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921.
The Myriogramme group of Kylin was found to contain two distinct clusters of genera that merit recognition at the tribal level. In this paper, we establish the tribe Myriogrammae based on a study of the type species of Myriogramme, M. livida, from the Southern Hemisphere. The Myriogrammae is characterized by 1) marginal and diffuse intercalary meristems; 2) nuclei arranged in a ring bordering the side walls of vegetative cells; 3) microscopic veins absent; 4) procarps scattered, formed opposite one another on both sides of the blade posterior to one or more vegetative pericentral cells (cover cells) and consisting of a carpogonial branch, a one-/to two-celled lateral sterile group and a one-celled basal sterile group; 5) auxiliary cell diploidized by a connecting cell cut off posteriolaterally from the fertilized carpogonium; 6) gonimoblast initial cut off distally from the auxiliary cell, generating one distal and one to two lateral gonimoblast filaments that branch in the plane of the expanding cystocarp cavity and later fuse to from an extensive, branched fusion cell; 7) spermatangial and tatrasporangial sori formed inside the margin on both sides of the blade by resumption of meristematic activity; and 8) tetrasporangia produced primarily from the central cells. The Myriogrammae currently includes Myriogramme Kylin , Gonimocolax Kylin , Haraldiophyllum A. Zinova , Hideophyllum A. Zinova, and a possible undescribed genus from Pacific North and South America. Genera are separated based primarily on features of gonimoblast and carposporangial development . 相似文献
922.
Here we describe a simple method for pulse-labeling tree seedlings with 13CO2(gas), and then apply the method in two related experiments: t (i) comparison of carbon allocation patterns between t Betula papyrifera Marsh. and t Pseudotsuga menziesii (Mirb.) Franco, and t (ii) measurement of one-way belowground carbon transfer from t B. papyrifera to t P. menziesii. Intraspecific carbon allocation patterns and interspecific carbon transfer both influence resource allocation, and consequently development, in mixed communities of t B. papyrifera and t P. menziesii.In preparation for the two experiments, we first identified the appropriate 13CO2(gas) pulse-chase regime for labeling seedlings: a range of pulse (100-mL and 200-mL 99 atom%13 CO2(gas)) and chase (0, 3 and 6 d) treatments were applied to one year-old t B. papyrifera and t P. menziesii seedlings. The amount of 13CO2 fixed immediately after 1.5 h exposure was greatest for both t B. papyrifera (40.8 mg excess 13C) and t P. menziesii (22.9 mg excess 13C) with the 200-mL pulse, but higher 13C loss and high sample variability resulted in little difference in excess13 C content between pulse treatments after 3 d for either species. The average excess 13C root/shoot ratio of t B. papyrifera and t P. menziesii changed from 0.00 immediately following the pulse to 0.61 and 0.87 three and six days later, which reflected translocation of 75% of fixed isotope out of foliage within 3 d following the pulse and continued enrichment in fine roots over 6 d. Based on these results, the 100-mL CO2(gas) and 6-d chase were considered appropriate for the carbon allocation and belowground transfer experiments.In the carbon allocation experiment, we found after 6 d that t B. papyrifera allocated 49% (average 9.5 mg) and t P. menziesii 41% (average 5.8 mg) of fixed isotope to roots, of which over 55% occurred in fine roots in both species. Species differences in isotope allocation patterns paralleled differences in tissue biomass distribution. The greater pulse labeling efficiency of t B. papyrifera compared to t P. menziesii was associated with its two-fold and 13- fold greater leaf and whole seedling net photosynthetic rates, respectively, 53% greater biomass, and 35% greater root/shoot ratio.For the carbon transfer experiment, t B. papyrifera and t P. menziesii were grown together in laboratory rootboxes, with their roots intimately mingled. A pulse of 100 mL13 CO2(gas) was applied to paper birch and one-way transfer to neighboring t P. menziesii was measured after 6 d. Of the excess 13C fixed by t B. papyrifera, 4.7% was transferred to neighboring t P. menziesii, which distributed the isotope evenly between roots and shoots. Of the isotope received by t P. menziesii, we estimated that 93% was taken up through belowground pathways, and the remaining 7% taken up by foliage as13 CO2(gas) respired by t B. papyrifera shoots. These two experiments indicate that t B. papyrifera fixes more total carbon and allocates a greater proportion to its root system than does t P. menziesii, giving it a competitive edge in resource gathering; however, below-ground carbon sharing is of sufficient magnitude that it may help ensure co-existence of the two species in mixed communities. 相似文献
923.
Abrams SR Rose PA Cutler AJ Balsevich JJ Lei B Walker-Simmons MK 《Plant physiology》1997,114(1):89-97
We report here the synthesis and biological activity of a new persistent abscisic acid (ABA) analog, 8[prime]-methylene ABA. This ABA analog has one additional carbon atom attached through a double bond to the 8[prime]-carbon of the ABA molecule. (+)-8[prime]-Methylene ABA is more active than the natural hormone (+)-ABA in inhibiting germination of cress seed and excised wheat embryos, in reducing growth of suspension-cultured corn cells, and in reducing transpiration in wheat seedlings. The (+)-8[prime]-methylene analog is slightly weaker than (+)-ABA in increasing expression of ABA-inducible genes in transgenic tobacco, but is equally active in stimulating a transient elevation of the pH of the medium of corn cell cultures. In corn cells, both (+)-ABA and (+)-8[prime]-methylene ABA are oxidized at the 8[prime] position. ABA is oxidized to phaseic acid and (+)-8[prime]-methylene ABA is converted more slowly to two isomeric epoxides. The alteration in the ABA structure causes the analog to be metabolized more slowly than ABA, resulting in longer-lasting and more effective biological activity relative to ABA. 相似文献
924.
925.
B. M. Degnan Duncan Souter Sandie M. Degnan Suzanne C. Long 《Development genes and evolution》1997,206(6):370-376
Increased K+ concentration in seawater induces metamorphosis in the ascidian Herdmania
momus. Larvae cultivated at 24°C exhibit highest rates of metamorphosis when treated with 40 mM KCl-elevated seawater at 21°C. At 24°C, H. momus larvae develop competence to respond to KCl-seawater and initiate metamorphosis approximately 3 h after hatching. Larval trunks
and tails separated from the anterior papillae region, but maintained in a common tunic at a distance of greater than 60 μm,
do not undergo metamorphosis when treated with KCl-seawater; normal muscle degradation does not occur in separated tails while
ampullae develop from papillae-containing anterior fragments. Normal programmed degradation of myofibrils occurs when posterior
fragments are fused to papillae-containing anterior fragments. These data indicate that H. momus settlement and metamorphosis only occurs when larvae have attained competence, and suggest that an anterior signalling centre
is stimulated to release a factor that induces metamorphosis.
Received: 15 May 1996 / Accepted: 19 September 1996 相似文献
926.
Suzanne Eaton 《Current opinion in cell biology》1997,9(6):860-866
Our understanding of the actin and microtubule rearrangements that generate planar polarity in Drosophila and in vertebrate epithelia has been extended by recent discoveries. Three different Rho family proteins have been shown to mediate polarization in the wing and the eye of Drosophila. In vertebrates, the importance of myosin Vlla has been uncovered by mutations that cause defects in planar polarization in the ear. Advances in our understanding of the Frizzled pathway, which coordinates planar polarization in Drosophila, are moving the field closer to understanding the links between signal transduction and polarized cytoskeletal reorganization. 相似文献
927.
Egg quality in fish: what makes a good egg? 总被引:15,自引:0,他引:15
Brooks Suzanne Tyler Charles R. Sumpter John P. 《Reviews in Fish Biology and Fisheries》1997,7(4):387-416
Factors affecting egg quality are determined by the intrinsic properties of the egg itself and the environment in which the egg is fertilized and subsequently incubated. Egg quality in fish is very variable. Some of the factors affecting egg quality in fish are known, but many (probably most) are unknown. Components that do affect egg quality include the endocrine status of the female during the growth of the oocyte in the ovary, the diet of the broodfish, the complement of nutrients deposited into the oocyte, and the physiochemical conditions of the water in which the eggs are subsequently incubated. In captive broodfish, the husbandry practices to which fish are subjected are probably a major contributory factor affecting egg quality. Our knowledge of the genetic influences on egg quality is very limited indeed. We know that parental genes strongly influence both fecundity and egg quality, but almost nothing is known about gene expression and/or mRNA translation in fish oocytes/embryos. This is surprising because the products synthesized in ovoand the mechanisms controlling their expression are likely to play a central role in determining egg quality. The genetic mechanisms underpinning oocyte and embryo growth and development are a priority for research 相似文献
928.
James K. Porter Charles W. Bacon Horace G. Cutler Richard F. Arrendale Joe D. Robbins 《Phytochemistry》1985,24(7):1429-1431
Balansia epichloë, a systemic plant pathogen isolated from Sporobolus poiretii, was shown to produce the plant growth regulators 3-indole acetic acid, 3-indole ethanol, 3-indole acetamide and methyl-3-indole carboxylate when grown on a medium containing tryptophan. When grown on a tryptophan deficient medium 3-substituted indole derivatives were not detected. However, extracts of the medium in lower doses increased and in higher doses inhibited the growth of wheat coleoptiles. 相似文献
929.
Relationships between the diversity and abundance of native versus exotic species underpin management of disturbance regimes for conservation. Theory predicts negative, positive or neutral relationships depending on respective drivers, with greatest potential benefit when natives and exotics show opposing responses to management. We examined drivers of exotic plant cover and relationships with native plant richness using 12-year burning, mowing and grazing experiments in two representative temperate grassy eucalypt woodlands with contrasting histories of frequent versus infrequent disturbance. We hypothesized that disturbance and high resources favour exotics, and assessed whether natives and exotics covary positively due to common external drivers or negatively due to contrasting external drivers and/or competition. Positive relationships with rainfall and disturbance explained >80 % of the variation in exotic cover at both sites, supporting our first hypothesis. Native–exotic relationships were non-linear, with native richness first increasing rapidly with increasing exotic cover, then levelling and beginning to decrease. Common external drivers, particularly inter-annual rainfall, explained initial positive relationships, highlighting a prevalence of positive relationships at long temporal (as well as large spatial) scales. At the historically frequently-burnt site, a concomitant increase in native richness and exotic cover after fire contributed to the positive relationship, indicating a management trade-off. At the long-unburnt site, exotics increased but natives decreased with fire, suggesting dual benefits of low fire frequency. We conclude that relationships between exotic cover and native richness emerge from interactions among external drivers and competitive responses, with responses to external drivers dominating at low resources and negative interactions gaining importance as resources increase. 相似文献
930.