R_A of P the most limiting nutrient, which was supplied at P/N 3% instead of an optimal 15%. Therefore, Ingestad's theory can also be used succesfully in sand culture and this may be particularly important for future studies of root and rhizosphere exudates. Low levels of Al (< 0.2 mM) in combination with low P supply significantly lowered the R_G of the birch seedlings by further reducing P supply. However, previous studies of birch seedling growth and nutrient uptake using Ingestad's solution culture technique with optimumal P supply did not show any effect of Al on growth untill the Al was in excess of 3 mM. Aluminium was not directly toxic to the plants and therefore roots could respond to the ³²P bioassay.     

Growth response to step-decrease in nutrient availability in small birch (Betula pendula Roth)

Abstract Changes in the uptake and allocation of carbon and nitrogen, after a step-decrease in nutrient availability, were investigated in small birch (Betula pendula Roth). By demonstrating stable nutrition, before and after the decrease in nutrient supply, it was possible to eliminate the effects of plant size and age. Immediately following the step-decrease in nutrient availability, net nitrogen uptake to leaves and the relative rate of increase in shoot area tended to zero. Although photosynthetic rate per shoot area decreased, carbon uptake remained in excess of that used in structural growth and respiration. More of the excess carbon was accumulated as starch in leaves than in roots. After a lag phase, the relative rates of increase in plant dry matter, starch amount, net nitrogen uptake to leaves and shoot area development equalled that of the reduced rate of nutrient supply. It is concluded that the reduction in plant relative growth rate was much more attributable to the reduced allocation of photosynthate to leaf area growth than to the reduction in photosynthesis per shoot area.