摘要:

We examined factors that limit diurnal and seasonal photosynthesis inLeymus cinereus, a robust tussock grass from shrub‐steppes of western North America. Data from plants in a natural stand and in experimental field plots indicate that this bunchgrass has 1) a high photosynthetic capacity, 2) high leaf nitrogen content and high nitrogen‐use efficiency, 3) a steep leaf‐to‐air diffusion gradient for carbon dioxide, which enhances intrinsic water‐use efficiency, and 4) photosynthetic tissues that tolerate severe water stress and recover quickly from moderate water stress. Midday depressions of CO2assimilation (A) and stomatal conductance were slight in plants with plentiful water, but marked in plants subject to moderate water stress. Midday stomatal closure in moderately stressed plants reduced intercellular carbon dioxide concentration (ci) by ≈40μl liter‐1. The maximum rate ofAachieved during the day for severely stressed plants (predawn water potential = ‐4 MPa) was one‐third and daily carbon gain per unit leaf area was about one‐fourth that of well‐watered plants. For plants in the natural stand, CO2‐saturated photosynthesis declined almost linearly with decreasing soil water availability over the growing season, whereas there was little effect onAat CO2ambient levels or on carboxylation efficiency until predawn water potentials reached ‐1.8 MPa. Nitrogen‐use efficiency declined with diminishing soil moisture, but there was no seasonal change in stomatal limitation or instantaneous water‐use efficiency as estimated fromAvs.cicurves at optimal leaf temperature and moderate atmospheric evaporative demand. Thus, reduced stomatal conductance in response to increased evaporative demand may increase stomatal limitation diumally, but over the growing season, stomatal limitation estimated fromAvs.cicurves is relatively constant because maximum stomatal conductance is closely tuned to the CO2assimilatory capacity of the mesophyll.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1995.tb15680.x

出版商:Wiley    

年代:1995

数据来源: WILEY

摘要:

The dynamic equilibrium hypothesis proposes that hybrid zones are stabilized by a balance between dispersal and selection against hybrids. A key prediction of this hypothesis is that hybrids should have lower fitness than either parental taxon, regardless of habitat. Hybrid big sagebrush (Artemisia tridentatassp.tridentata× ssp.vaseyana) in two narrow hybrid zones do show greatly decreased recruitment. Hybrids in one zone also show increased browsing by grasshoppers, while those in the other zone have increased numbers of aphids. Overall herbivore loads, however, are not greater on the hybrids than on the parental subspecies. Browsing by mule deer is greatest on ssp.vaseyanain both hybrid zones. Incidence of galls is also greatest on ssp.vaseyanain one hybrid zone. Moreover, browsing by Artemisia weevils is greatest on ssp.tridentatain one hybrid zone. Hybrids produce more flowers than either ssp.tridentataor ssp.vaseyana, while seed production rates of hybrids do not differ from those of the parental taxa. Finally, hybrid seeds germinate as well as those of ssp.tridentataand better than those of ssp.vaseyana. Thus, our data do not support the dynamic equilibrium hypothesis.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1995.tb15681.x

出版商:Wiley    

年代:1995

数据来源: WILEY

摘要:

We performed mixed pollinations, involving self vs. within‐ or between‐population pollen, and used starch gel electrophoresis to assay 2,210 progeny to detect pollen competition inTurnera ulmifolia. Within populations there was no evidence for a competitive advantage to outcross pollen. Between populations a marked advantage to self pollen occurred for most populations, and the extent of this competitive advantage was correlated with increasing morphological divergence of the outcross pollen donor population. A comparison of rates of ovule abortion revealed that the biased paternity ratios cannot be the result of ovule abortion alone, and that pollen competition must be the major contributing factor. We suggest that reproductive divergence among the populations, perhaps through adaptation to different pollinators or through the evolution of increased selling, has resulted in the evolution of reproductive isolation through pollen competition.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1995.tb15682.x

出版商:Wiley    

年代:1995

数据来源: WILEY

摘要:

In a pair of similar experiments, we examined the effects of varying substrate nutrient levels on several measures of male and female function inAbutilon theophrasti(Malvaceae). In the first trial, plants adjusted allocation to female function at several levels: number of flowers initiated, locule number, percentage of fruits matured, and percentage of ovules matured. In the second trial, plants were much larger, opened more flowers, and matured more fruit (in total but not on a percentage basis). However, in response to nutrient level, plants adjusted maternal investment at fewer levels than in the first trial. Seed mass varied between, but not within, trials. Little to no adjustment was seen in male characters within either trial, although flower number differed within trials and stamen number differed between trials. The lack of adjustment of paternal investment may be due to the fact thatA. theophrastiis primarily an autogamous species. Between the two trials, growing conditions, primarily light intensity, varied and plants adjusted their allocation to reproduction differently, indicating plasticity in this trait.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1995.tb15683.x

出版商:Wiley    

年代:1995

数据来源: WILEY

摘要:

Natural populations ofThymelaea hirsutahave previously been shown to comprise four distinct sexual morphs: males, females, protogynous individuals, i.e., first female then male, and protandrous individuals, i.e., first male then female. The objective of the present study has been to confirm the genetic basis of this sexual tetramorphism by quantifying morph ratios in the open‐pollinated progeny of the four sexual phenotypes growing in a natural population. All four phenotypes were recovered in the progeny of each morph. All observed plants displayed a single sexual phenotype, thus confirming the genetic basis of the tetramorphism. The progeny sex ratios indicate that the genetic determination of sex in this species may be influenced by cytoplasmic factors, while the observed levels of functional female fertility suggest a near‐dioecious system. The evolutionary significance of this tetramorphism as a transitional stage in the evolution of dioecy is discussed.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1995.tb15684.x

出版商:Wiley    

年代:1995

数据来源: WILEY

摘要:

Protandry is a common form of dichogamy in species with hermaphroditic flowers, yet the temporal separation of male and female organs is rarely synchronized enough to prevent self‐pollination among flowers on the same plant. Here we report an unusual mechanism for synchronized protandry within umbels ofBomarea acutifolia, a hummingbird‐pollinated vine in Costa Rica. Flowers in the same umbel open over a period of several days and spend approximately 9 days in the male phase while the six anthers dehisce one by one. Each flower then enters a week‐long neuter phase during which the spent stamens move away from the elongating style and nectar production continues. This is followed by a short female phase when the stigma finally becomes receptive. The long neuter phase ensures that most umbels are temporally unisexual, thereby limiting opportunities for self‐pollination among flowers in the same umbel. A neuter phase has been reported in a few other dichogamous species, but to our knowledge this is the first description of a phase in which the flowers remain open and continue to provide nectar without dispersing or receiving pollen.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1995.tb15685.x

出版商:Wiley    

年代:1995

数据来源: WILEY

摘要:

A mechanistic model of one hormone regulating both sexes in flowering plants was tested in buffalograss (Buchloe dactyloides). This model assumes that one hormone has male and female cell receptors to inhibit one sex and induce the other independently. Three components—the normal range of hormone level in the plant and the sensitivity levels of the two receptors—interact to regulate sex expression. The study organism, buffalograss, is usually considered dioecious, but natural populations consist of varying proportions of male, female, and monoecious plants. Prior research with growth regulators had shown that only gibberellin (GA) had consistent and significant effects on sex expression in this species. To test the model assumption of a hormone with a dual function, GA and a GA inhibitor (paclobutrazol, PAC) were applied to three monoecious genotypes; in two of the genotypes the GA treatment yielded a significantly higher proportion of male inflorescences, and this transition involved both inducing male and inhibiting female. PAC treatment produced exclusively female inflorescences, illustrating the dual effects of GA. To test the predictability of the model, GA was applied to two dwarf female genotypes. These plants were transformed into neuter and near‐neuter plants with normal height and vegetative growth, as predicted by our model for genotypes with a physiologically wide overlapping of male and female sterile regions. The model also predicts that male or female plants would be induced to produce inflorescences of the other sex if the hormone level could be shifted from one side of the overlapping sterile regions to the other. This was verified by applying high levels of GA to a normal female genotype that resulted in the production of male inflorescences. However, this is the only normal female that has responded to GA application by producing male inflorescences, and males lose vigor and/or die without producing female inflorescences at high levels of PAC. The model suggests that the constancy of these males and females is due to the relative location of the sensitivity levels in relation to each other and to the hormone range. We conclude that the one‐hormone model can facilitate both applied and basic research.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1995.tb15686.x

出版商:Wiley    

年代:1995

数据来源: WILEY

摘要:

Life history theory predicts that trade‐offs between growth and reproduction should be dictated by a population's mortality schedule. We tested this prediction withArabis fecunda, a short‐lived perennial that occurs in many different habitats in southwest Montana. Individuals produce either or both axillary or terminal inflorescences. Axillary‐flowering plants are usually iteroparous and have smaller reproductive bouts, while terminal‐flowering plants have larger reproductive bouts, and tend to be semelparous. We recorded size and fecundity ofA. fecundaindividuals from 1989 to 1993 in three different habitats. There was great variation in demographic and life history traits among the populations. A wide range of life history strategies among populations ofA. fecundais achieved through different proportions of axillary‐ and terminal‐flowering plants.Arabis fecundademonstrated a lower recruitment rate, higher survivorship, slower growth, and lower annual fecundity at the low‐elevation site compared to the high‐elevation site. At the low‐elevation site population size was more stable, and elasticity analysis of matrix projection models indicated that adult survivorship was the most important demographic parameter contributing to population growth. This association of life history characters conforms to theoretical predictions.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1995.tb15687.x

出版商:Wiley    

年代:1995

数据来源: WILEY

摘要:

Litocarpon beardiiis a multifolliculate fruit from the Upper Cretaceous Haslam Formation (late Santonian‐early Campanian) of British Columbia. About 19 follicles are helically arranged on an elongate, slender receptacle. Follicles are winged with the wing extending beyond the locule. Placentation is parietal along the ventral suture, and dehiscence was along the dorsal suture. Seeds are also winged and were derived from anatropous ovules. Affinities with the Magnoliaceae are suggested withLitocarponcombining characters ofMagnoliaandLiriodendron.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1995.tb15688.x

出版商:Wiley    

年代:1995

数据来源: WILEY

摘要:

Quantitative changes in cell pattern in the pith, cortex, cortical collenchyma, and epidermis were followed in developing internodes ofLiquidambarto examine the cellular basis of compressive and tensile stresses in organized shoot growth. Initially, the highest rates of cell multiplication occur in the pith, followed successively by the epidermis, cortex, and cortical collenchyma. As internodes enter the phase of maximum elongation growth, mitotic activity begins to shift acropetally, accompanied by pronounced changes in cell pattern. The highest rates of cell multiplication now occur in the pith and cortex and continue until the cessation of internode growth. Concomitantly, reduced rates of cell division in peripheral tissues result in rapid increases in rates of cell elongation in the cortical collenchyma and epidermis. Attention is focused on the role of continued cell division in developing internodes with emphasis on differences in rates of cell multiplication between inner and outer tissues affecting patterns of tissue stress. For example, rapid and sustained increases in cell number in the pith, accompanied by growth of readily extensible pith cells, result in the development of compressive forces driving the growth of internodes. Conversely, continuing divisions in less extensible collenchyma and epidermal cells can relieve threshold tensile stresses resulting from the continuous stretching of these tissues by the developing pith. The concept that the passive extension of peripheral tissues, especially the epidermis, control the rate of internode elongation is viewed as an oversimplification of the interacting role of compressive and tensile forces in organized growth and development.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1995.tb15689.x

出版商:Wiley    

年代:1995

数据来源: WILEY