Sun/shade adaptations of the photosynthetic apparatus of <Emphasis Type="Italic">Hoya carnosa</Emphasis>, an epiphytic CAM vine,in a subtropical rain forest in northeastern Taiwan

Most past work on the ecophysiology of the Crassulacean acid metabolism (CAM) plant, Hoya carnosa, in the lab and in situ in Australia indicates that this epiphytic vine is better adapted to shaded, not exposed, locations, although a recent study of this species in Taiwan presents findings that run counter to this conclusion. Thus, photosynthetic characteristics of shaded and exposed individuals of H. carnosa were compared in situ in a subtropical rain forest in northeastern Taiwan in order to determine whether this CAM epiphyte is better adapted to the shade or the sun. Although leaves of shade plants had much greater chlorophyll concentrations than did those of sun plants, chlorophyll a/b ratios did not differ between the two groups of plants. Fluorescence measurements revealed some ability of leaves to acclimate to both shade and sun, although some evidence for photoinhibition (photoprotection) was observed in more exposed plants. Despite the latter, both exposed and shaded plants exhibited CAM, measured as diel fluctuations in leaf acidity, and CAM was more consistently found in the exposed plants. Furthermore, some evidence for more CAM at higher light availabilities was found. Overall, the results of this investigation reveal that H. carnosa in this subtropical rain forest in Taiwan exhibits adaptations to both high and low light levels, which should prove adaptive for an epiphytic vine with leaves on the same individual exposed to a wide range of exposure and shade in the host tree canopy.     

不同微生境中水光温变化对毛尖紫萼藓叶绿素荧光特性的影响

毛尖紫萼藓（Grimmia pilifera）多生长在裸岩表面且具有多种微生境,其生长过程受到水分、光照和温度等环境因素的交互影响,但其光合生理特征如何响应这种变化的环境条件尚不清楚。开展原位（荫蔽和向阳裸岩2种微生境）和室内模拟实验,分析了不同水分（模拟降水量和降水频次）、光照、温度及其复合梯度处理对毛尖紫萼藓叶绿素荧光特性的影响。原位实验结果显示荫蔽生境原位生长毛尖紫萼藓光化学效率显著高于向阳生境。在室内相同培养条件下,脱水过程中来自荫蔽生境植株有效光合效率保持时间比向阳生境明显缩短,表现出较低的脱水耐受性。水-光-温复合模拟实验显示,降水频次和光-温变化对毛尖紫萼藓光化学效率均有极显著影响且存在一定的交互作用,而降水量的影响相对较弱;弱光低温及1次/（2 d）的降水频率条件下毛尖紫萼藓具有最高的光合活性。总体来看,荫蔽生境、弱光低温条件及中等频次降水有利于毛尖紫萼藓生长,但向阳生境毛尖紫萼藓则具有更强的环境耐受性。     

COMPARISON OF INTRASPECIFIC VARIATIONS IN THE REPRODUCTION AND PHOTOSYNTHESIS OF AN UNDERSTORY HERB,ARNICA CORDIFOLIA

Intraspecific variations in the reproduction of individual ramets and monospecific patches of the understory herb Arnica cordifolia Hook. were compared with variations in photosynthesis and understory light conditions. Ramets and patches were compared from three microhabitats (open, intermediate, and shaded) that differed in daily integrated irradiance. Individual ramets from open microhabitats (> 12 MJ m⁻² d⁻¹) had 23% more total dry wt and produced twice as many seeds, when compared to ramets from shaded locations (< 5 MJ m⁻² d⁻¹). In addition, monospecific patches from open locations were 63% more dense, and estimates of seed and vegetative patch reproductive effort were 4 and 2 times greater, respectively, when compared to shaded plant patches. For all measurements, ramets and patches from intermediate understory locations (6–10 MJ m⁻² d⁻¹) were intermediate in reproductive capacity between those of open and shaded locations. In addition, A. cordifolia seeds from open microhabitats germinated significantly better (45%) than either intermediate or shaded location seeds under high light and only seeds from shaded microhabitats germinated (14%) in the dark. Compared to shaded location plants, the greater total dry weight and seed production of individual ramets and the greater estimated reproductive effort of patches from open locations corresponded to a greater maximum photosynthetic rate (16.9 μmoles m⁻² s⁻¹) and daily carbon gain (12.2 g m⁻² d⁻¹). Possibly, a greater photosynthetic capacity may make more photosynthetic resources available for reproduction by A. cordifolia plants in open locations. Thus, intraspecific variation in physiology may contribute to intraspecific variation in reproduction.     

Research Trails Affect the Abundance of an Epiphytic Tropical Bromeliad

Concerns have been expressed that research methods and research infrastructure may affect the systems under study, which could lead to biased results. Effects of research activities on seedlings, saplings, and understory plants that are subjected to trampling and injury have been demonstrated. However, as of yet, no effects on epiphytic plants have been reported. In this paper, we demonstrate the impact of research trails on the abundance of an epiphytic tropical bromeliad, Guzmania vittata, at a research site in Peruvian Amazonia. Compared to the interior of the forest, the abundance of this bromeliad is significantly higher along the research trails. While we do not know the exact cause for the increased bromeliad abundance along the trails, we discuss enhanced dispersal of anemochorous seeds as a potential mechanism that generates this impact.     

Survival, Growth, and Ecosystem Dynamics of Displaced Bromeliads in a Montane Tropical Forest1

Epiphytes generally occupy arboreal perches, which are inherently unstable environments due to periodic windstorms, branch falls, and treefalls. During high wind events, arboreal bromeliads are often knocked from the canopy and deposited on the forest floor. In this study, we used a common epiphytic tank bromeliad, Guzmania berteroniana (R. & S.) Mez, to determine if fallen bromeliads can survive, grow, and reproduce on the forest floor and evaluate the potential impact of adult dispersal on plant and soil nutrient pools. Bromeliads were transplanted to and from tree stems and the forest floor and monitored intensively for six months; survival, growth, and impacts on ecosystem nutrient pools were followed on a subset of plants for 16 months. Six months after transplanting, bromeliad mortality was low (3%), and 19 percent of study individuals had flowered and produced new juvenile shoots. Mortality on the subset of plants followed for 16 months was 14–30 percent. Although survival rates were relatively high in all habitats, bromeliads transplanted to trees grew significantly more root length (x?± SE: 189 ± 43 cm) than those moved to the forest floor (53 ±15 cm) and experienced lower rates of leaf area loss. All transplanted bromeliads rapidly altered the substrate they occupied. Individuals transplanted to and among trees rapidly decreased base cation concentrations but significantly increased P concentrations of their underlying substrate. On the ground, bromeliads increased C, N, and P concentrations within nine months of placement. Our results suggest that in this montane tropical forest, bromeliads respond rapidly to displacement, locally modify their substrates, and can access the resources needed for survival regardless of habitat.     

Improved growth and water use efficiency of cherry saplings under reduced light intensity

Cherry (Prunus avium L.) saplings were grown under natural sunlight (controls) or moderate shading (up to 30%, depending on the incident light intensity and the hour of the day). Reduced light intensity increased the dry mass of each of the plant components studied. Consequently, the total dry mass of shaded plants was significantly greater than that of controls at the end of the growing season. However, the diurnal trend in the level of photosynthesis (per unit of leaf area) of shaded plants was similar to the controls in August, but lower in September. As the growing season proceeded, reduced photosynthetic rates, thinner mesophyll and larger specific leaf area in the shaded plants indicated that leaf development had adapted to shaded conditions throughout the growing season. It is suggested that increased growth of shaded plants was caused by a higher initial relative growth rate and a greater whole-plant photosynthesis. Shading consistently reduced transpiration over the season, therefore improving water use efficiency of shaded leaves. Our results suggest that a moderate reduction in light intensity can be a useful method for improving growth and saving water in hot and dry environments.     

Seasonal patterns of acid fluctuations and resource storage in the arborescent cactus Opuntia excelsa in relation to light availability and size

Summary We investigated relationships between light availability, diel acid fluctuation, and resource storage in the arborescent cactus Opuntia excelsa growing in western Mexico. We compared canopy and understory individuals from a deciduous forest as well as open-grown plants of the same approximate size as those in the understory. During the wet season light availability and daily fluctuations in titratable acidity (an index of carbon uptake) were lower in the understory than in unshaded habitats. In the dry season all plants had reduced levels of acid fluctuation, with the smallest individuals, regardless of habitat, showing the greatest reduction. These data suggest that light availability in the forest understory constrains carbon assimilation during the wet season, but that a factor associated with plant size, possibly water status, limits carbon gain during the dry season. Plants in all habitats remained physiologically active for at least five months into the dry season. We suggest that this was possible due to the maintenance of constant concentrations of water and nitrogen in the photosynthetically active chlorenchyma. Parenchyma in terminal cladodes showed a different seasonal pattern of resource storage; water content and nitrogen concentration were reduced from the wet to the dry season in the parenchyma. Using the parenchyma to supply photosynthetic tissues during times of reduced resource availability allows O. excelsa to assimilate carbon during times of the year when most other trees in the forest are leafless.     

Do cloud forest tree species differ in their suitability as a substrate for epiphytic bromeliads?

There is evidence for the existence of varying degrees of host preference in vascular epiphytes; certain tree species can be positively, neutrally, or negatively associated with epiphytes. The objective of this study was to evaluate whether tree species of the cloud forest differ in their suitability as a substrate for epiphytic bromeliads. To evaluate the association between epiphytic bromeliad cover and host tree species, we sampled 62 plots (each of 200 m²) in four cloud forest fragments in Veracruz, Mexico. For all trees ≥10 cm in diameter at breast height (DBH), we recorded species name, DBH, and percentage cover of bromeliads in categories of tree coverage. In total, 587 trees belonging to 52 species were recorded. All of the 10 tree species used to assess differences in epiphyte cover (each with a minimum of nine individuals) supported bromeliads, but mean bromeliad cover differed significantly among the tree species. The tree species that concentrated the highest bromeliad cover were Quercus sartorii (29.86%) and Liquidambar styraciflua (21.72%). Our results indicate that, while none of the tree species analyzed was a limiting host for epiphytic bromeliads in general, varying levels of bromeliad cover occur depending on the host species in tropical montane cloud forest fragments suggesting that certain tree species are better hosts than others. The implications for conservation efforts of differential tree species suitability as epiphyte hosts are discussed.     

Microhabitats and Water Relations of Epiphytic Cacti and Ferns in a Lowland Neotropical Forest1

The pattern of canopy distribution and some water relations characteristics of the epiphytic cacti Epiphyllum phyllanthus and Rhipsalis baccifera and the epiphytic ferns Polypodium crassifolium and Polypodium phyllitidis were examined in the tropical forest of Barro Colorado Island, Panama. The epiphytic cacti were 6-fold more succulent than the epiphytic ferns, which had a 9-fold higher root: shoot ratio. The four species, especially the cacti, were more abundant on the deciduous trees Ceiba pentandra, Platypodium elegans. and Tabebuia guayacan than on the evergreen Anacardium excelsum. The water-holding capacity of the bark was 2-fold higher for A. excelsum and T. guayacan than for C. pentandra and P. elegans; the main crotches of the latter species intercepted 2-fold more rainfall than those of the former. Well-watered plants of all epiphytic species showed similar total daily transpiration. After 2 weeks of drought, daily transpiration decreased an average of 48 percent for the cacti and 73 percent for the ferns. After 4 weeks of drought, daily transpiration decreased 80 percent for the cacti and was eliminated for the ferns; succulence then decreased an average of 29 percent for the cacti and 98 percent for the ferns, but all species recovered fully within 2 d after rewetting. The epiphytic ferns had a low relative capacitance (0.16 M/Pa), causing these presumably C₃ plants often to be under water stress. A high relative capacitance (0.50 M/Pa) and crassulacean acid metabolism allowed the epiphytic cacti to occur most frequently on the driest sites of this tropical forest.     

Genetic effects of tank-forming bromeliads on the associated invertebrate community in a tropical forest ecosystem

Within the area of community genetics there is an expanding volume of literature demonstrating how within-species genetic variation in temperate trees can have important effects on structuring animal and plant communities. The influence of intraspecific genetic variation on associated communities in relatively more complex ecosystems is only starting to be appreciated. Within tropical forests, epiphytic bromeliad plants often grow high in the canopy and create unique nutrient-rich microhabitats on which many invertebrate and vertebrate species depend. We investigated the influence of within-species genetic variation in the bromeliad Aechmea bracteata on the invertebrate microhabitat community. We found that more genetically similar bromeliad plants were host to more similar communities of juvenile-stage invertebrates, but not adult invertebrates. We discuss possible mechanisms for this, including differential survival and active female oviposition choice. Our work shows that the impact of within-species genetic variation on associated ecological communities may be more general than previously considered. These results agree with recent research suggesting that within-species genetic variation may perform a supporting ecosystem service for maintaining community and ecological processes.     

遮荫和全光下生长的棉花光合作用和叶绿素荧光特征

 遮荫条件下（遮荫下光强相当于自然光强的40％左右）棉花(Gossypium hirsutum)叶片光合速率明显降低,仅为自然光强下生长叶片的30%～40％,叶片中RuBP羧化酶活性降低,而表观量子效率(AQY)较高。不同光照条件下生长的棉花叶片对短时间持续光强的光合诱导过程有明显的差异,由弱光转到强光下,自然光强下生长的叶片的Pn、Gs、ΦPSⅡ及非光化学猝灭系数(NPQ)都能在较短的时间内达到最大值,而遮荫叶片需要的时间较长;遮荫下生长的棉花叶片的实际光化学效率,随光强的增加下降幅度较大,而自然光照下生长的叶片下降幅度较小;自然光照下生长的叶片的NPQ随光强的升高达到较高水平,而遮荫叶片在较低的光强下即达到最大值,此时NPQ较低,遮荫叶片依赖于叶黄素循环的能量耗散水平较低。遮荫叶片较低的光合速率以及过剩光能耗散能力是其转入自然强光后光抑制严重的主要原因。     

Microclimate, light adaptation and desiccation tolerance of epiphytic bryophytes in two Venezuelan cloud forests

Aim Analysis of microclimate factors and physiological responses determining survival and growth of epiphytic bryophytes in the lower canopy and trunk space of north‐Andean cloud forests. Location Two cloud forests at 2000–2400 m in the northern Andes near Mérida, Venezuela. Methods Data‐logging of dry and wet‐season temperature, relative humidity (r.h.) and photosynthetically‐active radiation (PAR) for month‐long periods, and laboratory measurements of desiccation tolerance and light responses of selected epiphytic bryophytes. Results Rainfall averages 20 mm or less in January and February, and 200 mm or more from August to October, but is very variable at all seasons. The proportion of time ‘wet’ (continuous 100% r.h.) in the months sampled ranged from 8.5% to 52.2% or more; a dry/wet‐season range between 20% and 40% is probably commoner. The length of ‘wet’ and ‘dry’ periods approximated log‐normal distributions, with mid‐points for wet periods ranging from 2.8 to 10.7 h, and dry periods from 6.2 to 17.1 h. The longest recorded dry period was 143 h. Humidity typically rose during the night to > 90% r.h., reaching 100% for significant periods (implying cloudwater (fog) deposition) on about one night in two in all seasons. Of six bryophytes of pendulous growth form, all survived periods of at least a few days’ desiccation; most recovered better from high than low humidities. Measured 95% light‐saturation values ranged from 110 to 256 μmol m^?2 s^?1, somewhat but not greatly higher than ambient light levels Main conclusions Environmental conditions in the cloud forests are probably near‐optimal for epiphytic bryophytes, but in even the wettest forest these plants must tolerate at least short periods of drying at any time of year, and longer periods seasonally. Interception of cloudwater droplets from moving air is likely to be an important source of water for bryophytes of pendant and other diffuse life forms, especially in periods of low rainfall. Absorption of water from near‐saturated air is probably of little physiological significance. Bryophytes of these life‐forms are notably conspicuous in tropical‐montane cloud forests. They remain prominent into humid temperate regions such as southern Chile, New Zealand and Macaronesia, but progressively disappear at higher latitudes with the stresses of increasing seasonality.     

Composition of kermes oak browse as affected by shade and stage of maturity

Kermes oak (Quercus coccifera L.) is a sclerophyllous evergreen shrub of the Mediterranean region. In Greece, it grows either alone or in a mixture with other sclerophyllous species in shrublands or as the understory in Pinus brutia forest.The nutritive value and digestibility of kermes oak browse were investigated under 2 light conditions: an an understory species in a P. brutia forest and in adjacent open shrublands. The comparison was made at 3 phenological stages: during the season of rapid growth (in April), after rapid growth was terminated (in May) and when growth had ceased and stems had hardened (in June).Crude protein content of the leaves and twigs was higher in the shaded than in the unshaded plants during the growing season, while the concentrations of total non-structural carbohydrate, cell contents and soluble protein were higher in the unshaded than in the shaded plants. Tannins and lignin content were higher in shaded than in unshaded plants. Similarly, dry matter digestibility was higher in unshaded plants, but declined more drastically in the shaded plants during the growing season. Production was also higher in unshaded than in shaded plants at the end of the growing season.     

Photoinhibitory responses of the epiphytic bromeliad Guzmania monostachia during the dry season in Trinidad maintain photochemical integrity under adverse conditions

The C₃–CAM epiphytic bromeliad Guzmania monostachia var. monostachia may be exposed to high incident photosynthetically active radiation (PAR) during the dry season in Trinidad, and resultant variations in photochemical efficiency have been investigated for ‘exposed’ (receiving ～100% incident PAR), ‘semi-exposed’ (～60% PAR) and shaded populations under natural conditions. The more succulent leaves of the plants growing fully exposed within the canopy had higher overall CAM activity (measured as ΔH⁺, the dawn-dusk titratable acidity), a smaller proportion of chlorenchyma and lower total chlorophyll content. There was a gradation of morphological and physiological characteristics between these and shaded leaves. Diurnal time-courses of photosynthetic light responses (as O₂ evolution) showed marked variations in apparent quantum yield (AQY) and light-saturated rates for both exposed and semi-exposed populations, dependent on incident PAR during the day. Similar measurements of photosystem II fluorescence characteristics showed that F_v/F_m declined from 0·70 to 0·42 at midday for exposed plants (on a day when total incident PAR was 44 mol photon m^?2), indicating non-photochemical quenching (qNP) of photosynthesis. However, in contrast to AQY determinations, F_v/F_m recovered during the afternoon. The decrease in F_v/F_m was reduced from 0·72 to 0·64 under 24 mol photon m^?2 d^?1. The long–term recovery of photo-synthetic efficiency was assessed for exposed plants placed under three shading regimes (60, 30 and 3% of incident PAR) over a 17-d period. During this time, total chlorophyll content increased from 228 to 515 and 585 μg g^?1 fresh weight (for 3 and 30%, respectively) and chlorophyll a:b declined. While AQY recovery was much longer under the lowest PAR (17d), under 30% PAR both AQY and F_v/F_m had recovered after 2d shading. The differences between timing of recovery for F_v/F_m during diurnal time courses and in the long term suggest that, while quenching associated with PSII recovers rapidly, enzyme activation and/or protein synthesis of other photosynthetic components may be limiting under low PAR. However, it is suggested that the occurrence of qNP on a daily basis may preclude long-term photoinhibitory damage under natural conditions during the dry season.     

Demographic consequences of drought in the herbaceous perennial Cryptantha flava: effects of density,associations with shrubs,and plant size

The demographic consequences of a severe drought year were examined for two experimental plantings of the herbaceous desert perennial Cryptantha flava(Boraginaceae) in northeastern Utah, United States. A total of 6680 nutlets were planted individually or in clusters of four both under shrubs and in open microhabitats within two natural populations. Survival, growth, and flowering as a function of density and microhabitat were followed for 7 years, including 1 year when precipitation just before and during the growing season was 74.5% below normal. The design permitted assessment of how intraspecific density and shrub cover affect demographic response to drought. Mortality increased and flowering decreased dramatically during drought but neither varied with density or between shrub and open microhabitats. For plants growing under shrubs, survival (at Site 1) and growth (at Site 2) varied with shrub species. Average aboveground plant size also decreased during drought. Population size hierarchies were rearranged because larger plants lost leaf rosettes while many smaller plants grew. Density and microhabitat affected plant performance in non-drought years but more often at Site 1 than at Site 2. Individuals growing alone often were more likely to flower and/or produced more inflorescences when they did flower than did individuals growing with at least one other plant. However, for 2 years, survival rates at Site 1 were higher for plants growing in clumps than for single individuals. Shrubs also had mixed effects on plant performance. In some years, survival was higher under shrubs, but at Site 1 plants in the open often were more likely to flower and/or produced more inflorescences. Thus despite severe demographic consequences of drought, the study provided no evidence that intraspecific competition, interference by shrubs, or facilitation by shrubs increases under limited soil water.     

Effects of coppicing on demographic structure, fruit and seed set in Orchis mascula

Opening of the forest canopy may result in higher fitness of understory plants due to increased light intensity on the forest soil and increased nutrient availability. Especially, tuberous orchids may profit from increased light as photosynthetic assimilation products and nutrients are accumulated in their belowground storage organs. We investigated the effects of coppicing on demographic structure and fruit and seed set in 15 populations of the tuberous, perennial orchid Orchis mascula, seven of which were located in undisturbed, shaded woodland and eight in recently coppiced woodland. Coppicing resulted in a massive increase in flowering and in increased fruit set. On average, 42.9% of all individuals consisted of flowering plants in coppiced woodland, whereas in undisturbed woodland only 20.8% of all individuals flowered. The percentage fruit set varied between 20% and 55% in coppiced woodland and was strongly related to population size (measured as the number of flowering individuals). Mean fruit weight and fruit size were also significantly higher in populations located in coppiced woodland than in undisturbed woodland, whereas the proportion of viable seeds was not related to woodland type. Given that O. mascula is dependent on seed set to fill space with new individuals, these results suggest that the long-term survival of this species strongly depends on frequent opening of the canopy.     

The Effect of Light Intensity on Certain Biological Characters of Rauvolfia

The effect of light intensity on Rauvolfia yunnanensis was studied under four different light conditions, namely: 1. 100% daylight (shadeless); 2.73% (27% shaded); 3. 48% (52% shaded) ; 4. 28% (72% shaded). The most suitable light intensity required for the growth of plant, both on the plant height and diameter, was varying from 28% daylight (at dry season) to 73% daylight (at foggy season) at different seasons. Under the half shaded condition (48% light intensity), the annual production of roots, leaves and stems was greater than under any other light conditions (100%, 73% or 28% light intensity). The total number of flowers was depressed markedly by shading, but the number of fruits yielded was higher under 73% light intensity. The number of embryoless seeds was as high as 96% under full light condition (100% light intensity), but decreased apparently under the shading of forest canopy. The size of leaves, fruits and seeds was varying quite a bit with different light intensities. Finally, we have to mention that the alkaloid contents in the leaves tended to increase with the decreasing of the light intensity.     

Survival and growth of Virola surinamensis yearlings: Water augmentation in gap and understory

Summary Factors affecting seedling Virola surinamensis (Myristicaceae) survival and growth were investigated on Barro Colorado Island, Panama. Seedlings planted 3 months after germination were monitored in treefall gaps and understory using 2.25 ha irrigated and control plots through the first dry season. During the dry season, irrigated plants in gaps increased total leaf area significantly more than did irrigated plants in the shaded understory. Over the same dry season, control plants in gaps and in the shaded understory lost similar amounts of leaf area. Seedlings in understory were suppressed in stem height and biomass in both irrigated and control plots; these measures were greater in gaps and greatest in irrigated gaps (height). Roots were similar in length in all treatments, but greater in biomass in gaps than understory due to greater proliferation of secondary roots in control and irrigated gaps than in control and irrigated understory. This experiment demonstrates both water and light limitation during the first dry season after germination. V. surinamensis seedlings are capable of survival and modest growth of leaf area in the deep shade of the understory in moist locations; they are severely disadvantaged in shaded understory subject to drought, where most seeds fall and most seedlings establish. The broken canopy of a gap allows shoot and consequently root growth that permits seedlings to survive seasonal drought.     

哀牢山湿性常绿阔叶林地生、树干及树枝 附生苔藓生活型组成及其水分特性

为探讨林内不同垂直高度生境下及不同生活型苔藓植物水分特征的差异,该文对哀牢山湿性常绿阔叶林内地生苔藓、林下树干附生和林冠层树枝附生苔藓优势或常见种的生活型组成、持水力、失水特征和水分利用效率进行了研究。结果表明:地生、树干附生和树枝附生苔藓植物类群分别以交织型、扇型、悬垂型苔藓植物占据优势;地生、树干附生、枝条附生苔藓植物的饱和持水率分别为476%DW、210.98%DW、238.95%DW;地生苔藓植物的持水率和失水速率均高于附生苔藓,树干附生苔藓植物持水率低于树枝附生苔藓,而失水速率高于树枝附生苔藓。在不同生活型苔藓的水分特性上,交织型苔藓具有较高的持水率和失水速率,保水性能较弱,其次为悬垂型苔藓,扇型苔藓的持水率最小,失水速率也较快;地生和树干附生苔藓的水分利用效率均显著高于树干附生苔藓,交织型和扇型苔藓的水分利用效率显著高于悬垂型苔藓。在三种生境下,地生苔藓持水力高,水分利用效率较高而保水能力低;枝条附生苔藓持水力低,水分利用效率低而保水能力较高;树干附生苔藓水分利用效率较高而持水力和保水能力均较差。因此,不同生境下苔藓植物生活型组成及其水分变化特性在一定程度上反映了它们对不同生境的适应策略。     

Physiological responses of the CAM epiphyte Tillandsia usneoides L. (Bromeliaceae) to variations in light and water supply

In an effort to understand the mechanisms that sustain rootless atmospheric plants, the modulation of Crassulacean acid metabolism (CAM) in response to variations in irradiance and water supply was investigated in the epiphyte Tillandsia usneoides. Plants were acclimated to three light regimes, i.e. high, intermediate and low, with integrated photon flux densities (PFD) of 14.40, 8.64 and 4.32 mol m-2 d-1 equivalent to an instantaneous PFD of 200, 100, and 50 mumol m-2 s-1, respectively. Daily watering was then withdrawn from half of the plants at each PFD for 7 d prior to sampling. In response to the three PFD treatments, chlorophyll content increased in plants acclimated to lower irradiances. Light response curves using non-invasive measurements of chlorophyll fluorescence demonstrated that photosystem II efficiency (phi PSII) was maintained in high PFD acclimated plants, as they exhibited a larger capacity for non-photochemical dissipation (NPQ) of excess light energy than low PFD acclimated plants. Net CO2 uptake increased in response to higher PFD, reflecting enhanced carboxylation capacity in terms of phosphoenolpyruvate carboxylase (PEPc) and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) activities. After water was withdrawn, nocturnal net CO2 uptake and accumulated levels of acidity declined in all PFD treatments, concomitant with increased respiratory recycling of malate. Examining the strategies employed by epiphytes such as T. usneodies to tolerate extreme light and water regimes has demonstrated the importance of physiological mechanisms that allow flexible carboxylation capacity and continued carbon cycling to maintain photosynthetic integrity.