The effects of seed weight on growth and competitive ability of Rumex acetosella from two successional old-fields

Summary The effects of seed size on growth, biomass allocation and competitive ability of Rumex acetosella plants grown either individually or in competition were studied in two populations (6 months and 15 years old respectively) sampled from a postcultivation successional gradient. For plants grown individually there were highly significant effects of seed weight on growth after 43 days, with a higher relative growth rate (RGR) observed for plants raised from heavier seeds. However at the end of the experiment, seedlings developed from lighter seeds had a RGR 2 times greater than those from heavier seeds. Final biomass of the two types was not significantly different after 73 days of growth. When plants were grown individually, there were only slight differences between populations, but when grown in monocultures of 4 plants per pot, plants from the old population had higher root and leaf biomass per pot whereas those from the young population had a higher reproductive effort per pot. This suggests that a trade-off between allocation to sexual and vegetative reproduction occurs over successional time. In mixtures of light and heavy seeds, plants from light seeds were shorter, had fewer leaves and lower biomass than plants from heavy seeds, which were also taller and produced more dry matter than plants grown from heavy seeds in monoculture. The significant effects of seed weight and population on biomass parameters persisted unit the end of the experiment. Seedlings from heavy seeds were strong competitors: those from the young population grew better in the presence of neighbors than in monoculture and those from the late successional population suppressed the more the growth of their partners. Seedlings from light seeds were subordinate competitors. These results suggest that seedlings from seeds of different sizes benefit from contrasting ecological conditions and that selection acts on reproductive output along successional gradients.     

Phenology of Chondrus ocellatus in Cheongsapo Near Busan, Korea

The reproductive phenology of Chondrus ocellatus and the effects of temperature and light on its growth were examined in Cheongsapo near Busan, Korea, from September 1994 to August 1995. The vegetative plants dominated over the year, with a peak occurrence in January. Gameto- and tetrasporophytes were most abundant in November and August. All vegetative and reproductive plants had a peak both in length and weight in October, when seawater temperature was highest (24°C). In laboratory culture, the maximum relative growth rate (RGR) of 2.94% day⁻¹ was obtained at 20°C and 100 μmol photons m⁻² s⁻¹, whereas the lowest value was recorded at 25°C and 100 μmol photons m⁻² s⁻¹ in a 12: 12 h LD photoperiod regime. Among the three photoperiod regimes (8:16 h, 12:12 h, 16:8 h LD) tested, there was evidence of a higher RGR in the 12:12 h LD cycle. This result suggests that the growth and reproduction of C. ocellatus are correlated with the seawater temperature based on laboratory culture and field observations.     

Chemical characterization of cell-wall polysaccharides from tank-cultivated and wild plants ofDelesseria sanguinea (Hudson) Lamouroux (Ceramiales,delesseriaceae): culture patterns and potent anticoagulant activity

Tank cultivation ofDelesseria sanguinea was investigated in order to manipulate conditions for vegetative growth and to provide biomass for the analysis of cell wall polysaccharides. Seasonality is subject to short-day photoperiodic control. Night-break or long-day conditions prevented fertility in tetrasporophytes and gametophytes and triggered outgrowth of new blades. Long-day illuminations allowed a 1% daily growth rate. Seawater temperature below 13 °C was necessary for inducing formation of new blades. Both wild and cultivated ofD. sanguinea plants contained a non gelling sulfated heteropolysaccharide composed of a galactosyl backbone branched with xylosyl residues. The hot water extract at neutral pH displayed the highest anticoagulant activity (5 μg ml^-1 polysaccharide concentration in APTT clotting assay). No obvious differences were found in polysaccharide chemical composition and properties between gametophytes and sporophytes or between cultivated and wild plants.     

唐古特雪莲花部特征及生殖分配的海拔差异

为探究高山植物生殖分配策略以及分析唐古特雪莲花部特征对海拔梯度的响应机制,该研究利用采样调查法和烘干称重法,对分布在青藏高原东缘不同海拔14个居群的唐古特雪莲花部特征和生殖分配进行了研究。结果表明:(1)繁殖分配随个体大小(地上生物量和株高)的增大呈线性递减趋势。(2)花期植株地上生物量、株高、管状小花数目、繁殖器官及营养器官生物量均随海拔升高呈线性递减趋势,管状小花生物量随海拔升高呈线性递增趋势。(3)花期管状小花数量及大小、繁殖器官生物量与营养器官生物量、雄蕊重量和雌蕊重量以及花粉数目与花丝长度之间均存在权衡现象。由此推论:(1)海拔作为外界因子对唐古特雪莲花部特征具有显著的影响,个体大小对其繁殖分配也存在潜在调控作用。(2)在不同海拔梯度上,唐古特雪莲有效地整合了有限的资源,其适应性特征之一就是通过减小个体大小来削弱营养生长以达到促进生殖生长的目的。     

Phenology ofLemanea fucina (Rhodophyta) in a Rhode Island river,USA

In a fourth-order river in Rhode Island, USA, cover, frequency of occurrence, length and node number of the red algaLemanea fucina were greatest in July and August, at which time current velocity and water depth were minimum and temperature was maximum. Population abundance and plant size were lowest in fall-early winter. The length of reproductive region and carpospore frequency were also highest in June to August, whereas percent plants with open tips was least in July and highest in February. It appears that growth and reproduction are confined to a period from April to August, after which there is thallus deterioration and carpospore release; between September and March remnants of this population remain.Lemanea plants are epiphyte-free from September to January. Subsequently, they are colonized by caddisflies and then the red algaAudouinella violacea; by August, 100% of theLemanea plants are covered byAudouinella.     

Effects of windspeed on the growth and biomass allocation of white mustard Sinapis alba L.

Summary We examined how different wind speeds and interactions between plant age and wind affect growth and biomass allocation of Sinapis alba L. (white mustard). Physiological and growth measurements were made on individuals of white mustard grown in controlled-environment wind tunnels at windspeeds of 0.3, 2.2 and 6.0 ms^–1 for 42 days. Plants were harvested at four different dates. Increasing wind speed slightly increased transpiration and stomatal conductance. We did not observe a significant decline in the photosynthetic rate per unit of leaf area. Number of leaves, stem length, leaf area and dry weights of total biomass and plant parts were significantly lower in plants exposed at high wind speed conditions. There were no significant differences in the unit leaf rate nor relative growth rates, although these were always lower in plants grown at high wind speed. Allocation and architectural parameters were also examined. After 42 days of exposure to wind, plants showed higher leaf area ratio, root and leaf weight ratios and root/shoot ratio than those grown at control treatment. Only specific leaf area declined significantly with wind speed, but stem and reproductive parts also decreased. The responses of plants to each wind speed treatment depended on the age of the plant for most of the variables. It is suggested that wind operates in logarithmic manner, with relatively small or no effect at lower wind speeds and a much greater effect at higher speeds. Since there is no evidence of a significant reduction in photosynthetic rate of Sinapis with increasing wind speed it is suggested that the effect of wind on plant growth was due to mechanical effects leading to changes in allocation and developmental patterns.     

Changes in percentage organic carbon content during ontogeny

Changes in percentage organic carbon content were assessed during the first five weeks of growth of Uniculm barley (Hordeum vulgare) and Brussels sprouts (Brassica oleracea) plants grown in controlled-environment conditions at two constant temperatures, 16° and 22°C. Foliage (leaf laminae), stem, and root material was assayed in both species, together with leaf sheaths of barley and cotyledon laminae of Brussels sprouts. In barley, there was a decline in percentage organic carbon content with increasing foliage age in plants grown at 22°C, but in sheath material there was no significant change at either temperature. Root material showed a decline in percentage carbon content at both growth temperatures, whereas stems showed the opposite trend. Similar results were found in Brussels sprouts, with an overall decline in percentage carbon content in foliage at 22°C and a rise in stem material at both growth temperatures. However, roots showed no significant change in percentage carbon content over the experimental period. The results demonstrate that percentage organic carbon content may change during plant growth.     

不同密度猪毛菜形态结构性状及生物量分配策略的异速关系

植物的生长特性随环境条件的变化具有可塑性,而不同的环境因素对植物可塑性的影响也不尽相同。利用异速分析的方法,通过模拟退化草地恢复过程中猪毛菜（Salsola collina）的不同种群密度（16、44、100、400株/m²）,研究其形态结构性状及生物量分配策略的异速关系在种群密度间的差异。结果表明,种群密度增大能抑制猪毛菜的生长,而且对猪毛菜的株高、根长、一级分枝数、二级分枝数、三级分枝数以及总分枝长均产生了极显著的影响,表明种群密度的变化使得植物的高生长和侧向生长发生了显著变化。种群密度的变化也引起了植物生物量的变化,其中植物根、茎、叶间的生物关系是一种表观可塑性,植物的生长策略未发生改变,只是植物个体大小发生改变引起的生物量分配的变化。植物株高、总分枝长、一级分枝数及繁殖分配的变化,是由种群密度变化引起的,植物的适应策略发生了改变,是真正的可塑性。种群密度改变了植物的繁殖分配策略,而未改变植物叶的分配策略,说明当环境发生变化时,植物调整了其繁殖策略以适应环境因素的改变,以保证种群的生存繁衍。     

Field response of wheat to arbuscular mycorrhizal fungi and drought stress

Mycorrhizal plants often have greater tolerance to drought than nonmycorrhizal plants. This study was conducted to determine the effects of arbuscular mycorrhizal (AM) fungi inoculation on growth, grain yield and mineral acquisition of two winter wheat (Triticum aestivum L.) cultivars grown in the field under well-watered and water-stressed conditions. Wheat seeds were planted in furrows after treatment with or without the AM fungi Glomus mosseae or G. etunicatum. Roots were sampled at four growth stages (leaf, tillering, heading and grain-filling) to quantify AM fungi. There was negligible AM fungi colonization during winter months following seeding (leaf sampling in February), when soil temperature was low. During the spring, AM fungi colonization increased gradually. Mycorrhizal colonization was higher in well-watered plants colonized with AM fungi isolates than water-stressed plants. Plants inoculated with G. etunicatum generally had higher colonization than plants colonized with G. mosseae under both soil moisture conditions. Biomass and grain yields were higher in mycorrhizal than nonmycorrhizal plots irrespective of soil moisture, and G. etunicatum inoculated plants generally had higher biomass and grain yields than those colonized by G. mosseae under either soil moisture condition. The mycorrhizal plants had higher shoot P and Fe concentrations than nonmycorrhizal plants at all samplings regardless of soil moisture conditions. The improved growth, yield and nutrient uptake in wheat plants reported here demonstrate the potential of mycorrhizal inoculation to reduce the effects of drought stress on wheat grown under field conditions in semiarid areas of the world.     

LATITUDINAL TRENDS IN THE GROWTH AND REPRODUCTIVE SEASONALITY OF DELESSERIA SANGUINEA,MEMBRANOPTERA ALATA,AND PHYCODRYS RUBENS (RHODOPHYTA)1

The seasonality of Delesseria sanguinea, Membranoptera alata, and Phycodrys rubens (Rhodophyta) was studied at Helgoland (North Sea, Germany) and Roscoff (Brittany, France). Plants were collected at bimonthly intervals, and growth and reproduction were monitored. Growth of blades was observed mainly in spring, although small blades were found on plants of M. alata and P. rubens all year round. In summer, plants started to degenerate and in autumn they became fertile. The reproductive season of D. sanguinea lasted from October to February/April at both locations, whereas reproductive plants of M. alata and P. rubens were found until April at Helgoland and until June and August, respectively, in Roscoff_: Lower winter temperatures at Helgoland than at Rascoff may have caused these differences in the duration of the reproductive season. Using published data, the seasonal patterns at Helgoland and Roscoff were compared to those found at other locations (e.g. Barents Sea; Maine, USA; Isle of Man, UK) and local temperature/daylength conditions. Blade growth was synchronized across all populations and occurred in spring, when temperatures were usually still suboptimal for growth. Maximum reproduction was generally found in the colder half of the year but started earlier in autumn in the Barents Sea. Adaptive strategies in the seasonal control of growth and reproduction are discussed. Adequate timing of the history events (e.g. appearance of juveniles in spring) appears more important than maximal growth and reproduction of adults during the season with the most favorable temperatures.     

Effects of simulated herbivory in three old field Compositae with different inflorescence architectures

The effects of simulated herbivory (early or late defoliation and cutting of the flowering shoot) on the growth and reproduction of three species of monocarpic composite forbs (Crepis pulchra, Picris hieracioides and C. foetida) with different inflorescence architectures were studied in experimental plots. For the three species studied, early defoliation had no significant effect on subsequent growth. In contrast, late defoliation, occurring at the start of the season of drought, had a negative effect on growth and reproduction in the two Crepis species, particularly C. foetida, but had less effect on P. hieracioides. Sexual biomass was more clearly affected by late defoliation than was vegetative biomass, although the effects differed markedly among species possibly as a result of differences in phenology. Clipping the flowering shoot removed about 3 times less biomass than late defoliation and had little effect on vegetative biomass. It had much greater effects on the sexual biomass in P. hieracioides and C. pulchra, and resulted in the production of many shoots sprouting from the rosette, allowing the treated plants to regain a vegetative biomass close to that of control plants. Clipping did however lead to the production of shorter shoots and a reduction in the number of capitula formed. In C. foetida, much branching occurred even when the main shoot was not cut; the architecture of individual plants was therefore only slightly changed by clipping the apical bud and the sexual biomass of this species was not affected by ablation of the flowering shoot. Overcompensation was found in only two families of C. pulchra for vegetative biomass. No over-compensation was found for sexual biomass, despite an increase in the number of flowering shoots in C. pulchra and P. hieracioides following clipping. However situations close to compensation for the vegetative biomass in the three species and in P. hieracioides for the sexual biomass were recorded. The response of the three study species to simulated herbivory were related to their architecture and to the time of defoliation.     

Utilization of seaweed Ulva sp. in Paracas Bay (Peru): experimenting with compost

In Paracas Bay (Peru), large quantities of Ulva sp. interfere with various important activities of the zone. In this context, the present study was undertaken to evaluate the effect of adding Ulva sp. in compost piles and the quality of the resulting composts. Six compost piles were prepared using, straw, green material mixture, cow manure and Ulva in different quantities (9, 17 and 28% of volume) and forms. Several variables were monitored during the process and the chemical characteristics of the final composts were determined. Aerial biomass achieved by maize plants was also evaluated for each compost at different compost/sand proportions. Results show that the compost pile made with Ulva powder registered the highest temperature and the longest thermophilic phase. Piles prepared with 28% washed Ulvapresented anoxic conditions at the beginning and had higher electrical conductivity values. Carbon/Nitrogen ratio diminished appropriately in all piles and there was no negative effect observed on the pH of the piles. Compost prepared with Ulva had lower contents of Total Kjeldahl Nitrogen, and produced maize plants aerial biomass.     

Cotton compensatory growth after loss of reproductive organs as affected by availability of resources and duration of recovery period

Damaged cotton plants in which reproductive organs were manually removed to simulate shedding induced by Helicoverpa spp. (Lepidoptera) were compared with undamaged controls grown under contrasting availability of resources. Plant growth and partitioning were analysed and fruit mass was taken as a measure of compensation. Under high availability of resources (low plant density, high fertility) damaged plants had a large potential compensatory capacity due to increased vegetative growth that enhanced their ability to assimilate carbon and nitrogen with respect to undamaged controls. These plants shifted from vegetative to reproductive growth when they were allowed to set fruit in the recovery period. Actual compensation was complete, however, only when the duration and conditions of the recovery period were favourable. Under multiple stresses (high plant density, low fertility, low temperature), damage triggered a marked increase in the allocation of biomass to roots which was not reversed when plants were allowed to set fruit. The apparent shift in the allocation pattern of damaged plants under stress-which matches well the survival strategy described for many perennials-probably restricted compensatory fruit growth.     

EFFECTS OF SALINITY,NITROGEN, AND POPULATION DENSITY ON THE SURVIVAL,GROWTH, AND REPRODUCTION OF ATRIPLEX TRIANGULARIS (CHENOPODIACEAE)

Populations of Atriplex triangularis were grown under laboratory conditions in a growth chamber and manipulated in an inland Ohio saline pond in order to examine the relative effects of salinity, nitrogen fertilization, and population density on growth, reproduction, and survival. For laboratory plants, nitrogen fertilization was the most important variable, with biomass and reproductive effort being greatest at the high nitrogen level. As salinity increased, biomass decreased only in plants not limited by nitrogen. Increasing density caused biomass per plant to decrease at both high and low nitrogen levels. For field plants, density was the most important variable, with biomass per plant and survival both decreasing as density increased. As density increased, size inequality among individuals increased but biomass per unit area and individual reproductive effort remained relatively constant. Nitrogen fertilization slightly enhanced survival, but did not affect biomass. It is suggested that density-dependent processes may be significant even in relatively harsh physical environments.     

Time course of plant-plant interactions in experimental mixtures of annuals: density,frequency, and nutrient effects

Summary Stellaria media and Poa annua were grown in a greenhouse over a wide range of mixed densities, and at three nutrient levels. The relative performances and interactions between individuals of both species were assessed over four interharvest growth periods using a new approach based on response functions (Connolly 1987). Species relative performances and interactions were complex, varying through time, and with mixed density and the nutrient environment. Studying the dynamics of the individual, interharvest growth periods led to a different, more accurate mechanistic interpretation of the final yield results, than did studying only one overall period of growth. Starting from considerably smaller seeds, Stellaria averaged twice as much biomass as young Poa plants by harvest 2 (H2). Stellaria achieved this early dominance with little suppression of Poa. Interspecific interference became more significant, and species proportional growth (K) became more similar in the second interharvest period (H2-3). During the final period of growth (H3-4), species relative performances largely reversed, Poa showing greater proportional growth than Stellaria. This superior performance by Poa occured despite individual plants being, on average, one half the size of Stellaria. Lack of interference early on is attributed to complementary above ground resource use resulting from species different shoot architectures (prostrate vs. cespitose). This complementarity disappeared as plants aged and became larger. The decline in Stellaria's superior relative growth performance through time was mainly due to its earlier and more substantial diversion of resources to reproduction. Species perception of each others influence, relative to themselves, as quantified by substitution rates, varied with both mixed density and nutrient environments. Stellaria consistently perceived Poa as less influential at higher nutrient levels than at lower levels. High nutrients favored the production of biomass for Stellaria more than for Poa, particularly in H2-3. While accelerating the switch to reproduction for Stellaria, nutrients did not increase its final reproductive yield per unit biomass. After H2, proportional growth for both species was relatively independent of frequency and density, indicating that a species acted as an aggregate of similarly behaving units of biomass whether arranged on many small, or a few large plants. An example shows that the conclusions from the response function approach applied within can be qualitatively different from those derived from a substitutive approach such as replacement series.     

Effects of fungal endophytes on the seed and seedling biology of Lolium perenne and Festuca arundinacea

Summary Many grasses are infected by endophytic fungi that grow intercellularly in leaves, stems, and flowers and are transmitted maternally by hyphal growth into ovules and seeds. The seed biology and seedling growth of endophyte-infected and uninfected perennial ryegrass (Lolium perenne) and tall fescue (Festuca arundinacea) were investigated under controlled environmental conditions. The percentage of filled seeds produced by infected tall fescue was over twice of uninfected tall fescue; infected and uninfected perennial reegrass had similar percentages. Weights of seeds from infected and uninfected plants were similar in both species. Seeds from infected plants of both species exhibited a higher rate of germination than seeds from uninfected plants. Shoot growth in the greenhouse was compared by making three sequential harvests of above-ground plant parts from infected and uninfected plants of both species. Infected perennial ryegrass plants produced significantly more biomass and tillers than uninfected plants after 6 and 10 weeks of growth and significantly more biomass after 14 weeks of growth. Infected tall fescue plants produced significantly more biomass and tillers than uninfected plants after 10 and 14 weeks of growth. The physiological mechanism of enhancement of growth is not known. The results of this study suggest that infected plants may have a selective advantage in populations with uninfected members.     

土壤灭菌处理对入侵植物南美蟛蜞菊及其伴生种生长的影响

该研究以温室盆栽法对南美蟛蜞菊重度入侵土壤进行高温高压湿热灭菌、添加杀真菌剂灭菌和添加杀细菌剂灭菌的处理后,将三种植株定植96 d后测定各生理指标参数,研究重度入侵土壤中各微生物类群对南美蟛蜞菊及其伴生种金腰箭和狗肝菜生长的影响。结果表明:在杀真菌、杀细菌以及高温高压湿热灭菌和未处理的南美蟛蜞菊重度入侵土壤中,三种植物生长情况均存在较大差异。在高温高压湿热灭菌土壤中南美蟛蜞菊的生长受到显著抑制,与未处理土壤中的生长情况相比,株高降低了17.59%,叶片数降低了38.10%,生物量降低了56.00%,电子传递速率变化不明显。在杀真菌土壤和杀细菌土壤中金腰箭的生长也受到显著抑制,与未处理土壤中的生长情况相比较,杀真菌土壤中的金腰箭株高降低最多(为42.28%),叶片数降低了38.89%,生物量降低了16.99%,电子传递速率变化不明显;在杀细菌土壤中金腰箭株高降低了36.64%,叶片数降低最多(为38.89%),生物量降低了33.67%,电子传递速率升高了11.11%。由此可见,不含微生物的土壤对南美蟛蜞菊生长有较强的抑制作用,不含真菌和细菌的土壤对金腰箭的生长有明显抑制作用。南美蟛蜞菊重度入侵土壤不仅适合南美蟛蜞菊的生长,也适合金腰箭的生长,对狗肝菜影响不大。     

Life cycle of Egeria densa planch., an aquatic plant naturalized in Japan

The ecophysiological life cycle of Egeria densa Planch., in a lotic irrigation ditch was evaluated. Phenological and quantitative measurements were made from August 1984 to July 1985. Lateral shoots with roots developed and elongated to the surface of the water when the bottom-water temperature increased above c. 15°C. Root crown developed simultaneously as the plant biomass increased. Dense shoot crown was formed under the water surface after the shoot reached the water surface. Plant biomass had two maxima in August and December-January. The bimodal curve in plant biomass was caused by the difference of growth attributed to the winter-type and summer-type plants. Relative growth rate, in length, of summer-type plants occurred at an optimum temperature of 20.7°C in culture. The seasonal activity of photosynthesis and respiration was measured in March, August and December. The optimum temperature of net photosynthesis of the summer-type plants reached a high 35°C similar to that of the C₄ plant. The compensation for light intensity at 35°C was 340 lux. Each photosynthesis-temperature curve suggested that Egeria had the ability to adapt to the seasonal changes in temperature in the natural habitat. The maximum starch concentrations reached 25.4% in the leaf and 22.6% in the stem in December. The shortage in the balance of organic matter for overwintering was found to be maintained by stored starch in the leaf and the stem.     

Root herbivores,pathogenic fungi,and competition between Centaurea maculosa and Festuca idahoensis

We used a common garden experiment to evaluate the isolated and combined effects of a biocontrol agent, the insect (Agapeta zoegana, Lepidoptera), and a native North American fungal pathogen (Sclerotinia sclerotiorum) on competition between the noxious weed Centaurea maculosa and the native Festuca idahoensis. In 0.5-m² plots with 24 plants per plot, competition between Centaurea and Festuca was highly asymmetrical, with Centaurea strongly reducing the final biomass and reproduction of Festuca, and Festuca having no effect, or possibly a positive effect, on Centaurea. The direct effects of the biological control agents differed entirely. All Centaurea individuals died in plots receiving Sclerotinia, but Agapeta did not significantly reduce the growth of Centaurea, and apparently stimulated a compensatory reproductive response in the weed. Individual Centaurea plants that had been damaged by Agapeta produced more flowerheads, and the number of Centaurea plants with Agapeta root damage in a plot was positively correlated with total Centaurea biomass. These differences in the direct effects of the consumers were reflected in their indirect effects. In plots where Sclerotinia killed Centaurea (strong direct effects) Festuca growth and reproduction was equal to that in Festuca plots without Centaurea and the reproductive output of Festuca increased substantially (strong indirect effects). However, in the absence of Sclerotinia, the application of Agapeta did not significantly decrease Centaurea biomass (weak direct effects) and actually stimulated small, but significant decreases in Festuca reproduction and trends towards lower Festuca biomass (weak and opposite indirect effects –Agapeta does not eat Festuca). If the direct effects of biocontrol agents on Centaurea are weak, as suggested by our results, natives are unlikely to be released from the competitive effects of Centaurea, and natives may suffer from Centaurea's compensatory response to herbivory. This revised version was published online in August 2006 with corrections to the Cover Date.     

The decline in N2 fixation rate in common bean with the onset of pod-filling: Fact or artifact

It is not clear to what extent genetic, environmental and measurement factors are responsible for the commonly reported decline in nitrogenase activity with the onset of pod-filling in grain legumes. We address this question by observing nitrogenase activity and assimilate partitioning throughout the life span of an indeterminate variety of common bean (GN 1140) under controlled-environment and field conditions. Nitrogenase activity per plant was maintained well into pod-filling in GN 1140 under high-light conditions in growth cabinets. In contrast, plants exposed to a gradual reduction in light intensity during early reproductive growth had a decline in nitrogenase activity on a whole plant basis with the onset of pod-filling. However, the decline was due to an inability to maintain nodule growth, rather than a decrease in specific nitrogenase activity. Under field conditions, acetylene reduction assay of root crowns appeared to indicate a rapid decline in nitrogenase activity with the onset of pod-filling in GN 1140. This decline was not correlated with the water status of the soil or the plant. In contrast, acetylene reduction activity of root cores taken from outside the root crown region (non-crown) and N accumulation by above-ground biomass during pod-filling suggested that whole plant nitrogenase activity was maintained longer than that indicated by root crown assays. We conclude that although the occurrence of a decline in nitrogenase activity with the onset of pod-filling in grain legumes can be genetically determined, in many cases the decline can be the result of growing conditions and improper measurement techniques.