Growth and nitrate uptake properties of plants grown at different relative rates of nitrogen supply. II. Activity and affinity of the nitrate uptake system in Pisum and Lemna in relation to nitrogen availability and nitrogen demand

Abstract Net nitrate uptake rates were measured and the kinetics calculated in non-nodulated Pisum sativum L. cv. Marma and Lemna gibba L. adapted to constant relative rates of nitrate-N additions (R_A), ranging from 0.03 to 0.27 d^?1 for Pisum and from 0.05 to 0.40 d^?1 for Lemna, V_max of net nitrate uptake (measured in the range 10 to 100 mmol m^?3 nitrate, i.e. ‘system I’) increased with R_A in the growth limiting range but decreased when R_A exceeded the relative growth rate (RGR), K_m was not significantly related to changes in R_A. On the basis of previous ¹³N-flux experiments, it is concluded that the differences in V_max at growth limiting R_A are attributable to differences in influx rates. Linear relationships between V_max and tissue nitrogen concentrations were obtained in the growth limiting range for both species, and extrapolated intercepts relate well with the previously defined minimal nitrogen concentrations for plant growth (Oscarson, Ingemarsson & Larsson, 1989). Analysis of V_max for net nitrate uptake on intact plant basis in relation to nitrogen demand during stable, nitrogen limited, growth shows an increased overcapacity at lower R_A values in both species, which is largely explained by the increased relative root size at low R_A. A balancing nitrate concentration, defined as the steady state concentration needed to sustain the relative rate of increase in plant nitrogen (R_N), predicted by R_A, was calculated for both species. In the growth limiting range, this value ranges from 3.5 mmol m^?3 (R_A 0.03 d^?1) to 44 mmol m^?3 (R_A 0.21 d^?1) for Pisum and from 0.2 mmol m^?3 (R_A 0.05 d^?1) to 5.4 mmol m^?3 (R_A 0.03 d^?1) for Lemna. It is suggested that this value can be used as a unifying measure of the affinity for nitrate, integrating the performance of the nitrate uptake system with nitrate flux and long term growth and demand for nitrogen.     

Growth retardation induced by nutritional deficiency or abscisic acid in Lemna gibba: The relationship between growth rate and endogenous cytokinin content

The relationship between endogenous cytokinin content and relative growth rate (RGR) was studied in cultures of Lemna gibba L. G3 supplied with daily doses of mineral nutrients that were increased exponentially over time. At the optimal level of nutrient supply the RGR was 30–35% day^-1. The RGR was regulated by adjusting the rate of nitrogen supply, or it was restricted by addition of 0.5 M abscisic acid (ABA). Another approach used to investigate the specific roles of nitrogen (N) and phosphorus (P), was to transfer optimally growing plants to media without N or P but otherwise complete. The plants were harvested at regular intervals for determination of the RGR and levels of cytokinins of the isopentenyladenosine (iPA) and zeatinriboside (ZR) types with an enzyme-linked immunosorbent assay (ELISA). Levels of both iPA- and ZR-type cytokinins decreased when nitrogen was applied to cultures in growth limiting amounts. The cytokinin levels decreased more rapidly than the RGR when either N or P was lacking in the medium, suggesting an early influence of nutrient availability on cytokinin levels which in turn may induce adaptive response by the plant. RGR retardation induced by ABA did not affect cytokinin levels during the first 4 days of the treatment, and the later effects were small. The experiments gave no indication that ABA is involved in the adaptation response of Lemna plants to nutritional stress.Abbreviations ABA - abscisic acid - BAP - benzylaminopurine - ELISA - enzyme-linked-immunosorbent-assay - iP - isopentenyladenine - iPA - isopentenyladenosine - PBS - phosphate-buffered saline - PVP - polyvinylpyrrolidone - RGR - relative growth rate - R_N - relative nitrogen addition rate - Z - trans-zeatin - ZR - trans-zeatin riboside     

Temperature Controls a Dependence of Barley Plant Growth on Mineral Nutrition Level

The effect of temperature regime on growth and other morphological characteristics of barley plants (Hordeum distichum L., cv. Andrei) as dependent on the level of mineral nutrition was investigated in a controlled experiment. Plants were raised hydroponically at a high (0.22 g/(g day)) and low (0.05 g/(g day)) relative rates of the addition of mineral nutrients (R _A). Mineral nutrients were daily added to the nutrient solutions in exponentially increased amounts to provide steady-state plant growth. At the optimum temperature regime (21/17°C, day/night), the plant relative growth rate (RGR) was proportional to the preset R _A during the entire exponential period. Low R _A led to a decrease in the nitrogen content in plants, plant weight, and respiratory activity, as well as to the increase in the relative root weight. Biomass accumulation at lowered temperature regime (13/8°C) and a high R _A was 1.8-fold lower than at optimum temperature regime. Although under these conditions, the ratio of respiration to gross photosynthesis reduced threefold due to the decrease in the respiration rate, RGR of plants was equal to 0.11 ± 0.02 g/(g day), which was twice lower than the preset R _A. These pointed to the decrease in plant ability to maintain a certain ratio of photosynthesis to respiration within a day. At a deficiency of mineral nutrition and low temperature, RGR reached the preset R _A. Plants adapted to lowered temperature by a shift of the temperature optimum of their metabolism (heat production) to lower values. As a whole, a low variability of such growth parameters as RGR, C/N, and root to shoot weight ratio at different R _A and lowered temperatures testified to the lessening of growth limitation by the mineral nutrition.__________Translated from Fiziologiya Rastenii, Vol. 52, No. 3, 2005, pp. 384–391.Original Russian Text Copyright © 2005 by Garmash.     

Growth Cycles in Lemna gibba Cultures and Their Effects on Growth Rate and Ultrastructure

A long-established axenic culture of Lemna gibba G3 was maintained in exponential growth phase under controlled conditions. Weekly analyses for 2 years showed that the individual plants of the Lemna gibba clone fluctuated between two forms. One extreme consisted of plants light in weight, small in size, and with a high relative growth rate (RGR), the other of heavy, large, and more slowly growing plants. At intervals plants having intermediate characteristics dominated in the stock culture. Indication of an annual growth-cycle was also found. The magnitude of growth response (weight, RGR, area, and dry matter content) after treatment with abscisic acid (ABA), 6-benzylaminopurine (BAP), and a combination of the two was different for low-weight and heavy plants. The heavy plants were more sensitive to ABA and BAP treatment than the low-weight ones. The accumulation of starch was least in small untreated plants and greatest in ABA treated plants. Large electron transparent globules were found in the chloroplasts of the ABA treated plants and in heavy plants regardless of how they had been treated. The different physiological and ultrastructural characteristics of the two forms of Lemna plants probably reflect an ageing-rejuvenation cycle. Emphasis is placed on the importance of this cycle when Lemna is used as a model plant in physiological experiments.     

Photosynthesis and nitrogen utilization in exponentially growing nitrogen-limited cultures of Lemna gibba

The photosynthetic performance and nitrogen utilization of Lemna gibba L. G3 adapted to limited nitrogen supply was studied. The plants were adapted to two levels of nitrogen limitation where the nitrogen addition rates were calculated to sustain relative growth rates (RGR) of 0.15 day^?1 and 0.25 day^?1, respectively. The photosynthetic performance of these cultures was compared to nitrogen-sufficient cultures with an average RGR of 0.32 day^?1. Plants transferred from nitrogen-sufficient conditions attained RGR values corresponding to the nitrogen addition rates after 6 to 10 days. Light-saturated net photosynthesis declined during adaptation according to the drop in growth rate, and a concomitant decrease in the respiration rate was recorded. The efficiency of net photosynthesis on a dry weight basis increased with increased nitrogen supply, whereas it was the same in all cultures when expressed on a chlorophyll basis. The light compensation point was unaffected by the nitrogen regime. Limited nitrogen supply resulted in an increased proportion of dry matter in the roots, which led to decreased leaf area ratios. The net assimilation rates also decreased, but not to the same extent as the leaf area ratios. Growth-limiting amounts of nitrogen were added to the cultures once daily, and the net influx of N was higher than the requirement for N, also in adapted cultures with a steady growth rate. This resulted in transient, periodic fluctuations in the NO₃^?, NH₄⁺ and amino acid pools. Also the rates of NO₃^? reduction and NH₄⁺ assimilation fluctuated as did the amino acid assimilation which paralleled NH₄⁺ assimilation. The role of flux rates over the plasmalemma and tonoplast for control of nitrogen assimilation rates are discussed.     

川西亚高山森林土壤呼吸和微生物生物量碳氮对施氮的响应

随着全球大气氮沉降的明显增加,将有可能显著影响我国西部地区受氮限制的亚高山森林生态系统。土壤微生物是生态系统的重要组成部分,是土壤物质循环和能量流动的重要参与者。由于生态系统类型、土壤养分、氮沉降背景值等的差异,土壤呼吸和土壤生物量碳氮对施氮的响应存在许多不确定性。而施氮会不会促进亚高山森林生态系统中土壤呼吸和微生物对土壤碳氮的固定?基于此假设,选择了川西60年生的四川红杉(Larix mastersiana)亚高山针叶林为研究对象,通过4个水平的土壤施氮控制试验(CK:0 g m~(-2) a~(-1)、N1:2 g m~(-2)a~(-1)、N2:5 g m~(-2) a~(-1)、N3:10 g m~(-2)a~(-1)),监测了土壤呼吸及土壤微生物生物量碳氮在一个生长季的动态情况。结果表明:施氮对土壤呼吸各指标和土壤微生物碳氮都有极显著的影响,施氮能促进土壤全呼吸、自养呼吸、异养呼吸通量和土壤微生物生物量碳氮的增长,施氮使土壤呼吸通量提高了11%—15%,土壤微生物量碳提高了5%—9%,土壤微生物量氮提高了23%—34%。在中氮水平下(5 g m~(-2) a~(-1))对土壤呼吸的促进最显著。相关分析发现,土壤呼吸与微生物生物量碳氮和微生物代谢商极呈显著正相关,微生物量碳氮与土壤温度呈极显著的正相关,与土壤湿度呈极显著负相关。通过一般线性回归拟合土壤呼吸速率与土壤10 cm温湿度的关系,发现土壤呼吸速率与土壤温度呈极显著的正相关,与土壤湿度极显著负相关(P0.001),中氮水平下土壤温度敏感性系数Q_(10)值(7.10)明显高于对照(4.26)。     

Effect of plant growth regulators and medium composition on cell growth and saponin production during cell-suspension culture of mountain ginseng (Panax ginseng C. A. mayer)

We have established cell-suspension cultures of mountain ginseng (Panax ginseng G A. Mayer), and have attempted to increase the yield of saponin by manipulating our processing method and culturing factors (e.g., media strengths; the presence of plant growth regulators or sucrose; ratios of NO⁺ ₃/ NH^- ₄). Maximum biomass yield was obtained in media containing 2,4-D. However, saponin productivity was much higher in a medium comprising either IBA or NAA; 7.0 mg/L IBA was optimal for promoting both cell growth (10.0 g/L dry weight) and saponin production (7.29 mg/g DW total ginsenoside). Although the addition of cytokinins (BA and kinetin) did not affect cell growth, the level of saponin (particularly in the Rb group) was enhanced when the media were supplemented with either 0.5 mg/L BA or 0.5 mg/L kinetin. Half- and full-strength MS media were equally suitable for inducing both biomass as well as saponin production. We also investigated the effect of various concentrations of sucrose and nitrogen, and found that 30 g/L sucrose enhanced biomass yield as well as saponin content However, further increases (i.e., up to 70 g/L) led to a decrease in saponin accumulation and biomass production. Maximum growth and saponin productivity were reported from treatments with an initial nitrogen concentration of 30 mM. In general, the amount of saponin increased when the test media had high NO⁺ ₃/ NH^- ₄ ratios; in fact, saponin production was greatest when nitrate was the sole nitrogen source.     

Organic nitrogen content and nitrate and nitrite reductase activities in tritordeum and wheat grown under nitrate or ammonium

Tritordeum is a fertile amphiploid derived from durum wheat (Triticum turgidum L. conv. durum) × a wild barley (Hordeum chilense Roem. et Schultz.). The organic nitrogen content of tritordeum grain (34 mg g^-1 DW) was significantly higher than that of its wheat parent (25 mg g^-1 DW). Leaf and root nitrogen content became higher in tritordeum than in wheat after four weeks of growth, independently of the nitrogen source (either NO₃ ^- or NH₄ ⁺). Under NO₃ ^- nutrition, tritordeum generally exhibited higher levels of nitrate reductase (NR) activity than wheat. Nitrite reductase (NiR) levels were however lower in tritordeum than in its wheat parent. In NH₄ ⁺-grown plants, both NR and NiR activities progressively decreased in the two species, becoming imperceptible after 3 to 5 weeks of growth. Results indicate that, in addition to a higher rate of NO₃ ^- reduction, other physiological factors must be responsible for the greater accumulation of organic nitrogen in tritordeum grain.     

A laboratory study of phosphorus and nitrogen excretion of Euchlanis dilatata lucksiana

Phosphorus (PO₄-P) and nitrogen (NH₄-N) excretion rates of Euchlanis dilatata lucksiana, a rotifer, isolated from Lake Loosdrecht (The Netherlands) and cultured in the lake water at 18–19 °C, were measured in the laboratory.In a series of experiments, the effects of experiment duration on the P and N excretion rates were examined. The rates measured in the first half-hour were about 2 times higher for P and 2–4 times for N than the rates in the subsequent three successive hours which were quite comparable.Eight experiments were carried out in triplicate, 4 each for P and N excretion measurements, using animals of two size ranges: 60–125 µm and > 125 µm. The specific excretion rates varied from 0.06 to 0.18 µg P.mg^–1 DW.h^–1 and 0.21 to 0.76 µg N.mg^–1 DW.h^–1. Generally an inverse relationship was observed between the specific excretion rates and the mean individual weight. The excretion rates of Euchlanis measured by us are lower than those reported for several other rotifer species, most of which are much smaller than Euchlanis.Extrapolating the excretion rates of Euchlanis to the other rotifer species in Lake Loosdrecht, and accounting for their density, size and temperature, rotifer excretion appears to be a significant, potential nutrient (N,P) source for phytoplankton growth in the lake. The excretion rates for the rotifers appear to be about two thirds of the total zooplankton excretion, even though the computed rotifer mean biomass is about one-third of the total zooplankton biomass.     

Source of nitrogen associated with recovery of relative growth rate in Arabidopsis thaliana acclimated to sustained cold treatment

To determine (1) whether acclimation of carbon metabolism to low temperatures results in recovery of the relative growth rate (RGR) of plants in the cold and (2) the source of N underpinning cold acclimation in Arabidopsis thaliana, we supplied plants with a nutrient solution labelled with ¹⁵N and subjected them to a temperature shift (from 23 to 5 °C). Whole‐plant RGR of cold‐treated plants was initially less than 30% of that of warm‐maintained control plants. After 14 d, new leaves with a cold‐acclimated phenotype emerged, with the RGR of cold‐treated plants increasing by 50%; there was an associated recovery of root RGR and doubling of the net assimilation rate (NAR). The development of new tissues in the cold was supported initially by re‐allocation of internal sources of N. In the longer term, the majority (80%) of N in the new leaves was derived from the external solution. Hence, both the nutrient status of the plant and the current availability of N from external sources are important in determining recovery of growth at low temperature. Collectively, our results reveal that both increased N use efficiency and increases in nitrogen content per se play a role in the recovery of carbon metabolism in the cold.     

Growth and shoot:root partitioning of spinach plants as affected by nitrogen supply

Spinach plants (Spinacia oleracea L.) were grown hydroponically in fixed environmental conditions either at full nitrate availability (11·8mol m^-3) or at a suboptimum relative nitrate addition rate of 0·20d^-1, 0·15d^-1 or 0·10d^-1 respectively, the other nutrients being adequately provided. The relative growth rate (RGR) of the plants varied significantly with the nutrition treatment and decreased during development in all treatments. The concentration of reduced nitrogen in the plants grown at full nitrate availability did not change significantly during the experimental growth period and nitrate accumulation was substantial. After an adaptation period, the concentration of reduced nitrogen in the plants at the suboptimum nitrate addition rates increased during growth and was lowest at the lowest relative nitrate addition rate. Nitrate uptake was almost complete in the suboptimum treatments and nitrate accumulation was negligible as long as the concentration of reduced nitrogen was below 2·0 mmol (g dry weight)^-1. The RGR of all plants was proportional to the concentration of reduced nitrogen in the plant minus a minimal tissue concentration required for growth. However, the proportionality factor was inversely related to the plant mass. This relationship was summarized in an empirical model which explained 98·7% of the variance of the dry weight (log scale) data of all treatments at all harvests. The model was compared with other growth models found in the literature. The shoot/root weight ratio increased from 2 to 4 if nitrate provision was not limiting, and initially, this ratio decreased at suboptimum nitrate provision but increased at higher growth stages. Possible explanations of the dynamics of dry matter partitioning are discussed in relation to models.     

Growth and nutrition of small Betula pendula plants at different relative addition rates of iron

Summary Small birch plants (Betula pendula Roth.) were cultivated in a hydroponic spray solution where the relative addition rate of iron (R_Fe; g g^–1 day^–1), was the growth-controlling variable. All other elements were added in free access. An additional treatment was performed where all nutrients, including iron, were in free access (FA). The plants showed deficiency symptoms at steady-state growth and severe limitation of iron, R_Fe 0.05 and 0.10 day^–1. There were few symptoms at R_Fe of 0.15 or above. Plant relative growth rate (R_G; g g^–1 day^–1), equalled the relative rate of increase in iron supply, R_Fe. Internal iron concentration of the plants ranged from 40 to 70 g g^–1 dry weight (DW) over the range for which iron supply was limiting growth. At FA, the internal concentration was approximately 200 g g^–1 DW without further increase in R_G, demonstrating that iron may be taken up in excess without affecting growth. Internal concentrations of macronutrients were stable at the different R_Fe, except for Ca and Mg in shoots which were higher at low iron supply. Uptake rates of iron, calculated per root growth rate (mol g^–1 root DW), were approximately twice as high at R_Fe 0.20 as at 0.05 day^–1. The effect of iron limitation on dry matter allocation to leaves was small, with increases in the root fraction being largely at the expense of the stem. Leaf area ratio was constant regardless of R_Fe and the specific leaf area tended to increase with increasing iron limitation. Net assimilation rate decreased by a factor of 6 from free access to severe iron limitation, largely accounting for the differences in plant R_G.     

Carbon and nitrogen content of congeneric annual and perennial grass species: relationships with growth

Seven annual-perennial pairs of grass species (six congeneric and one pair taken at random) were grown under productive conditions in the laboratory in order to investigate which plant characters were responsible for the higher relative growth rate (RGR) of annuals as compared to perennials under these conditions. The nitrogen and carbon concentrations of shoot organs and of the whole plant were higher in annuals than in perennials. This was also the case for the specific absorption rate for nitrate and nitrogen productivity (on whole plant and leaf basis). The range of RGR displayed by the 14 species was large enough (0.15–0.33d⁻¹) to examine the general relationships between RGR and the various parameters measured in the present study. RGR was positively related to plant, leaf blade and sheath nitrogen concentrations, but there was no relationship between RGR and any of the carbon concentrations. RGR also strongly correlated with specific absorption rate for nitrate and with nitrogen productivity. A new factorization of this latter parameter led to the definition of the ‘leaf nitrogen productivity’ (N_LP), which is likely to depend on photosynthetic nitrogen use efficiency. RGR was shown to be strongly correlated with N_LP, but not with the second term of the factorization, namely the proportion of plant nitrogen allocated to the leaves.     

Culture ofGelidium sesquipedale (Clem.) Born.et Thur. in a chemostat system. Biomass production and metabolic responses affected by N flow

Gelidium sesquipedale is the most important raw material used for extraction of agar in Spain. Based on chemostats, a system of culture for macroalgae with a continuous flow of culture medium has been developed. A stressed morphotype from the South of Spain was cultured, and the effects of different rates of NO ₃ ^– flow on growth and internal constituents were investigated in the laboratory. Cultivation was successful after optimizing factors affecting growth, such as irradiance level, renewal rate and water movement. Mass production was dependent on N supply. With a flow of 35 mol NO₃ ^– g^–1 DW d^–1, optimal values of growth (2.1% d^–1) and biomass yield were obtained. In these conditions, biomass yield resembled the values observed in natural populations (about 500 g DW m^–2 y^–1). When the flow of N was reduced to 15 mol NO ₃ ^– g^–1 DW d^–1, growth rate and biomass yield were reduced three-fold, and were null when N was supplied as 7 mol NO ₃ ^– g^–1 DW d^–1. C:N ratio was an index of the physiological status of the tissue, remaining low when N was sufficient and raised to critical values when N supply was limited. Phycobiliproteins, kept at a constant irradiance level, were affected by N supply, acting as an internal nitrogen reserve, unlike chlorophylla. An effective phycobiliprotein synthesis took place when the flow of N was sufficient. Agar yield, on dry weight basis, was similar as a function of N flow, whereas agar yield of the culture was higher when N was sufficient as a result of growth not being limited by N.This system of culture, commonly used in microalgal studies, may have an important use in macroalgae as a system to obtain biomass of high quality as well as a good tool for physiological studies in conditions of continuous and controlled flow of nutrients.     

Microbial biomass and nitrogen cycling responses to fertilization and litter removal in young northern hardwood forests

The influence of site fertility on soil microbial biomass and activity is not well understood but is likely to be complex because of interactions with plant responses to nutrient availability. We examined the effects of long-term (8 yr) fertilization and litter removal on forest floor microbial biomass and N and C transformations to test the hypothesis that higher soil resource availability stimulates microbial activity. Microbial biomass and respiration decreased by 20–30 % in response to fertilization. Microbial C averaged 3.8 mg C/g soil in fertilized, 5.8 mg C/g in control, and 5.5 mg C/g in litter removal plots. Microbial respiration was 200 µg CO₂-C g^–1 d^–1 in fertilized plots, compared to 270 µg CO₂-C g^–1 d^–1 in controls. Gross N mineralization and N immobilization did not differ among treatments, despite higher litter nutrient concentrations in fertilized plots and the removal of substantial quantities of C and N in litter removal plots. Net N mineralization was significantly reduced by fertilization. Gross nitrification and NO₃ ^– immobilization both were increased by fertilization. Nitrate thus became a more important part of microbial N cycling in fertilized plots even though NH₄ ⁺ availability was not stimulated by fertilization.Soil microorganisms did not mineralize more C or N in response to fertilization and higher litter quality; instead, results suggest a difference in the physiological status of microbial biomass in fertilized plots that influenced N transformations. Respiration quotients (qCO₂, respiration per unit biomass) were higher in fertilized plots (56 µg CO₂-C mg C^–1 d^–1) than control (48 µg CO₂-C mg C^–1 d ^–1) or litter removal (45 µg CO₂-C mg C^–1 d^–1), corresponding to higher microbial growth efficiency, higher proportions of gross mineralization immobilized, and lower net N mineralization in fertilized plots. While microbial biomass is an important labile nutrient pool, patterns of microbial growth and turnover were distinct from this pool and were more important to microbial function in nitrogen cycling.     

Relative growth rate in relation to physiological and morphological traits for northern hardwood tree seedlings: species,light environment and ontogenetic considerations

The influence of ontogeny, light environment and species on relationships of relative growth rate (RGR) to physiological and morphological traits were examined for first-year northern hardwood tree seedlings. Three Betulaceae species (Betula papyrifera, Betula alleghaniensis and Ostrya virginiana) were grown in high and low light and Quercus rubra and Acer saccharum were grown only in high light. Plant traits were determined at four ages: 41, 62, 83 and 104 days after germination. In high light (610 mol m^–2 s^–1 PPFD), across species and ages, RGR was positively related to the proportion of the plant in leaves (leaf weight ratio, LWR; leaf area ratio, LAR), in situ rates of average canopy net photosynthesis (A) per unit mass (A_mass) and per unit area (A_area), and rates of leaf, stem and root respiration. In low light (127 mol m^–2 s^–1 PPFD), RGR was not correlated with A_mass and A_area whereas RGR was positively correlated with LAR, LWR, and rates of root and stem respiration. RGR was negatively correlated with leaf mass per area in both high and low light. Across light levels, relationships of CO₂ exchange and morphological characteristics with RGR were generally weaker than within light environments. Moreover, relationships were weaker for plant parameters containing a leaf area component (leaf mass per area, LAR and A_area), than those that were solely mass-based (respiration rates, LWR and A_mass). Across light environments, parameters incorporating the proportion of the plant in leaves and rates of photosynthesis explained a greater amount of variation in RGR (e.g. LWR^*A_mass, R²=0.64) than did any single parameter related to whole-plant carbon gain. RGR generally declined with age and mass, which were used as scalars of ontogeny. LWR (and LAR) also declined for seven of the eight species-light treatments and A declined in four of the five species in high light. Decreasing LWR and A with ontogeny may have been partially responsible for decreasing RGR. Declines in RGR were not due to increased respiration resulting from an increase in the proportion of solely respiring tissue (roots and stems). In general, although LWR declined with ontogeny, specific rates of leaf, stem, and root respiration also decreased. The net result was that whole-plant respiration rates per unit leaf mass decreased for all eight treatments. Identifying the major determinants of variation in growth (e.g. LWR^*A_mass) across light environments, species and ontogeny contributes to the establishment of a framework for exploring limits to productivity and the nature of ecological success as measured by growth. The generality of these relationships both across the sources of variation we explored here and across other sources of variation in RGR needs further study.     

Short-term effects of fertilization on loblolly pine (Pinus taeda L.) physiology

Fertilization commonly increases biomass production in loblolly pine (Pinus taeda L.). However, the sequence of short‐term physiological adjustments allowing for the establishment of leaf area and enhanced growth is not well understood. The effects of fertilization on photosynthetic parameters, root respiration, and growth for over 200 d following the application of diammonium phosphate were intensively investigated in an effort to establish a relative sequence of events associated with improved growth. Root respiration, foliar nitrogen concentration [N]_f, and light‐saturated net photosynthesis (A_sat) temporarily increased following fertilization. A_sat was correlated positively with [N]_f when non‐fertilized and fertilized treatments were pooled (R² = 0.47). Increased photosynthetic capacity following fertilization was due to both improved photochemical efficiency and capacity and enhanced carboxylation capacity of Rubisco. Positive effects of fertilization on growth were observed shortly after A_sat increased. Fertilized seedlings had 36.5% more leaf area and 36.5% greater total dry weight biomass at 211 d following fertilization. It is concluded that fertilization temporarily increased photosynthetic capacity, which resulted in a pool of photo‐assimilate used to build leaf area. The N from fertilizer initially invested in photosynthetic structures and enzymes probably re‐translocated to newly developing foliage, explaining the reduction in [N]_f and A_sat that was observed after peak levels were achieved following fertilization.     

Effect of mineral nitrogen on nitrogen nutrition and biomass partitioning between the shoot and roots of pea (Pisum sativum L.).

The effect of mineral N availability on nitrogen nutrition and biomass partitioning between shoot and roots of pea (Pisum sativum L., cv Baccara) was investigated under adequately watered conditions in the field, using five levels of fertiliser N application at sowing (0, 50, 100, 200 and 400 kg N ha^–1). Although the presence of mineral N in the soil stimulated vegetative growth, resulting in a higher biomass accumulation in shoots in the fertilised treatments, neither seed yield nor seed nitrogen concentration was affected by soil mineral N availability. Symbiotic nitrogen fixation was inhibited by mineral N in the soil but it was replaced by root mineral N absorption, which resulted in optimum nitrogen nutrition for all treatments. However, the excessive nitrogen and biomass accumulation in the shoot of the 400 kg N ha^–1 treatment caused crop lodging and slightly depressed seed yield and seed nitrogen content. Thus, the presumed higher carbon costs of symbiotic nitrogen fixation, as compared to root mineral N absorption, affected neither seed yield nor the nitrogen nutrition level. However, biomass partitioning within the nodulated roots was changed. The more symbiotic nitrogen fixation was inhibited, the more root growth was enhanced. Root biomass was greater when soil mineral N availability was increased: root growth was greater and began earlier for plants that received mineral N at sowing. Rooting density was also promoted by increased mineral N availability, leading to more numerous but finer roots for the fertilised treatments. However, the maximum rooting depth and the distribution of roots with depth were unchanged. This suggested an additional direct promoting effect of mineral N on root proliferation.     

Growth and physiological responses of <Emphasis Type="Italic">Picea asperata</Emphasis> seedlings to elevated temperature and to nitrogen fertilization

Picea asperata is a dominant species in the subalpine coniferous forests distributed in eastern edges of Tibetan Plateau and upper reaches of the Yangtze River. The paper mainly identified the short-term influences of experimental warming, nitrogen fertilization, and their combination on growth and physiological performances of Picea asperata seedlings. These seedlings were subjected to two levels of temperature (ambient; infrared heater warming) and two nitrogen levels (0; 25 g m⁻² a⁻¹ N) for 6 months. We used a free air temperature increase of overhead infrared heater to raise both air and soil temperature by 2.1 and 2.6°C, respectively. The temperature increment induced an obvious enhancement in biomass accumulation and the maximum net photosynthetic rate, and decreased AOS and MDA level under ambient nitrogen conditions. Whereas, negative effects of experimental warming on growth and physiology was observed under nitrogen fertilization condition. On the other hand, nitrogen fertilization significantly improved plant growth in unwarmed plots, by stimulating total biomass, maximum net photosynthetic rate (A _max), antioxidant compounds, as well as reducing the content of AOS and MDA. However, in warmed plots, nitrogen addition clearly decreased A _max, antioxidant compounds, and induced higher accumulation of AOS and MDA. Obviously, the beneficial effects of sole nitrogen on growth and physiology of Picea asperata seedlings could not be magnified by artificial warming.     

Hemp (Cannabis sativa L.) leaf photosynthesis in relation to nitrogen content and temperature: implications for hemp as a bio‐economically sustainable crop

Hemp (Cannabis sativa L.) may be a suitable crop for the bio‐economy as it requires low inputs while producing a high and valuable biomass yield. With the aim of understanding the physiological basis of hemp's high resource‐use efficiency and yield potential, photosynthesis was analysed on leaves exposed to a range of nitrogen and temperature levels. Light‐saturated net photosynthesis rate (A_max) increased with an increase in leaf nitrogen up to 31.2 ± 1.9 μmol m^?2 s^?1 at 25 °C. The A_max initially increased with an increase in leaf temperature (T_L), levelled off at 25–35 °C and decreased when T_L became higher than 35 °C. Based on a C₃ leaf photosynthesis model, we estimated mesophyll conductance (g_m), efficiency of converting incident irradiance into linear electron transport under limiting light (κ_2LL), linear electron transport capacity (J_max), Rubisco carboxylation capacity (V_cmax), triose phosphate utilization capacity (T_p) and day respiration (R_d), using data obtained from gas exchange and chlorophyll fluorescence measurements at different leaf positions and various levels of incident irradiance, CO₂ and O₂. The effects of leaf nitrogen and temperature on photosynthesis parameters were consistent at different leaf positions and among different growth environments except for κ_2LL, which was higher for plants grown in the glasshouse than for those grown outdoors. Model analysis showed that compared with cotton and kenaf, hemp has higher photosynthetic capacity when leaf nitrogen is <2.0 g N m^?2. The high photosynthetic capacity measured in this study, especially at low nitrogen level, provides additional evidence that hemp can be grown as a sustainable bioenergy crop over a wide range of climatic and agronomic conditions.