ECTODESMATA AND FOLIAR ABSORPTION

Franke , Wolfgang . (U. Bonn, Germany.) Ectodesmata and foliar absorption. Amer. Jour. Bot. 48(8): 683–691. Illus. 1961.—Plasmodesmata, called ectodesmata, in the outer walls of epidermal cells, have been investigated. Their occurrence and distribution in the epidermis of leaves of Plantago major and Helxine soleirolii have been examined in connection with foliar absorption. Leaf structures such as guard-cells, conical hairs, anticlinal walls and the epidermal cells adjacent to the leaf veins have been shown consistently to contain large numbers of ectodesmata, while in neighboring cells ectodesmata may be low in number or lacking. The same areas also are known to be especially pervious to water and dissolved dyes applied to the surface of the leaf. From special investigations, it appears that certain solutions that form visible crystals and precipitates in the outer wall enter the epidermal cell wall in localized pathways. The localization of these bodies coincides with that of ectodesmata. Therefore, it is concluded that the ectodesmata may be the pathways for transport of substances from the outside to the interior of tissues and vice versa. Nutrients applied to the surface of leaves are thought to enter by the same pathways, i.e., the ectodesmata, as those in which the penetration can be visibly detected. Some phenomena of foliar absorption which confirm this theory are explained in connection with the presence of ectodesmata.     

SIEVE TUBES IN FOLIAR VEIN ENDINGS: REVIEW AND QUANTITATIVE SURVEY OF RUDBECKIA LACINIATA (ASTERACEAE)

Sieve tube distribution in foliar vein endings of angiosperms is mentioned in 33 publications. Their collective judgment is that sieve tubes may be lacking, may end partway along, or may extend to the vein tip. Most reports conclude that all vein endings are the same within a species, but a few studies mention variation. Only three studies include quantitative data, all based on reconstruction from microtome sections. The present quantitative study, the most extensive one to date, surveyed sieve tubes in vein endings from cleared and stained leaf pieces of Rudbeckia laciniata (Asteraceae, tribe Heliantheae), a North American perennial herb. Of 203 areoles sampled, 32 (15.8%) lacked vein endings whereas 171 (84.2%) had one to six endings per areole. There were 385 individual vein endings in the 171 areoles: 115 lacked sieve tubes, 38 had them to the vein tip, but in most (232) they ended at some intermediate point. Serial cross sections of the latter showed phloem parenchyma continuing beyond the sieve tubes for some distance, beyond which occurred a distal zone consisting solely of tracheary elements. R. laciniata vein endings exhibit the same range of variation reported for dicots in general. More studies combining clearings with selected sections are needed to establish the range of vein ending patterns among angiosperms.     

FOLIAR INITIATION AND THE FATE OF THE DWARF-SHOOT APEX IN PINUS MONOPHYLLA

The dwarf-shoot apex of Pinus monophylla has a cytohistological zonation typical of pines. Cataphylls are initiated by this apex both before and after the dormant winter period. The needle is initiated in the spring of the same season in which it matures. The cytological details of cataphyll initiation are identical to that of needle initiation; however, the two primordia can be distinguished very early on the basis of their shape and direction of growth. After needle initiation the stem apex becomes inactive, is crowded to one side, and eventually disappears as the base of the needle enlarges. The results of the present work are discussed in relation to earlier conflicting interpretations regarding needle initiation and the subsequent existence of the dwarf-shoot apex of P. monophylla.     

RIBOSOMAL RNA GENE DIVERSITY,EFFECTIVE POPULATION SIZE,AND EVOLUTIONARY LONGEVITY IN ASEXUAL GLOMEROMYCOTA

Arbuscular mycorrhizal fungi (phylum Glomeromycota) are among the oldest and most successful symbionts of land plants. With no evidence of sexual reproduction, their evolutionary success is inconsistent with the prediction that asexual taxa are vulnerable to extinction due to accumulation of deleterious mutations. To explore why Glomeromycota defy this prediction, we studied ribosomal RNA (rRNA) gene evolution in the Claroideoglomus lineage and estimated effective population size, N_e, in C. etunicatum. We found that rRNA genes of these fungi exhibit unusual and complex patterns of molecular evolution. In C. etunicatum, these patterns can be collectively explained by an unexpectedly large N_e combined with imperfect genome‐wide and population‐level rRNA gene repeat homogenization. The mutations accumulated in rRNA gene sequences indicate that natural selection is effective at purging deleterious mutations in the Claroideoglomus lineage, which is also consistent with the large N_e of C. etunicatum. We propose that in the near absence of recombination, asexual reproduction involving massively multinucleate spores typical for Glomeromycota is responsible for the improved efficacy of selection relative to drift. We postulate that large effective population sizes contribute to the evolutionary longevity of Glomeromycota.     

DEMOGRAPHIC GENETICS OF A PIONEER TROPICAL TREE SPECIES: PATCH DYNAMICS,SEED DISPERSAL,AND SEED BANKS

We consider whether changes in population-genetic structure through the life cycle of Cecropia obtusifolia, a tropical pioneer tree, reflect its gap-dependent demography and the role of evolutionary processes that are important for this species. We asked whether the spatial scale at which population-genetic subdivision occurs corresponds to the scale of habitat patchiness created by gap dynamics; whether patterns of seed dispersal and storage in the soil affect spatial genetic patterns; and whether spatial genetic patterns change through the species life cycle. We estimated Wright's F-statistics for six successive life-history stages for individuals grouped into subpopulations according to occurrence in natural gaps, physical proximity, or occurrence within large quadrats. For each life stage, F_ST-statistics were significantly higher when individuals were grouped by gaps, although concordant patterns across life stages for the three grouping methods were obtained. This supports the hypothesis that patchy recruitment in gaps or among-gap heterogeneity influences the species' genetic structure. F-statistics of seeds collected from females before dispersal (tree seeds), seed-rain seeds, soil seeds, seedlings, juveniles, and adults grouped by gaps, were, respectively: F_IT = 0.004, 0.160, 0.121, 0.091, –0.0002, –0.081; F_IS = –0.032, 0.124, 0.118, 0.029, –0.016, –0.083; and F_ST = 0.035, 0.041, 0.003, 0.063, 0.015, 0.002. Spatial genetic differentiation in rain seeds was not significantly lower than that of tree seeds. The loss of genetic structure in the soil seed bank, relative to that found in the seed rain may be due to sampling artifacts, but alternative explanations, such as microsite selection or temporal Wahlund effect are also discussed. If structure among soil seeds is unbiased, the peak in seedling F_ST may be due to microsite selection. F_IS of seeds in the rain and soil were significantly greater than zero. A Wahlund effect is the most likely cause of these positive F_IS values. Such fine-scale substructuring could be caused by correlated seed deposition by frugivores. The decrease in F_IS from seedlings to adults could result from loss of fine-scale genetic structure during stand thinning or from selection.     

REPRODUCTIVE ISOLATION AND INTROGRESSION BETWEEN NOTROPIS CORNUTUS AND NOTROPIS CHRYSOCEPHALUS (FAMILY CYPRINIDAE): COMPARISON OF MORPHOLOGY,ALLOZYMES, AND MITOCHONDRIAL DNA

Hybrid zones in fluvial fishes may be heterogeneous from drainage to drainage. The comparison of data from morphology, allozymes, and mitochondrial DNA (mtDNA) indicates variability in the causes and degree of restriction of gene flow between Notropis cornutus and Notropis chrysocephalus. Allozyme marker loci show frequency-dependent introgression; i.e., the rarer species, whichever it is at a particular locality, tends to exhibit a higher proportion of introgressed alleles. Unlike allozymes, introgression of mtDNA haplotypes varies geographically. In westward-flowing Michigan drainages, N. cornutus mtDNA haplotypes are more common in F₁ hybrids and backcrosses, independent of parental frequencies. In eastward-flowing Michigan drainages, N. chrysocephalus mtDNA is more common in F₁ hybrids and backcrosses; this pattern may be due to local ecological effects or frequency-dependent introgression. Morphological data alone are not sufficient to distinguish all classes of hybrids. The lack of concordance of morphological, allozymic, and mtDNA introgression patterns implies operation of one or two factors: 1) geographically variable patterns of selection against different hybrid and backcross combinations or 2) genetic differences between Michigan populations inhabiting eastward- and westward-flowing drainage systems accumulated during historical isolation.     

COMPARATIVE FOLIAR HISTOGENESIS IN ACORUS CALAMUS AND ITS BEARING ON THE PHYLLODE THEORY OF MONOCOTYLEDONOUS LEAVES

A comparative histogenetic investigation of the unifacial foliage leaves of Acorus calamus L. (Araceae; Pothoideae) was initiated for the purposes of: (1) re-evaluating the previous sympodial interpretation of unifacial leaf development; (2) comparing the mode of histogenesis with that of the phyllode of Acacia in a re-examination of the phyllode theory of monocotyledonous leaves; and (3) specifying the histogenetic mechanisms responsible for morphological divergence of the leaf of Acorus from dorsiventral leaves of other Araceae. Leaves in Acorus are initiated in an orthodistichous phyllotaxis from alternate positions on the bilaterally symmetrical apical meristem. During each plastochron the shoot apex proceeds through a regular rhythm of expansion and reduction related to leaf and axillary meristem initiation and regeneration. The shoot apex has a three- to four-layered tunica and subjacent corpus with a distinctive cytohistological zonation evident to varying degrees during all phases of the plastochron. Leaf initiation is by periclinal division in the second through fourth layers of the meristem. Following inception early growth of the leaf primordium is erect, involving apical and intercalary growth in length as well as marginal growth in circumference in the sheathing leaf base. Early maturation of the leaf apex into an attenuated tip marks the end of apical growth, and subsequent growth in length is largely basal and intercalary. Marked radial growth is evident early in development and initially is mediated by a very active adaxial meristem; the median flattening of this leaf is related to accentuated activity of this meristematic zone. Differentiation of the secondary midrib begins along the center of the leaf axis and proceeds in an acropetal direction. Correlated with this centralized zone of tissue specialization is the first appearance of procambium in the center of the leaf axis. Subsequent radial expansion of the flattened upper leaf zone is bidirectional, proceeding by intercalary meristematic activity at both sides of the central midrib. Procambial differentiation is continuous and acropetal, and provascular strands are initiated in pairs in both sides of the primordium from derivatives of intercalary meristems in the abaxial and adaxial wings of the leaf. Comparative investigation of foliar histogenesis in different populations of Acorus from Wisconsin and Iowa reveals different degrees of apical and adaxial meristematic activity in primordia of these two collections: leaves with marked adaxial growth exhibit delayed and reduced expression of apical growth, whereas primordia with marked apical growth show, correspondingly, reduced adaxial meristematic activity at equivalent stages of development. Such variations in leaf histogenesis are correlated with marked differences in adult leaf anatomy in the respective populations and explain the reasons for the sympodial interpretation of leaf morphogenesis in Acorus and unifacial organs of other genera by previous investigators. It is concluded that leaf development in Acorus resembles that of the Acacia phyllode, thereby confirming from a developmental viewpoint the homology of these organs. Comparison of development with leaves of other Araceae indicates that the modified form of the leaf of Acorus originates through the accentuation of adaxial and abaxial meristematic activity which is expressed only slightly in the more conventional dorsiventral leaf types in the family.     

FOLIAR MORPHOLOGY AND ANATOMY OF THE GIGANTOPTERID PLANT DELNORTEA ABBOTTIAE,FROM THE LOWER PERMIAN OF WEST TEXAS

Delnortea is a monotypic genus (type-species: D. abbottiae) of Lower Permian gymnosperms based on leaves from uppermost Leonardian deltaic sediments exposed in the Del Norte Mountains, West Texas. The leaves are simple, symmetrical, mostly oblong or elliptical, and vary in length from 1.2 to about 35 cm. The petioles are short and stout, with a basally enlarged abscission zone. The margins are crenate, with a narrow, indurated border. Venation is in 4 orders: the secondaries and tertiaries are robust, unbranched, and pinnately arranged in a precise “herringbone” pattern, with the secondaries ending in the marginal sinuses; the quaternaries divide sparingly and fuse with others to form a dense reticulum of small meshes. Permineralized petiole and midrib material reflects a bifacial cambium, shown by a semicircular vascular arc, irregularly divided into several collateral bundles with secondary xylem and phloem. Delnortea is referable to the Gigantopteridaceae, a probably artificial family of gymnosperms incertae sedis with important venation features in common, but without known diagnostic reproductive organs. With Delnortea, the North American gigantopterids now include 5 genera, but Gigantopteris itself is lacking. Delnortea holds a relatively advanced evolutionary position among the American gigantopterids; its leaf morphology and gymnospermous anatomy entail intriguing points of comparison with Gnetum. The limited geographic and stratigraphic ranges and morphological distinctiveness of the American gigantopterids and associated taxa attest to rapid evolution and dispersal from a small area of origin in the southwestern United States during Leonardian time.     

THE FOLIAR AND FLORAL NECTARIES OF TURNERA ULMIFOLIA L.

The floral and foliar nectaries of Turnera ulmifolia are specialized and are representative of others found in the Turneraceae. The foliar and floral nectary systems must be treated independently. Foliar nectaries are organized into a definite structure (composed of a base, rim, secretory tissue, modified epidermis) and are supplied with vascular tissue composed of both xylem and phloem. Nectar from foliar nectaries contained equal concentrations of glucose, fructose, and sucrose. Floral nectaries are an integral part of the basal portion of each filament. The nectariferous tissue is not supplied with vascular tissue and secretion lasts only a few hours. Nectar from these staminal nectaries yielded a sucrose-dominant nectar containing also fructose, glucose, an unknown, and a trace amount of melezitose. Observations of flowering confirmed the reported short duration of the individual flowers.     

COMPARISONS OF NITRATE UPTAKE, STORAGE, AND REDUCTION IN MARINE DIATOMS AND FLAGELLATES

Diatoms, but not flagellates, have been shown to increase rates of nitrogen release after a shift from a low growth irradiance to a much higher experimental irradiance. We compared NO₃ ^? uptake kinetics, internal inorganic nitrogen storage, and the temperature dependence of the NO₃ ^? reduction enzymes, nitrate (NR) and nitrite reductase (NiR), in nitrogen‐replete cultures of 3 diatoms (Chaetoceros sp., Skeletonema costatum, Thalassiosira weissflogii) and 3 flagellates (Dunaliella tertiolecta, Pavlova lutheri, Prorocentrum minimum) to provide insight into the differences in nitrogen release patterns observed between these species. At NO₃ ^? concentrations <40 μmol‐N·L ^{? 1}, all the diatom species and the dinoflagellate P. minimum exhibited saturating kinetics, whereas the other flagellates, D. tertiolecta and P. lutheri, did not saturate, leading to very high estimated K _s values. Above ～60 μmol‐N·L ^{? 1}, NO₃ ^? uptake rates of all species tested continued to increase in a linear fashion. Rates of NO₃ ^? uptake at 40 μmol‐N·L ^{? 1}, normalized to cellular nitrogen, carbon, cell number, and surface area, were generally greater for diatoms than flagellates. Diatoms stored significant amounts of NO₃ ^? internally, whereas the flagellate species stored significant amounts of NH₄ ⁺ . Half‐saturation concentrations for NR and NiR were similar between all species, but diatoms had significantly lower temperature optima for NR and NiR than did the flagellates tested in most cases. Relative to calculated biosynthetic demands, diatoms were found to have greater NO₃ ^? uptake and NO₃ ^? reduction rates than flagellates. This enhanced capacity for NO₃ ^? uptake and reduction along with the lower optimum temperature for enzyme activity could explain differences in nitrogen release patterns between diatoms and flagellates after an increase in irradiance.     

FOLIAR ONTOGENY AND HISTOGENESIS IN MAGNOLIA GRANDIFLORA L. I. APICAL ORGANIZATION AND EARLY DEVELOPMENT

Foliar ontogeny of Magnolia grandiflora was studied to elucidate possible unique features of evergreen leaves and their development. The apex of Magnolia grandiflora is composed of a biseriate or triseriate tunica overlying a central initial zone, a peripheral zone and a pith rib meristem. Leaf primordia are initiated by periclinal divisions on the apical flank of the tunica in its second layer. This initiation and expansion is seasonal just as in related deciduous magnolias. Following leaf initiation, a foliar buttress is formed and the leaf base gradually extends around the apex. As growth continues, separation of the leaf blade primordium from the stipule proceeds by intensified anticlinal divisions in the surface and subsurface layers near the base. Marginal growth begins in the blade primordium when it reaches approximately 200 μm in height and results in the formation of two wing-like extensions, the lamina. This young blade remains in a conduplicately folded position next to the stipule until bud break.     

UPTAKE AND TRANSLOCATION OF TI FROM NANOPARTICLES IN CROPS AND WETLAND PLANTS

Bioavailability of engineered metal nanoparticles affects uptake in plants, impacts on ecosystems, and phytoremediation. We studied uptake and translocation of Ti in plants when the main source of this metal was TiO₂ nanoparticles. Two crops (Phaseolus vulgaris (bean) and Triticum aestivum (wheat)), a wetland species (Rumex crispus, curly dock), and the floating aquatic plant (Elodea canadensis, Canadian waterweed), were grown in nutrient solutions with TiO₂ nanoparticles (0, 6, 18 mmol Ti L^?1 for P. vulgaris, T. aestivum, and R. crispus; and 0 and 12 mmol Ti L^?1 for E. canadensis). Also examined in E. canadensis was the influence of TiO₂ nanoparticles upon the uptake of Fe, Mn, and Mg, and the influence of P on Ti uptake. For the rooted plants, exposure to TiO₂ nanoparticles did not affect biomass production, but significantly increased root Ti sorption and uptake. R. crispus showed translocation of Ti into the shoots. E. canadensis also showed significant uptake of Ti, P in the nutrient solution significantly decreased Ti uptake, and the uptake patterns of Mn and Mg were altered. Ti from nano-Ti was bioavailable to plants, thus showing the potential for cycling in ecosystems and for phytoremediation, particularly where water is the main carrier.     

CORRELATION OF PAIRWISE GENETIC AND GEOGRAPHIC DISTANCE MEASURES: INFERRING THE RELATIVE INFLUENCES OF GENE FLOW AND DRIFT ON THE DISTRIBUTION OF GENETIC VARIABILITY

Attempts to relate estimates of regional F_ST to gene flow and drift via Wright's (1931) equation F_ST ≈ 1/ (4Nm + 1) are often inappropriate because most natural sets of populations probably are not at equilibrium (McCauley 1993), as assumed by the island model upon which the equation is based, or ineffective because the influences of gene flow and drift are confounded in the product Nm. Evaluations of the association between genetic (F_ST) and geographic distances separating all pairwise populations combinations in a region allows one to test for regional equilibrium, to evaluate the relative influences of gene flow and drift on population structure both within and between regions, and to visualize the behavior of the association across all degrees of geographic separation. Tests of the model using microsatellite data from 51 populations of eastern collared lizards (Crotaphytus collaris collaris) collected from four distinct geographical regions gave results highly consistent with predicted patterns of association based on regional differences in various historical and ecological factors that affect the amount of drift and gene flow. The model provides a prerequisite for and an alternative to regional F_ST analyses, which often simply assume regional equilibrium, thus potentially leading to erroneous and misleading inferences regarding regional population structure.     

GENETIC DRIFT AND COLLECTIVE DISPERSAL CAN RESULT IN CHAOTIC GENETIC PATCHINESS

Chaotic genetic patchiness denotes unexpected patterns of genetic differentiation that are observed at a fine scale and are not stable in time. These patterns have been described in marine species with free‐living larvae, but are unexpected because they occur at a scale below the dispersal range of pelagic larvae. At the scale where most larvae are immigrants, theory predicts spatially homogeneous, temporally stable genetic variation. Empirical studies have suggested that genetic drift interacts with complex dispersal patterns to create chaotic genetic patchiness. Here we use a coancestry model and individual‐based simulations to test this idea. We found that chaotic genetic patterns (qualified by global F_ST and spatio‐temporal variation in F_ST's between pairs of samples) arise from the combined effects of (1) genetic drift created by the small local effective population sizes of the sessile phase and variance in contribution among breeding groups and (2) collective dispersal of related individuals in the larval phase. Simulations show that patchiness levels qualitatively comparable to empirical results can be produced by a combination of strong variance in reproductive success and mild collective dispersal. These results call for empirical studies of the effective number of breeders producing larval cohorts, and population genetics at the larval stage.     

THE TEMPORAL ORGANIZATION OF DAILY TORPOR AND HIBERNATION: CIRCADIAN AND CIRCANNUAL RHYTHMS

Mammals and birds have evolved the ability to maintain a high and constant body temperature T_b over a wide range of ambient temperatures T_a using endogenous heat production. In many, especially small endotherms, cost for thermoregulatory heat production can exceed available energy; to overcome these energetic bottlenecks, they enter a state of torpor (a regulated reduction of T_b and metabolic rate). Since the occurrence of torpor in many species is a seasonal event and occurs at certain times of the day, we review whether circadian and circannual rhythms, important in the timing of biological events in active animals, also play an important role during torpor when T_b is reduced substantially and may even fall below 0°C. The two distinct patterns of torpor, hibernation (prolonged torpor) and daily torpor, differ substantially in their interaction with the circadian system. Daily torpor appears to be integrated into the normal circadian rhythm of activity and rest, although torpor is not restricted only to the normal rest phase of an animal. In contrast, hibernation can last for several days or even weeks, although torpor never spans the entire hibernation season, but is interrupted by periodic arousals and brief normothermic periods. Clearly, a day is no longer divided in activity and rest, and at first glance the role of the circadian system appears negligible. However, in several hibernators, arousals not only follow a regular pattern consistent with a circadian rhythm, but also are entrainable by external stimuli such as photoperiod and T_a. The extent of the interaction between the circadian and circannual system and hibernation varies among species. Biological rhythms of hibernators for which food availability appears to be predictable seasonally and that hibernate in deep and sealed burrows show little sensitivity to external stimuli during hibernation and hence little entrainability of arousal events. In contrast, opportunistic hibernators, which some times use arousals for foraging and hibernate in open and accessible hibernacula, are susceptible to external zeitgebers. In opportunistic hibernators, the circadian system plays a major role in maintaining synchrony between the normal day-night cycle and occasional foraging. Although the daily routine of activity and rest is abandoned during hibernation, the circadian system appears to remain functional, and there is little evidence it is significantly affected by low T_b. (Chronobiology International, 17(2), 103–128, 2000)     

PHOTOSYNTHESIS AND CARBON ALLOCATION IN TIPULARIA DISCOLOR (ORCHIDACEAE), A WINTERGREEN UNDERSTORY HERB

Seasonal patterns of photosynthesis and carbon allocation were determined for Tipularia discolor, a summer-deciduous wintergreen orchid of the southeastern United States, to assess the effects of environmental conditions and leaf age on carbon acquisition and allocation patterns. There was no shift in the optimum temperature for photosynthesis (T_opt) on a seasonal basis and T_opt (≈26 C) was at least 10 C higher than daily maximum air temperature during most of the growing season. Lack of photosynthetic adjustment in Tipularia to seasonal fluctuations in temperature and light suggested that the photosynthetic characteristics of this wintergreen were more similar to those of spring ephemerals than to those of evergreens and summer-active herbs. The decline in photosynthetic capacity during the winter growing season for Tipularia, largely due to leaf age effects, gradually reduced net photosynthetic rates in the field despite more favorable light and temperature conditions. Photosynthesis in the field was primarily limited by environmental conditions in early- and mid-season and by photosynthetic capacity in late-season. A ¹⁴CO₂ labelling experiment demonstrated that patterns of carbon allocation to vegetative structures were affected by the season of photosynthetic carbon fixation, whereas reproductive structures received 21% of the recovered labelled carbon regardless of the period of labelling. Carbon acquired and stored during all periods of the growing season was used to produce new vegetative and reproductive structures.     

STOMATAL PENETRATION: EFFECT OF SURFACTANTS AND ROLE IN FOLIAR ABSORPTION

Stomatal penetration of aqueous solutions into pear (Pyrus communis L. cv. Bartlett) leaves was induced with surfactants. The effectiveness of surfactants in promoting stomatal penetration was related to their effectiveness in decreasing surface tension. Vatsol OT (dioctyl ester of sodium sulfosuccinic acid) was most effective, X-77 (compounded product of alkyl aryl polyethoxy ethanol, free fatty acids and isopropanol) intermediate, and Tween 20 (polyoxyethylene sorbitan monolaurate) least effective. The percentage of stomata penetrated was limited, ranging from 0.5–4.5 %. There was no evidence of stomatal penetration of aqueous solutions with surface tensions of approximately 70 dyne cm⁻¹. The importance of stomatal penetration in foliar absorption under conditions of plant culture is discussed.     

EFFECT OF CYSTEINE AND METHIONINE ON SULFATE UPTAKE AND PRIMARY PRODUCTIVITY BY AXENIC ALGAL CULTURES AND LAKE MICROPLANKTON COMMUNITIES1

The presence of up to 500 μg sulfur·l^?1 of an equimolar mixture of cysteine and methionine had virtually no effect on the SO₄^2- uptake rate of Navicula pelliculosa, (Bréb.) Hilse whereas the rate of Ankistrodesmus falcatus (Corda) Ralfs was decreased by the presence of 500 μg S· l^?1 and Anabaena flos-aquae (Lyngbye) Bréb. by 50 μg S·l^?1. Primary productivity in these axenic cultures was affected (decreased) only in A. falcatus. The C:S uptake ratio was lowest in N. pelliculosa and highest in A. falcatus. Considering these species as representative of groups of naturally occurring algae, patterns of SO₄^2- uptake and primary productivity in a eutrophic and a moderately oligotrophic lake reflected the results of the algal culturing experiments: SO₄^2- uptake rates, relative to primary productivity, were higher in the presence of diatoms and bluegreen algae and lower when green algae were present; the addition of the cysteine I methionine mixture to the lake waters decreased the rate of microplankton SO₄^2- uptake in correlation with the makeup of the algal community; primary productivity decreased upon the addition of cysteine I methionine when green algae were relatively abundant. It is concluded that, in most fresh water systems, the effects of organic sulfur pollution on algal SO₄^2- uptake and primary productivity are insignificant as compared to other ecological changes that occur due to that pollution.     

DEVELOPMENT OF FOLIAR DIAPHRAGMS IN SPARGANIUM EURYCARPUM

Three types of diaphragms are produced in regular sequence by the basal intercalary meristem in the leaf of Sparganium eurycarpum Engelm. (Sparganiaceae). They bridge compartments formed by the collapse and disintegration of rib meristem derivatives. The adaptive nature of diaphragms, intercalary meristems, and linear photosynthetic organs is considered for emergent aquatic plants.     

LANDSCAPE GENOMICS OF POPULUS TRICHOCARPA: THE ROLE OF HYBRIDIZATION,LIMITED GENE FLOW,AND NATURAL SELECTION IN SHAPING PATTERNS OF POPULATION STRUCTURE

Populus trichocarpa is an ecologically important tree across western North America. We used a large population sample of 498 accessions over a wide geographical area genotyped with a 34K Populus SNP array to quantify geographical patterns of genetic variation in this species (landscape genomics). We present evidence that three processes contribute to the observed patterns: (1) introgression from the sister species P. balsamifera, (2) isolation by distance (IBD), and (3) natural selection. Introgression was detected only at the margins of the species’ distribution. IBD was significant across the sampled area as a whole, but no evidence of restricted gene flow was detected in a core of drainages from southern British Columbia (BC). We identified a large number of F_ST outliers. Gene Ontology analyses revealed that F_ST outliers are overrepresented in genes involved in circadian rhythm and response to red/far‐red light when the entire dataset is considered, whereas in southern BC heat response genes are overrepresented. We also identified strong correlations between geoclimate variables and allele frequencies at F_ST outlier loci that provide clues regarding the selective pressures acting at these loci.