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1.
M. Méthy  P. Alpert  J. Roy 《Oecologia》1990,84(2):265-271
Summary Plant canopy shade reduces photosynthetic photon flux density (PPFD) and ratio of red to far-red light (z). Both effects can cause plants to increase potential for light acquisition through vertical growth and leaf area expansion. Clonal plants such as Eichhornia crassipes might alternatively increase light interception via horizontal growth of stolons or rhizomes and placement of new ramets in less shaded microsites. Effect of simulated canopy shade and component effects of PPFD and z were tested by filtering or adding light uniformly, to a whole group of connected ramets, or locally, to individual ramets within a group. In uniform treatments, low PPFD reduced total growth but low z did not. Low PPFD and low z independently reduced stolon and ramet production and caused etiolation of petioles; effect of low PPFD plus low z on ramet production was greater than that of either factor alone. Lateral clonal growth thus did not seem to be a response to uniform shading; instead, uniformly low PPFD or low z increased partitioning to established ramets. Low z changed partitioning without changing total growth. In local treatments, reduction of growth of individual ramets due to low PPFD and inhibition of new ramet production attributable to spectral composition of light were mitigated when connected ramets were unshaded; plants may respond differently to patchy than to uniform shade.  相似文献   

2.
Lamoureaux  S.L.  Kelly  D.  Barlow  N.D. 《Plant Ecology》2003,165(2):263-273
The dynamics of a mature population of Hieracium pilosella under four different treatments (± irrigation, ± fertiliser, ± defoliation, and ± fungicide & insecticide, in a full factorial design) were compared in the Mackenzie Basin, New Zealand. H. pilosella had been growing at the site for at least 30 years. Recorded variables included rosette density, inflorescence and stolon production, recruitment and mortality, and plant cover. The population appears to be regulated through density-dependent mortality interacting with density-independent reproduction. In the treated plots, irrigation and fertiliser had the greatest effects, increasing both the proportion of established rosettes producing stolons and stolon length. Irrigation also increased the likelihood of a daughter rosette becoming reproductive and decreased inflorescence abortion. Fertiliser increased the probability of an established rosette reproducing and the number of daughter rosettes produced. In unmanipulated plots H. pilosella appeared to be at or near an equilibrium density of ca. 3200 rosettes m−2, but over longer time scales the low reproductive rates shown in this study may presage slow population decline due to plant-induced environmental degradation. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

3.
Summary Humidity, at the young nodes of white clover stolones, varied by enclosing nodes in the atmosphere above a range of saturated solutions, inhibited root initiation at 85% RH or less. The threshold humidity for root initiation increased to about 93% on young nodes subject to moisture stress or old nodes on well watered plants in which root initiation had been previously suppressed by low humidity.Roots at old nodes and at the three youngest on stolons were either subject to moisture stress or adequately watered. Growth of young roots and N2-fixation were more adversely affected by the direct effects of drought than by subjecting old roots to drought. Although old roots under stress affected new root growth and N2-fixation, length of roots and lateral root number were little affected. By contrast stolon growth was affected more by stress to old roots than to young nodes, although after 6 weeks the contribution made by young roots to stolon growth was almost as high as old roots.The data suggest that deep roots at old nodes will allow clover stolons to grow during drought due to the high acropetal movement of water but initiation of roots and functioning of young roots at the soil surface will be adversely affected, with possible implications on the persistence of clover.  相似文献   

4.
Sack  Lawren  Grubb  Peter J.  Marañón  Teodoro 《Plant Ecology》2003,168(1):139-163
It has been hypothesized that plants cannot tolerate combined shade and drought, as a result of morphological trade-offs. However, numerous plant species are reportedly widespread in shaded forest understories that face drought, whether seasonal or occasional. We studied juveniles of six plant species that cope with strong summer drought in the understoreys of mixed Quercus forests in southern Spain: the tall-shrubs Phillyrea latifolia and Viburnum tinus, the perennial herb Rubia peregrina, the small shrub Ruscus aculeatus, and climbers Hedera helix and Smilax aspera. All of these species persist in evergreen shade (c. 3% daylight). Two other species were studied as comparators, Ruscus hypoglossum, less tolerant of drought, and Ceratonia siliqua, less tolerant of shade. Morphological and chemical variables relevant to shade and drought tolerance were measured for juveniles in a range of sizes, and also for the leaves of mature plants. The species converge in features that confer tolerance of shade plus drought by reducing demand for resources. Demand for water is reduced through a moderate to high below-ground mass fraction and low to moderate specific leaf area (respectively 0.22–0.52 and 112–172 cm2 g–1 at 1.00 g total dry mass). Demand for both irradiance and water is reduced through a low to moderate foliar nitrogen concentration and long-lived, physically protected leaves (2 yr). The species also converge in features that confer tolerance of either low irradiance or drought through specialized capture of resource, without precluding the other tolerance. These features include deep roots relative to shoot size, moderately higher specific leaf area in shade (1.2–2.0 × that in sun) and higher chlorophyll:nitrogen ratio in shade. Foliar chlorophyll per unit mass was higher in shade, but chlorophyll was not necessarily synthesized in greater amounts; rather, it was higher apparently due to shade effects on structural features linked with specific leaf area. In contrast, N per unit mass was higher in sun leaves independently of specific leaf area. Despite these convergences, the species diverge considerably in their root mass allocation and architecture, leaf saturated water content, density of stomata and guard cell size. No single narrowly defined functional type is needed for tolerance of shade plus drought.  相似文献   

5.
Bourguyia hamata females oviposit almost exclusively inside the rosette formed by the curled leaves of the epiphytic bromeliad Aechmea nudicaulis. We investigated whether the architecture of the individual bromeliads influences oviposition site selection by this harvestman species. We collected data on the presence of clutches inside bromeliads, rosette length, rosette slope in relation to tree trunks, and the amount of debris inside the rosette. Additionally, we measured the water volume inside the rosettes as well as the variation in the humidity inside and outside bromeliads with long and short rosettes. Longer rosettes were preferred as oviposition site possibly because they accumulate more water and maintain lower internal humidity variation than the external environment. Although the slope of the rosettes did not influence the occurrence of oviposition, the probability of debris accumulation inside the rosettes increased with their slope, and the frequency of clutches was greater in bromeliads with small amounts of debris. A field experiment showed that bromeliads with water inside the rosette were more frequently used as oviposition sites than bromeliads without water. In conclusion, females oviposit predominantly in bromeliads that accumulate more water and have small amounts of debris inside the rosettes, probably because these characteristics promote a more adequate microhabitat for egg development.  相似文献   

6.
Summary In youngFunaria protonemata the influence of various inhibitors and treatments on cell elongation, fine-structure, and particle rosettes within the plasma membrane, putative parts of cellulose synthase complexes, was investigated. Cycloheximide (3×10–5M) inhibited growth, reduced the number of rosettes and evened the gradient of rosette distribution at the beginning of treatment. The cell fine-structure was unaffected. Actinomycin D (10–5M and 10–4) caused an initial but transient decrease in rosette number. Alterations in cell elongation and fine-structure have not been observed. Application of 2.6-dichlorobenzonitrile (10–5 M) for some minutes reduced the number of rosettes remarkably, while cell elongation seemed to be normal after the filaments had been transferred back to normal medium. An incubation of 2 h or longer stopped growth and caused cells to burst. The number of rosettes then rose to about 50% of the control values. When applied for 7 h biofluor (5×10–4 M) promoted growth slightly, but generally it retarded it when used for a longer time. It did not markedly affect the number of rosettes. A short heat stock stopped elongation, caused the disappearance of rosettes and affected the structure of the mitochondria and of the Golgi apparatus. Plasmolysed cells did not grow and, initially, did not have rosettes. At reduced turgor, wider cells are formed. Freeze fracturing under UHV conditions and shadowing at very low specimen temperature revealed a small, central depression in the 8 nm rosette particles, suggesting that they are composed of subunits. Our results provide further evidence that the rosettes are parts of the cellulose synthase complexes. Their existence clearly depends on protein synthesis and on the constitution of the plasma membrane, but not on cellulose crystallization.  相似文献   

7.
Studying the response to drought stress of keystone epiphytes such as tank bromeliads is essential to better understand their resistance capacity to future climate change. The objective was to test whether there is any variation in the carbon, water and nutrient status among different leaf ontogenetic stages in a bromeliad rosette subjected to a gradient of drought stress. We used a semi-controlled experiment consisting in a gradient of water shortage in Aechmea aquilega and Lutheria splendens. For each bromeliad and drought treatment, three leaves were collected based on their position in the rosette and several functional traits related to water and nutrient status, and carbon metabolism were measured. We found that water status traits (relative water content, leaf succulence, osmotic and midday water potentials) and carbon metabolism traits (carbon assimilation, maximum quantum yield of photosystem II, chlorophyll and starch contents) decreased with increasing drought stress, while leaf soluble sugars and carbon, nitrogen and phosphorus contents remained unchanged. The different leaf ontogenetic stages showed only marginal variations when subjected to a gradient of drought. Resources were not reallocated between different leaf ontogenetic stages but we found a reallocation of soluble sugars from leaf starch reserves to the root system. Both species were capable of metabolic and physiological adjustments in response to drought. Overall, this study advances our understanding of the resistance of bromeliads faced with increasing drought stress and paves the way for in-depth reflection on their strategies to cope with water shortage.  相似文献   

8.
Transpiration, xylem water potential and water channel activity were studied in developing stolons and leaves of strawberry (Fragaria × ananassa Duch.) subjected to drought or flooding, together with morphological studies of their stomata and other surface structures. Stolons had 0.12 stomata mm–2 and a transpiration rate of 0.6 mmol H2O m–2 s–1, while the leaves had 300 stomata mm–2 and a transpiration rate of 5.6 mmol H2O m–2 s–1. Midday water potentials of stolons were always less negative than in leaves enabling nutrient ion and water transport via or to the strawberry stolons. Drought stress, but not flooding, decreased stolon and leaf water potential from –0.7 to –1 MPa and from –1 to –2 MPa, respectively, with a concomitant reduction in stomatal conductance from 75 to 30 mmol H2O m–2 s–1. However, leaf water potentials remained unchanged after flooding. Similarly, membrane vesicles derived from stolons of flooded strawberry plants showed no change in water channel activity. In these stolons, turgor may be preserved by maintaining root pressure, an electrochemical and ion gradient and xylem differentiation, assuming water channels remain open. By contrast, water channel activity was reduced in stolons of drought stressed strawberry plants. In every case, the effect of flooding on water relations of strawberry stolons and leaves was less pronounced than that of drought which cannot be explained by increased ABA. Stomatal closure under drought could be attributed to increased delivery of ABA from roots to the leaves. However, stomata closed more rapidly in leaves of flooded strawberry despite ABA delivery from the roots in the xylem to the leaves being strongly depressed. This stomatal closure under flooding may be due to release of stress ethylene. In the relative absence of stomata from the stolons, cellular (apoplastic) water transport in strawberry stolons was primarily driven by water channel activity with a gradient from the tip of the stolon to the base, concomitant with xylem differentiation and decreased water transport potential from the stolon tip to its base. Reduced water potential in the stolons under drought are discussed with respect to reduced putative water channel activity.  相似文献   

9.
In tropical forests light and water availability are the most important factors for seedling growth and survival but an increasing frequency of drought may affect tree regeneration. One central question is whether drought and shade have interactive effects on seedling growth and survival. Here, we present results of a greenhouse experiment, in which seedlings of 10 Ghanaian tree species were exposed to combinations of strong seasonal drought (continuous watering versus withholding water for nine weeks) and shade (5% irradiance versus 20% irradiance). We evaluated the effects of drought and shade on seedling survival and growth and plasticity of 11 underlying traits related to biomass allocation, morphology and physiology. Seedling survival under dry conditions was higher in shade than in high light, thus providing support for the “facilitation hypothesis” that shade enhances plant performance through improved microclimatic conditions, and rejecting the trade-off hypothesis that drought should have stronger impact in shade because of reduced root investment. Shaded plants had low biomass fraction in roots, in line with the trade-off hypothesis, but they compensated for this with a higher specific root length (i.e., root length per unit root mass), resulting in a similar root length per plant mass and, hence, similar water uptake capacity as high-light plants. The majority (60%) of traits studied responded independently to drought and shade, indicating that within species shade- and drought tolerances are not in trade-off, but largely uncoupled. When individual species responses were analysed, then for most of the traits only one to three species showed significant interactive effects between drought and shade. The uncoupled response of most species to drought and shade should provide ample opportunity for niche differentiation and species coexistence under a range of water and light conditions. Overall our greenhouse results suggest that, in the absence of root competition shaded tropical forest tree seedlings may be able to survive prolonged drought.  相似文献   

10.
Summary Fragaria chiloensis is a stoloniferous perennial herb that grows on coastal sand dunes where scattered shrubs create small patches of lower photon flux density (PFD) but higher soil nitrogen availability. The potential effects of resource transport between ramets when PFD and soil nitrogen are negatively associated in space were tested by comparing the growth of pairs of ramets in which the vascular connection between ramets was either severed or left intact. One ramet in each pair was given high PFD but a low level of soil nitrogen and the other ramet was given low PFD but high N. The analogous effects of resource transport likely to be realized in nature were tested by substituting a more realistic medium soil nitrogen level in place of the high level. Results suggested that connected ramets exchanged carbon and nitrogen under both regimes of soil nitrogen heterogeneity. In the low versus high nitrogen regime, connected ramets had higher combined dry biomass and different patterns of dry mass partitioning from those of severed ramets; effect of connection was greater on ramets given low PFD and high N and on younger ramets. In the low versus medium nitrogen regime, connected ramets had different patterns of partitioning only. Apparent reciprocal resource transport between ramets can enhance the growth of ramets with complementary resource deficiencies, but may affect growth in dry mass only when maximum resource levels are high.  相似文献   

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