Architecture and growth of an annual plant <Emphasis Type="Italic">Chenopodium album</Emphasis> in different light climates

Light climates strongly influence plant architecture and mass allocation. Using the metamer concept, we quantitatively described branching architecture and growth of Chenopodium album plants grown solitarily or in a dense stand. Metamer is a unit of plant construction that is composed of an internode and the upper node with a leaf and a subtended axillary bud. The number of metamers on the main-axis stem increased with plant growth, but did not differ between solitary and dense-stand plants. Solitary plants had shorter thicker internodes with branches larger in size and number than the plant in the dense stand. Leaf area on the main stem was not different. Larger leaf area in solitary plants was due to a larger number of leaves on branches. Leaf mass per area (LMA) was higher in solitary plants. It did not significantly differ between the main axis and branches in solitary plants, whereas in the dense stand it was smaller on branches. Dry mass was allocated most to leaves in solitary plants and to stems in the dense stand in vegetative growth. Reproductive allocation was not significantly different. Branch/main stem mass ratio was higher in solitary than dense-stand plants, and leaf/stem mass ratio higher in branches than in the main axis. Nitrogen use efficiency (NUE) (dry mass growth per unit N uptake) was higher and light use efficiency (LUE) (dry mass growth per unit light interception) was lower in the plant grown solitarily than in the dense stand.     

藜个体在高密度种群中的氮素利用效率

 氮素利用效率（NUE）是植物养分策略研究中的一项重要内容。该文利用Berendse和Aerts提出的氮素利用效率概念和原理研究了高密度的藜(Chenopodium album)种群中不同植物个体在种内竞争条件下的氮素利用效率。结果表明,由于植株的氮素吸收速率与其个体大小成非线性关系,说明不同植株个体对氮素的竞争属于非对称竞争。个体较大的植株氮素输入较高,而个体较小的植株氮素输出较高,因而较大个体植株的氮素净增加也较高。植株的氮素损失随着个体大小的增加而增加,较大植株个体的氮素浓度随着生长而下降,而较小植株个体的氮素浓度随时间的变化不大,说明个体较小的植株的生长受光照的限制比受氮素的限制更大,而对较大的植株个体而言,它们的生长受氮素的限制更大。高密度藜种群中的不同植物个体具有不同的养分策略,氮素利用效率及其组成部分氮素生产力（NP）和氮素滞留时间（MRT）均不同。植株的NP和MRT与其个体大小正相关,较大的植物个体具有较高的NP和较长的MRT,因而氮素利用效率也高于个体较小的植株。在个体水平上,种内不同植株的NP与MRT不存在权衡关系（Trade-off）。因此,Berendse和Aerts提出的氮素利用效率概念不仅适用于研究种间的养分策略,对于研究种内不同植株的养分策略也同样适用。     

Leaf-level nitrogen use efficiency: definition and importance

Nitrogen use efficiency (NUE) has been widely used to study the relationship between nitrogen uptake and dry mass production in the plant. As a subsystem of plant nitrogen use efficiency (NUE), I have defined leaf-level NUE as the surplus production (gross production minus leaf respiration) per unit amount of nitrogen allocated to the leaf, with factorization into leaf nitrogen productivity (NP) and mean residence time of leaf nitrogen (MRT). These concepts were applied to two herbaceous stands: a perennial Solidago altissima stand and an annual Amaranthus patulus stand. S. altissima had more than three times higher leaf NUE than A. patulus due to nearly three times longer MRT of leaf N. In both species, NUE and NP were higher at the leaf level than at the plant level, because most leaf N is involved directly in the photosynthetic activity and because leaf surplus production is higher than the plant net production. MRT was longer at the plant level. The more than twice as long MRT at the plant level as at the leaf level in S. altissima was due to a large contribution of nitrogen storage belowground in the winter in this species. Thus, comparisons between a perennial and an annual system and between plant- and leaf-level NUE with their components revealed the importance of N allocation, storage, recycling, and turnover of organs for leaf photosynthetic production and plant dry mass growth.     

向日葵种群中植株个体大小对其氮素利用策略的影响

我们利用Berendse和Aerts提出的氮素利用效率(NUE)概念及原理研究了高密度一年生草本植物向日葵(Helianthus annuus L.)种群中植株个体大小对其氮素吸收利用的影响,并对种内竞争进行了分析.结果表明,植株对氮素的吸收与其个体大小不成线性关系,说明种群内不同植株个体对土壤氮素的竞争属于非对称竞争.植株的氮素损失随着个体大小的增加而增加.个体较大的植株具有较高的氮素输入率和较低的氮素输出率,因而具有较高的氮素净增加值.植株的氮素生产力(NP)和氮素平均滞留时间(MRT)均与植株个体大小呈正相关.较大的植物个体具有较高的NP和较长的MRT,由于NUE为NP和MRT二者的乘积,因而较大个体植株的NUE高于个体较小的植株.同种植物的不同个体的NP和MRT之间不存在协衡关系.氮素回收效率(NRE)与植株个体大小密切相关.在个体水平上,较大的植株个体具有较高的NUE与其较高的NRE有关.种群内植株个体对土壤氮素的非对称竞争主要由于植株对氮素的吸收和利用效率不同所致.因此,Berendse和Aerts提出的氮素利用效率概念不仅适用于研究种间的养分利用策略,对于种内不同植株的养分策略研究也同样适用.     

Effect of Nitrogen Supply on the Nitrogen Use Efficiency of an Annual Herb, Helianthus annuus L.

Nitrogen use efficiency (NUE) is the product of nitrogen productivity (NP) and the mean residence time of nitrogen (MRT). Theory suggests that there should be a trade-off between both components,but direct experimental evidence is still scarce. To test this hypothesis, we analyzed the effect of varying nitrogen supply levels on NUEand its two components (NP, MRT) in Helianthus annuus L., an annual herb.The plants investigated were subjected to six nitrogen levels (0, 2, 4, 8, 16, and 32 g N/m2). Total plant production increased substantially with increasing nitrogen supply. Nitrogen uptake and loss also in creased with nitrogen supply. Nitrogen influx (rin) and outflux (rout) were defined as the rates of nitrogen uptake and loss per unit aboveground nitrogen, respectively. Both rin and rout increased with increasing nitrogen supply. In addition, rin was far higher than rout. Consequently, the relative rate of nitrogen incre ment (rin- rout) also increased with nitrogen supply. There were marked differences between treatments with respect to parameters related to the stress resistance syndrome: nitrogen pool size, leaf nitrogen concentration,and net aboveground productivity increased with nitrogen supply. Plants at high nitrogen levels showed a higher NP (the growth rate per unit aboveground nitrogen) and a shorter MRT (the inverse of rout), whereas plants at low nitrogen levels displayed the reverse pattern. Shorter MRT for plants at high nitrogen levels was caused by the abscission of leaves that contained relatively large fractions of total plant nitrogen. We found a negative relationship between NP and MRT, the components of NUE, along the gradient of nitrogen availability, suggesting that there was a trade-off between NP and MRT. The NUE increased with increasing nitrogen availability, up to a certain level, and then decreased. These results offer support for the hypoth esis that adaptation to infertile habitats involves a low nitrogen loss (long MRT in the plant) rather than a high NUE per se. The higher NUE at the plant level was a result, in part, of greater nitrogen resorption during senescence. We suggest that a long MRT (an index of nitrogen conservation) is a potentially successful strategy in nitrogen-poor environments.     

向日葵种群中植株个体大小对其氮素利用策略的影响

我们利用Berendse和Aerts提出的氮素利用效率(NUE)概念及原理研究了高密度一年生草本植物向日葵(Helianthus annuus L.)种群中植株个体大小对其氮素吸收利用的影响,并对种内竞争进行了分析。结果表明,植株对氮素的吸收与其个体大小不成线性关系,说明种群内不同植株个体对土壤氮素的竞争属于非对称竞争。植株的氮素损失随着个体大小的增加而增加。个体较大的植株具有较高的氮素输入率和较低的氮素输出率,因而具有较高的氮素净增加值。植株的氮素生产力(NP)和氮素平均滞留时间(MRT)均与植株个体大小呈正相关。较大的植物个体具有较高的NP和较长的MRT,由于NUE为NP和MRT二者的乘积,因而较大个体植株的NUE高于个体较小的植株。同种植物的不同个体的NP和MRT之间不存在协衡关系。氮素回收效率(NRE)与植株个体大小密切相关。在个体水平上,较大的植株个体具有较高的NUE与其较高的NRE有关。种群内植株个体对土壤氮素的非对称竞争主要由于植株对氮素的吸收和利用效率不同所致。因此,Berendse和Aerts提出的氮素利用效率概念不仅适用于研究种间的养分利用策略,对于种内不同植株的养分策略研究也同样适用。     

Nitrogen Uptake and Plant Growth I. Effect of Nitrogen Removal on Growth of Polygonum cuspidatum

Polygonun cuspidatum was grown hydroponically to examine theeffect of nitrogen removal from the nutrient solution upon plantgrowth and the partitioning of dry matter and nitrogen amongorgans. Nitrogen removal reduced the growth rate mainly dueto the reduced growth of leaf area. Accelerated root growthwas observed only in plants which earlier had received highlevels of nitrogen. Nitrogen removal caused almost exclusiveallocation of available nitrogen to root growth. Nitrogen fluxfrom the shoot to the root occurred in plants which had receivedlow nitrogen. Not only was net assimilation rate (NAR) littleaffected by nitrogen removal, but it also was not correlatedwith the concentration of leaf nitrogen on an area basis. Light-saturatedCO₂ exchange rate (CER) was highly correlated with the concentrationof leaf nitrogen. Nitrogen use efficiency (NUE) in CER (CERdivided by leaf nitrogen) remained constant against leaf nitrogen,indicating efficient use of nitrogen under light saturation,while NUE in terms of NAR decreased with higher concentrationof leaf nitrogen. Polygonum cuspidatum Sieb. et Zuce., CO₂ exchange rate, growth analysis, leaf nitrogen, net assimilation rate, nitrogen use efficiency, partitioning of dry matter and nitrogen     

Nitrogen use efficiency revisited

Nitrogen use efficiency (NUE) was originally defined as the dry mass productivity per unit N taken up from soil. The term was subsequently redefined as the product of nitrogen productivity (NP) and mean residence time of nitrogen (MRT). However, this redefinition was found to contradict the original definition under certain conditions, and confusion arose when the MRT defined for a steady-state system was applied to a system that was actually not at steady state. As MRT is the expected length of time that a unit of N newly taken up from soil is retained before being lost, it can be translated into the plant nitrogen duration (PND) divided by the total N uptake. This MRT is determined equally well for a steady state- and a non-steady state system and is in accordance with the original definition of NUE. It can be applied to a herbaceous perennial stand (that was at a steady state) and to an annual stand (that was not at a steady state) to determine NUE. NUE is also applicable when plant growth and reproduction are analyzed in relation to N use.     

Does leaf shedding increase the whole-plant carbon gain despite some nitrogen being lost with shedding?

When old leaves are shed, part of the nitrogen in the leaf is retranslocated to new leaves. This retranslocation will increase the whole-plant carbon gain when daily C gain : leaf N ratio (daily photosynthetic N-use efficiency, NUE) in the old leaf, expressed as a fraction of NUE in the new leaf, becomes lower than the fraction of leaf N that is resorbed before shedding (R(N)). We examined whether plants shed their leaves to increase the whole-plant C gain in accord with this criterion in a dense stand of an annual herb, Xanthium canadense, grown under high (HN) and low (LN) nitrogen availability. The NUE of a leaf at shedding expressed as a fraction of NUE in a new leaf was nearly equal to the R(N) in the LN stand, but significantly lower than the R(N) in the HN stand. Thus shedding of old leaves occurred as expected in the LN stand, whereas in the HN stand, shedding occurred later than expected. Sensitivity analyses showed that the decline in NUE of a leaf resulted primarily from a reduction in irradiance in the HN stand. On the other hand, it resulted from a reduction in irradiance and also in light-saturated photosynthesis : leaf N content ratio (potential photosynthetic NUE) in the LN stand.     

Nitrogen uptake and use by competing individuals in a Xanthium canadense stand

We studied differences in nitrogen uptake and use for plant growth among individuals competing in a natural dense stand of an annual herb, Xanthium canadense. Larger individuals took up more nitrogen than proportionately to their size, indicating that the competition for soil nitrogen was asymmetric among individuals, although it was more symmetric than the competition for light. The rate of nitrogen loss of individuals also increased with plant size. While smaller individuals shared smaller fractions of total plant nitrogen in the stand, they had higher nitrogen concentrations per unit mass. "Turnover" rates of nitrogen influx (r_in) and outflux (r_out) were defined as the rates of nitrogen uptake and loss per unit aboveground nitrogen, respectively. r_in was higher in larger individuals, whereas r_out was higher in smaller individuals. Consequently, the relative rate of nitrogen increment (r_in-r_out) was higher in larger individuals, whereas it was around zero in the smallest individuals. The mean residence time of nitrogen (MRT), defined as the inverse of r_out, was longer in larger individuals. Nitrogen productivity (NP), i.e. the growth rate per unit aboveground nitrogen, was higher in larger individuals. As the product of lifetime MRT and NP gives the nitrogen use efficiency (NUE), defined as biomass production per unit flux of nitrogen, higher MRT and NP observed in larger individuals would have contributed to their higher lifetime NUE. Shorter MRT in smaller individuals was caused by the abscission of leaves which contained relatively large fractions of total plant nitrogen. Xanthium canadense, as a competitive ruderal, tended to produce leaves at higher positions to acquire higher light levels at the expense of older leaves rather than to modify their productive structure to efficiently use low light levels as observed in shade-tolerant species.     

Effects of mechanical stress and plant density on mechanical characteristics, growth, and lifetime reproduction of tobacco plants

Plastic increases in stem elongation in dense vegetation are generally believed to be induced by canopy shading, but because plants protect each other from wind, shielding (reduced mechanical stress) could also play a role. To address this issue, tobacco Nicotiana tabacum plants were subjected to two levels of mechanical stress, 0 (control) or 40 (flexed) daily flexures, and grown solitarily, in a dense monostand (with plants of only one mechanical treatment), or in a mixed stand (flexed and control plants grown together). Flexed plants produced shorter and thicker stems with a lower Young's modulus than control plants, while dense-stand plants had relatively taller and thinner stems than solitary ones. Flexing effects on stem characteristics were independent of stand density. Growth, reproduction, and survival of solitary plants were not affected by flexing, while in the monostand growth was slightly reduced. But in the mixed stand, flexed plants were readily shaded by controls and had considerably lower growth, survival, and reproduction rates. These results suggest that wind shielding indeed plays a role in the plastic increase in stem elongation of plants in dense vegetation and that this response can have important consequences for competitive ability and lifetime seed production.     

Geographical variation in the response to nitrogen deposition in Arabidopsis lyrata petraea

The adaptive responses to atmospheric nitrogen deposition for different European accessions of Arabidopsis lyrata petraea were analysed using populations along a strong atmospheric N-deposition gradient. Plants were exposed to three N-deposition rates, reflecting the rates at the different locations, in a full factorial design. Differences between accessions in the response to N were found for important phenological and physiological response variables. For example, plants from low-deposition areas had higher nitrogen-use efficiencies (NUE) and C : N ratios than plants from areas high in N deposition when grown at low N-deposition rates. The NUE decreased in all accessions at higher experimental deposition rates. However, plants from high-deposition areas showed a limited capacity to increase their NUE at lower experimental deposition rates. Plants from low-deposition areas had faster growth rates, higher leaf turnover rates and shorter times to flowering, and showed a greater increase in growth rate in response to N deposition than those from high-deposition areas. Indications for adaptation to N deposition were found, and results suggest that adaptation of plants from areas high in N deposition to increased N deposition has resulted in the loss of plasticity.     

Photosynthesis, growth and nutrient changes in non-nodulated Phaseolus vulgaris grown under atmospheric and elevated carbon dioxide conditions

The response of Phaseolus vulgaris L. cv. Contender grown under controlled environment at either ambient or elevated (360 and 700 μmol mol^-1, respectively) CO₂ concentrations ([CO₂]), was monitored from 10 days after germination (DAG) until the onset of senescence. Elevated CO₂ had a pronounced effect on total plant height (TPH), leaf area (LA), leaf dry weight (LD), total plant biomass (TB) accumulation and specific leaf area (SLA). All of these were significantly increased under elevated carbon dioxide with the exception of SLA which was significantly reduced. Other than high initial growth rates in CO₂-enriched plants, relative growth rates remained relatively unchanged throughout the growth period. While the trends in growth parameters were clearly different between [CO₂], some physiological processes were largely transient, in particular, net assimilation rate (NAR) and foliar nutrient concentrations of N, Mg and Cu. CO₂ enrichment significantly increased NAR, but from 20 DAG, a steady decline to almost similar levels to those measured in plants grown under ambient CO₂ occurred. A similar trend was observed for leaf N content where the loss of leaf nitrogen in CO₂-enriched plants after 20 DAG, was significantly greater than that observed for ambient-CO₂ plants. Under enhanced CO₂, the foliar concentrations of K and Mn were increased significantly whilst P, Ca, Fe and Zn were reduced significantly. Changes in Mg and Cu concentrations were insignificant. In addition. high CO₂ grown plants exhibited a pronounced leaf discoloration or chlorosis, coupled with a significant reduction in leaf longevity.     

Growth analysis of UV-B-irradiated cucumber seedlings as influenced by photosynthetic photon flux source and cultivar

A growth analysis was made of ultraviolet-B (UV-B)-sensitive (Poinsett) and insensitive (Ashley) cultivars of Cucuumis satives L. grown in growth chambers at 600 μmol m⁻² s⁻¹ of photosynthetic photon flux (PPF) provided by red- and far-red-deficient metal halide (MH) or blue- and UV-A-deficient high pressure sodium/deluxe f HPS/DX) lamps. Plants were irradiated 6 h daiiy with 0.2 f-UV-B) or 18.2 C+UV-B) kJ m⁻² day⁻¹ of biologically effective UV-B for 8 or 15 days from time of seeding. In general, plants given supplemental UV-B for 15 days showed lower leaf area ratio (LARs, and higher specific leaf mass (SLM) mean relative growth rate (MRGR) and net assimilation rate (NAR) than that of control plants, but they showed no difference in leaf mass ratio (LMR), Plants grown under HPS/DX lamps vs MH lamps showed higher SLM and NAR. lower LAR and LMR. hut no difference in MRGR. LMR was the only growth parameter affected by cultivar: at 15 days, it was slightly greater in Poinsett than in Ashley. There were no interactive effects of UV-B. PPF source or cultivar on any of the growth parameters determined, indicating that the choice of either HPS/DX or MH lamps should not affect growth response to UV-B radiation. This was true even though leaves of UV-B-irradiated plants grown under HPS/DX lamps have been shown to have greater chlorosis than those grown under MH lamps.     

Growth temperature influences the underlying components of relative growth rate: an investigation using inherently fast- and slow-growing plant species

We examined the effect of growth temperature on the underlying components of growth in a range of inherently fast‐ and slow‐growing plant species. Plants were grown hydroponically at constant 18, 23 and 28 °C. Growth analysis was conducted on 16 contrasting plant species, with whole plant gas exchange being performed on six of the 16 species. Inter‐specific variations in specific leaf area (SLA) were important in determining variations in relative growth rate (RGR) amongst the species at 23 and 28 °C but were not related to variations in RGR at 18 °C. When grown at 18 °C, net assimilation rate (NAR) became more important than SLA for explaining variations in RGR. Variations in whole shoot photosynthesis and carbon concentration could not explain the importance of NAR in determining RGR at the lower temperatures. Rather, variations in the degree to which whole plant respiration per unit leaf area acclimated to the different growth temperatures were responsible. Plants grown at 28 °C used a greater proportion of their daily fixed carbon in respiration than did the 18 and 23 °C‐grown plants. It is concluded that the relative importance of the underlying components of growth are influenced by growth temperature, and the degree of acclimation of respiration is of central importance to the greater role played by NAR in determining variations in RGR at declining growth temperatures.     

The Nitrogen Use Efficiency of C(3) and C(4) Plants: I. Leaf Nitrogen, Growth, and Biomass Partitioning in Chenopodium album (L.) and Amaranthus retroflexus (L.)

The effect of applied nitrogen (N) on the growth, leaf expansion rate, biomass partitioning and leaf N levels of Chenopodium album (C₃) and Amaranthus retroflexus (C₄) were investigated. At a given applied N level, C. album had 50% greater leaf N per unit area (N_a) than A. retroflexus. Nitrate accumulated at lower N_a in A. retroflexus than C. album. A. retroflexus was more productive than C. album at high N, but C. album was more productive at low N. At high applied N, nitrogen use efficiency (NUE), expressed either as net assimilation rate (NAR) per unit N or relative growth rate per unit N, was greater in A. retroflexus than C. album. However, at low applied N, C. album had a greater NUE on both an NAR and growth basis than A. retroflexus. The leaf area partitioning coefficient was similar in the species at high N, but was greater in A. retroflexus than C. album at low N. At low N, greater leaf area partitioning apparently lowered leaf N in A. retroflexus to levels at which necrosis occurred. In C. album by contrast, leaf area partitioning declined to a greater degree with declining N than it did in A. retroflexus, so that leaf N did not decline as much. Consequently, low N C. album plants did not lose leaf area to necrosis and had a greater NAR and NUE at low applied N than A. retroflexus.     

Branching, nitrogen distribution, and carbon gain in solitary plants of an annual herb, Xanthium canadense

The theory of optimal nitrogen (N) distribution predicts that the carbon gain of plants will be maximised when leaves of higher irradiance have higher N content per area (N _area). Most previous studies have examined optimal N distribution without explicitly considering the branching status of plants. I investigated light environment, N distribution and photosynthetic traits of individual leaves of an herbaceous species, Xanthium canadense. X. canadense was grown solitary under high (HN) and low nutrients (LN). Light availability, leaf mass per unit area and N _area were measured for all leaves within plants. Daily photosynthesis of the plants of actual N distribution was compared with those of optimal and constant N distribution. Branch production was facilitated in HN but not in LN plants. N _area was correlated more with leaf order than with leaf light environment. Although N was more limited and the light environment was less heterogeneous within crowns in LN than in HN plants, leaf N distribution was closer to optimal in the latter. These results suggest that leaf N distribution was not optimised in solitary plants of X. canadense. Because this species often regenerates in a dense stand, leaf N distribution might be selected to maximise carbon gain only in such a stand. Leaf N distribution might thus be constrained by the regeneration strategy of the species.     

Nitrogen response efficiency increased monotonically with decreasing soil resource availability: a case study from a semiarid grassland in northern China

The concept of nutrient use efficiency is central to understanding ecosystem functioning because it is the step in which plants can influence the return of nutrients to the soil pool and the quality of the litter. Theory suggests that nutrient efficiency increases unimodally with declining soil resources, but this has not been tested empirically for N and water in grassland ecosystems, where plant growth in these ecosystems is generally thought to be limited by soil N and moisture. In this paper, we tested the N uptake and the N use efficiency (NUE) of two Stipa species (S. grandis and S. krylovii) from 20 sites in the Inner Mongolia grassland by measuring the N content of net primary productivity (NPP). NUE is defined as the total net primary production per unit N absorbed. We further distinguished NUE from N response efficiency (NRE; production per unit N available). We found that NPP increased with soil N and water availability. Efficiency of whole-plant N use, uptake, and response increased monotonically with decreasing soil N and water, being higher on infertile (dry) habitats than on fertile (wet) habitats. We further considered NUE as the product of the N productivity (NP the rate of biomass increase per unit N in the plant) and the mean residence time (MRT; the ratio between the average N pool and the annual N uptake or loss). The NP and NUE of S. grandis growing usually in dry and N-poor habitats exceeded those of S. krylovii abundant in wet and N-rich habitats. NUE differed among sites, and was often affected by the evolutionary trade-off between NP and MRT, where plants and communities had adapted in a way to maximize either NP or MRT, but not both concurrently. Soil N availability and moisture influenced the community-level N uptake efficiency and ultimately the NRE, though the response to N was dependent on the plant community examined. These results show that soil N and water had exerted a great impact on the N efficiency in Stipa species. The intraspecific differences in N efficiency within both Stipa species along soil resource availability gradient may explain the differences in plant productivity on various soils, which will be conducive to our general understanding of the N cycling and vegetation dynamics in northern Chinese grasslands.     

Variation in relative growth rate of 20 Aegilops species (Poaceae) in the field: The importance of net assimilation rate or specific leaf area depends on the time scale

Field experiments reporting the relative growth rate (RGR) patterns in plants are scarce. In this study, 22 herbaceous species (20 Aegilops species, Amblyopyrum muticum and Triticum aestivum) were grown under field conditions to assess their RGR, and to find out if the differences in RGR amongst species were explained by morphological or physiological traits. Plants were cultivated during two months, and five harvests (every 13–19 days) were carried out. Factors explaining between-species differences in RGR varied, depending on whether short (13–19 days) or longer periods (62 days) were considered. RGR for short periods (4 growth periods of 13–19 days each) showed a positive correlation with net assimilation rate (NAR), but there was no significant correlation with leaf area ratio (LAR) (with the exception of the first growth period). In contrast, when growth was investigated over two months, RGR was positively correlated with morphological traits (LAR, and specific leaf area, SLA), but not with physiological traits (NAR). A possible explanation for these contrasting results is that during short growth periods, NAR exhibited strong variations possibly caused by the variable field conditions, and, consequently NAR mainly determined RGR. In contrast, during a longer growth period (62 days) the importance of NAR was not apparent (there was no significant correlation between RGR and NAR), while allocation traits, such as LAR and SLA, became most relevant.     

Mean residence time of leaf number, area, mass, and nitrogen in canopy photosynthesis

Mean residence time (MRT) of plant nitrogen (N), which is an indicator of the expected length of time N newly taken up is retained before being lost, is an important component in plant nitrogen use. Here we extend the concept MRT to cover such variables as leaf number, leaf area, leaf dry mass, and nitrogen in the canopy. MRT was calculated from leaf duration (i.e., time integral of standing amount) divided by the total production of leaf variables. We determined MRT in a Xanthium canadense stand established with high or low N availability. The MRT of leaf number may imply longevity of leaves in the canopy. We found that the MRT of leaf area and dry mass were shorter than that of leaf number, while the MRT of leaf N was longer. The relatively longer MRT of leaf N was due to N resorption before leaf shedding. The MRT of all variables was longer at low N availability. Leaf productivity is the rate of canopy photosynthesis per unit amount of leaf variables, and multiplication of leaf productivity by MRT gives the leaf photosynthetic efficiency (canopy photosynthesis per unit production of leaf variables). The photosynthetic efficiency of leaf number implies the lifetime carbon gain of a leaf in the canopy. The analysis of plant-level N use efficiency by evaluating the N productivity and MRT is a well-established approach. Extension of these concepts to leaf number, area, mass, and N in the canopy will clarify the underlying logic in the study of leaf life span, leaf area development, and dry mass and N use in canopy photosynthesis.