LIGHT EFFECTS ON LEAF MORPHOLOGY IN WATER HYACINTH (EICHHORNIA CRASSIPES)

Water hyacinth leaves in natural populations vary from being long and thin-petioled to being short with inflated petioles. A variety of factors has been used experimentally to alter water hyacinth leaf shape, but what controls the development of leaf morphology in the field has not been established. We measured photosynthetic photon flux density (PPFD) and spectral distribution of radiation in a natural water hyacinth population. PPFD in the center of the water hyacinth mat was reduced to 2.7% of full sunlight, and the red to far red (R:FR) ratio was reduced to 0.28. When shoot tips of plants were exposed to artificial light environments, only plants in the treatment with a R:FR ratio comparable to that in the natural population produced leaves with long, thin petioles. Shoot tips in full sun or covered with clear plastic bags or bags that reduced light quantity without greatly altering light quality produced shorter leaves with inflated petioles. We hypothesize that the altered light quality inside a mat is a major environmental control of water hyacinth leaf morphology.     

THE DEVELOPMENTAL BASIS OF TRISTYLY IN EICHHORNIA PANICULATA (PONTEDERIACEAE)

Eichhornia paniculata is a tristylous, self-compatible, emergent aquatic. A given plant produces flowers with either long, mid or short styles and two levels of stamens equal in length to the styles not found in that flower. Flowers of each morph have two whorls of three tepals, six stamens and three fused carpels. The six stamens differentiate into two sets of three stamens each. A relatively short set, having either short- or mid-level stamens, occurs on the upper side of the flower, while a relatively long set, having either mid- or long-level stamens, occurs on the lower side. Stamen level depends on differences among stamens in filament length and position of insertion on the floral tube. Floral parts arise in whorls of three, but the two stamen whorls do not form the two sets of stamens found in each mature flower. Instead, stamens from both whorls make up a given set. Floral differences among morphs are not present at flower origin or floral organ initiation. Morphological differences arise first among stamen sets. The two sets within a flower differ prior to meiosis in the size, number, and timing of comparable developmental events in the sporogenous cells. After these initial differences arise, anther size diverges. In later developmental stages differences in filament and floral tube length, cell size, and cell number, as well as differences in the length, cell size, and cell number of styles, develop among morphs. This sequence of developmental events suggests that the genes controlling development in different morphs do not control flower and floral organ initiation but are first morphologically visible in sporogenous cell differentiation.     

VARIATION IN THE MATING SYSTEM OF EICHHORNIA PANICULATA (SPRENG.) SOLMS. (PONTEDERIACEAE)

A multilocus procedure was used to estimate outcrossing rates from allozyme data in nine populations of Eichhornia paniculata from NE Brazil and Jamaica. The populations were chosen to represent stages in a proposed model of the evolutionary breakdown of tristyly to semi-homostyly; they differed in morph structure (trimorphic, dimorphic, or monomorphic) and floral traits likely to influence the mating system. The interpopulation range in outcrossing rate, t, was 0.96–0.29. Two additional populations from Jamaica, composed exclusively of self-pollinating, semi-homostylous, mid-styled plants, were invariant at 21 isozyme loci, precluding estimates of outcrossing frequency. Trimorphic populations from Brazil had uniformly high outcrossing rates of 0.96–0.88. Values for the floral morphs within populations were not significantly different. A controlled pollination experiment, comparing the competitive ability of self and cross pollen using the Got-3 marker locus, provided evidence that the maintenance of high outcrossing rates in trimorphic populations results from the prepotency of cross pollen and/or the selective abortion of selfed zygotes. Morph-dependent variation in t was detected within a dimorphic population with the L morph outcrossing with a frequency of 0.76 in comparison with 0.36 in the M morph. The difference in the mating system of floral morphs results from modifications in position of short-level stamens in flowers of the M morph resulting in automatic self-pollination. The occurrence of E. paniculata populations composed exclusively of self-pollinating, mid-styled variants is thought to be associated with the spread of genes modifying stamen position. The high level of self-fertilization demonstrated in the M morph would allow automatic selection of these genes, augmented by fertility assurance in the absence of specialized pollinators.     

THE DEVELOPMENT OF SPATIAL PATTERN IN CLONES OF AN AQUATIC PLANT,EICHHORNIA CRASSIPES SOLMS

The spatial geometry of clonal growth of Eichhornia crassipes, a plant which increases in number by the vegetative production of stolon offshoots (or ramets), was assessed by the use of a vector analysis. Growth of water hyacinth clones was found to be asymmetric, asymmetry increasing with time. The direction of asymmetry was independent of either population density or the position of nearest neighbors. The analysis revealed the importance of geometric relationships within the parental growth axis for determining patterns of clonal spread. Meristem growth and differentiation appear to be controlled so as to maximize clonal growth rate. This contrasts with the patterns observed in rhizomatous terrestrial species in which the control of meristem growth and differentiation appears to result in efficient use of space at the expense of greater increase in clonal size.     

TRANSPORT OF CARBON AMONG CONNECTED RAMETS OF EICHHORNIA CRASSIPES (PONTEDERIACEAE) AT NORMAL AND HIGH LEVELS OF CO2

The floating, stoloniferous plant, Eichhornia crassipes, has high rates of productivity and rapidly invades new sites. Because the transport of carbon among connected ramets is known to increase the growth of clonal plants, we asked whether there is intraclonal carbon transport in E. crassipes. Because net photosynthesis of E. crassipes is significantly higher at high levels of atmospheric CO₂, we also asked if high CO₂ can change patterns of carbon transport in ways that might modify clonal growth. We exposed individual ramets within groups of connected ramets to ¹⁴CO₂ for 15–45 min and measured the distribution of ¹⁴C in the group after 4 days of growth at 350, 700, 1,400, or 2,800 μ1 1^−-1 CO₂. At 350 μ1 1^−-1 CO₂, a parent ramet exported approximately 10% of the ¹⁴C that it assimilated to its first rooted offspring ramet. The offspring exported a similar percentage of the ^l4C it assimilated toward the parent; two-thirds of this ¹⁴C was retained by the parent, and one-third moved into new offspring of the parent. In all ramets, imported carbon moved into leaves as well as roots. At the higher levels of CO₂, the percentage of assimilated carbon exported from a parent ramet to the leaf blades of its first offspring was lower by half. High CO₂ had little other effect on carbon transport. E. crassipes maintains bidirectional transport of carbon between ramets even under uniform and favorable environmental conditions and when external CO₂ levels are very high.     

THE DISSOLUTION OF A COMPLEX GENETIC POLYMORPHISM: THE EVOLUTION OF SELF-FERTILIZATION IN TRISTYLOUS EICHHORNIA PANICULATA (PONTEDERIACEAE)

Eichhornia paniculata (Pontederiaceae) displays a wide range of outcrossing levels as a result of the dissolution of the tristylous genetic polymorphism and the evolution of semihomostyly. Population surveys, comparison of fitness components of the style morphs, and computer simulations were used to investigate the breakdown of tristyly and the selective mechanisms responsible for the evolution of self-fertilization. Of 110 populations surveyed in northeast Brazil and Jamaica, 53% were trimorphic, 25% were dimorphic, and 22% were monomorphic for style morph. The short (S) morph was underrepresented in trimorphic populations and absent from nontrimorphic populations. The mid (M) morph predominated in dimorphic populations and was the only morph in monomorphic populations. Stamen modifications promoting selfing, associated with semihomostyle evolution, were largely confined to the M morph. They were rare in trimorphic populations, common in dimorphic populations, and often fixed in monomorphic populations. Stochastic simulations and comparisons of fruit set in natural populations indicate that founder events, population bottlenecks, and lowered fertility of the S morph due to an absence of long-tongued pollinators can each account for loss of the S morph from trimorphic populations. A reduced level of disassortative mating can accentuate the rate at which the S morph is lost by both random and deterministic processes. Nontrimorphic populations occur at the geographical margins of the region surveyed and tend to be smaller and less dense than trimorphic populations. These observations and the higher fruit set of the M morph relative to the L morph in dimorphic populations suggest that reproductive assurance, favoring selfing variants of the M morph under conditions of low pollinator service, has been of primary importance in the origin of most monomorphic populations. Where pollinator service is reliable, however, automatic selection of selfing genes, aided by mating asymmetries between the morphs, can cause the M morph to spread to fixation in dimorphic populations.     

SOME CASES OF DELAYED OR INDUCED DEVELOPMENT OF AXILLARY BUDS FROM PERSISTING DETACHED MERISTEMS IN CONIFERS

In the apparently “empty” axils of the needles of Taxus baccata, Sequoia sempervirens, Sequoiadendron giganteum, Cryptomeria japonica, Thuja occidentalis, and Thujopsis dolabrata persisting detached meristems were found, which are derived from superficial layers of the apical eumeristem. In T. baccata delayed development of minute axillary buds occurs from these meristems after 1–4 yr on the intact plant. In the other conifers, development of additional axillary buds from these meristems was induced by natural frost damage or by artificial pruning and disbudding. The discovery of these detached meristems is discussed with regard to the regenerative capacity of the conifers in comparison to other plants.     

DEVELOPMENTAL POTENTIAL OF EXCISED PRIMORDIAL AND EXPANDING LEAVES OF COLEUS BLUMEI BENTH.

Coleus blumei Benth. primordial leaves 1 through 4 and expanding leaves 5 to 8 were isolated and cultured to examine the effects of auxin and kinetin on development. Without the plant growth regulators in the medium, expanding leaves 7 and 8 developed as leaves; younger leaf primordia did not develop. With 0.01 to 5.0 mg/1 IAA, 2–7% of the youngest pair of primordial leaves (1 and 2) developed as roots. Small leaf blade development occurred on IAA at 0.5 to 5.0 mg/1 with 10–12% of the explants, and shoots developed from 2% of the youngest primordia explants at 3 mg/1 IAA. With 2–28% of the second set of primordial leaves (3 and 4), a leaf with a root developed with 0.01 to 5.0 mg/1 IAA. At 3.0 mg/1 IAA, in addition to leaf formation, 2% of the explants formed a rosette of leaves and 1% formed a shoot. With the highest level of IAA (5 mg/1), 2% of the explants formed a root. Expanding leaves 5 through 8 developed mostly into leaves without petioles on IAA and kinetin. Plant development occurred from 2% of the youngest primordial leaves on 0.03 mg/1 kinetin; otherwise, these primordia on 0.003 to 2 mg/1 kinetin developed into abnormal leaves. Primordia 3 and 4 developed into normal appearing leaves at levels of kinetin between 0.03 and 2 mg/1. At lower levels the leaves were abnormal.     

EFFECT OF WATER POTENTIAL ON A MICROCOLEUS (CYANOPHYCEAE) FROM A DESERT CRUST1

The effect of water potential, on the growth and photosynthesis of a species of Microcoleus forming a desert crust was determined, using both osmotic and matric variations in water potential. The alga was quite sensitive to moisture stress, partial inhibition of growth being observed at -7 bars, and complete inhibition at -18 bars. Photosynthesis was markedly inhibited at -18 bars, and virtually completely at, -28 bars (water potential of seawater) and lower. The alga was more sensitive to matric reduction in water potential than osmotic. By comparisons of these results with those obtained with other algae, it is concluded that this desert crust alga is not especially adapted to grow and photosynthesize at low water potentials, although it shows considerable ability to survive severe drought conditions.     

INFLUENCE OF IRRADIATION,SOIL WATER POTENTIAL,AND LEAF TEMPERATURE ON LEAF MORPHOLOGY OF A DESERT BROADLEAF,ENCELIA FARINOSA GRAY (COMPOSITAE)

Laboratory experiments were performed to evaluate observed seasonal changes in leaf morphology of the desert perennial shrub, Encelia farinosa Gray. Plants were grown under low or high conditions of photosynthetically active irradiation, soil water potential (Ψ^soil), and leaf temperature (8 different experimental regimes). The relative growth rate, leaf water vapor conductance, leaf water potential, and leaf length were all greater for the high Ψ^soil regimes, the largest leaves occurring at low irradiation. High irradiation during growth led to thicker leaves with a higher internal to external leaf area ratio (A^mes/A); low Ψ^soil tended to increase A^mes/A somewhat. High irradiation also led to decreased absorptance to solar irradiation caused by increased pubescence. High leaf temperature during development resulted in slightly smaller, thicker leaves with higher A^mes/A. Thus, irradiation appeared to have its major influence on leaf thickness, A^mes/A, and absorptance, with a secondary effect on leaf length; Ψ^soil affected primarily leaf length, growth rate, and water status, and secondarily A^mes/A. Results are discussed with regard to recent ecophysiological studies on the observed seasonal changes in leaf morphology of E. farinosa.     

CONTROL OF IN VITRO SEPAL DEVELOPMENT OF EXCISED FLORAL BUDS OF AQUILEGIA (RANUNCULACEAE)

Excised floral buds of Aquilegia formosa Fisch. were grown on a coconut-milk medium containing the minerals and vitamins of Murashige and Skoog, sucrose, and kinetin. The plant growth regulators indoleacetic acid (IAA, 0.5 mg/liter) and gibberellic acid (GA, 2.0 mg/liter) were added singly and in combinations; both were omitted from the control medium. The addition of GA to the basal medium was required to support sepal development on flowers at all phases of development. The formation of stomatal complexes in the epidermis of the sepals occurred only in the presence of GA. Sepals grown in the presence of GA also contained trichoblasts and developing trichomes, while none were formed in the absence of GA. The role of IAA in the development of these idioblasts was not clear but it appeared to have no effect. The hormones GA and IAA had different effects on the growth of the sepals. In the presence of GA the sepals increase in length until comparable with sepals grown in vivo. However, the sepals remained small when GA was omitted from the medium. Upon closer examination of this effect, it was determined that there was a direct proportionality between an increase in cell number in the epidermis and an increase in sepal length. The role of the two hormones in increasing epidermal cell length in sepals was distinct and separate. Exogenous IAA had no effect upon cell division but was required for cell elongation, while GA was required for cell division and had no effect on cell elongation. The GA effect in promoting cell division in the sepals was substantiated by use of autoradiography. If the buds were grown on media with GA, twice as many epidermal cells along the central file incorporated significant amounts of tritiated thymidine. The cell cycle of the epidermal cells of the GA-treated sepals was ca. 8.7 hr in duration and ca. 13.0 hr if GA was deleted from the medium.     

MORPHOLOGY,SEASONAL VARIATION,AND FUNCTION OF RESIN GLANDS ON BUDS AND LEAVES OF POPULUS DELTOIDES (SALICACEAE)

When buds form in summer or early fall, modified stipules act as bud scales and their adaxial epidermis secretes a resin that fills the bud. This secretory layer collapses in the dormant bud. Immature leaves, stipules, and leaf primordia occupy the center of the bud; all lack functional resin glands. In spring, stipules of emerging leaves develop an adaxial palisadelike secretory epidermis that becomes more ridged longitudinally in successive stipules. Marginal teeth of the first leaves to emerge are covered with trichomes and lack a secretory epidermis. In successive leaves the teeth become glandular and secrete resin as the lamina unrolls. Later in the season, marginal leaf glands account for much of the resin. Unspecialized hydathodes or extrafloral nectaries occur proximal to each glandular tip. Guttation of water or nectar occurs here through stomata located above a vein ending. On the basis of field observations and a laboratory feeding experiment, the resin seems to function mainly as an insect repellent. It may also reduce water loss from young leaves.     

Water Relations of Tropical Epiphytes: I. RELATIONSHIPS BETWEEN STOMATAL RESISTANCE, RELATIVE WATER CONTENT AND THE COMPONENTS OF WATER POTENTIAL

The water relations of five species of tropical vascular epiphytesnative to Malaysia were studied. The species were ferns: Pyrrosiaadnascens (Forst.) Ching. and Pyrrosia angustata (Sw.) Ching.;orchids: Eria velutina Lindl., Dendrobium tortile Lindl. andDendrobium crumenatum Sw. Leaf resistance as a function of leafwater potential was measured for the two ferns. The criticalwater potential at which stomata closed was found to be highin each case; –0.75 MPa and –0.5 MPa respectively.The components of water potential were estimated with the pressurechamber as functions of relative water content. For each speciescell sap was found to be dilute, pressure potential low at fullturgor, and the change in relative water content between fullturgor and wilting point small. Small values of solute potentialat full turgor were also found for the ferns and E. velutinausing a vapour pressure osmometer. Values of the bulk modulusof elasticity of the leaf tissue for each species lay withinthe range of published data. The significance of these resultsfor the epiphytic way of life is discussed. Key words: Water potential, Epiphytes, Diffusive resistance, Orchid, Fern