Showing papers in "Botanical Review in 1994"

Bateman's principle and plant reproduction: The role of pollen limitation in fruit and seed set

Abstract: Bateman’s principle states that male fitness is usually limited by the number of matings achieved, while female fitness is usually limited by the resources available for reproduction. When applied to flowering plants this principle leads to the expectation that pollen limitation of fruit and seed set will be uncommon. However, if male searching for mates (including pollen dissemination via external agents) is not sufficiently successful, then the reproductive success of both sexes (or both sex functions in hermaphroditic plants) will be limited by number of matings rather than by resources, and Bateman’s principle cannot be expected to apply. Limitation of female success due to inadequate pollen receipt appears to be a common phenomenon in plants. Using published data on 258 species in which fecundity was reported for natural pollination and hand pollination with outcross pollen, I found significant pollen limitation at some times or in some sites in 159 of the 258 species (62%). When experiments were performed multiple times within a growing season, or in multiple sites or years, the statistical significance of pollen limitation commonly varied among times, sites or years, indicating that the pollination environment is not constant. There is some indication that, across species, supplemental pollen leads to increased fruit set more often than increased seed set within fruits, pointing to the importance of gamete packaging strategies in plant reproduction. Species that are highly self-incompatible obtain a greater benefit relative to natural pollination from artificial application of excess outcross pollen than do self-compatible species. This suggests that inadequate pollen receipt is a primary cause of low fecundity rates in perennial plants, which are often self-incompatible. Because flowering plants often allocate considerable resources to pollinator attraction, both export and receipt of pollen could be limited primarily by resource investment in floral advertisement and rewards. But whatever investment is made is attraction, pollinator behavioral stochasticity usually produces wide variation among flowers in reproductive success through both male and female functions. In such circumstances the optimal deployment of resources among megaspores, microspores, and pollinator attraction may often require more flowers or more ovules per flower than will usually be fertilized, in order to benefit from chance fluctuations that bring in large number of pollen grains. Maximizing seed set for the entire plant in a stochastic pollination environment might thus entail a packaging strategy for flower number or ovule number per flower that makes pollen limitation of fruit or seed set likely. Pollen availability may limit female success in individual flowers, entire plants (in a season or over a lifetime), or populations. The appropriate level must be distinguished depending on the nature of the question being addressed.

Strategies of seed dispersal and germination in plants inhabiting deserts

Abstract: Massive seed consumption is typical in many deserts. The “escape” or “protection” strategies of seed dispersal are important, as they prevent massive seed consumption. The more extreme the desert, the more unpredictable the low amounts and distribution of the rains as well as the beginning and length of the season or seasons with rains. Seeds, which have the highest resistance to extreme environmental conditions, develop during germination into seedlings, which are the most sensitive. Therefore, germination of parts of the seed population at their respective proper times spreads the risk over time and is thus very important for plant survival, especially in those plants inhabiting the more extreme deserts. Each of the plant species studied was found to have its own survival strategies of seed dispersal and germination. At least two extreme seed-dispersal and germination strategies have been observed: 1) the “escape” seed dispersal and “opportunistic” portioned seed-germination strategies, such as inSchismus arabicas andSpergularia diandra, and 2) the “cautious” portioned dispersal by rain of the protected seeds, such as inAsteriscus hierochunticus and portioned dispersal and rapid germination strategies such as inBlepharis spp. The fate of future generations, as far as the germinability of seeds of some species is concerned, depends on the influences of maternal and environmental factors when the seeds are still on the mother plant, mostly during the final stage of seed maturation, as inTrigonella arabica. It may even depend on the position of the caryopsis from which the mother plant originated, as inAegilops geniculata. The dry post-maturation conditions and the environmental factors during seed wetting and germination may also affect the percentage of seed germination, as inSchismus arabicus.

Effects of cobalt on plants

Abstract: Cobalt, a transition element, is an essential component of several enzymes and co-enzymes. It has been shown to affect growth and metabolism of plants, in different degrees, depending on the concentration and status of cobalt in rhizosphere and soil. Cobalt interacts with other elements to form complexes. The cytotoxic and phytotoxic activities of cobalt and its compounds depend on the physico-chemical properties of these complexes, including their electronic structure, ion parameters (charge-size relations) and coordination. Thus, the competitive absorption and mutual activation of associated metals influence the action of cobalt on various phytochemical reactions. The distribution of cobalt in plants is entirely species-dependent. The uptake is controlled by different mechanisms in different species. Biosorption involves ion-exchange mechanism in algae, but in fungi both metabolism-independent and -dependent processes are operative. Physical conditions like salinity, temperature, pH of the medium, and presence of other metals influence the process of uptake and accumulation in algae, fungi, and mosses. Toxic concentrations inhibit active ion transport. In higher plants, absorption of Co2+ by roots involves active transport. Transport through the cortical cells is operated by both passive diffusion and active process. In the xylem, the metal is mainly transported by the transpirational flow. Distribution through the sieve tubes is acropetal by complexing with organic compounds. The lower mobility of Co2+ in plants restricts its transport to leaves from stems. Cobalt is not found at the active site of any respiratory chain enzymes. Two sites of action of Co2+ are found in mitochondrial respiration since it induces different responses toward different substrates like α-keto glutarate and succinate. In lower organisms, Co2+ inhibits tetraphyrrole biosynthesis, but in higher plants it probably participates in chlorophyll b formation. Exogenously added metal causes morphological damage in plastids and changes in the chlorophyll contents. It also inhibits starch grain differentiation and alters the structure and number of chloroplasts per unit area of leaf. The role of cobalt in photosynthesis is controversial. Its toxic effect takes place by inhibition of PS2 activity and hence Hill reaction. It inhibits either the reaction centre or component of PS2 acceptor by modifying secondary quinone electron acceptor Qb site. Co2+ reduces the export of photoassimilates and dark fixation of CO2. In C4 and CAM plants, it hinders fixation of CO2 by inhibiting the activity of enzymes involved. Cobalt acts as a preprophase poison and thus retards the process of karyokinesis and cytokinesis. The action of cobalt on plant cells is mainly turbagenic. Cobalt compounds act on the mitotic spindle, leading to the formation of chromatin bridges, fragmentation, and sticky bridges at anaphase and binucleate cells. High concentrations of cobalt hamper RNA synthesis, and decrease the amounts of the DNA and RNA probably by modifying the activity of a large number of endo- and exonucleases. The mutagenic action of cobalt salts results in mitochondrial respiratory deficiency in yeasts. In cytokinesis-deficient mutant of Chlamydomonas it increases the amount of sulfhydryl compounds. Cobalt has been shown to alter the sex of plants like Cannabis sativa, Lemna acquinoclatis, and melon cultivars. It decreases the photoreversible absorbance of phytochrome in pea epicotyl and interferes with heme biosynthesis in fungi. Low concentration of Co2+ in medium stimulates growth from simple algae to complex higher plants. Relatively higher concentrations are toxic. A similar relationship is seen with crop yield when the metal is used in the form of fertilizer, pre-seeding, and pre-sowing chemicals. Toxic effect of cobalt on morphology include leaf fall, inhibition of greening, discolored veins, premature leaf closure, and reduced shoot weight. Being a component of vitamin B12 and cobamide coenzyme, Co2+ helps in the fixation of molecular nitrogen in root nodules of leguminous plants. But in cyanobacteria, CoCl2 inhibits the formation of heterocyst, ammonia uptake, and nitrate reductase activity. The interaction of cobalt with other metals mainly depends on the concentration of the metals used. For example, high levels of Co2+ induce iron deficiency in plants and suppress uptake of Cd by roots. It also interacts synergistically with Zn, Cr, and Sn. Ni overcomes the inhibitory effect of cobalt on protonemal growth of moss, thus indicating an antagonistic relationship. The beneficial effects of cobalt include retardation of senescence of leaf, increase in drought resistance in seeds, regulation of alkaloid accumulation in medicinal plants, and inhibition of ethylene biosynthesis. In lower plants, cobalt tolerance involves a cotolerance mechanism. The mechanism of resistance to toxic concentration of cobalt may be due to intracellular detoxification rather than defective transport. In higher plants, only a few advanced copper-tolerant families showed cotolerance to Co2+. Tolerance toward Co2+ may sometimes determine the taxonomic shifting of several members of Nyssaceae. Due to the high cobalt content in serpentine soil, essential element uptake by plants is reduced, a phenomenon known as “serpentine problem,” for New Caledonian families like Flacourtiaceae. Large amounts of calcium in soil may compensate for the toxic effects of heavy metals in adaptable genera grown in this type of soil. The biomagnification of potentially toxic elements, such as cobalt from coal ash or water into food webs, needs additional study for effective biological filtering.

The role of seed coats in seed viability

Abstract: The seed coat is the seed’s primary defense against adverse environmental conditions. A hard seed coat protects the seed not only from mechanical stress but also from microorganism invasion and from temperature and humidity fluctuations during storage. Phenolic compounds in the seed coat contribute to seed hardness and inhibition of microorganism growth. During germination, the seed coat protects the seed from hydration stress and electrolyte leakage.

Physiological ecology of the Bromeliaceae

Abstract: The physiological ecology of members of the Bromeliaceae is reviewed with an emphasis on photosynthesis and water relations. Terrestrial and epiphytic species are, for the most part, treated separately. Water relations, photosynthetic pathways, and photosynthetic responses to light, temperature, drought, atmospheric moisture, elemental nutrients, and pollutants are considered from an ecological perspective. In addition, appendices provide values of numerous ecophysiological parameters for all species studied thus far. Results of this review include the following: (1) the ecophysiology of terrestrial and epiphytic species is surprisingly similar; (2) approximately two-thirds of bromeliads are CAM plants and occupy arid sites or are epiphytic; (3) many species are adapted to full or partial shade, yet can grow in full sunlight; (4) photosynthesis is optimal when day temperatures are warm and night temperatures are cool; (5) species with heavy trichome indumenta on their leaf surfaces are capable of absorbing atmospheric water vapor, yet improvement of tissue water relations is unlikely; (6) heavy trichome covers also suppress CO2 exchange when leaf surfaces are wetted; (7) high levels of recycling of respiratory CO2 via CAM occur in many species, especially under stress; and (8) tissue osmotic and water potentials of nearly all bromeliads investigated are seldom more negative than -1.0 MPa. A potential explanation of the mechanisms underlying maintenance of high tissue water potentials despite large water losses during droughts is discussed. In summary, the diversity of physiological adaptations to the environment in the few bromeliads studied thus far is impressive, but likely will be surpassed with investigation of more species in the Bromeliaceae.

The excitability of plant cells: With a special emphasis on characean internodal cells

Abstract: This review describes the basic principles of electrophysiology using the generation of an action potential in characean internodal cells as a pedagogical tool. Electrophysiology has proven to be a powerful tool in understanding animal physiology and development, yet it has been virtually neglected in the study of plant physiology and development. This review is, in essence, a written account of my personal journey over the past five years to understand the basic principles of electrophysiology so that I can apply them to the study of plant physiology and development. My formal background is in classical botany and cell biology. I have learned electrophysiology by reading many books on physics written for the lay person and by talking informally with many patient biophysicists. I have written this review for the botanist who is unfamiliar with the basics of membrane biology but would like to know that she or he can become familiar with the latest information without much effort. I also wrote it for the neurophysiologist who is proficient in membrane biology but knows little about plant biology (but may want to teach one lecture on "plant action potentials"). And lastly, I wrote this for people interested in the history of science and how the studies of electrical and chemical communication in physiology and development progressed in the botanical and zoological disciplines.

Seed germination of the Australian desert shrubEremophila (Myoporaceae)

Abstract: Eremophila is an Australian genus of 212 species ranging from prostrate shrubs to small trees, the great majority of which occur in Western Australia. Recent interest in the genus’s germination strategies developed out of a need to seek rehabilitation techniques for mine-site and rangeland areas. The genus is of special interest because of its broad geographic range and prominence in vegetation associations of the arid zone, especially in Western Australia, where it often dominates or codominates over wide areas.

A consensus classification for the order gentianales with additional details on the suborder apocynineae

Abstract: The Gentianales as circumscribed by Benson, Cronquist, Dahlgren, Goldberg, Hey wood et al., Melchior, Stebbins, Takhtajan, and Thorne is investigated. From these a consensus classification (not in the cladistic sense) or classificatory model for the order is proposed. This classification is discussed, as are the relationships of the taxa within it. Excluded taxa are also discussed. In particular, the Apocynineae is examined in detail and it is pointed out that for this suborder the present consensus classification, at the family level, is congruous only if the taxa involved are monophyletic. Data are then supplied to show that, as yet, neither monophyly nor paraphyly can be proved conclusively for the taxa of the Apocynineae.

The role of botanists during World War II in the Pacific theatre

Abstract: During World War II some professional botanists and graduate students who were drafted, enlisted, or commissioned in the armed forces were fortunate to be able to use their training directly or indirectly. This was especially true for the Pacific theatre. Others served their country as civilians. The roles of botanists in the military ranged from teaching or research to participation in combat or support operations. A few botanists in uniform, in spite of their occupational obligations, were able to collect botanical specimens and were encouraged to do so by civilian museum personnel. The best known projects for botanists as civilians involved the search for native supplies of strategic raw materials, particularlyCinchona andHevea, whileCryptostegia and Guayule, as possible sources of latex, were grown on plantations and studied in detail. Tropical problems of fungal deterioration of fabrics and optical equipment involved primarily civilian botanists in both military and academic laboratories. Some older botanists and those deferred for marital, dependent, or physical reasons served as instructors in regular academic programs or the special college programs for military personnel. This paper is a summary of the contributions of botanists from the United States, Canada, Australia, and New Zealand to the war effort in the Pacific theatre during World War II.