Growth and survival of cork oak (Quercus suber) seedlings after simulated partial cotyledon consumption under different soil nutrient contents

Aims

We examined the importance of partial seed consumption (cotyledon loss) by rabbits in the early establishment of seedlings of cork oaks restricted to nutrient-impoverished soils.

Methods

To determine the importance of cotyledons in the growth and development of seedlings, we simulated two levels of predation [light (30 % cotyledon loss) and heavy (60 % loss) partial consumption] and two soil nutrient contents (nutrient-poor soil, nutrient-rich soil). Seedlings height, root length, dry root and shoot biomass, specific leaf mass, leaf density, gas exchange, chlorophyll fluorescence parameters and photosynthetic pigment concentrations were determined.

Results

Results indicated that effect of nutrient level on the growth of the oak seedlings was more important than that of cotyledon biomass. However, in nutrient–poor soils, cotyledon biomass played a major role in the early performance of cork oaks. Acorns grown in nutrient-rich substrate, despite having greater aerial vigor, were slower to develop a vertical root, and hence less likely to reach permanent moisture. Cotyledon loss caused a decrease in the biomass of roots and shoots when acorns were heavily consumed, and as a result experienced a reduction in net photosynthetic rate, stomatal conductance and chlorophyll concentration. Survival of seedlings was unaffected by either soil type or cotyledon loss.

Conclusions

Our results show that effects of soil type on the survival of oak seedlings were more important than those of cotyledon biomass. However, in a competitive situation, cotyledon biomass, as an indicative of growth nutrient support rather than an energy source, could be vital in a nutrient-poor environment, particularly in Mediterranean climate regions and for species with little inherent drought tolerance (as is the case of Quercus spp.), where rapid root growth is required to ensure that contact with soil moisture is maintained over the first summer.     

Photosynthetic nitrogen-use efficiency of species that differ inherently in specific leaf area

Factors that contribute to interspecific variation in photosynthetic nitrogen-use efficiency (PNUE, the ratio of CO₂ assimilation rate to leaf organic nitrogen content) were investigated, comparing ten dicotyledonous species that differ inherently in specific leaf area (SLA, leaf area:leaf dry mass). Plants were grown hydroponically in controlled environment cabinets at two irradiances (200 and 1000 μmol m^–2 s^–1). CO₂ and irradiance response curves of photosynthesis were measured followed by analysis of the chlorophyll, Rubisco, nitrate and total nitrogen contents of the leaves. At both irradiances, SLA ranged more than twofold across species. High-SLA species had higher in situ rates of photosynthesis per unit leaf mass, but similar rates on an area basis. The organic N content per unit leaf area was lower for the high-SLA species and consequently PNUE at ambient light conditions (PNUE_amb) was higher in those plants. Differences were somewhat smaller, but still present, when PNUE was determined at saturating irradiances (PNUE_max). An assessment was made of the relative importance of the various factors that underlay interspecific variation in PNUE. For plants grown under low irradiance, PNUE_amb of high-SLA species was higher primarily due to their lower N content per unit leaf area. Low-SLA species clearly had an overinvestment in photosynthetic N under these conditions. In addition, high SLA-species allocated a larger fraction of organic nitrogen to thylakoids and Rubisco, which further increased PNUE_amb. High-SLA species grown under high irradiance showed higher PNUE_amb mainly due to a higher Rubisco specific activity. Other factors that contributed were again their lower contents of N_org per unit leaf area and a higher fraction of photosynthetic N in electron transport and Rubisco. For PNUE_max, differences between species in organic leaf nitrogen content per se were no longer important and higher PNUE_max of the high SLA species was due to a higher fraction of N in␣photosynthetic compounds (for low-light plants) and a higher Rubisco specific activity (for high-light grown plants). Received: 11 October 1997 / Accepted: 9 April 1998     

Leaf anatomical structures of <Emphasis Type="Italic">Paphiopedilum</Emphasis> and <Emphasis Type="Italic">Cypripedium</Emphasis> and their adaptive significance

Paphiopedilum and Cypripedium are closely related in phylogeny, but have contrasting leaf traits and habitats. To understand the divergence in leaf traits of Paphiopedilum and Cypripedium and their adaptive significance, we analyzed the leaf anatomical structures, leaf dry mass per area (LMA), leaf lifespan (LL), leaf nitrogen concentration (N _mass), leaf phosphorus concentration (P _mass), mass-based light-saturated photosynthetic rate (A _mass), water use efficiency (WUE), photosynthetic nitrogen use efficiency (PNUE) and leaf construction cost (CC) for six species. Compared with Cypripedium, Paphiopedilum was characterized by drought tolerance derived from its leaf anatomical structures, including fleshy leaves, thick surface cuticles, huge adaxial epidermis cells, lower total stoma area, and sunken stomata. The special leaf structures of Paphiopedilum were accompanied by longer LL; higher LMA, WUE, and CC; and lower N _mass, P _mass, A _mass, and PNUE compared with Cypripedium. Leaf traits in Paphiopedilum helped it adapt to arid and nutrient-poor karst habitats. However, the leaf traits of Cypripedium reflect adaptations to an environment characterized by rich soil, abundant soil water, and significant seasonal fluctuations in temperature and precipitation. The present results contribute to our understanding of the divergent adaptation of leaf traits in slipper orchids, which is beneficial for the conservation of endangered orchids.     

遮阴对三种木兰科幼苗生长和光合特性的影响

为探讨观光木、山白兰和灰木莲三种木兰科植物对不同光环境的光合适应机制,该文以其幼苗为材料,设置透光率分别为100%NS、72.3%NS、48.6%NS、24.9%NS的四种光照处理,测定其光合特性参数、生物量分配比例、叶绿素含量等,研究不同遮阴处理对其生长和光合作用的影响。结果表明:(1)过强或过弱的光照环境限制了幼苗株高及地径的生长,与全光照相比,72.3%NS有利于幼苗的形态生长。(2)随着遮阴程度的增加,观光木、山白兰和灰木莲幼苗的最大光合速率、光补偿点、光饱和点、暗呼吸速率、蒸腾速率逐渐降低,叶绿素含量增加,水分利用效率先升后降。(3)随着光强的减弱,观光木和山白兰地上部分的生物量积累增加,灰木莲的根生物量积累增加。(4)观光木的比叶面积随着遮阴程度的增加而先增后减;灰木莲的比叶面积显著减小;山白兰的比叶面积差异不显著;观光木、山白兰和灰木莲对低光环境响应和适应的差异主要表现在生物量分配比例及叶的形态特征上。综上结果认为,72.3%NS最有利于幼苗的生长,观光木、山白兰和灰木莲在遮阴条件下可以通过降低P_max、LSP、LCP、R_d、T...     

不同组成群落3种共有植物光合生理特征研究

为研究物种在不同群落中光合生理特征的变化,以亚高寒草甸围封恢复地为研究对象,对样地内3个不同组成群落进行样方调查,测定了物种高度及各群落垂直方向上光照强度以及群落中3个共有种披碱草(Elymus dahuricus)、刺儿菜(Cirsium setosum)和紫花苜蓿(Medicago sativa)的净光合速率(Aarea)、叶片氮含量(Nmass)、比叶重(LMA)及光合氮利用效率(PNUE)。结果表明:(1)3个样地的群落组成有明显的差异,豆科植物的增多可以一定程度上改善群落氮养分状况,但植物叶片Nmass还受到群落优势种竞争的影响。(2)同一物种在不同群落的高度不同,不同群落垂直方向上光照强度也不相同,导致同一物种在不同群落中能够获得的光照强度有一定差异。(3)在养分、光照强度有差异的情况下,不同植物的Aarea、LMA及PNUE在不同群落中的变化趋势不尽相同,而Narea与Aarea的关系在总体上、群落间及物种间变化不大,基本上显示了较强的正相关关系。由此可见,群落组成、结构引起的光照及氮素差异是导致同一物种光合生理特征在不同群落中变化的重要因素,但不同物种光合生理特征对光照及氮素变化的响应不同。     

混交比例对桉树-乡土树种混交林优势树种叶片资源获取性状的影响

桉树-乡土树种混交林在提高林分生产力和生态系统功能等方面具有较大潜力。该研究以南亚热带4种桉树-乡土树种混交林(桉树与乡土树种混交比例分别为5:5、6:4、7:3、8:2)和桉树纯林为研究对象,研究了3种优势乡土树种华润楠(Machilus chinensis)、阴香(Cinnamomum burmannii)、灰木莲(Manglietia glauca)和速生树种尾叶桉(Eucalyptus urophylla)的叶片生理、结构和化学性状在不同比例混交林中的差异。结果表明,4优势造林树种的叶片性状存在明显的种间差异,其中灰木莲的比叶面积(SLA)、光合磷利用效率(PPUE)、单位质量叶片最大光合速率(A_mass)和蒸腾速率(T_mass)以及叶片养分含量最高,说明灰木莲采取资源获取型的生态策略;尾叶桉的SLA、A_mass、T_mass及叶片养分含量最低,但具有最高的PPUE,说明尾叶桉兼顾了资源获取型和保守型的物种特征。灰木莲与尾叶桉在SLA、A_mass、T_mass     

Photosynthetic plasticity of two rain forest shrubs across natural gap transects

Summary Photosynthetic plasticity of two congeneric shrub species growing under natural field conditions was compared along transects spanning two canopy gaps in a Costa Rican rain forest. Piper arieianum is a shadetolerant species common in successional and mature forests, whereas P. sancti-felicis is a pioneer species abundant in abandoned clearings and large gaps. Twenty potted cuttings of each species were placed at regular intervals along two east-west transects crossing a small branch-fall gap and a large tree-fall gap. Along the transects, the percent of full sun photon flux density varied from less than 2% to 45%. After six months of growth under these conditions, leaves were monitored for incident photon flux density, photographic measures of light availability, photosynthetic capacity (A_max), leaf nitrogen content, leaf chlorophyll content, and specific leaf mass. Although both species demonstrated considerable plasticity in A_max across gap transects, P. sancti-felicis leaves had a superior capacity to track closely variation in light availability, particularly in the larger gap. For regressions of A_max on measures of light availability, P. sancti-felicis consistently showed a 3.5 to 5-fold higher coefficient of determination (R²) and a 3 to 4-fold higher slope than P. arieianum. In both species leaf nitrogen content per leaf area increased significantly with light availability, although P. sancti-felicis, again, showed a much stronger relationship between these variables. Across the transects, mean chlorophyll content per unit leaf area did not differ significantly between the species, whereas mean chlorophyll content per unit leaf dry mass was 3-times greater in leaves of P. sancti-felicis. Piper arieianum exhibited highly significant increases in chlorophyll a:b ratio with increased light availability, whereas P. sancti-felicis lacked significant variation in this trait across a gradient of light availability. Mean specific leaf mass did not vary significantly between species across the gap transects. The nature of the light acclimatory response differs quantitatively and qualitatively between these species. An important constraint on light acclimation of the shade-tolerant P. arieianum is its inability to increase photosynthetic nitrogen-use efficiency under conditions of high light availability. The lack of plasticity in chlorophyll a:b ratios does not restrict light acclimation of A_max in P. sancti-felicis. Leaves of P. arieianum exhibited symptoms of chronic photoinhibition in exposed microsites within the large gap. Species differences in the capacity to finely adjust A_max across a wide range of light conditions may be attributed to their maximum growth potential. Light acclimation in species with low maximum growth potential may be constrained at the cellular level by rates of protein and chlorophyll synthesis and at the whole-plant level by low maximum rates of uptake and supply of nutrients and water. For P. arieianum, restriction of photosynthetic plasticity is likely to limit competitive abilities of plants in high-light conditions of large gaps and clearings, whereas observed habitat restrictions for P. sancti-felicis do not appear to depend upon the highly-developed capacity for adjustment of A_max observed in this species.     

Effects of nutrient addition on leaf chemistry,morphology, and photosynthetic capacity of three bog shrubs

Plants in nutrient-poor environments typically have low foliar nitrogen (N) concentrations, long-lived tissues with leaf traits designed to use nutrients efficiently, and low rates of photosynthesis. We postulated that increasing N availability due to atmospheric deposition would increase photosynthetic capacity, foliar N, and specific leaf area (SLA) of bog shrubs. We measured photosynthesis, foliar chemistry and leaf morphology in three ericaceous shrubs (Vaccinium myrtilloides, Ledum groenlandicum and Chamaedaphne calyculata) in a long-term fertilization experiment at Mer Bleue bog, Ontario, Canada, with a background deposition of 0.8 g N m⁻² a⁻¹. While biomass and chlorophyll concentrations increased in the highest nutrient treatment for C. calyculata, we found no change in the rates of light-saturated photosynthesis (A _max), carboxylation (V _cmax)_, or SLA with nutrient (N with and without PK) addition, with the exception of a weak positive correlation between foliar N and A _max for C. calyculata, and higher V _cmax in L. groenlandicum with low nutrient addition. We found negative correlations between photosynthetic N use efficiency (PNUE) and foliar N, accompanied by a species-specific increase in one or more amino acids, which may be a sign of excess N availability and/or a mechanism to reduce ammonium (NH₄) toxicity. We also observed a decrease in foliar soluble Ca and Mg concentrations, essential minerals for plant growth, but no change in polyamines, indicators of physiological stress under conditions of high N accumulation. These results suggest that plants adapted to low-nutrient environments do not shift their resource allocation to photosynthetic processes, even after reaching N sufficiency, but instead store the excess N in organic compounds for future use. In the long term, bog species may not be able to take advantage of elevated nutrients, resulting in them being replaced by species that are better adapted to a higher nutrient environment.     

异质光环境下克隆整合对白夹竹光合氮分配格局的影响

采用盆栽试验,以单轴散生型竹类植物——白夹竹(Phyllostachys bissetii)克隆片段为对象,使近端分株处于自然光照环境,远端分株处于遮荫环境中,对根状茎作切断或不切断处理,研究克隆整合对远端分株和近端分株光合氮分配格局的影响。结果表明:(1)根状茎保持连接的远端分株较根状茎切断的远端分株具有更高的最大净光合速率、叶片氮含量、光合色素含量、叶片光合氮分配系数。(2)相对根状茎切断处理,根状茎保持连接的远端分株将更多的氮分配到光合系统的羧化系统、生物力能学组分,而分配至捕光系统组分的比例较小。(3)比较处于自然光照条件的近端分株,比叶重、叶片氮含量等并没有因根状茎切断与否表现出显著性差异,根状茎连接的近端分株部分指标甚至高于根状茎切断的近端分株。研究认为,克隆整合作用影响处于遮荫环境的白夹竹远端分株的光合氮分配格局,使得处于遮荫生境中的克隆分株依旧维持较高的光合能力,保证了处于逆境条件下克隆分株的生存与生长。     

Improvement of growth conditions and gas exchange of<Emphasis Type="Italic"> Fagus sylvatica</Emphasis> L. seedlings planted below a recently thinned<Emphasis Type="Italic"> Pinus sylvestris</Emphasis> L. stand

Abstract Gas exchange and growth of beech seedlings planted in the understory of a recently thinned pinewood were recorded for 2 years. Relative irradiance was assessed by hemispherical photographs taken just after the thinning. Predawn water potential (_pd), daily gas exchange and chlorophyll fluorescence were measured several times during the two growing seasons. Maximum values of photosynthesis (A _max) and stomatal conductance to water vapour (g _wvmax) were established from daily data. Maximum quantum efficiency of PS II was recorded at dawn by taking the variable to maximum chlorophyll fluorescence ratio on dark adapted leaves (F _v/F _m). In the middle of each summer, leaf nitrogen content and leaf mass per area were evaluated, and height growth and basal area increment were recorded at the end of the season. The thinning treatment removed half the trees and generated around 10% more available relative irradiance (GLF). This was followed by an increase in net photosynthesis at saturating PPFD (A _sat) and in maximum stomatal conductance to water vapour (g _wvmax). Moreover, specific leaf mass (SLM) and mass based nitrogen content (N_m) showed higher values for seedlings in the thinned stand. In both years, a positive relationship was established between the area based nitrogen content (N_a) and maximum net photosynthesis (A _max). In 1998, a year with a dry summer, seedlings suffered a significant drop in daily A _max irrespective of the thinning regime. This was a response to an increase in stomatal limitation to net photosynthesis, g _wvmax reaching the lowest value on dates with the highest drought. A lack of decrease of Fv/Fm confirmed the absence of significant non-stomatal limitation to A as a consequence of photoinhibition after opening the pinewood. A higher maximum quantum efficiency of open PS II centres (Fv/Fm) was registered in seedlings in the thinned stand. The significance of the differences between the treatments was stronger in the second year after thinning. In 1999, a year with frequent summer storms, water availability increased for seedlings growing under the thinned pinewood. Overall, the reduced pine overstory had a positive effect on physiological responses of beech seedlings, which was translated into improved seedling growth.     

Photosynthetic Characteristics,Dark Respiration,and Leaf Mass Per Unit Area in Seedlings of Four Tropical Tree Species Grown Under Three Irradiances

We investigated the effect of growth irradiance (I) on photon-saturated photosynthetic rate (P _max), dark respiration rate (R _D), carboxylation efficiency (CE), and leaf mass per unit area (LMA) in seedlings of the following four tropical tree species with contrasting shade-tolerance. Anthocephalus chinensis (Rubiaceae) and Linociera insignis (Oleaceae) are light-demanding, Barringtonia macrostachya (Lecythidaceae) and Calophyllum polyanthum (Clusiaceae) are shade-tolerant. Their seedlings were pot-planted under shading nets with 8, 25, and 50 % daylight for five months. With increase of I, all species displayed the trends of increases of LMA, photosynthetic saturation irradiance, and chlorophyll-based P _max, and decreases of chlorophyll (Chl) content on both area and mass bases, and mass-based P _max, R _D, and CE. The area-based P _max and CE increased with I for the light-demanders only. Three of the four species significantly increased Chl-based CE with I. This indicated the increase of nitrogen (N) allocation to carboxylation enzyme relative to Chl with I. Compared to the two shade-tolerants, under the same I, the two light-demanders had greater area- and Chl-based P _max, photosynthetic saturation irradiance, lower Chl content per unit area, and greater plasticity in LMA and area- or Chl-based P _max. Our results support the hypothesis that light-demanding species is more plastic in leaf morphology and physiology than shade-tolerant species, and acclimation to I of tropical seedlings is more associated with leaf morphological adjustment relative to physiology. Leaf nitrogen partitioning between photosynthetic enzymes and Chl also play a role in the acclimation to I.     

Effects of soil warming during spring on photosynthetic recovery in boreal Norway spruce stands

The effect of soil thawing and soil temperature on postwinter recovery of photosynthetic capacity was studied, during late spring and early summer, in Norway spruce stands in northern Sweden. Soil temperature was manipulated by means of buried heating cables. The warming treatment was applied to stands with low (natural) and high (fertilized) availability of nutrients. Soil thawing, expressed as water availability, was followed by means of sapflow in stems, and shoot water potentials. The recovery of photosynthetic capacity was assessed by measuring the rate of light-saturated photosynthesis (A_max), and maximum photochemical efficiency of photosystem II in detached shoots, and chlorophyll a fluorescence. Accumulation of starch reserves in the needles was followed as an independent indicator of photosynthetic performance in situ. Snowmelt and soil thawing occurred more than one month earlier in heated than in unheated plots. This was expressed both as sapflow and as differences in shoot water potential between treatments. During May, the rates of A_max were significantly higher on heated than on control plots. The effect of soil warming on A_max was, however, not reflected in chlorophyll fluorescence or needle starch content. The time course of the recovery of photosynthetic capacity was mainly controlled by mean air temperature and by the frequency of severe night frosts, and to a lesser extent by earlier soil thawing and higher soil temperatures.     

Herbivory alters plant carbon assimilation,patterns of biomass allocation and nitrogen use efficiency

Herbivory can trigger physiological processes resulting in leaf and whole plant functional changes. The effects of chronic infestation by an insect on leaf traits related to carbon and nitrogen economy in three Prunus avium cultivars were assessed. Leaves from non-infested trees (control) and damaged leaves from infested trees were selected. The insect larvae produce skeletonization of the leaves leaving relatively intact the vein network of the eaten leaves and the abaxial epidermal tissue. At the leaf level, nitrogen content per mass (N_mass) and per area (N_area), net photosynthesis per mass (A_mass) and per area (A_area), photosynthetic nitrogen-use efficiency (PNUE), leaf mass per area (LMA) and total leaf phenols content were measured in the three cultivars. All cultivars responded to herbivory in a similar fashion. The N_mass, A_mass, and PNUE decreased, while LMA and total content of phenols increased in partially damaged leaves. Increases in herbivore pressure resulted in lower leaf size and total leaf area per plant across cultivars. Despite this, stem cumulative growth tended to increase in infected plants suggesting a change in the patterns of biomass allocation and in resources sequestration elicited by herbivory. A larger N investment in defenses instead of photosynthetic structures may explain the lower PNUE and A_mass observed in damaged leaves. Some physiological changes due to herbivory partially compensate for the cost of leaf removal buffering the carbon economy at the whole plant level.     

Do photosynthetic limitations of evergreen Quercus ilex leaves change with long‐term increased drought severity?

Seasonal drought can severely impact leaf photosynthetic capacity. This is particularly important for Mediterranean forests, where precipitation is expected to decrease as a consequence of climate change. Impacts of increased drought on the photosynthetic capacity of the evergreen Quercus ilex were studied for two years in a mature forest submitted to long‐term throughfall exclusion. Gas exchange and chlorophyll fluorescence were measured on two successive leaf cohorts in a control and a dry plot. Exclusion significantly reduced leaf water potential in the dry treatment. In both treatments, light‐saturated net assimilation rate (A_max), stomatal conductance (g_s), maximum carboxylation rate (V_cmax), maximum rate of electron transport (J_max), mesophyll conductance to CO₂ (g_m) and nitrogen investment in photosynthesis decreased markedly with soil water limitation during summer. The relationships between leaf photosynthetic parameters and leaf water potential remained identical in the two treatments. Leaf and canopy acclimation to progressive, long‐term drought occurred through changes in leaf area index, leaf mass per area and leaf chemical composition, but not through modifications of physiological parameters.     

喀斯特石漠化地区土壤养分对泡核桃功能性状的影响

为了探究喀斯特石漠化地区植物叶片功能性状及影响因素,以及揭示其对石漠化环境的适应机理,该文以中国南方喀斯特高原峡谷地区的泡核桃(Juglans sigillata)为对象,揭示土壤养分对叶片结构和光合性状的影响效应。结果表明：(1)泡核桃叶功能性状随石漠化等级增加,叶面积减小,比叶面积增大,叶干物质含量和叶组织密度先降后升,蒸腾速率、胞间CO₂浓度、气孔导度和光能利用率先下降后升高,其他性状变化趋势不显著。(2)冗余分析表明土壤养分能够解释37.4%的光合性状变异与53.4%的结构性状变异,其中全磷和溶解性有机碳对光合性状影响最大,而对结构性状影响最显著的是碱解氮和速效磷。(3)比叶面积分别与叶干物质含量极显著负相关,与净光合速率极显著正相关,叶厚度与叶组织密度极显著负相关,蒸腾速率与胞间CO₂浓度、气孔导度极显著正相关,水分利用速率与蒸腾速率、胞间CO₂浓度、气孔导度极显著负相关,光能利用率与净光合速率显著正相关。研究结果表明,泡核桃为适应喀斯特石漠化的特殊生境采取增强生长功能性状,同时提高资源获取能力的开拓型生长策略...     

Comparisons of nutrient recovery and specific leaf area variation betweenCarex lasiocarpa var.occultans andCarex thunbergii var.appendiculata with reference to nutrient conditions and shading byPhragmites australis

The distribution of two sedge species was studied in two mires which differ in abiotic environments and in distribution ofPhragmites australis. Carex lasiocarpa var.occultans dominated in nutrient-poor valley mire, andCarex thunbergii var.appendiculata dominated in nutrient-rich flood plain subject to water fluctuations.Phragmites australis grew well in nutrient-rich conditions. The distribution ofC. lasiocarpa showed a strong negative correlation withP. australis coverage, whereasC. thunbergii coverage was not affected byP. australis. The leaf area per dry leaf mass (specific leaf area: SLA) ofC. thunbergii increased with shading byP. australis, but that ofC. lasiocarpa was stable. The SLA flexibility ofC. thunbergii to light interception might enable this species to invadeP. australis patches in nutrient-rich environments. The residual nutrient ratio of nitrogen and phosphorus (the ratio of the residual nutrient content at the end of the growing season to peak nutrient content) in the vegetative ramet ofC. thunbergii was 1.7 times higher than that ofC. lasiocarpa. This low residual ratio may indicate effective nutrient recovery to storage organs. The effective nutrient recovery inC. lasiocarpa might enable this species to grow even in nutrient-poor environments. However, it may be difficult forC. lasiocarpa to expand its habitat to nutrient-rich areas whereP. australis dominates as it is not shade tolerant.     

Seasonal response of photosynthesis to elevated CO2 in loblolly pine (Pinus taeda L.) over two growing seasons

Trees growing in natural systems undergo seasonal changes in environmental factors that generate seasonal differences in net photosynthetic rates. To examine how seasonal changes in the environment affect the response of net photosynthetic rates to elevated CO₂, we grew Pinus taeda L. seedlings for three growing seasons in open-top chambers continuously maintained at either ambient or ambient + 30 Pa CO₂. Seedlings were grown in the ground, under natural conditions of light, temperature nd nutrient and water availability. Photosynthetic capacity was measured bimonthly using net photosynthetic rate vs. intercellular CO₂ partial pressure (A-C_i) curves. Maximum Rubisco activity (Vc_max) and ribulose 1,5-bisphosphate regeneration capacity mediated by electron transport (J_max) and phosphate regeneration (PiRC) were calculated from A-C_i curves using a biochemically based model. Rubisco activity, activation state and content, and leaf carbohydrate, chlorophyll and nitrogen concentrations were measured concurrently with photosynthesis measurements. This paper presents results from the second and third years of treatment. Mean leaf nitrogen concentrations ranged from 13.7 to 23.8 mg g^?1, indicating that seedlings were not nitrogen deficient. Relative to ambient CO₂ seedlings, elevated CO₂ increased light-saturated net photosynthetic rates 60–110% during the summer, but < 30% during the winter. A relatively strong correlation between leaf temperature and the relative response of net photosynthetic rates to elevated CO₂ suggests a strong effect of leaf temperature. During the third growing season, elevated CO₂ reduced Rubisco activity 30% relative to ambient CO₂ seedlings, nearly completely balancing Rubisco and RuBP-regeneration regulation of photosynthesis. However, reductions in Rubisco activity did not eliminate the seasonal pattern in the relative response of net photosynthetic rates to elevated CO₂. These results indicate that seasonal differences in the relative response of net photosynthetic rates to elevated CO₂ are likely to occur in natural systems.     

Ecophysiological traits of plant functional groups in forest and pasture ecosystems from eastern Amazônia,Brazil

The plant functional group approach has the potential to clarify ecological patterns and is of particular importance in simplifying the application of ecological models in high biodiversity ecosystems. Six functional groups (pasture grass, pasture sapling, top-canopy tree, top-canopy liana, mid canopy tree, and understory tree) were established a priori based on ecosystem inhabited, life form, and position within the forest canopy profile on eastern Amazonian region. Ecophysiological traits related to photosynthetic gas exchange were then used to characterize such groups. The ecophysiological traits evaluated showed considerable variations among groups. The pasture grass functional group (a C₄ photosynthetic pathway species) showed high instantaneous water use efficiency (A _max/g _s@A _max), high photosynthetic nitrogen use efficiency (A _max/N _area), and high ratio of A _max to dark respiration (A _max/R _d). Among the species with the C₃ photosynthetic pathway, the top-canopy liana group showed the highest mean of A _max/g _s@A _max, statistically distinct from the lowest average presented by the understory tree group. Furthermore, the pasture sapling group showed the lowest average of A _max/R _d, statistically distinct from the high average observed for the understory tree group. Welch-ANOVAs followed by Games–Howell post hoc tests applied to ecophysiological traits produced reasonable distinctions among functional groups, although no significant distinction was detected between the groups top-canopy tree and pasture sapling. Species distribution within the functional groups was accurately reproduced by discriminant analyses based on species averages of ecophysiological traits. The present work convincingly shows that the functional groups identified have distinct ecophysiological characteristics, with the potential to respond differently to environmental factors. Such information is of great importance in modeling efforts that evaluate the effects of dynamic changes in tropical plant communities over ecosystem primary productivity.     

Leaf traits of natural populations of <Emphasis Type="Italic">Adiantum reniforme</Emphasis> var. <Emphasis Type="Italic">sinensis</Emphasis>, endemic to the Three Gorges region in China

Leaf mass per unit area (LMA), carbon and nitrogen contents, leaf construction cost, and photosynthetic capacity (P _max) of Adiantum reniforme var. sinensis, an endangered fern endemic to the Three Gorges region in southwest China, were compared in five populations differing in habitat such as soil moisture and irradiance. The low soil moisture and high irradiance habitat population exhibited significantly higher LMA, area-based leaf construction (CC_A), and carbon content (C_A), but lower leaf nitrogen content per unit dry mass (N_M) than the other habitat populations. The high soil moisture and low irradiance habitat populations had the lowest CC_A, but their cost/benefic ratios of CCA/P _max were similar to the medium soil moisture and irradiance habitat population due to their lower leaf P _max. Hence A. reniforme var. sinensis prefers partially shaded, moist but well-drained, slope habitats. Due to human activities, however, its main habitats now are cliffs or steeply sloped bare rocks with poor and thin soil. The relatively high energy requirements and low photosynthetic capacity in these habitats could limit the capability of the species in extending population or interspecific competition and hence increase its endangerment.