Relationship between photosynthetic phosphorus‐use efficiency and foliar phosphorus fractions in tropical tree species

How plants develop adaptive strategies to efficiently use nutrients on infertile soils is an important topic in plant ecology. It has been suggested that, with decreasing phosphorus (P) availability, plants increase photosynthetic P‐use efficiency (PPUE) (i.e., the ratio of instantaneous photosynthetic carbon assimilation rate per unit foliar P). However, the mechanism to increase PPUE remains unclear. In this study, we tested whether high PPUE is explained by an optimized allocation of P in cells among P‐containing biochemical compounds (i.e., foliar P fractions). We investigated the relationships among mass‐based photosynthetic carbon assimilation rate (A_mass), PPUE, total foliar P concentration, and foliar P fractions in 10 tree species in two tropical montane rain forests with differing soil P availability (five species on sedimentary soils and five species on P‐poorer ultrabasic serpentine soils) on Mount Kinabalu, Borneo. We chemically fractionated foliar P into the following four fractions: metabolic P, lipid P, nucleic acid P, and residual P. A_mass was positively correlated with the concentrations of total foliar P and of metabolic P across 10 tree species. Mean A_mass and mean concentrations of total foliar P and of each foliar P fraction were lower on the P‐poorer ultrabasic serpentine soils than on the sedimentary soils. There was a negative relationship between the proportion of metabolic P per total P and the proportion of lipid P per total P. PPUE was positively correlated with the ratio of metabolic P to lipid P. High PPUE is explained by the net effect of a relatively greater investment of P into P‐containing metabolites and a relatively lesser investment into phospholipids in addition to generally reduced concentrations of all P fractions. We conclude that plants optimize the allocation of P among foliar P fractions for maintaining their productivity and growth and for reducing demand for P as their adaptation to P‐poor soils.     

氮磷添加对金露梅叶片化学计量及光合特征的影响

通过三种养分添加处理,氮添加(5、10和15 g??m-2)、磷添加(梯度同氮添加)、氮磷同时添加[(5 g N＋5 g P)??m-2、(10 g N＋10 g P)??m-2、(15 g N＋15 g P)??m-2],对照(无养分添加),探讨养分添加对金露梅叶片性状氮含量(Nmas )、磷含量(Pmas )、氮磷比(N∶P)、比叶重(LMA)、净光合速率(Pn )和光合氮利用效率(PNUE)的影响,以及各性状之间的相互关系.结果表明：在处理水平上,除N或P显著提高金露梅叶片的N∶P外,氮、磷添加对叶片其它性状无显著影响;不同氮、磷处理下添加水平对金露梅叶片的Nmas、N∶P、Pn和PNUE均有显著影响,随着养分水平提高,各性状的变化模式各不相同,叶片Pmas无明显变化,而叶片LMA虽有降低的趋势但不显著.回归分析表明,叶片Pmas与Nmas之间呈显著正相关(R2＝0．347,P＜0．001),叶片Nmas 与N∶P之间也呈显著正相关(R2＝0．018,P＜0．05),而叶片Pmas与N∶P呈显著负相关(R2＝0．505,P＜0．001);叶片LMA与Pn之间显著负相关(R2＝0．02,P＜0．05),而与PNUE之间显著正相关(R2＝0．077,P＜0．001).这表明在一定范围内,环境变化可以改变金露梅叶片的养分保持能力、光合能力以及养分利用效率.     

Different leaf cost–benefit strategies of ferns distributed in contrasting light habitats of sub-tropical forests

Background and Aims Ferns are abundant in sub-tropical forests in southern China, with some species being restricted to shaded understorey of natural forests, while others are widespread in disturbed, open habitats. To explain this distribution pattern, we hypothesize that ferns that occur in disturbed forests (FDF) have a different leaf cost–benefit strategy compared with ferns that occur in natural forests (FNF), with a quicker return on carbon investment in disturbed habitats compared with old-growth forests.Methods We chose 16 fern species from contrasting light habitats (eight FDF and eight FNF) and studied leaf functional traits, including leaf life span (LLS), specific leaf area (SLA), leaf nitrogen and phosphorus concentrations (N and P), maximum net photosynthetic rates (A), leaf construction cost (CC) and payback time (PBT), to conduct a leaf cost–benefit analysis for the two fern groups.Key Results The two groups, FDF and FNF, did not differ significantly in SLA, leaf N and P, and CC, but FDF had significantly higher A, greater photosynthetic nitrogen- and phosphorus-use efficiencies (PNUE and PPUE), and shorter PBT and LLS compared with FNF. Further, across the 16 fern species, LLS was significantly correlated with A, PNUE, PPUE and PBT, but not with SLA and CC.Conclusions Our results demonstrate that leaf cost–benefit analysis contributes to understanding the distribution pattern of ferns in contrasting light habitats of sub-tropical forests: FDF employing a quick-return strategy can pre-empt resources and rapidly grow in the high-resource environment of open habitats; while a slow-return strategy in FNF allows their persistence in the shaded understorey of old-growth forests.     

Nutritional status of Abies pinsapo forests along a nitrogen deposition gradient: do C/N/P stoichiometric shifts modify photosynthetic nutrient use efficiency?

Chronic atmospheric N deposition has modified relative N availability, altering the biogeochemical cycles of forests and the stoichiometry of nutrients in trees, inducing P limitation, and modifying the N:P ratios of plant biomass. This study examines how the variation in the foliar stoichiometry of Abies pinsapo across an N deposition gradient affects foliar traits and photosynthetic rate. We measured the maximum net assimilation rates (A _max) and the foliar nitrogen (N) and phosphorus (P) concentrations in A. pinsapo needles of five age classes. The leaf mass per area and photosynthetic N and P use efficiencies (PNUE and PPUE, respectively) were also estimated. The results from the N-saturated stand (Sierra Bermeja, B) differed from the comparable N-limited stands under investigation (Yunquera, Y, and Sierra Real, SR). The trees from Y and SR exhibited a reduction in the N content in older needles, whereas the foliar N concentration at the B site increased with needle age. N and P were positively correlated at Y and SR, but not at B, suggesting that the overload of N in the trees at site B has exceeded the homeostatic regulation capacity of the N-saturated stand in terms of foliar stoichiometry. A _max and PNUE were correlated positively with P and negatively with the N/P ratio at the three study sites. The foliar N concentration was positively correlated with A _max at Y and SR. However, this relationship was negative for the B site. These findings suggest that the nutritional imbalance caused by increased chronic deposition of N and an insufficient supply of P counteracts the potential increase in net photosynthesis induced by the accumulation of foliar N.     

Light and VPD gradients drive foliar nitrogen partitioning and photosynthesis in the canopy of European beech and silver fir

While foliar photosynthetic relationships with light, nitrogen, and water availability have been well described, environmental factors driving vertical gradients of foliar traits within forest canopies are still not well understood. We, therefore, examined how light availability and vapour pressure deficit (VPD) co-determine vertical gradients (between 12 and 42 m and in the understorey) of foliar photosynthetic capacity (Amax), 13C fractionation (∆), specific leaf area (SLA), chlorophyll (Chl), and nitrogen (N) concentrations in canopies of Fagus sylvatica and Abies alba growing in a mixed forest in Switzerland in spring and summer 2017. Both species showed lower Chl/N and lower SLA with higher light availability and VPD at the top canopy. Despite these biochemical and morphological acclimations, Amax during summer remained relatively constant and the photosynthetic N-use efficiency (PNUE) decreased with higher light availability for both species, suggesting suboptimal N allocation within the canopy. ∆ of both species were lower at the canopy top compared to the bottom, indicating high water-use efficiency (WUE). VPD gradients strongly co-determined the vertical distribution of Chl, N, and PNUE in F. sylvatica, suggesting stomatal limitation of photosynthesis in the top canopy, whereas these traits were only related to light availability in A. alba. Lower PNUE in F. sylvatica with higher WUE clearly indicated a trade-off in water vs. N use, limiting foliar acclimation to high light and VPD at the top canopy. Species-specific trade-offs in foliar acclimation to environmental canopy gradients may thus be considered for scaling photosynthesis from leaf to canopy to landscape levels.     

Phosphorus limitation on photosynthesis of two dominant understory species in a lowland tropical forest

Aims Elevated nitrogen (N) deposition in tropical regions may accelerate ecosystem phosphorus (P) limitation. However, it is not explicitly addressed that how changes in soil N and P availability affect foliar nutrients and photosynthesis of plants in tropical forests. In this study, we examined the effects of N and P additions on foliar nutrients and net photosynthesis of two dominant understory species, Randia canthioides (R. canthioides) and Cryptocarya concinna (C. concinna) in an N-saturated old-growth tropical forest (>400-year-old) in southern China.Methods A full factorial NP addition experiment (2×2) was established in 2007 and continued through August 2010. Four treatments, including control, N addition (150kg N ha^-1 year^-1), P addition (150kg P ha^-1 year^-1) and NP addition (150kg N ha^-1 year^-1 plus 150kg P ha^-1 year^-1) were set up in this experiment. Photosynthetic traits (maximum photosynthetic CO₂ assimilation (A max), stomatal conductance (g s), leaf transpiration (E), light saturating point, concentrations of chlorophyll a/b and foliar nutrients (N and P) of the two species were measured with standard methods.Important findings Three years of N addition had no significant effects on any measured photosynthetic parameter of either species. However, N addition significantly elevated foliar N and P concentrations of one species (R. canthioides), resulting in lower photosynthetic nitrogen use efficiency (PNUE). N treatments decreased foliar P concentration of the other (C. concinna), resulting in increased photosynthetic phosphorus use efficiency, which was potentially related to N-induced P shortage. In contrast, positive effects of P treatments on g s of R. canthioides, A max and chlorophyll a+b of C. concinna were observed. P treatments also elevated foliar P and PNUE of both species, implying P induced more efficient use of N. Our results suggested a more important role of P than N on influencing photosynthetic traits of these two understory species. Alleviation of P shortage through P addition may enhance photosynthetic performances of some understory species in N-rich tropical forests.     

华南地区固氮与非固氮豆科树种叶片养分利用策略的对比研究

为探究富氮环境中固氮(nitrogen-fixing leguminous trees,NLT)与非固氮豆科树种(non-nitrogen-fixing leguminous trees,n-NLT)的叶片养分利用策略差异,以华南地区5种NLT植物[水黄皮(Pongamia pinnata)、大叶相思(Acacia auriculiformis)、朱樱花(Calliandra haematocephala)、海南红豆(Ormosia pinnata)、台湾相思(Acacia confusa)]和3种n-NLT植物[油楠(Sindora glabra)、中国无忧花(Saraca dives)、银珠(Peltophorum tonkinense)]为对象,测定其单位质量叶片碳(C)、氮(N)和磷(P)含量及其比值、单位面积叶片最大净光合速率(Aarea)和叶片光合氮、磷利用效率(PNUE、PPUE)等功能性状。结果表明,NLT的单位质量叶片N、P含量和Aarea均显著高于n-NLT,而两者PNUE和PPUE无显著差异;尽管两类植物单位质量叶片C含量无显著差异,但NLT的叶片C:N和C:P显...     

Ecophysiological response in leaves of Caragana microphylla to different soil phosphorus levels

Phosphorus (P) is one of the limiting mineral nutrient elements in the typical steppe of Inner Mongolia, China. In order to find out the adaptive strategy of Caragana microphylla to low soil P status, we grew plants in P-deficient soil in April 2009 and gave a gradient of P addition ranging from 0 to 60 mg(P) kg^?1(soil) from May 2010. Leaf traits were measured in September 2010. Both leaf growth and light-saturated photosynthetic rate (P _max) were similar among different groups. Leaf nitrogen (N):P ratio indicated that the growth of C. microphylla was not P-limited in most of the Inner Mongolia typical steppe, which had an average soil available P content equal to 3.61 mg kg^?1. The optimal P addition was 20 mg(P) kg^?1(soil) for two-year-old plants of C. microphylla. Leaf mass area (LMA) and leaf dry matter content (LDMC) were enhanced with low P, and significantly negatively correlated with photosynthetic N-use efficiency (PNUE). Photosynthetic P-use efficiency (PPUE) increased with decreasing soil P and increasing leaf inorganic P (Pi): organic P (Po) ratio, and showed no significant negative correlation with LMA or LDMC. P _max of C. microphylla did not decline so sharply as it was anticipated. The reason for this phenomenon might be due to the increased PPUE through regulating the leaf total P allocation. C. microphylla had high P-use efficiency via both high PPUE and long P-retention time at low-P supply. The adaptation of C. microphylla to low-P supply provided a new explanation for the increased distribution of the species in the degraded natural grassland in Inner Mongolia, China.     

天然次生林植物叶片生态化学计量特征及光合特性对长期N沉降的响应

2006年5月于吉林省抚松县露水河林业局实验林场布设了人工模拟氮沉降控制试验,共设置3个氮（N）添加梯度,分别为对照（CK 0 g·N·m^-2·a^-1）、低N（LN 2.5 g·N·m^-2·a^-1）和高N（HN 5.0 g·N·m^-2·a^-1）,旨在探讨N沉降对天然次生林先锋树种白桦（Betula platyphylla）和山杨（Populus davidiana）鲜叶、凋落叶化学计量特征、养分重吸收的影响,以及鲜叶光合特性的变化和各性状之间的相互关系。结果表明：（1）模拟N沉降处理下白桦、山杨鲜叶的C含量较对照均无显著影响,LN处理显著降低了山杨鲜叶N、P含量（P<0.05）,显著增加了C：N、C：P和N：P（P<0.05）;HN处理显著增加了白桦鲜叶N含量和N：P,显著降低了C：N（P<0.05）。（2）白桦、山杨鲜叶N、P重吸收率在两个梯度N添加下均显著下降（P<0.05）,且均为负值。山杨鲜叶N重吸收率与P重吸收率呈显著正相关关系（P<0.05）,与鲜叶C：N呈显著负相关关系（P<0.05）。（3）N添加可以提高2种树木叶片氮素光合利用效率（PNUE）（P<0.05）、净光合速率（P_n）（P<0.05）。白桦鲜叶N含量与P_n、PNUE呈显著正相关（P<0.05）;白桦、山杨鲜叶比叶重（LMA）与N含量呈显著负相关（P<0.05）;P_n与PNUE呈显著正相关（P<0.05）。本试验研究表明：在生长季,白桦、山杨鲜叶中N、P均表现为富集状态,土壤养分及外源N可供林木较快吸收并促进其生长,无需从凋落叶中吸收养分。N添加可以增强白桦、山杨鲜叶的光合性能,进而促进植物养分吸收和叶片发育。HN对长白山天然次生林的生长有促进作用。     

Nitrogen fertilization enhances water‐use efficiency in a saline environment

Effects of salinity and nutrients on carbon gain in relation to water use were studied in the grey mangrove, Avicennia marina, growing along a natural salinity gradient in south‐eastern Australia. Tall trees characterized areas of seawater salinities (fringe zone) and stunted trees dominated landward hypersaline areas (scrub zone). Trees were fertilized with nitrogen (+N) or phosphorus (+P) or unfertilized. There was no significant effect of +P on shoot growth, whereas +N enhanced canopy development, particularly in scrub trees. Scrub trees maintained greater CO₂ assimilation per unit water transpired (water‐use efficiency, WUE) and had lower nitrogen‐use efficiency (NUE; CO₂ assimilation rate per unit leaf nitrogen) than fringe trees. The CO₂ assimilation rates of +N trees were similar to those in other treatments, but were achieved at lower transpiration rates, stomatal conductance and intercellular CO₂ concentrations. Maintaining comparable assimilation rates at lower stomatal conductance requires greater ribulose 1·5‐bisphosphate carboxylase/oxygenase activity, consistent with greater N content per unit leaf area in +N trees. Hence, +N enhanced WUE at the expense of NUE. Instantaneous WUE estimates were supported by less negative foliar δ¹³C values for +N trees and scrub control trees. Thus, nutrient enrichment may alter the structure and function of mangrove forests along salinity gradients.     

黄土高原油松和刺槐叶片光合生理适应性比较

以黄土高原地区由南向北分布的杨凌、永寿、富县、安塞、米脂、神木等县为研究地点，研究不同地区油松和刺槐的光合特性与叶结构性状间的关系.结果表明，不同地区油松针叶和刺槐叶片的净光合速率（P_n）、光合氮利用效率（PNUE）、水分利用效率（WUE）、比叶质量（LMA）、氮含量（N_mass）和叶绿素相对含量（Chl）差异均达极显著水平（P＜0.001）,说明不同地区油松和刺槐的光合能力和叶结构性状参数差异很大.由南向北，油松的P_n、WUE和PNUE呈略微增加趋势，而刺槐则呈显著降低趋势，表明油松在干旱生境下仍能维持较高的光合能力，而刺槐光合能力明显受到抑制；油松和刺槐的LMA均呈略微上升趋势，而N_mass和Chl均呈略微下降趋势，且刺槐的变化幅度高于油松，说明油松从生理代谢和叶结构性状上对干旱环境的适应能力均强于刺槐.相关分析表明，不同地区油松和刺槐的LMA与N_mass整体上呈极显著负相关；P_n、PNUE与LMA、N_mass相关不显著，与Chl呈极显著正相关；WUE与LMA呈显著负相关（P＜0.05），与N_mass呈显著正相关.     

广西猫儿山不同海拔常绿树种和落叶树种光合速率与氮的关系

分析广西猫儿山不同海拔常绿和落叶树种的光合作用-氮关系,探讨光合氮利用效率(PNUE)是否受到叶片习性和海拔的影响。落叶树种的PNUE都显著高于常绿树种,这与前者有较低的比叶重(LMA)和较高的单位叶重光合速率(Amass)、氮含量和气孔导度(gs)有密切关系。高海拔树种的PNUE显著低于中低海拔树种的PNUE,这与前者较高的LMA和较低的Amass和gs相关。PNUE和相关的叶片特征的主成分分析表明常绿-落叶树种和低海拔-中海拔-高海拔树种的分布是一个自然过渡的过程。此外,PNUE与土壤碳:氮比没有显著相关性,但与年均温正相关,这表明温度气候是调节PNUE沿海拔变化的主要环境因素。因此,这种叶片习性和温度气候调节的PNUE变化可能是调节猫儿山常绿树种沿海拔形成双峰分布的一种机制。     

A test of the generality of leaf trait relationships on the Tibetan Plateau

Leaf mass per area (LMA), nitrogen concentration (on mass and area bases, N(mass) and N(area), respectively), photosynthetic capacity (A(mass) and A(area)) and photosynthetic nitrogen use efficiency (PNUE) are key foliar traits, but few data are available from cold, high-altitude environments. Here, we systematically measured these leaf traits in 74 species at 49 research sites on the Tibetan Plateau to examine how these traits, measured near the extremes of plant tolerance, compare with global patterns. Overall, Tibetan species had higher leaf nitrogen concentrations and photosynthetic capacities compared with a global dataset, but they had a slightly lower A(mass) at a given N(mass). These leaf trait relationships were consistent with those reported from the global dataset, with slopes of the standardized major axes A(mass)-LMA, N(mass)-LMA and A(mass)-N(mass) identical to those from the global dataset. Climate only weakly modulated leaf traits. Our data indicate that covarying sets of leaf traits are consistent across environments and biogeographic regions. Our results demonstrate functional convergence of leaf trait relationships in an extreme environment.     

Analysis of differences in photosynthetic nitrogen-use efficiency between four contrasting species

Causes of differences in photosynthetic nitrogen-use efficiency (PNUE), the rate of photosynthesis per unit leaf N, were investigated in four species. These were in order of decreasing PNUE; the two herbs Galinsoga ciliata and Origanum vulgare , and the two trees Populus nigra and Quercus robur . Plants were grown in pots outdoors at three levels of nutrient availability. The light- and CO₂ response of gas exchange of leaves were measured, and their nitrogen and chlorophyll contents were determined. Furthermore, the internal conductance for CO₂ diffusion was estimated. Nutrients did not have a large effect on PNUE except in Galinsoga . Leaf mass per unit area was negatively correlated with PNUE_max, which is likely to be partly caused by N present in cell wall proteins among other non-photosynthetic N compounds. The trees had a larger fraction of photosynthetic N in light harvesting components compared to the herbs. This contributed also substantially to the difference in PNUE at light saturation (PNUE_max) between the two groups, but not for PNUE calculated for an overcast day. Intercellular CO₂ concentration was high in Galinsoga and Populus , which contributed significantly to their higher PNUE_max, particularly at low nutrient availability. The large gradient in CO₂ concentration between intercellular spaces and chloroplasts was another factor that explained a substantial part of the differences in PNUE_max between Quercus and the other species that had smaller gradients. Stomatal and internal conductances for CO₂ explained most of the difference in PNUE_max between Quercus and Populus at high nutrient availability for which these data were available.     

Photosynthetic nitrogen and water use efficiency of acacia and eucalypt seedlings as afforestation species

The ecophysiological traits of acacia and eucalypt are important in assessing their suitability for afforestation. We measured the gas-exchange rate, the leaf dry mass per area (LMA) and the leaf nitrogen content of two acacia and four eucalypt species. Relative to the eucalypts, the acacias had lower leaf net photosynthetic rate (P _N), lower photosynthetic nitrogen-use efficiency (PNUE), higher water-use efficiency (WUE), higher LMA and higher leaf nitrogen per unit area (N _area). No clear differences were observed within or between genera in the maximum rate of carboxylation (V _cmax) or the maximum rate of electron transport (J _max), although these parameters tended to be higher in eucalypts. PNUE and LMA were negatively correlated. We conclude that acacias with higher LMA do not allocate nitrogen efficiently to photosynthetic system, explaining why their P _N and PNUE were lower than in eucalypts.     

Pseudoviviparous reproduction of Poa alpina var. vivipara L. (Poaceae) during long-term exposure to elevated atmospheric CO2

Pseudovivipary is an asexual reproductive strategy exhibited by some arctic/alpine grasses in which leafy plantlets are produced in place of seeds, with genetic conservation an advantage for stress tolerators in these nutrient-poor habitats. Photosynthetic metabolism and the development of this reproductive system were investigated under varying nutrient availability and predicted future CO(2) partial pressure (pCO(2)). Poa alpina var. vivipara L., grown at present ambient pCO(2) or ambient plus 340 micro mol mol(-1) CO(2) (elevated pCO(2)), was supplied with either 0.05 mol m(-3) phosphorus and 0.2 mol m(-3) nitrogen, or 0.2 mol m(-3) phosphorus and 1.0 mol m(-3) nitrogen. Gas exchange measurements and determination of total non-structural carbohydrate (TNC), nitrogen and phosphorus contents revealed that parent plant leaf blade tissues experienced acclimatory loss of photosynthetic capacity after long-term growth at elevated pCO(2) (particularly so when nutrient availability was low); there were associated reductions in photosynthetic nitrogen and phosphorus use efficiencies (PNUE and PPUE). In addition, decreased PNUE and PPUE were exhibited by plantlets grown at elevated pCO(2) with low nutrient availability. Decreased reproductive dry matter in this treatment also resulted from a lack of reproductive initiation in daughter tillers, and altered phenology. Pseudoviviparous P. alpina is likely to be at a disadvantage in both vegetative and reproductive phases at predicted future elevated atmospheric CO(2) concentrations, particularly where nutrients are scarce and when in competition with species experiencing less acclimatory loss of photosynthetic capacity.     

绒毛番龙眼对生长光强的形态和生理适应

在100％、50％、25％和8％自然光强下栽培绒毛番龙眼幼苗并研究了其对光环境的适应。100％生长光强下绒毛番龙眼通过增大叶片悬挂角(midrib angle,MA)和比叶重(lamina mass per unit area,LMA),减少叶氮在捕光组分中的分配等降低光能捕获;通过增加类胡萝卜素含量增加热耗散。虽如此,还是发生了比较严重的光抑制,加之叶氮在光合机构中的分配最少,导致光合能力最低,长势最差。8％生长光强下绒毛番龙眼通过降低MA、LMA以及叶片技转,增加叶氮在捕光组分中的分配等提高光能捕获能力,光能转换及利用效率较高,热耗散水平较低,但由于环境光较弱,限制了光合碳同化,植株生长也较慢。50％和25％生长光强下绒毛番龙眼有较强的光能捕获、利用和耗散能力,在几种光处理中长势最好。     

Effects of simulated herbivory on photosynthesis and N resorption efficiency in <Emphasis Type="Italic">Quercus</Emphasis><Emphasis Type="Italic">pyrenaica</Emphasis> Willd. saplings

We examined the effects of simulated folivory by caterpillars on photosynthetic parameters and nitrogen (N) resorption efficiency in Quercus pyrenaica saplings. We analyzed the differences between intact leaves in control plants, punched leaves in damaged plants, and intact leaves in damaged plants. We then established two levels of simulated folivory: low (≈13% of the leaf area of one main branch removed per plant) and high (≈26% of the leaf area of one main branch removed per plant) treatments. No differences were found in net assimilation rate and conductance between either leaf type or treatment during the most favourable period for photosynthesis. However, the N content was lower in punched than in intact leaves, and as a result PNUE was higher in damaged leaves from treated trees. In leaf-litter samples, N mass was significantly higher in punched than in intact leaves in treated plants, and LMA was significantly higher in damaged than in intact leaves of both the treated and control plants. Consequently, N resorption efficiency was around 15% lower in damaged leaves as compared with intact leaves from treated and control plants. Mechanical injury to leaves not only triggered no compensatory photosynthetic response to compensate a lower carbon uptake due to leaf area loss, but also affected the resorption process that characterizes leaf senescence.     

Relationships between photosynthetic activity and silica accumulation with ages of leaf in Sasa veitchii (Poaceae, Bambusoideae)

BACKGROUND AND AIMS: Bamboos have long-lived, evergreen leaves that continue to accumulate silica throughout their life. Silica accumulation has been suggested to suppress their photosynthetic activity. However, nitrogen content per unit leaf area (N(area)), an important determinant of maximum photosynthetic capacity per unit leaf area (P(max)), decreases as leaves age and senescence. In many species, P(max) decreases in parallel with the leaf nitrogen content. It is hypothesized that if silica accumulation affects photosynthesis, then P(max) would decrease faster than N(area), leading to a decrease in photosynthetic rate per unit leaf nitrogen (photosynthetic nitrogen use efficiency, PNUE) with increasing silica content in leaves. METHODS: The hypothesis was tested in leaves of Sasa veitchii, which have a life span of 2 years and accumulate silica up to 41 % of dry mass. Seasonal changes in P(max), stomatal conductance, N(area) and silica content were measured for leaves of different ages. KEY RESULTS: Although P(max) and PNUE were negatively related with silica content across leaves of different ages, the relationship between PNUE and silica differed depending on leaf age. In second-year leaves, PNUE was almost constant although there was a large increase in silica content, suggesting that leaf nitrogen was a primary factor determining the variation in P(max) and that silica accumulation did not affect photosynthesis. PNUE was strongly and negatively correlated with silica content in third-year leaves, suggesting that silica accumulation affected photosynthesis of older leaves. CONCLUSIONS: Silica accumulation in long-lived leaves of bamboo did not affect photosynthesis when the silica concentration of a leaf was less than 25 % of dry mass. Silica may be actively transported to epidermal cells rather than chlorenchyma cells, avoiding inhibition of CO2 diffusion from the intercellular space to chloroplasts. However, in older leaves with a larger silica content, silica was also deposited in chlorenchyma cells, which may relate to the decrease in PNUE.     

Needle traits of an evergreen, coniferous shrub growing at wind-exposed and protected sites in a mountain region: does Pinus pumila produce needles with greater mass per area under wind-stress conditions?

Snow depth is one of the most important determinants of vegetation, especially in mountainous regions. In such regions, snow depth tends to be low at wind-exposed sites such as ridges, where stand height and productivity are limited by stressful environmental conditions during winter. Siberian dwarf pine ( Pinus pumila Regel) is a dominant species in mountainous regions of Japan. We hypothesized that P. pumila produces needles with greater mass per area at wind-exposed sites than at wind-protected sites because it invests more nitrogen (N) in cell walls at the expense of N investment in the photosynthetic apparatus, resulting in increased photosynthetic N use efficiency (PNUE). Contrary to our hypothesis, plants at wind-exposed site invested less resources in needles, as exhibited by lower biomass, N, Rubisco and cell wall mass per unit area, and had higher photosynthetic capacity, higher PNUE and shorter needle life-span than plants at a wind-protected site. N partitioning was not significantly different between sites. These results suggest that P. pumila at wind-exposed sites produces needles at low cost with high productivity to compensate for a short leaf life-span, which may be imposed by wind stress when needles appear above the snow surface in winter.