PHOTOPERIODIC ADAPTATION TO LATITUDE IN XANTHIUM STRUMARIUM

Photoperiodic responses of collections of Xanthium strumarium L., grown from seed obtained in nature at various localities in North America between latitudes 20° and 45.5° N, were examined. The critical night length was found to vary noticeably with latitude of origin, from about 7.5 hr in the northernmost strains to above 10 hr in the southern strains. These differences are considered to represent genetic adaptation of the reproductive system to environmental variables as a result of natural selection. Several strains showed a quantitative short day response rather than the more usual qualitative response. Strains from Hawaii exhibited a surprisingly tardy and erratic short-day response with a critical night length of about 11 hr, which may be an adaptation to a tropical climate.     

THE QUANTITATIVE ULTRASTRUCTURE OF THE MERISTEMATIC CELLS OF XANTHIUM STRUMARIUM DURING THE TRANSITION TO FLOWERING

Ultrastructural changes occurring in the central part of the apical meristem of the SDP Xanthium strumarium, induced to flower by a single 16-hr long night, were quantitatively investigated using stereological methods and compared to the changes previously reported in other species, particularly the LDP Sinapis alba. Changes detected in Xanthium, which are also found in other species, included: increase in cellular, cytoplasmic, cytoplasmic matrix and nucleolar sizes, change in nucleolar structure; increase in mitochondrial number and chondriome size, increase in dictyosome number. These changes are believed to be essential for floral evocation because of their universality. Other changes were specific to Xanthium and not detected in Sinapis. Accordingly, they were thought to be accompanying nonessential events of floral evocation in Xanthium. These changes included an increase in the number of plastid profiles and in plastidome size. The size of the nucleus, chromatin and vacuolar apparatus, as well as the number of vacuolar profiles, did not change in Xanthium, contrary to what was observed in other plants.     

VARIATION AMONG POPULATIONS OF XANTHIUM STRUMARIUM (COMPOSITAE) FROM NATURAL AND RUDERAL HABITATS

We assessed the mechanisms underlying the ability of cocklebur to spread from its natural riverside habitats and establish weedy populations in urban waste areas. We collected fruits from plants growing in natural and urban ruderal habitats and planted 2 maternal families from each of 9 habitat populations in 3 experimental gardens. The gardens were all in full sunlight but differed in the availability of water and nutrient resources. Plant performance in the gardens was measured by numbers and size of fruits produced. Traits known to be associated with cocklebur reproductive success were also measured: times to emergence and anthesis, photosynthetic capacity, mean stomatal conductance, relative growth rate, and biomass allocation to leaves and stems. Although there were significant differences among populations in the tradeoff between rate of growth in height and timing of anthesis, these population differences were not associated with habitat. Apart from a tendency to produce larger fruits in ruderal populations, there were no detectable differences in the characteristics of plants from natural vs. ruderal habitats. Plants from both habitats did have substantial and significant plastic responses to growth environment. In the three experimental gardens, fruit numbers increased with resource availability but fruit size did not differ significantly. As resource availability increased, plants from both habitats sustained growth longer and became more branchy. Canonical discriminant analysis of all the somatic and reproductive traits together supported the idea that natural and ruderal populations do differ in their overall plastic response to resource availability. The subtly different plastic responses of plants from the two habitats do not arise by substantial adjustments in a few dominant traits, but instead by relatively minor adjustments in a host of functionally interrelated phenological, morphological, and physiological traits. It is these small but coordinated differences in the plastic responses of many traits that appear to differentiate cocklebur from natural vs. weedy urban habitats.     

CHLOROPHYLL LEVELS AND LEAF ULTRASTRUCTURE AS ECOTYPIC CHARACTERS IN XANTHIUM STRUMARIUM L.

Comparison of six different populations of Xanthium strumarium grown under controlled laboratory conditions revealed a general pattern of greater chlorophyll levels with increase in latitude of origin. Indications of ultrastructural differences were found, with plants containing greater chlorophyll levels having more chloroplasts with greater membrane development compared to plants of this species from more southern or lower latitude habitats.     

EXPERIMENTAL HYBRIDIZATION OF XANTHIUM STRUMARIUM (COMPOSITAE) FROM ASIA AND AMERICA. II. SESQUITERPENE LACTONES OF F1 HYBRIDS

Effects of hybridization on sesquiterpene lactones in Xanthium strumarium were studied for clues to the relationship of Old and New World populations. In crosses between indigenous Asiatic plants in the “strumarium” morphological complex and various American complexes that produce xanthinin as a major sesquiterpene lactone, the F₁ hybrids contained xanthinin and the related compounds, xanthanol, xanthatin, and xanthinosin. In other crosses with various American complexes that produce xanthumin, the stereoisomer of xanthinin, the F₁ hybrids contained xanthinin and xanthumin as well as xanthinin-related compounds and their stereoisomers, xanthumanol, deacetoxylxanthumin, and tomentosin. The Asiatic plants of “strumarium” from Hong Kong involved in the crosses produce approximately equal percentages of xanthinin, xanthatin and xanthinosin, but those from India contain only xanthinosin. The putative introduction of the American morphological complex, “chinense,” contains xanthumin as the major component but shows chemical diversity that indicates genes derived from the indigenous “strumarium” complex.     

THE EARLY ACTION OF THE FLORAL STIMULUS ON MITOTIC ACTIVITY AND DNA SYNTHESIS IN THE APICAL MERISTEM OF XANTHIUM STRUMARIUM

Vegetative plants of Xanthium strumarium (a short-day species) were induced to flower by exposure to a single 16-hr long night. By cutting off the induced leaf (half-expanded leaf) at various times, it was established that, by 8 hr after the end of the long night, a sufficient amount of floral stimulus had reached the meristem to induce a flowering response. The following sequence of events occurred in both the peripheral and central zones of the apical meristem of induced plants: 1) a rise in the mitotic index beginning at 28 hr after the end of the long night and culminating at 36 and 56 hr; 2) a stimulation of DNA synthesis starting at 32–36 hr and reaching a maximum at 60 hr; 3) an increase in nucleolus diameter starting at 32 hr. The cell population in the meristems of both vegetative and induced plants displayed a similar distribution, with about 80 % of the nuclei with the 2C amount of DNA. The comparison of the kinetic data concerning the mitotic index and DNA synthesis indicated that one of the early effects of the floral stimulus in the peripheral and central zones was the release in mitosis of cells whose nuclei were in the postsynthetic (G₂) phase of the mitotic cycle. In the pith-rib meristem, the following events were recorded: 1) a stimulation of DNA synthesis starting at 20 hr; 2) a rise of the mitotic index beginning at 28 hr; 3) the vacuolation and elongation of cells starting at 48 hr. All these events occurred well before the initiation of bract and flower primordia, which began at 96 and 136 hr, respectively. Neither stimulation of mitotic activity nor flowering occurred in the meristems of plants subjected to a long night interrupted at its midpoint by a 5-min light break. The results are discussed in relation to the early events which are known to occur in the meristems of other photoperiodic species in transition to flowering.     

MOVEMENT OF THE FLOWER STIMULUS IN XANTHIUM

A distribution pattern of the flower stimulus is clearly demonstrated. The direction of the movement of the flower stimulus could be altered either by application of GA or by removal of the plant organs concerned. Dormant buds responded to the flower stimulus provided that the flower stimulus was forced down by means of decapitation and disbudding or was raised to a high level by increasing the number of the inductive cycles. A photoperiodically insensitive leaf which had been exposed to short days can still exert the inhibitory effect on flowering.     

牛蒡叶绿素的性质研究

对菊科植物牛蒡所含叶绿素的性质进行了初步研究,目的在于为牛蒡植物的深加工提供可靠信息。内容包括叶绿素含量的测定及其与光,温度,食品添加剂的关系。结果表明,叶绿素总含量为２．３１９５ｍｇ／ｇ,在散射光环境中叶绿素降解速度随时间延长而加快;随温度的升高也加快了叶绿素的降解速度;食品添加剂除糖外,都或多或少对该色素有影响,故以生产中应采取使其稳定的措施。     

INCORPORATION OF H3-THYMIDINE IN LEAF NUCLEI OF XANTHIUM PENNSYLVANICUM

The basic kinetics and the pattern of incorporation of H³-thymidine was studied in the leaf lamina of Xanthium pennsylvanicum. A method of foliar absorption was used to incorporate the radioisotope into leaf nuclei. The autoradiographic techniques employed provided data on the amount of the isotope incorporated. It was determined that 10 μc/ml (sp. act. 6.7 c/mmole) of H³-thymidine with 1–8 hr of isotopic growth and 4 hr of postisotopic growth gave the most satisfactory results. The percent of labelled nuclei and the number of grains per nucleus were presented as functions of isotopic and postisotopic growth periods. Distribution of grains in the nuclei approximated the Poisson distribution at 1 hr of isotopic growth. Increased time of isotopic growth changed the pattern of grain distribution. No deleterious effects were observed using an 8-hr period of isotopic growth, but prolonged incubation time significantly decreased the proportion of mitotic figures in the lamina. The amount of incorporation of the DNA precursor expressed as percent of labelled nuclei was linear to about 16 hr of isotopic growth and thereafter decreased gradually. As indicated by the average number of grains per nucleus, H³-thymidine incorporation increased to about 16 hr, and soon after reached a saturation level. The percent of labelled nuclei and the number of grains per nucleus decreased as a function of the postisotopic growth period. However, they were significantly greater in the lamina near the vein than in the lamina region at some distance from the vein. The radioactive precursor was initially absorbed by the cells of the lamina and was subsequently translocated into the vascular system. There it was circulated and made available to the dividing cells near veins of the lamina. This region may be a metabolically distinct part of the lamina with significantly higher rates of incorporation and mitotic turnover.     

FINE STRUCTURE OF EPITHELIAL CANAL CELLS IN PETIOLES OF XANTHIUM PENSYLVANICUM

Secretory canals, lined with an epithelium, occur in many families, e.g., Umbelliferae, Compositae. These canals are said to extend continuously through the root and shoot systems and are known, in some cases, to secrete resins, essential oils, etc. In Xanthium the canals arise schizogenously from cells derived from canal initials. Subsequent divisions lead to a ring of 7–12 epithelial cells surrounding a central cavity. During maturation the epithelium becomes crushed and obliterated. Canals were examined in petioles of Xanthium pensyhanicum (Cocklebur) grown under long day illumination to maintain vegetative growth. The fine structure of the canal and its epithelium was studied by electron microscopy of thin sections cut transverse to the principal axis of petioles from leaves in an early stage of development. The canal proper is delimited by walls of epithelial cells which protrude into a scallop shaped cavity. In comparison to the surrounding parenchyma, the epithelial cells are smaller, cytoplasmically more dense, and less vacuolate. The epithelium contains pleomorphic starch-free plastids with planar thylakoids frequently stacked into grana; thus, the plastids are presumed photosynthetically active. Mitochondria are abundant and often dense. The cytoplasm is rich in free polysomes, and smooth endoplasmic reticulum predominates over the rough form. Spheroidal granules averaging about 530 nm in diameter are numerous in the epithelium and appear at lower concentration in neighboring cells. Many features of fine structure of the epithelial cells suggest that a high metabolic activity is present in this tissue during this early stage of development. A possible function of the canals is defense against insect predation and animal grazing.     

REGIONS OF CELL DIVISION AND ELONGATION DURING STEM GROWTH OF XANTHIUM

Regions of cell division and cell elongation were established during stem growth of Xanthium pensylvanicum (cocklebur). From percent mitosis it was determined that the region of cell division in a Xanthium stem of Plastochron Index (PI) 13.89 was 20 mm long, starting from the stem apex and proceeding in a basipetal direction. Measurements of cortical cell lengths demonstrated that mature cell length in the stem of the same plant was reached at about 55 mm distance from the stem apex. Between 20 and 55 mm distance from the apex, shoot growth occurred by cell elongation alone. The maximum rate of change in cell length (dC/dX) was at about 15 mm distance from the stem apex. The length of the apical growth region was a function of the age of the plants. The older the plant, the longer its apical region of elongation. Growth of Xanthium stems was due primarily to elongation of internodes; the nodal regions did not seem to elongate.     

STUDIES ON OSCINELLA FRIT LINN.

Data, collected in Sweden during the year 1927, indicate the existence of considerable variation in extent of infestation of the grain of different varieties, sown at the same time. Also they suggest that sterility or "blindness" of grain may be a varietal character.