Fluctuations in nitrate reductase activity,and nitrate and organic nitrogen concentrations of succulent plants under different nitrogen and water regimes

The CAM (Crassulacean acid metabolism) succulent species Kalanchoe daigremontiana, K. tubiflora and Crassula argentea, and the succulent C₃ species Peperomia obtusifolia, were cultivated in pure culture in open-air conditions under two different regimes of nitrogen and water supply. At specified intervals during the course of vegetative growth, biomass, nitrate reductase activity (NRA), nitrate concentration, and organic nitrogen concentration of whole plants were measured. After 100 days of cultivation the leaf conductance of Crassula and Peperomia was measured at intervals for the duration of a day. Behaviour of all four species was strongly influenced by the cultivation regime. This was apparent in terms of productivity and variable flucturations in NRA, nitrate concentration, and organic nitrogen concentration during the vegetative period. Increase in biomass was mostly connected with a decrease in all other investigated parameters, especially under conditions of water and/or nitrogen deficiency. The typical reaction of the CAM species Crassula to limited netrogen but adequate soil water was to reduce leaf conductance during light, whereas the C₃ plant Peperomia increased conductance in comparison with plants having a nitrogen suppy. The NRA of all plant species was reduced by both soil nitrate deficiency and drought. The succulent plant species, which are specially adapted to drought, neither took up nor used nitrate when water was limited. This was particularly the case for the CAM species, but less so for the C₃ Peperomia, which showed very high concentrations of nitrate and organic nitrogen, but low NRA and biomass gain. A formula was derived to express the nitrogen use efficiency (NUE) of the species, i.e. the ability of a plant to use nitrogen over a specific period of growth. NUE was shown to increase with age for the crassulacean species but to decrease for the C₃ Peperomia. Furthermore, NUE varied with the different nutrient levels in a species-specific manner, with high values for NUE not necessarily coupled to high productivity, and with NUE of the C₃ species generally higher than that of CAM species.     

Effect of nitrogen supply on growth,allocation and gas exchange characteristics of two perennial grasses from inland dunes

Summary We studied the effects of nitrogen supply on growth, allocation, and gas exchange characteristics of two perennial grasses of dry, nutrient-poor inland dunes: Corynephorus canescens (L.) Beauv. and Agrostis vinealis Schreber. C. canescens invests more biomass in leaves and less in roots, but has less leaf area and more root length per unit plant weight than A. vinealis. A. vinealis invests more nitrogen per unit leaf weight, but less per unit leaf area, despite a similar relative nitrogen investment in leaves and plant nitrogen concentration. Between-species differences in the rate of net photosynthesis, transpiration and shoot respiration are positively related to leaf nitrogen content per unit leaf area. The rate of net photosynthesis per unit plant weight is higher for A. vinealis at both levels of nitrogen supply, due to differences in leaf area ratio (LAR), and despite the reverse differences in the rate of net photosynthesis per unit leaf area. The water use efficiency of the two species is similar and increases significantly with an increase in nitrogen supply. The photosynthetic nitrogen use efficiency on the other hand is not affected by nitrogen supply, while at both low and high nitrogen supply A. vinealis has a 10% higher photosynthetic nitrogen use efficiency than C. canescens.     

The influences of increased CO2 and water supply on growth,biomass allocation and water use efficiency of Sinapis alba L. grown under different wind speeds

We examined how independent and interactive effects of CO₂ concentrations, water supply and wind speed affect growth rates, biomass partitioning, water use efficiency, diffusive conductance and stomatal density of plants. To test the prediction that wind stress will be ameliorated by increased CO₂ and/or by unrestricted water supply we grew Sinapis alba L. plants in controlled chambers under combinations of two levels of CO₂ (350 ppmv, 700 ppmv), two water regimes and two wind speeds (0.3 ms^–1, 3.7 ms^–1). We harvested at ten different dates over a period of 60 days. A growth analysis was carried out to evaluate treatment effects on plant responses. Plants grown both in increased CO₂ and in low wind conditions had significantly greater stem length, leaf area and dry weights of plant parts. Water supply significantly affected stem diameter, root weight and leaf area. CO₂ enrichment significantly increased the rate of biomass accumulation and the relative ratio of biomass increase to leaf area expansion. High wind speed significantly reduced plant growth rates and the rate of leaf area expansion was reduced more than the rate of biomass accumulation. Regression analysis showed significant CO₂ effects on the proportion of leaf and stem dry weight to total dry weight. A marked plant-age effect was dependent on water supply, wind speed and CO₂ concentration. A reduced water supply significantly decreased the stomatal conductance, and water use efficiency significantly increased with a limited water supply, low wind and increased CO₂. We found significant CO₂ x wind effects for water diffusion resistance, adaxial number of stomata and water use efficiencies and significant wind x water effect for water use efficiency. In conclusion, wind stress was ameliorated by growing in unrestricted water but not by growing in increased CO₂.     

Biomass production and nitrate metabolism of Atriplex hortensis L. (C3 plant) and Amaranthus retroflexus L. (C4 plant) in cultures at different levels of nitrogen supply

Summary Pure and mixed cultures of the dicotyledons Atriplex hortensis L. (C₃ plant) and Amaranthus retroflexus L. (C₄ plant) were maintained under open air conditions in standard soil at low and high nitrogen supply levels.A comparison of shoot dry weight and shoot length in the various series shows that the growth of the aboveground parts of both species was severely reduced under low N conditions. In both pure and mixed cultures the differences resulting from low N vs. high N conditions was less pronounced with Atriplex (C₃ plant) than with Amaranthus (C₄ plant). The root dry weight of the two species was not reduced so much under low N conditions as was the shoot dry weight. The low N plants were found to contain a larger proportion of their biomass in the roots than did the high N plants. In general the root proportion of Atriplex was greater than that of Amaranthus. The contents of organic nitrogen and nitrate and the nitrate reductase activity (NRA) per g dry weight of both species decreased continually throughout the experiments. With the exception of young plants, the low N plants always had tower contents of organic nitrogen and nitrate and nitrate reductase activities than did the high N plants. The highest values of NRA were measured in the leaf laminae. The eaves also exhibited the highest concentrations of organic nitrogen. The highest nitrate concentrations, however, were observed in the shoot axis, and in most cases the lowest nitrate values were found in the laminae. At the end of ne growing season this pattern was found to have been reversed with Atriplex, but not with Amaranthus. Thus Atriplex was able to maintain a higher NRA in the laminae than Amaranthus under low N conditions.The transpiration per leaf area of the C₄ plant Amaranthus during the course of a day was substantially lower than that of the C₃ plant Atriplex. There were no significant differences in transpiration between the low N and high N series of Amaranthus. The low N plants of Atriplex, however, clearly showed in most cases higher transpiration rates than the corresponding high N plants. These different transpiration rates of the high N and the low N Atriplex plants were also reflected in a distinct ¹³C discrimination.The sum of these results points to the conclusion that the C₃ plant Atriplex hortensis can maintain a better internal inorganic nitrogen supply than the C₄ plant Amaranthus retroflexus under low N conditions and an ample water supply, due to the larger root proportion and the more pronounced and flexible transpiration of the C₃ plant.Dedicated to Prof. Dr. Karl Mägdefrau, Deisenhofen, on the ocasion of his 80th birthday     

Carbon and nitrogen economy of 24 wild species differing in relative growth rate

The relation between interspecific variation in relative growth rate and carbon and nitrogen economy was investigated. Twentyfour wild species were grown in a growth chamber with a nonlimiting nutrient supply and growth, whole plant photosynthesis, shoot respiration, and root respiration were determined. No correlation was found between the relative growth rate of these species and their rate of photosynthesis expressed on a leaf area basis. There was a positive correlation, however, with the rate of photosynthesis expressed per unit leaf dry weight. Also the rates of shoot and root respiration per unit dry weight correlated positively with relative growth rate. Due to a higher ratio between leaf area and plant weight (leaf area ratio) fast growing species were able to fix relatively more carbon per unit plant weight and used proportionally less of the total amount of assimilates in respiration. Fast growing species had a higher total organic nitrogen concentration per unit plant weight, allocated more nitrogen to the leaves and had a higher photosynthetic nitrogen-use efficiency, i.e. a higher rate of photosynthesis per unit organic nitrogen in the leaves. Consequently, their nitrogen productivity, the growth rate per unit organic nitrogen in the plant and per day, was higher compared with that of slow growing species.     

遮光和施氮对暗紫贝母形态特征和生物量分配的影响

根据最优分配理论( optimal partitioning theory,OPT),在自然环境中,由于地上和地下资源在空间上的分离,陆生植物需要调整其地上和地下部分的生物量分配以平衡对资源的吸收。研究结果[1-2]显示：在植物地上资源严重受限时,叶片的生物量分配相对增加;而地下资源严重受限时,根的生物量分配相对增加。然而,植物的生物量分配对资源的响应较为复杂。首先,除了生物量分配,形态变化也是植物调节资源吸收的重要手段,例如,植物通过增加比叶面积提高对光照的吸收,通过调整比根长响应土壤养分的变化[3-4];并且,一些植物甚至以形态响应为主,生物量分配的变化并不明显[3,5]。此外,某些植物还有一些特殊的资源平衡方式,例如,克隆植物通过构件之间的养分传输平衡资源在空间上的分配不均[6];拥有特化储藏器官的植物可依靠大量储藏物质来平衡资源在时间上的分配不均[7]。植物所具有的丰富的资源响应方式决定了其多样的资源策略。     

施氮和减水对中亚热带壳斗科三种幼树生物量及其分配的影响

土壤氮素和土壤含水量是森林生态系统中限制植物生长的重要因子。为探讨常绿阔叶树种对模拟氮沉降增加和降水减少的响应,以同质园中自然生长的壳斗科(Fagaceae)丝栗栲(Castanopsis fargesii)、大叶青冈(Cyclobalanopsis jensenniana)和麻栎(Quercus acutissima)五年生幼树为对象,比较了对照(CK)、施氮(+N,施氮60 kg hm~(-2) a~(-1))、减水(-W,减少自然降水的50%)以及施氮和减水(+N-W,施氮60 kg hm~(-2) a~(-1)和减少自然降水的50%)对总生物量、各器官生物量及其生物量分配的影响。研究表明:(1)施氮显著增加了三种幼树的总生物量及各器官生物量(P0.05)。(2)减水未引起三种幼树的总生物量及各器官生物量的显著下降(P0.05)。(3)除麻栎树叶生物量、树枝生物量外,施氮和减水交互作用对三种幼树总生物量及各器官生物量没有明显影响(P0.05)。(4)施氮显著提高了丝栗栲幼树的干重比、大叶青冈幼树的干重比和枝重比(P0.05),降低了大叶青冈的叶重比和根重比(P0.05)。(5)减水导致丝栗栲幼树和麻栎幼树的根重比、麻栎幼树的根冠比显著增加(P0.05),引起丝栗栲幼树的枝重比、麻栎幼树的叶重比、枝重比和干重比显著降低(P0.05)。(6)施氮和减水交互作用显著提高了丝栗栲幼树的根重比(P0.05),降低了麻栎幼树的干重比(P0.05)。     

The effects of light and nitrogen on photosynthesis,leaf characteristics,and dry matter allocation in the chaparral shrub,Diplacus aurantiacus

Summary Plants of Diplacus aurantiacus, a successional shrub common in California chaparral, were grown under controlled conditions in which either quantum flux density or nitrogen availability was varied. Photosynthesis and leaf nitrogen content were determined on a leaf area and a leaf weight basis, and whole plant growth was monitored.There was a direct relationship between photosynthesis and leaf nitrogen content on both area and weight bases. Reduced light intensity of the growth environment resulted in reductions in light-saturated photosynthesis and nitrogen content on an area basis, but not on a weight basis. With reduced nitrogen availability, photosynthesis and leaf nitrogen content per unit leaf weight decreased.Resource use efficiency increased as the resource became more limiting. The results are consistent with a model of plant growth in which net carbon gain of the leaf is maximized. Abbreviations. For brevity, the following set of abbreviations is used in presenting and discussing the results. P/area and N/area are, respectively, photosynthesis and leaf nitrogen content per unit leaf area. P/wt and N/wt are the same quantities per unit leaf dry weight. SLW (specific leaf weight) is dry weight per unit leaf area. RGR (relative growth rate) is the relative rate of increase in shoot dry matter per day     

Effect of litter nitrogen on decomposition and microbial biomass inSpartina alterniflora

The effect on decomposition of 4 different levels of nitrogen in aerial tissue ofSpartina alterniflora, collected at the end of its growing season litter, was studied in laboratory percolators for 56 days at 20C. The CO₂ evolution and the release of organic nitrogen and organic carbon were monitored. From these data, the ash-free dry weight (AFDW), nitrogen (N) content, and carbonnitrogen (C/N) ratio were calculated at various times during decomposition. Fungal biomass, bacterial biomass, and the relative autoradiographic activity of bacteria were measured at the end of the study. Decomposition was significantly affected by the nitrogen content of the litter. A 55% increase in plant N increased overall weight loss and k by 50% and 40%, respectively. Furthermore, k (calculated from time course weight loss data) responded linearly to the 4 different levels of nitrogen inSpartina tissue. Fungi appear to dominate the microbial community. At the end of the experiment, fungal biomass was between 2.23 and 3.08% of the AFDW, and was calculated to contain 12 to 22% of the nitrogen in the litter. Bacterial biomass was 1/10 of the fungal biomass, and 12–17% of the bacteria were active. The total microbial biomass was not affected by increased plant nitrogen. In the course of decomposition, the organic nitrogen and carbon were highest in the effluent water in all treatments during the first 8 days. The respiration rate (CO₂ evolution) first increased to a maximum at day 18 and then decreased to a constant rate (1–2 mg C/day/g detritus). Respiration was highest in the high N litter. The C/N ratio in all treatments increased from the start to day 8, then decreased to day 20. In low N litter, C/N then increased again as a result of increased total organic nitrogen (TON) loss relative to carbon mineralization. In the high N, this was reversed.     

Interaction of carbon and nitrogen metabolism in the productivity of maize

Five maize (Zea mays L.) hybrids, FS854, B73 × Mo17, B84 × Mo17, B73 × B77, and P3382, grown under field conditions, were sampled at intervals during the grain-filling period. Plants were subdivided into stalks (including sheaths), leaves, and kernels. These parts were assayed for dry weight, reduced nitrogen, and extractable nonstructural carbohydrates. The duration and rates of net nitrate reduction and photosynthesis were approximated by the changes over time in the accumulation of reduced nitrogen and dry weight by the plant (total, above ground), respectively.

Data on the accumulation of reduced nitrogen and dry weight by the plant show that decreases in nitrate reduction preceded (in time and extent for four of the hybrids and in extent for FS854) decreases or cessation of photosynthesis. FS854 continued to accumulate reduced nitrogen and dry matter throughout the grain-filling period.

The patterns of change in stalk carbohydrate and reduced nitrogen during the early stages of ear development show the stalk serves as a storage reservoir and that these reserves were remobilized during the final stages of grain development. The marked increase and maintenance of dry weight and carbohydrate content of stalks until 34 days after anthesis, shows the capacity of the leaves to produce photosynthate through the first half of the grain-filling period exceeds the needs of the ear and/or the transport system. In contrast, stalk nitrogen content shows a slight increase up to 12 days after anthesis and decreases continually thereafter. Leaf nitrogen was lost continuously throughout grain development. The potential capacity of the plant to supply newly reduced nitrogen was inadequate to support initiation and early development of the kernels without remobilization of vegetative nitrogen. Of the two hybrids having delayed leaf senescence, FS854 with its initially higher concentration and content of reduced nitrogen in the stalk, initiated and developed a bigger ear than P3382, which had lower levels of stalk nitrogen.

Three of the five hybrids had `near linear' rates of accumulation of kernel dry weight, whereas none of the hybrids had linear rates of gain in kernel nitrogen. All hybrids had maximum or near maximum rates of gain of kernel nitrogen between 26 and 34 days after anthesis and a marked reduction (41-52%) of rates in the following sampling interval. These decreases are concurrent with decreases in rates of nitrate reduction (nitrogen accumulation) by the whole plant for four of the hybrids and with decreases in remobilization of nitrogen from the vegetation of FS854. Data for the ratio of rates of accumulation of dry weight/reduced nitrogen by the kernels versus time after anthesis, show that the accumulation of dry weight and reduced nitrogen are independent of each other. The variations in the ratio values appear best related to variations in the availability of nitrogen from the vegetation.

     

Effects of water stress and high nocturnal temperature on photosynthesis and nitrogen level of a perennial grass Leymus chinensis

Water deficit and high temperature are important environmental factors restricting plant growth and photosynthesis. The two stresses often occur simultaneously, but their interactions on photosynthesis and nitrogen level have been less studied. In the present experiment, we measured photosynthetic parameters, stomatal density, and nitrogen levels, as well as soluble sugar content of leaves of a perennial grass, Leymus chinensis, experiencing two day/night temperature regimes of 30/20 °C and 30/25 °C, and five different soil moisture contents (the soil relative-water content ranged from 80% to 25%). Leaf relative water content, leaf biomass, whole plant biomass, the ratio between the leaf biomass and total plant biomass, and the photosynthetic rate, as well as water-use efficiency decreased at high night temperature, especially under severe water stress conditions. Stomatal index was also increased by soil water stress except very severe water stress, and high nocturnal temperature decreased the leaf stomatal index under soil water stress. Nocturnal warming decreased nitrogen concentration in the leaves and increased it in the roots, particularly when plants were subjected to severe water stress. There were significant positive correlations between the photosynthetic rate and both soluble sugar concentration and nitrogen concentration at low nocturnal temperature. It is suggested that nocturnal warming significantly exacerbates the adverse effects of soil water stress, and their synergistic interactions might reduce the plant productivity and constrain its distribution in the region dominated by L. chinensis, based on predictions of global climate change.     

Partitioning of nitrogen and biomass at a range of N-addition rates and their consequences for growth and gas exchange in two perennial grasses from inland dunes

This paper describes the effects of nitrgen supply on the partitioning of biomass and nitrogen of Agrostis vinealis (L.) Schreber and Corynephorus canescens (L.) Beauv., two perennial grasses of dry, nutrient-poor inland dunes, and their consequences for growth and gas exchange. At a given plant nitrogen concentration (PNC) the two species allocate the same relative amount of dry matter and nitrogen to their leaves. However, A. vinealis allocates more dry matter and nitrogen to its roots and less to its above-ground support tissue than C. canescens . Both the leaf weight ratio and leaf nitrogen ratio increase with increasing PNC. Despite species-specific differences in growth form and leaf morphology, the leaf area ratio and specific leaf area of the two species are similar, both at high and low PNC. At intermediate nitrogen supply, and thus intemediate PNC, however, A. vinealis has a higher leaf area ratio and specific leaf area than C. canescens .
The two species exhibit a similar positive relationship when either the rate of net photosynthesis or the rate of shoot respiration are compared to the leaf nitrogen concentration, all expressed per unit leaf weight. The rate of net photosynthesis per unit Jeafnitrogen (PNUE) of the two species increases with decreasing leaf nitrogen concentration per unit leaf weight. C. canescens has a higher PNUE at low, and a lower PNUE at high leaf nitrogen concentration per unit leaf weight than A. vinealis . At non-limiting nitrogen supply, A. vinealis has a higher nitrogen productivity and net assimilation rate and a similar PNC and leaf area ratio as compared to C. canescens , which explains the higher relative growth rate (RGR_max) of A. vinealis. At growth-limiting nitrogen supply C. canescens achieves a similar relative growth rate at a lower PNC than A. vinealis.     

Comparison of photosynthetic acclimation to elevated CO2 and limited nitrogen supply in soybean

Plants grown at elevated CO₂ often acclimate such that their photosynthetic capacities are reduced relative to ambient CO₂-grown plants. Reductions in synthesis of photosynthetic enzymes could result either from reduced photosynthetic gene expression or from reduced availability of nitrogen-containing substrates for enzyme synthesis. Increased carbohydrate concentrations resulting from increased photosynthetic carbon fixation at elevated CO₂ concentrations have been suggested to reduce the expression of photosynthetic genes. However, recent studies have also suggested that nitrogen uptake may be depressed by elevated CO₂, or at least that it is not increased enough to keep pace with increased carbohydrate production. This response could induce a nitrogen limitation in elevated-CO₂ plants that might account for the reduction in photosynthetic enzyme synthesis. If CO₂ acclimation were a response to limited nitrogen uptake, the effects of elevated CO₂ and limiting nitrogen supply on photosynthesis and nitrogen allocation should be similar. To test this hypothesis we grew non-nodulating soybeans at two levels each of nitrogen and CO₂ concentration and measured leaf nitrogen contents, photosynthetic capacities and Rubisco contents. Both low nitrogen and elevated CO₂ reduced nitrogen as a percentage of total leaf dry mass but only low nitrogen supply produced significant decreases in nitrogen as a percentage of leaf structural dry mass. The primary effect of elevated CO₂ was to increase non-structural carbohydrate storage rather than to decrease nitrogen content. Both low nitrogen supply and elevated CO₂ also decreased carboxylation capacity (V_cmax) and Rubisco content per unit leaf area. However, when V_cmax and Rubisco content were expressed per unit nitrogen, low nitrogen supply generally caused them to increase whereas elevated CO₂ generally caused them to decrease. Finally, elevated CO₂ significantly increased the ratio of RuBP regeneration capacity to V_cmax whereas neither nitrogen supply nor plant age had a significant effect on this parameter. We conclude that reductions in photosynthetic enzyme synthesis in elevated CO₂ appear not to result from limited nitrogen supply but instead may result from feedback inhibition by increased carbohydrate contents.     

The influence of nitrogen availability on carbon and nitrogen storage in the biennial Cirsium vulgare (Savi) Ten. I. Storage capacity in relation to resource acquisition, allocation and recycling

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.     

Biomass production and nitrogen content of C3- and C4- grasses in pure and mixed culture with different nitrogen supply

Summary Two C₃ grasses (Hordeum vulgare L., Avena sativa L.) and two C₄ grasses (Panicum miliaceum L., Panicum crus-galli L.) were cultivated in standard soil in the open air in pure cultures and in various mixed cultures at low and high nitrogen fertilization levels. After three months the dry weight, length and nitrogen content of the aboveground and below-ground parts of the plants and the shoot/root ratios were determined. Hordeum vulgare was the most successful species irrespective of the nitrogen fertilization level, and also exhibited in most cases the highest nitrogen concentrations. Panicum miliaceum, on the other hand, was the species least able to compete. The production of biomass was reduced in cultures growing under nitrogen starvation conditions, this phenomenon being more pronounced with respect to the C₄ than to the C₃ species. The decrease in the production of biomass at low N conditions was most drastic with Panicum crus-galli, the species with the lowest nitrogen content and thus assumed to be best adapted to nitrogen starvation conditions. In cultures growing at low nitrogen fertilization levels the shoot/root ratios of all species.shifted in favour of an increasing root proportion. The extent of this shift, however, differed from species to species.     

沙质草地不同生活史植物的生物量分配对氮素和水分添加的响应

以氮素和水分(冬季增雪和夏季增雨)为控制因子, 开展相关田间控制实验, 分析不同功能群(以生活史为划分依据)尺度和群落尺度植物生物量分配格局对氮素和水分的响应, 得出以下结论: 1)一年生植物的繁殖生物量比重明显高于多年生植物, 而多年生植物种的叶/地上生物量比值显著高于一年生植物; 2)一年生植物对氮素和水分添加的响应剧烈, 氮添加耦合夏季增雨、氮添加耦合冬季增雪显著增加了一年生植物的繁殖生物量比重和叶生物量比重。多年生植物对氮素和水分添加的响应不敏感, 表现为多年生植物的各器官生物量分配格局对氮素添加和水分添加的响应不明显。3)氮素添加和水分处理改变了群落尺度生物量分配格局: 氮素添加耦合冬季增雪处理降低了群落植物的繁殖生物量比重和茎生物量比重, 提高了群落植物的叶生物量比重。4)冬季增雪和夏季增雨与氮素添加的交互作用对群落生物量分配格局的改变不同。夏季增雨耦合氮素添加处理下群落的茎生物量比重显著提高, 群落茎生物量分配的改变引起群落的垂直结构发生改变。冬季增雪氮素处理下群落的叶生物量比重增加, 但茎生物量比重增加不明显。冬季增雪也改变了群落的结构和功能。     

麻栎和闽楠幼苗叶功能性状及生物量对光照和施肥的响应

光照和养分条件是影响植物生长的重要环境因子,不同生活型植物对环境异质性的响应机制不同。以落叶阔叶树种麻栎和常绿阔叶树种闽楠幼苗为研究对象,设置2个光照梯度（全光照和45%全光照）和4个施肥梯度（不施肥、氮磷供应比为5、15和45）共8种处理,研究光照和施肥及其交互作用对麻栎和闽楠生物量和叶形态、生理及化学性状的影响,并探讨了叶功能性状和生物量的关系。结果表明：（1）光照、施肥及其交互作用对光合气体交换参数（除水分利用效率外）、叶绿素荧光参数、叶形态指标（除比叶面积外）、单位质量叶氮含量和根冠比影响显著（P<0.05）。此外,光照和施肥对地上生物量和总生物量影响显著（P<0.05）。（2）全光照显著增加了麻栎和闽楠单株总叶面积和地上、地下生物量及总生物量（P<0.05）,而遮荫降低了非光化学猝灭系数、光合氮利用率和根冠比,增加了单位质量叶氮含量。（3）在全光照处理中,施肥显著增加了麻栎和闽楠水分利用效率（P<0.05）;在遮荫处理中,氮磷供应比45显著增加了麻栎和闽楠净光合速率和水分利用效率（P<0.05）。（4）麻栎和闽楠在全光照中倾向于资源获取策略,在遮荫中偏向于资源保守策略。在光照和施肥处理中,麻栎和闽楠单株总叶面积与地上生物量均显著正相关（P<0.05）。总之,单株总叶面积是预测麻栎和闽楠幼苗地上生物量变化的稳定指标,施肥有助于增加低光环境下麻栎和闽楠幼苗的生态适应能力。     

Growth and carbon economy of a fast-growing and a slow-growing grass species as dependent on nitrate supply

In previous experiments systematic differences have been found in the morphology, carbon economy and chemical composition of seedlings of inherently fast- and slow-growing plant species, grown at a non-limiting nutrient supply. In the present experiment it was investigated whether these differences persist when plants are grown at suboptimal nutrient supply rates. To this end, plants of the inherently fast-growing Holcus lanatus L. and the inherently slow-growing Deschampsia flexuosa (L.) Trin. were grown in sand at two levels of nitrate supply. Growth, photosynthesis, respiration and carbon and nitrogen content were studied over a period of 4 to 7 weeks. At low N-supply, the potentially fast-growing species still grew faster than the potentially slow-growing one. Similarly, differences in leaf area ratio (leaf area:total dry weight), specific leaf area (leaf area:leaf dry weight) and leaf weight ratio (leaf dry weight:total dry weight), as observed at high N-supply persisted at low N-availability. The only growth parameter for which a substantial Species × N-supply interaction was found was the net assimilation rate (increase in dry weight per unit leaf area and time). Rates of photosynthesis, shoot respiration and root respiration, expressed per unit leaf, shoot and root weight, respectively, were lower for the plants at low N-availability and higher for the fast-growing species. Species-specific variation in the daily carbon budget was mainly due to variation in carbon fixation. Lower values at low N were largely determined by both a lower C-gain of the leaves and a higher proportion of the daily gain spent in root respiration. Interspecific variation in C-content and dry weight:fresh weight ratio were similar at low and high N-supply. Total plant organic N decreased with decreasing N-supply, without differences between species. It is concluded that most of the parameters related to growth, C-economy and chemical composition differ between species and/or are affected by N-supply, but that differences between the two species at high N-availability persist at low N-supply.     

Limiting effects of low leaf-water content on the nitrogen utilization,energy budget,and larval growth ofHyalophora cecropia (Lepidoptera: Saturniidae)

Summary Hyalophora cecropia larvae were reared on leaves of wild cherry,Prunus serotina, which contained variable amounts of leaf water but otherwise did not differ in fiber, total nitrogen, and caloric content. Larvae which were fed leaves low in leaf water grew more slowly and were less efficient at utilizing plant biomass, energy, and nitrogen than those larvae fed leaves which were fully supplemented with water.Experiments were performed using excised leaves under different regimes of relative humidity and leaf water supplementation in climatic control chambers maintained at identical temperatures and photoperiod. Foodplant biomass utilization efficiencies were severely reduced by decreasing amounts of leaf water. Growth rates were halved and the efficiency of conversion of assimilated dry matter into larval biomass was reduced from 82% in the treatment with fully supplemented leaves to 34% in the driest treatment. The nitrogen utilization efficiency (N.U.E.) was reduced from 75–80% to 48%, and the relative accumulation rate of nitrogen (N.A.R.) was suppressed nearly 2-fold for larvae on low-water leaves. Relative maintenance costs (calories expended in respiration/mg tissue/day) of larvae were nearly five times higher on dry leaves than on fully supplemented leaves. Larvae on leaves which were low in water content were themselves more desiccated, and metabolized greater portions of assimilated energy, perhaps in an attempt to supplement body water with metabolic water derived from respiration.The larval rates of consumption of biomass, energy, and nitrogen were the same for all treatments, indicating that leaf water affected larval growth primarily by restricting the efficiency of utilizing these nutrients. Where water was limiting (as in tree leaves), an increased consumption rate did not appear to be a successful means of increasing growth rates. There were daily and seasonal differences in leaf water content between different trees of the same species. Although absolute differences in leaf water exist between different trees and between young and old (fully expanded) leaves of a single tree, these differences are proportional and parallel each other through daily and seasonal cycles.In spite of evolutionary adaptations of herbivores to acquire adequate water and avoid desiccation, the leaf water content naturally encountered by cecropia larvae on cherry leaves may limit their growth, especially if the R.H. is low.     

Symbiotic nitrogen fixation and vegetative growth of cowpea (Vigna unguiculata (L.) walp.) in waterlogged conditions

Summary Various periods of waterlogging (up to 32 days duration) were imposed upon cowpea plants grown in pots under controlled glasshouse conditions. Particular attention was paid to treatment effects on nodule cortication, nitrogenase activity and fixation efficiency, and the consequent differences in plant dry weight and nitrogen content.All waterlogging treatments increased nodule cortication as compared with the unstressed controls; a 16-day stress period being of critical duration with respect to the bi-phasic nature of this anatomical response. Conspicuous lenticel-type protuberances were present on nodules formed under waterlogged conditions but were markedly reduced, or indeed absent, in the controls. Total dry weight of nodules per plant was reduced by 60 per cent after only 8 days waterlogging, but nitrogen fixation efficiency of nodules which persisted was only 18 per cent less than those on control plants; mean nodule cortex having increased from 39.8 (control) to 51.5 per cent. After 16 days waterlogging, total plant dry weight was decreased by ca 60 per cent as compared with control plants; reflecting similar adverse changes in leaf, stem and root dry weight. The most severe treatment (32 days waterlogging) did not further reduce plant dry weight but mean nodule cortex area increased from 55.9 (16 days) to a maximum of 59.3 per cent. With the exception of nodules, percentage nitrogen content of various plant components was unaffected by the treatments imposed.Both the formation of enlarged lenticles and increased nodule cortication are regarded as adaptive anatomical responses which facilitate continued symbiotic nitrogen fixation and vegetative growth of this legume under waterlogged conditions.One of a series of papers describing work undertaken in a collaborative project with the International Institute of Tropical Agriculture, Nigeria, sponsored by the U.K. Ministry for Overseas Development.