Showing papers on "Plant physiology published in 1992"

Seed dormancy and germination: the role of abscisic acid and gibberellins and the importance of hormone mutants

Abstract: Over the past decades many studies have aimed at elucidating the regulation of seed dormancy and germination. Many hypotheses have been proposed and rejected but the regulatory principle behind changes in dormancy and induction of germination is still a ‘black’ box. The majority of proposed mechanisms have a role for certain plant hormones in common. Abscisic acid and the gibberellins are the hormones most frequently suggested to control these processes. The development of hormone-deficient mutants made it possible to provide direct evidence for the involvement of hormones in germination and dormancy related processes. In the present paper an attempt is made to assess the role of abscisic acid and gibberellins in the transitions between dormant and non-dormant states and germination. First a conceptual framework is presented in which the different states of dormancy and germination are defined in order to contribute to a solution of the semantic confusion about these terms that has existed since the beginning of seed physiology. It is concluded that abscisic acid plays a pivotal role during the development of primary dormancy and gibberellins are involved in the induction of germination. Changes in sensitivity to these hormones occur during changes in dormancy. Both synthesis of and responsiveness to the hormones are controlled by natural environmental factors such as light, temperature and nitrate.

Physiological and growth responses of Centaurea maculosa (Asteraceae) to root herbivory under varying levels of interspecific plant competition and soil nitrogen availability.

Abstract: Centaurea maculosa seedlings were grown in pots to study the effects of root herbivory by Agapeta zoegana L. (Lep.: Cochylidae) and Cyphocleonus achates Fahr. (Col.: Curculionidae), grass competition and nitrogen shortage (each present or absent), using a full factorial design. The aims of the study were to analyse the impact of root herbivory on plant growth, resource allocation and physiological processes, and to test if these plant responses to herbivory were influenced by plant competition and nitrogen availability. The two root herbivores differed markedly in their impact on plant growth. While feeding by the moth A. zoegana in the root cortex had no effect on shoot and root mass, feeding by the weevil C. achates in the central vascular tissue greatly reduced shoot mass, but not root mass, leading to a reduced shoot/root ratio. The absence of significant effects of the two herbivores on root biomass, despite considerable consumption, indicates that compensatory root growth occurred. Competition with grass affected plant growth more than herbivory and nutrient status, resulting in reduced shoot and root growth, and number of leaves. Nitrogen shortage did not affect plant growth directly but greatly influenced the compensatory capacity of Centaurea maculosa to root herbivory. Under high nitrogen conditions, shoot biomass of plants infested by the weevil was reduced by 30% compared with uninfested plants. However, under poor nitrogen conditions a 63% reduction was observed compared with corresponding controls. Root herbivory was the most important stress factor affecting plant physiology. Besides a relative increase in biomass allocation to the roots, infested plants also showed a significant increase in nitrogen concentration in the roots and a concomitant reduction in leaf nitrogen concentration, reflecting a redirection of the nitrogen to the stronger sink. The level of fructans was greatly reduced in the roots after herbivore feeding. This is thought to be a consequence of their mobilisation to support compensatory root growth. A preliminary model linking the effects of these root herbivores to the physiological processes of C. maculosa is presented.

An amalgamation between hormone physiology and plant ecology: A review on flooding resistance and ethylene

Abstract: Both distribution of terrestrial plants and species composition in flood plain communities are strongly influenced by flooding (waterlogging, partial submergence, or submergence). The interaction between a plant's flooding resistance and the seasonal timing, duration, depth, or frequency of flooding often determines plant distribution in flood plains. Flooding may be accompanied by marked physical changes in light, carbon availability, diffusion rate of gases, and density of the environment. Various physiological processes may be affected by these flooding-induced physical changes, including aerobic respiration, photosynthesis, and processes in which light acts as a source of information (e.g., phytochrome photoequilibrium). Certain plant species acclimatize and adapt to these physical changes to relieve the constraints imposed by the flooded environment. Underwater photosynthesis, enhanced shoot elongation, adventitious roots, and aerenchyma formation are typical adaptive responses which are believed to improve the oxygen status of submerged plants. Ethylene and other plant hormones play a central role in the initiation and regulation of most of these adaptive responses, which permit “escape” from anaerobiosis. Mechanisms of direct tolerance of anaerobic conditions, such as a vigorous fermentative respiratory pathway, are of particular importance when the plant is very deeply submerged, or during the night and when the water is sufficiently turbid to exclude light. Studies on the cosmopolitan genus Rumex, distributed in a flooding gradient on river flood plains, have integrated plant hormone physiology with plant ecology. Rumex species showed a high degree of interspecific variation in ethylene production rates, endogenous ethylene concentrations, ethylene sensitivity, and ethylene-mediated growth responses. The field distribution of Rumex species in flooding gradients is explained in terms of a balance between endogenous ethylene concentrations and sensitivity towards this growth regulator (“ethylene economy”). Much data has been gathered using a recently developed laser-driven photoacoustic detection technique capable of detecting six parts of ethylene in 1012 parts air flowing continuously over the plant.

Control of nitrogen and carbon metabolism in root nodules

Abstract: Because legume root nodules have high rates of carbon and nitrogen metabolism, they are ideal for the study of plant physiology, biochemistry and molecular biology. Many plant enzymes involved in carbon and nitrogen assimilation have enhanced activity and enzyme protein in nodules as compared to other plant organs. For all intents and purposes the interior of the root nodule is O 2 limited. Both plant and bacterial components of effective root nodules have unique adaptive features for maximizing carbon and nitrogen metabolism in an O 2 -limited environment

Some reflections on the relationship between endogenous hormones and light-mediated seed dormancy

Abstract: The relationships between phytochrome and endogenous hormones in the light-mediated control of seed dormancy are discussed. It is concluded that gibberellins are primarily involved in post-dormancy metabolic processes leading to embryo growth and radicle emergence, such as food reserve mobilisation and endosperm softening. Evidence is considered that germination inhibitors, particularly abscisic acid, are involved in the establishment and maintenance of primary dormancy. The role of cytokinins not fully elucidated but there is considerable evidence to suggest that phytochrome control may involve cytokinin effects on transmembrane ion fluxes. In terms of hormonal control, phytochrome mediated dormancy is a complex phenomenon. There is a need for molecular studies of processes controlled by phytochrome, GAs, CKs and ABA during dormancy and germination to unravel the complexities of the dormancy mechanisms. Such studies would be facilitated by the availability of CK-deficient mutants of classical light-sensitive species.

The physiology of Salix

Abstract: Salix has many physiological features in common with other deciduous woody plants, e.g. C3 photosynthesis, occurrence of latitudinal photoperiodic ecotypes, and organic N (no ) flux to the shoot in the xylem. Special points about the physiology of Salix spp. which may have impact on their ecology and economic uses include: (i) relatively high (for woody plants) light-saturated rate of photosynthesis on a leaf area or leaf dry weight basis, (ii) sex differences in water (transpiration) costs of growth, (iii) very limited seed longevity and a wide range of temperature and light conditions permitting germination, and (iv) ready rooting and establishment of naturally or artificially detached twigs and branches. Areas in which work on Salix has been especially influential for the development of plant physiology include: (i) the analysis of phloem functioning using aphids, (ii) the role of photoinhibition under natural conditions, and (iii) the realisation that the woody habit need not constrain the achieved activity of enzymes and hence N-based metabolic rates.

Effect of nitrogen form on maize response to drought stress

Abstract: Two hybrids of maize (Zea mays L.) differing in resistance to drought, were grown in chernozem soil in a greenhouse and were fertilized with two different forms of nitrogen: Ca(NO3)2 and (NH4)2SO4 in concentrations corresponding to 100 kg of N ha-1. After emergence of the 4th leaf, plants were exposed to drought. During the drought period, the parameters of plant water status (water potential, osmotic potential, turgor pressure and relative water content) and chlorophyll a+b concentration were monitored every two days. N and K concentration and accumulation over the drought period were also monitored. Next to differences in adaptability of the two hybrids to drought, the results demonstrate different adaptability of NH4 and NO3-treated plants within each hybrid. NH4-plants of each hybrid maintain higher turgor pressure during the drought by better osmotic adaptation. Especially significant differences appear between chlorophyll (a+b) values of NH4 and NO3-treated plants and as affected by drought. Chlorophyll concentrations of NH4-plants are higher than those of NO3-plants both in control and droughted plants. NH4 plants show a characteristic initial chlorophyll increase at the beginning of the drought period while in NO3 plants chlorophyll constantly decreases throughout the whole drought period. The influence of the nitrogen form on chlorophyll concentration changes during drought does not appear to be affected by regulation of the K concentration.

Involvement of cytokinins in the germination of chick-pea seeds

Abstract: Eight cytokinins detected in germinated chick-pea (Cicer arietinum L. var. Castellana) seeds were first present in the embryonic axes but appeared in the cotyledons after 12h of germination. The cytokinins detected in the cotyledons originate in the embryonic axes, but no passage of these substances from the cotyledons to the axes was detected, except when the seeds were treated with red light.

Plant physiology, genetics, biotechnology and pathology in the Institute of Experimental Botany of the Czechoslovak Academy of Sciences (1962–1992)

Abstract: The main scientific results achieved in individual departments of the Institute of Experimental Botany during 30 years of its existence are briefly summarized. They include methods of studying photosynthesis, ontogenetic changes of photosynthetic characteristics, stress factors affecting photosynthetic activities, photosynthesis of transgenic plants and duringin vitro cultivation, roles of auxins and cytokinins in plant growth and development, development and testing of new plant growth regulators, models of organogenesisin vitro, metabolic and mutagenic activities of phenolic substances, hormonal regulation of flowering, activities of promutagens (nitrosamines, 7,12-dimethylbenzanthracene), model systems of genetic damage, repair synthesis and post-replication repair, developmental pollen biology and biotechnology, extracellular nucleolytic activity of pollen, selection of apple scab immune cultivars of apple tree, chemotaxonomy ofFabaceae andAllium species, selection pressures in embryoids, somatic embryogenesis and nuclear genome changes in plant cell and callus cultures, discoveries of new plant viruses, virus spread and persistence in crops, development of polyclonal and monoclonal antibodies, role of oxidative pentosephosphate cycle in biosynthesis of viral RNA, and virus diseases of forest trees.