Soil Physical Conditions And Plant Growth-Books Pdf

Soil physical conditions and plant growth
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conditions are optimal for their growth and performance Comparative experiments. with various plants show that the general shape of the curve representing the. response to the degree of severity of adverse conditions such as oxygen deficiency. soil compaction lowsoiltemperature high sodium chloride concentrations is. very similar to all plants whether or not they are adapted Minor differences. in a single soil factor are sufficient to cause minute variations in the. occurrence of plant species in the field Comparative experiments have also shown. not only that plants have a tremendous plasticity that enables them to survive. under adverse conditions but also that species develop different strategies in. order to survive We do not fully understand many plant soil relationships. because we do not have sufficient knowledge about, a the physical conditions of the soil in spa ce and time. b the differences in a plant s response at various developmental stages. c the plant s response to changes in the degree of adversi ty of conditions. d the extent to which a plant s response is determined by the interaction. of other factors, e methods to assess the effect of minor differences in response over a. long period in a plant s life cycle, One of the serious drawbacks of ecophysiological experimentation is that we. can hardly discern differences amounting to less than 5 10 per cent but in. nature even smaller differences may determine discriminationinthe long run. especially in interspecific competition, It is therefore with considerabIe diffidence that I present this paper. since it cannot solve the two main problems in plant ecology namely. 1 Why are certain types of vegetation species restricted to a certain. habitat whereas others clearly prefer another set of conditions. 2 How can diversity of species in a vegetation he maintained for a relatively. long period measured by human standards when so many individuals are. all dependent on the same resources i e light water and mineraIs. The second of these problems is the more challenging one since we know that. in contrast to the diversity in nature competition experiments almost invariably. resul t in survival of only one of the competing species DE WIT 1960 The niche. concept aseparation of interests in time and or space was introduced to. reconcile this discrepancy DE WIT VAN DEN BERGH 1965 VAN DEN BERGH BRAAK. HEKKE this volume but we shall have to learn much more about the ways in. which plants behave before these questions can he adequately answered. If we want to obtain satisfactory answers we must pay close attention to the. complete life cycle of the species in question since niche differentiation may. show up in only one of the life stages germination seedling establishment. vegetative growth generative growth and dissemination or seed longevity. Sometimes adaptation to a certain habitat can be due to relatively small. differences in a number of aspects PEGl EL 1976 PONS 1976 1977 none of which. alone would fully explain the species preference for a particular habitat. Since the ecophysiological approach is based on experience acquired in the. field of crop physiology possibly essential differences in behaviour between. natural vegetations and crops must be taken into account In both kinds of. populations the individual responds to the complex of conditions but agricultural. practice has selected for uniformity of response whereas natural selection. has of ten resulted in the maintenance of a certain degree of diversity and. plasticity within a population Moreover the external conditions are much. less predictable for natural vegetations, 2 SEED POPULATION AND SOIL PHYSICAL CONDITIONS.
2 1 INTRODUCTORY REMARKS, The number of seeds in a population on or in the soil depends on the ra te of. dissemination and on the rate at which seeds are lost through deterioration. germination and consumption Seed consumption will not be considered in this. paper The proceedings of a recent Nottingham Symposium on Seed Ecology. HEYDECKER 1973 have provided a considerable amount of information about the. behaviour of seeds in general the information is useful to both agriculturists. and ecologists The seed population is important not only because it determines. the timing of germination but also because in a given locality it may represent. a high percentage of the total number of individuals present in that locality. see also the paper by RABOTNOV in this volume Depending on the type of. dispersal seeds pass from the plant and the place where they have been. produced to a place on or in the soil where they will lie until conditions are. suitable for germination and for growth into new plants For the ultimate. success of the seeds both their longevity and their germination behaviour. are important, 2 2 LONGEVITY OF SEEDS, It is well known that several ambiguous factors such as cool temperatures low. oxygen tension and a low moisture content all of which tend to decrease metabolic. activity increase the length of time that seeds can be stored BARTON 1961. HARRISON 1966 ABDALLA ROBERTS 1968 This finding is surprising and makes. it difficult to explain why many seeds apparently retain their viability longer. when buried in moist soil than when kept in dry storage VILLIERS 1973 Fig 1. In an attempt to reconcile these different types of seed behaviour VILLIERS. 1973 showed that longevity is high at low water content depending on the. species 5 8 per cent and at high water content when the seed has fully imbibed. He assumed that ageing phenomena occurred at all levels of water content These. ageing processes are assumed to result in cross linkage of macromolecules which. render enzymes and membranes non functional and in a gradually increasing number. af somatic mutations manyof which may cause the production of defective proteins. 1110 P IM B BED, o 2 4 6 8 10 12 14 16, WEEKS OF STORAGE. FIG 1 Influence of the water content of seeds during storage on. longevity VILLIERS 1973, By comparing the results of germination experiments with those of electron. microscopical studies on the embryos Villiers concluded that the functioning. of macromolecule and organelle repair mechanisms was seriously impaired during. air dry storage at intermediate relative humidities of the air In practice. the degree of ageing can be related to the amounts and kinds of substances. lost by leaching from the seeds during soaking According to SIMON 1974. when dried seeds we re moistened again the initial leakage of electrolytes. which normally decreased rapidly when the seeds reached a water content of. about 15 per cent persisted due to the globular state of the plasma membrane. of these dry seeds This globular state is ineffecti ve in maintaining gradients. of ions and charge across the memhrane Af ter rewetting the normal non leaky. bilayer structure is restored a process which would take more time in aged. seeds In imbibed seeds stored in the soil leakage wil 1 not occur while the. seed remains viable provided the membranes remain in good condition Repeated. alternation of drying and wetting which of ten occurs in the upper soillayers. could ultimately lead to the complete exhaustion of accumulated solutes and. could therefore prove very harmful In this respect it is interesting to note. that the reverse response has been reported more frequently A number of authors. e g KOLLER et al 1962 HEGARTY 1970 PEGTEL 1976 demonstrated that both. germination and establishment improved af ter seeds of various species were. repeatedly wetted and dried Af ter studying the effects of the duration and. number of the drying wetting cycles BERRIE DRENNAN 1971 concluded that. germination was more rapid when the seeds were redried af ter short periods of. inbibition A prolonged wet period before redrying resulted in embryo damage. and poor germination the critical factor being whether or not active cell. division had begun in the imbibed embryo The duration of the periods of imbibition. that seeds can withstand depends on the species CARCELLOR SORIANO 1972 and. the stage of development of the embryo WOODRUFF 1969 Since these periods. cannot be controlled under field conditions repeated drying will result in a. reduction of the seed population, It may be concluded that seeds lying in or on the soil are subject to.
deterioration as a consequence of ageing The rate of deterioration increases. with increasing temperature and decreasing water content Reiterated drying. wetting cycles are especially harmful In fully imbibed seeds deterioration and. ultimate death are postponed by the continuous repair of damaged structures. Although seeds have a fairly efficient repair mechanism their ability to. produce vigorous seedlings gradually declines during the ageing process. 2 3 OORMANCY, The fate of a seed af ter dissemination depends largelyon external conditions. and its internal features In many cases seeds are not able to germinate. immediately af ter dissemination even when conditions seem to be favourable. These seeds are dormant GOROON 1973 The function of dormancy in determining. the timing of growth resumption whenever external conditions become suitable. is quite clear but the phenomenon itself is very complex For instance. dormancy may depend on a variety of internal features The seed coat may be. highly resistant to the diffusion of oxygen from the environment to the embryo. Furthermore the seeds coatings may contain substances that inhibit embryo. growth and have to be broken down or rinsed out before germination can start. Dormancy mayalso be governed by an internal hormonal balance between growth. inhibiting and growth promoting substances in the embryo itself i e a. balance which is inadequate for growth initiation HEMBERG1949 WAREING 1965. WAREING et al 1973, In same cases the degree of dormancy of the seeds depends on theconditions. to which the mother plant was exposed during frui t development Very of ten the. inability to germinate directly af ter dissemination disappears for na apparent. reason in a few weeks af ter ripering There is also a kind of dormancy that. can be induced in normally non dormant seeds by the application of special. treatments such as high temperatures thermo dormancy orosmotic stress. The breaking of dormancy requires a specific sequence of external factors. The li terature on dormancy and termination of dormancy is very extensi ve and at. first gives the impression that each case forms a separate problem Only. recently has some progress been made in formulating more unifying concepts. ROBERTS 1973 these new theories are mainly based on what the various. dormancy breaking agents have in common some of the latter play a role inthe. ecological situation whereas others are in use only in laboratory experiments. In a series of papers Roberts and co workers for references see ROBERTS 1973. developed the hypothesis that stimulation of the activity of the pentose. phosphate PP pathway leads to 1055 of dormancy These authors classified. the dormancy breaking agents into a number of categories and found a very. close resemblance between the stimulation of germination and the possible. stimulation of the PP pathway which for the relevant ecological factors can. be summarized as follows, nitrate is known to break seed dormancy in a large number of species. TOOLE et al 1956 STElNBAUER GRIGSBY 1957 WILLIAMS HARPER. 1965 and stimulates hydrogen acceptance by intermediates of the. PP pathway it shares this property with agents such as nitrite. oxygen and methylene blue which have been shown to stimulate. germination in certain cases, temperature has a number of quite different effects on dormancy depending on. whether the seed is dry or wet ROBERTS 1973 freshly shed. seeds that have been kept dry show a rapid 1055 of dormancy at. high temperatures af ter ripening when kept imbibed immediately. af ter harvesting they germinate only within a small temperature. range that is sometimes high 35 45 G C and in other cases low. 3 7 GC VEGIS 1964 another weIl known way of breaking dormancy. is to keep the imbibed seeds for a certain time at low temperatures. 3 5 GC stratification treatment in some cases fluctuating. temperatures are required to stimulate germination According to. ROBERTS 1973 in all these treatments a stimulation of the PP. pathway seemed to lead to a concomitant increase in the seed s. ability to germinate, light has effects on dormancy breaking invariably via the phytochrome.
system light7sensitive seeds respond to red light and their. germination is promoted whereas far red counteracts this. stimulation there does not seem to be enough evidence yet to. prove that the PP pathway is involved but there are indications. that this may be the case e g the changes in re spi rat ion seen. af ter exposure to light EVENARI 1961 and the involvement of. phytochrome in the stimulation of gibberellic acid synthesis. LOVEYS WAREING 1971, Although we still do not know the exact role of the PP pathway the above. mentioned working hypothesis is rather attractive and may help to reconcile a. number of experimental results from various sources Nevertheless differences. between species and within species between populations will have to be. quantitatively specified if we want to understand their behaviour in thefield. As an example of such a detailed approach mention should b. plant growth but much of the literature on the subject is qualitative or vague This is not surprising in view of the difficulty one encounters in attempting to di vide the edaphic factors unambiguously into physical chemical and biological classes Most physical phenomena have important effects on the chemical and

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