Agriculture and Food Systems - Theses

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    Efficiency of nitrogen fertilization of dry-seeded rice in south-east Australia
    Humphreys, Elizabeth ( 1986)
    The efficiency and fate of fertiliser nitrogen applied to combine-sown rice were investigated in field experiments in which fertiliser timing, water regime and soil type were varied. The information gained was used to predict strategies for optimising fertiliser efficiency using current techniques, and to predict and test improved fertilisation technologies. The stage of crop growth, water regime and soil properties all had large and interacting effects on agronomic efficiency. On an infertile alkaline grey clay soil, agronomic efficiency of urea applied at sowing was very low (8 kg kg-1) compared with later applications associated with continuous flooding (up to 56 kg kg-1). The low efficiency was due to nitrification and subsequent de-nitrification during the flushing period. Nitrogen-15 balance studies indicated that 80% of the urea nitrogen was lost from the soil-plant system. The rapid nitrification rate and high loss of nitrogen on the grey soil contrasted with the very low levels of nitrate and low losses of nitrogen (10-25%) from urea applied to rice growing on an infertile acidic red soil under alternating conditions of saturation and aeration (sprinkler-irrigation). Furthermore, on a fertile acidic red soil in the same region, other authors have measured large yield responses to nitrogen applied at sowing. These differences highlight the need to consider soil properties and water management when attempting to predict optimum fertilisation strategies. Yields of sprinkler-irrigated rice (managed to replace water lost by evaporation) were reduced by more than 50% compared with rice grown under continuous flood. However, this was not due to decreased plant uptake or increased loss of fertiliser nitrogen in the sprinkler-irrigated treatments. The low yields appeared to be associated with plant water stress, decreased nitrogen mineralization (by approximately half) and location of the fertiliser nitrogen near the soil surface where root activity was probably restricted due to inadequate moisture. On the grey soil, maximum agronomic efficiencies occurred with application shortly before permanent flood or at early panicle elongation. Efficiency was doubled when urea was applied before permanent flood compared with shortly after permanent flood. The greater efficiency appeared to be associated with the deeper transport of the applied nitrogen into the soil, and consequently lower losses by ammonia volatilisation and/or nitrification/de-nitrification. However, even with the most efficient fertilisation strategies, plant 15N recoveries were less than 40%, while losses exceeded 20%. When the 15N balance data were considered in conjunction with the agronomic data,. it appeared that it would be possible to further increase agronomic efficiency if plant recovery of applied nitrogen could be increased. In particular, minimisation of losses of nitrogen (via nitrification/de-nitrification) from fertiliser applied before permanent flood was a most attractive option. Potential methods identified for increasing agronomic efficiency by minimising losses of nitrogen applied before permanent flood were deep placement and the use of nitrification inhibitors and slow release nitrogen sources. Several experiments were conducted in an attempt to improve fertiliser efficiency by banding urea and modified urea sources 5-7 cm below the soil surface before permanent flood. Plant recovery of 15N was increased by up to 20% with banding compared with surface broadcasting. The best recoveries were from urea super-granules (USG). An experimental fertiliser rig and a commercial seeder fitted with a triple disc assembly were used to band the fertilisers in the main plots. There was no significant yield advantage with banded urea over broadcast urea applied before permanent flood. The fertiliser rig caused considerable plant damage. Using the triple disc applicator, it was possible to band fertiliser below the soil surface with minimal soil disturbance, and plant damage, but only under i ideal conditions of soil moisture. Furthermore, where the soil surface was dry and cracked, penetration with the triple discs was no greater than the depth of the cracks into which surface applied urea prills would be washed upon flooding. The yield test of USG was unsatisfactory because of the excessive plant damage with this method. With current technology, there appears to be little scope for improving the efficiency of urea applied before permanent flood by mechanical placement below the soil surface.
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    Lactation in mice as a model to study genetic improvement of dairy cattle in the tropics
    Westra, Paridjata ( 1989)
    In chapter one of this thesis aspects of reproduction and production, mainly milk production were reviewed. Components of reproductive efficiency such as the fertility components: age at calving, service periods, days open, calving interval and non return rate; were also investigated. The physiological relationship between those traits and milk production and persistency, in both temperate and tropical environments, was also considered, as well as longevity or stayability as other alternatives for evaluating reproduction. Attention was put on first and second lactation performance. With a view towards understanding physiological aspects of reproduction and production in dairy cows, the nature of lactation in mice was reviewed from the relatively few works that have been done. The papers reviewed included the role of pre-natal and post-natal maternal effects and the relationship between littersize and milk-yield. Daily yield and lactation from a number of studies were compared. Theories of genotype-environment interaction (GM) and adaptation, as well as their implications, were reviewed in the subsequent part of. chapter 1. Evidence of GEE in dairy cattle (Friesian, Holstein or Friesian-Holstein, crossbred with Friesian, and native) in both reproductive characters and milk production throughout the tropic regions were examined particularly closely. Mating systems and selection responses in genetic improvement programmes also were reviewed. Constraints and shortcomings for the achievement of genetic progress through natural mating, AI and progeny-testing in the tropics were examined in conjunction with attainment of high milk production in the temperate countries. The use of new technology (MOET) for manipulating reproductive efficiency was also introduced in the hope that it can be used to increase breeding efficiency in the tropics. The milk-yield and reproductive . characters, of two genotypes of mice (one genotype had been selected for high early reproductive and milking performance and the other was a random bred control) were measured in the normal and a hot environment to check if GEI were present. Both genotypes had been developed in the normal temperature. Evidence of interaction was found in the second parity and only in reproductive traits, e.g. fecundity, interval between mating and birth of second parity. These results are supported by many studies of GEI for both milk-yield and reproductive characters in dairy cattle in the tropics. Milk production does not show GEI, if the environment is defined merely as temperature. The previous selection of the improved genotype had resulted in positive correlated responses in: litter size and litter weight at birth in the first parity, production characters (growth characters and preweaning weight of the litters) in both parities, and on milk production but not on persistency. The selected line did maintain its superiority across environments. However females from the improved genotype performed worse in almost all characters in the hot environment compared to the normal environment. In an analogy with dairy cattle, the high production capacity of the selected line was not exhibited in the adverse environment. Problems of genetic improvement in the tropics were discussed. They included the use of appropriate selection criteria based on . more knowledge of both physiological and genetical relationships between milk-yield and other-characters, with the aim of increasing adaptability and productivity in the target environment. Other possibilities for genetic improvement of dairy cattle in the tropics and the use of synthetic breeds for overcoming the problems in the long term were also discussed.
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    Studies of the preovulatory LH surge and ovulation in the ewe
    Phillips, David James ( 1989)
    The studies presented in this thesis addressed the hypothesis that the induction of the preovulatory LH surge and ovulation in the ewe involves changes in the pattern of GnRH and gonadotrophin secretion, which can be compromised by the effects of glucocorticoids from the adrenal gland. A series of experiments tested the hypothesis that administration of exogenous GnRH or gonadotrophins could induce the growth of follicles to the point of ovulation in ewes deficient in endogenous gonadotrophins. A number of regimes were tested in hypophysectomized ewes where the timing, dose and type of exogenous gonadotrophin varied, but none of these could elicit ovulations consistently. When one of these regimes was administered to hypothalamic-pituitary disconnected (HPD) ewes, a greater response was obtained (33% of ewes ovulating versus 4% in hypophysectomized ewes). Administration of exogenous pulses of GnRH was able to elicit ovulations in HPD ewes consistently (88% of ewes ovulated). These findings imply that a pituitary factor besides the gonadotrophins may be important for the normal growth and ovulation of follicles. In another group of experiments, exogenous regimes of GnRH were administered to ovariectomized HPD ewes to test whether modifications in GnRH input could effect changes in LH secretion during an oestrogeninduced LH surge. It was found that `signal' pulses of GnRH, as either a single large pulse or a rapid series of smaller pulses, were required to initiate an LH surge in the presence of oestrogen. These `signal' pulses of GnRH were unable to cause a similar surge of LH in the absence of oestrogen. A continuous infusion of 250 ng/hour GnRH decreased the magnitude of the oestrogen-induced LH surge compared to 250 ng pulses of GnRH, but elicited a significantly greater response than if GnRH input was abolished, suggesting that the basal secretion of GnRH as well as the pulsatile format was important in eliciting an LH surge. When the continuous infusion was doubled to 500 ng/hour, this treatment was as effective as the 250 ng pulses of GnRH, whereas decreasing the GnRH pulse amplitude from 250 ng to 125 ng had no effect. These findings suggest that an increased baseline level of GnRH secretion can overcome the lack of pulsatile input, and that within the range tested, the amplitude of the GnRH pulses is not critical during the oestrogeninduced LH surge. Removal of GnRH inputs following the initiation of the oestrogen-induced LH surge significantly decreased in the amount of LH secreted compared to when GnRH pulses were maintained, showing that GnRH input to the pituitary gland is still required once the LH surge has begun. To investigate the hypothesis that gonadotrophin subunit mRNA levels are dynamic during an oestrogen-induced LH surge, ovariectomized and ovariectomized HPD ewes were treated with oestrogen and sacrificed at various times during the short-term negative and positive feedback events. In the ovariectomized ewes, all gonadotrophin mRNA levels decreased progressively, with LH/3 mRNA levels being significantly less than control values by the onset of the LH surge, whereas FSH/3 and a subunit mRNA levels declined significantly during the LH surge. These findings are at variance with those reported for ovary-intact ewes, suggesting that the mechanisms responsible for the LH surge in ovariectomized ewes treated with oestrogen may be different from those involved in the preovulatory LH surge. In the ovariectomized HPD ewes, the levels of a subunit mRNA levels diminished after treatment with oestrogen, whereas the levels of LHJ3 and FSH/3 mRNA levels were unchanged, implying that the a and f3 gonadotrophin subunits may be differentially regulated. Another series of experiments investigated the hypothesis that the synthetic glucocorticoid, dexamethasone, modified reproductive function. Chronic administration of dexamethasone at rates of up to 2 mg/day had little or no effect on gonadotrophin secretion, the incidence of behavioural oestrus or ovulation rate in either the breeding or non-breeding seasons. Based on these findings, it was revealed that in the ewe, the induction of the preovulatory LH surge involves changes in the pattern of GnRH and gonadotrophin secretion, but that glucocorticoids from the adrenal gland have, at most, only a minor role in modulating these processes.
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    The long-term effects of land use on a soil profile
    Lorimer, Malcolm Strickland ( 1989)
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    The activity of glyphosate and other herbicides in soil
    Eberbach, Philip ( 1989)
    The effects of herbicides on a legume Rhizobium symbiosis were studied in laboratory experiments. Root applications of all herbicides examined reduced nodulation of legumes grown in aqueous nutrient media. The growth of Rhizobium trifolii TA1 was reduced by 2 - 20 mg ai 1-1 of diquat, 2 mg ai 1-1 of paraquat, 10 mg ai 14 of glyphosate and 2 mg ail-1 of chlorsulfuron. No other herbicide affected rhizobia growth when applied at 2 - 20 mg ai l-1 of nutrient medium. Inoculation with TA1 pre-treated with amitrole, atrazine or glyphosate decreased nodulation of sub-clover plants indicating that these herbicides may affect the nodulation potential of certain strains of Rhizobium. Residues of 2,4-D, amitrole, diquat, trifluralin and glyphosate persisted in a Walpeup sandy loam in sufficient concentration for four months after application to soil to affect growth and symbiotic activity of sub-clover. The behaviour of glyphosate in soil, under various conditions was studied in the laboratory. Adsorption of glyphosate as depicted by Freundlich K constant was greater in an acid soil than in three alkaline soils and values for this constant ranged from 8 - 67.8 at 23.5C and 4.3 - 57.8 at 10C. Rate of decomposition of 14C-glyphosate at 25C decreased slowly over the experimental period in all soils. Two compartments of adsorbed glyphosate in soil were identified as labile glyphosate and non-labile glyphosate and these reflected the strength of adsorption of the chemical. The amount of glyphosate in the labile firm for the soils ranged from 24 - 34.5% of the total and half-life ranged from 6 - 9 days. The amount of glyphosate in the non-labile form for soils ranged from 67 . 75% of the total and half-life ranged from 222 to 835 days. At 10C, the two compartments of glyphosate adsorption were identified for the Walpeup and Rutherglen soils but only one compartment could be identified in the Wimmera and Culgoa soils. Methodology was developed to permit extraction and analysis of glyphosate and AMPA in soil. Recovery of glyphosate from soils where time between fortification and extraction was only 30 sec. was 84.6 - 104%. However where extraction was delayed 13 hours, recoveries were 47.6 - 66.8%. The extractant (0.1 M triethylamine) was shown to be unable to desorb adsorbed glyphosate. Studies revealed that at 25C, the pool of extractable glyphosate was rapidly depleted by decomposition. At this temperature, the pool of extractable glyphosate was supplemented by slow desorption of non-labile glyphosate for each soil. At 10C, depletion of the pool of extractable glyphosate was considerably slower. For the Walpeup and Rutherglen soils, the rate of desorption of glyphosate from the non-labile pool was less than the rate of loss by decomposition of the herbicide. Rate of desorption of non-labile glyphosate in the Wimmera soil was shown to be the same as the rate of loss by decomposition of the herbicide. Loss of extractable glyphosate in the Culgoa soil occurred by decomposition and by slow adsorption of extractable herbicide from the labile to the non-labile form. The effects of residues of glyphosate in the field following an autumn and a summer application were investigated at selected field sites. Following the autumn application, phytotoxic activity of glyphosate was observed in sub-clover plants growing at the Walpeup and Culgoa sites but not at the Wimmera site. Growth and nodulation of plants sown up to 4 weeks after herbicide treatment were reduced at the Walpeup site. Only nodulation of plants sown up to 4 weeks after treatment was reduced at the Culgoa site. Results suggest that residues of glyphosate are only likely to significantly affect the growth of susceptible plants during winter on sandy soils. Following summer application of glyphosate, no phytotoxic activity of the herbicide was observed for sub-clover plants grown in the Walpeup sandy loam. Results suggest that in a hot summer, it is unlikely that residues of glyphosate in any soil would cause significant damage to plant growth.
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    Root and top growth of the wheat plant as affected by water supply
    Alamoodi, Ahmed S ( 1987)
    Two experiments were conducted to study the growth, particularly the root growth, of wheat in relation to moisture supply. One was conducted in an igloo house at the Mt Derrimut Field Station of the School of Agriculture and Forestry, University of Melbourne and the other in a glass house on the main campus. The plants were grown in a mixture of soil, sand and ligna peat contained in plastic bags to give columns about 0.8 m deep. In the first experiment the effects on both above-ground and below-ground growth of withholding water at different stages of growth and for different times were studied on three cultivars of wheat. In the second experiment effects of supplying different amounts of water were studied. Plants were harvested periodically for measurement of dry weights of the various plant parts, measurement of leaf area and root length and counts of tillers, heads, grains and roots. The soil columns were dismantled 20 cm at a time to enable the distribution of roots in the profile to be observed. Withholding water at any stage of growth and for any period reduced the above-ground dry matter at maturity; the longer the period of non-watering the greater the reduction. Reduction in above-ground dry matter resulted from reductions in number of tillers, leaf area and grain yield. Tillering was most affected by non-watering during early and mid-season growth, and yield by non-watering after anthesis, especially when it occurred during the first two weeks after anthesis. Non-watering after anthesis reduced yield mainly by reducing grain size (1000 grain weight). The quantity of recoverable roots, measured either by weight or by length, reached a maximum about the time of anthesis and then declined as roots died and were lost during harvest. The quantity of roots formed was reduced when non-watering was imposed during early or mid-growth. The distribution of roots in the profile was also affected by the watering treatments. Withholding water during early and mid-growth resulted in a greater proportion being located in the deeper soil zones. However, with the soil initially below field capacity, withholding water from sowing onwards resulted in shallow rooting presumably because an absolute lack of water limited the plants' ability to produce deep roots. Watering the soil to field capacity every three days in Experiment 2 resulted in less above-ground dry matter than watering to field capacity once a week. It resulted also in a greater proportion of the roots being in the upper part of the profile suggesting that root penetration of the lower part was inhibited by poor aeration as a consequence of overwatering. Watering to field capacity once a week resulted in less above-ground dry matter than giving half the amount of water needed to restore the whole soil column to field capacity. Moreover, the proportion of roots in the bottom zone of the rofile was less under the former treatment than under the latter suggesting that watering to field capacity once a week was causing some restriction of root growth in the bottom zone, presumably through poor aeration. These two facts taken together suggest that in this experiment watering to field capacity once a week even amounted to over-watering.
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    The nutrition of multiple-bearing ewes in late pregnancy
    Hall, David Graham ( 1989)
    The nutrition of multiple-bearing ewes in late pregnancy and relationships with lamb survival are reviewed (Chapter 1). The frequent reductions in voluntary intake in late pregnancy, the reliance on maternal reserves and the key role of glucose and protein are highlighted. The sensitivity to nutritional manipulation of traits associated with lamb survival and their variation due to litter size are discussed. The traits include lamb birth weight, lamb vigour, ewe mammary growth and colostrum production, maternal behaviour and gestation length. A hypothesis 'That short-term supplementation of multiple-bearing ewes late in pregnancy will improve responses associated with lamb survival' was developed. Supplementing prolific ewes with lupin grain for the final 10 days of pregnancy has previously significantly increased lamb survival rates (Chapter 2). Experiments were designed to test the hypothesis. Treatments included supplements of different grains, nitrogen or rumen undegradable protein and direct infusions of glucose. Responses measured included production traits associated with lamb survival, glucose, urea and fatty acid production rates, some hormone concentrations, voluntary feed intake and mobilisation of maternal reserves. The variation in responses was compared at various litter sizes. Treatments were selected so that results could be modified and then applied directly to grazing sheep in southern Australia. In the first experiment (Chapter 3) a lupin grain supplement fed to Booroola (prolific) ewes in the final 17 days of pregnancy reduced condition score losses compared to no supplement or an oat grain supplement. -Live weight gains were highest with the lupin supplement and with no supplement. There was a trend for milk production at day two post-partum to be higher with the lupin supplement. Some of the multiple-born lambs died because of low colostrum intake, as indicated by low serum immunoglobulin concentrations. The possible importance of colostrum and initial milk production was demonstrated. Colostrum production and milk production on day one and nine were similar from single and multiple-bearing ewes fed a ration containing 100 g protein/10 MJ metabolizable energy and this ration supplemented with formaldehyde-protected casein or urea (Chapter 4). The ration was fed at 90% of the average estimated requirements of all ewes in the final five weeks of pregnancy. Gestation length was two days shorter with the casein supplemented diet which resulted in twin-born lambs from this diet being about 14% lighter than lambs born to ewes fed the basal ration. Glucose production on day 121 was 32% higher with multiple than single-bearing ewes even though intakes were comparable. Many ewes had low voluntary intakes on the grain/roughage ration and this resulted in large energy deficits. On a roughage/ oat grain ration fed for the final seven weeks of gestation, single and multiple-bearing ewes had consistently low and equivalent intakes (Chapter 5). Nonesterified fatty acid production rates on day 115 and 136 averaged 65 % higher for multiple than single-bearing ewes and rates were similar on both days. Glucose production increased by 32 % between these days and was 17 % greater for multiple than single-bearing ewes. The correlation coefficient of fatty acid and glucose production rates was about 0.7 on both days of measurement. The provision of a large glucose source late in pregnancy may be beneficial to multiple-bearing ewes when their voluntary intake potential seems low. Multiple-bearing ewes were infused at the abomasum from day 119 to 145 of gestation with nil, 106 or 207 g glucose /day (Chapter 6). The glucose was estimated to provide an additional 20 or 40 % energy compared to the basal ration of lucerne chaff. The ewes which were infused with glucose gained more weight during the treatment period (90, 159 and 267 g/d for basal, +20% and +40% energy respectively). Litter weight, colostrum yield and early milk production were insensitive to additional energy, as glucose, in the last 30 days of pregnancy in the circumstances where ewes were in low condition and fed a restricted roughage ration containing a high percentage of protein. The insensitivity seemed to lie with increased insulin levels leading to peripheral tissue accretion. Again high variation occurred in colostrum yields, although amounts were higher than in previous experiments. There were significant positive correlations between litter weight and both progesterone and plasma ovine placental lactogen levels in late pregnancy. Colostrum production was negatively correlated to progesterone concentrations measured in late pregnancy. Intake declined close to parturition even with the glucose infused ewes. When ewes were fed a medium quality roughage ad libitum, a lupin supplement in the final 10 days of pregnancy increased colostrum yield by 37% and milk production on day 1 by 28% when averaged over all ewes (Chapter 9). The largest response occurred with the triplet-bearing ewes, which had much lower production than single and twin-bearing ewes. Lambs born to lupin supplemented ewes had faster growth rates to day 1 and 3 post-partum. Benefits occurred either through additional substrates and /or by changing hormonal status, specifically progesterone. Colostrum levels at birth and total milk production to 24 hours were significantly improved when multiple-bearing ewes were supplied with supplements of glucose, lupin grain or undegradable rumen protein in the final 10 days of pregnancy. The basal ration was a medium quality roughage fed at a restricted level. The results provided strong evidence that additional protein which escapes rumen degradation will increase colostrum production near birth and advance the timing of copious milk production. The colostrum and milk yields were negatively correlated to progesterone concentrations near parturition. Prolactin and ovine placental lactogen concentrations measured before and after parturition were poorly correlated with the lactation results. Triplet bearing ewes had lower milk production than twin bearing ewes. There were no effects of the treatments on birth weight, but large effects on ewe live weight change and gestation length. The mechanisms could be due to extra fat mobilisation in late pregnancy with additional protein supplies, changes in hormone status, or possibly the supply of extra amino acids compared to the non-protein treatments. Variability in the colostrum and milk responses on the glucose treatment made it unclear what the role of glucose was for colostrum production. Milk production of multiple-bearing ewes is likely to be below that required by their litter in a cold and wet environment in the first 24 hours when the ewes are fed a medium quality roughage diet (Chapters 7 and 10). Lambs had a potential colostrum intake in the first two hours of an average 110 g and 230 g/kg birth weight during the first day (Chapter 8). This amount was usually above that available to all lambs in the litter from the dam. The potential intake was also above that required in most environments. Limited variation in intake between lambs within a litter may be crucial to survival of the complete litter. It is proposed that protein has a specific effect on the endocrinology of the ewe in late pregnancy. Thus a possible hypothesis consistent with these data is that increased amino acid supply at the tissue level results in a faster clearance of progesterone allowing lactogenic hormones and hormones involved with the initiation of parturition to act. These experiments have thus demonstrated that short-term supplementation of ewes can influence some traits associated with lamb survival, including colostrum production at birth and milk production during the following 24 hours. Multiplebearing ewes will often have lower amounts of colostrum and early milk production than single bearing ewes. Supplementing ewes on medium quality protein/energy pastures with feed sources which provide a high protein yield at the small intestine should increase the initial lactation output and possibly survival rates of multiple-born lambs.
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    Study of some aspects of elm root sucker growth (Ulmus x hollandica Mill.)
    Yau, Dickon Peter (1946-) ( 1989)
    Elm suckers developed from root buds and/or bud primordia formed on root surfaces, and originated from pericyclic tissues beneath the periderm, and are therefore of endogenous origin. Elm root sucker formation was an auxin mediated process, and was related to the level of endogenous auxin at that particular time. Root segments suckered most in spring and least in autumn. Application of exogenous anti-auxin and cytokinin stimulated sucker formation from root segments in autumn, and application of auxin or growth inhibitors suppressed sucker formation in spring. The elongation rate of root suckers of elms decreased as the soil dried out and the soil penetrometer resistance increased. The effect of the soil drying out on increasing the soil penetrometer resistance was not in itself sufficient to account for the reduction in elongation rate as the soil dried. It was suggested that the reduction in soil unsaturated hydraulic conductivity (and the consequent reduced water uptake by the sucker) as the soil dried out was an additional reason for the reduced elongation rate. The osmotic potential of root suckers decreased as the soil dried out and as soil penetrometer resistance increased. In this instance, the effect of the soil drying out on increasing the soil penetrometer resistance was sufficient in itself to account for the reduction in osmotic potential (osmoregulation). As a consequence of this osmoregulation, turgor pressure either increased or was maintained at a constant value as the soil dried and as the soil penetrometer resistance was increased. The soil penetrometer resistance was found to be three times the value of the soil mechanical resistance to sucker growth. The net growth pressure (turgor pressure less soil mechanical resistance) decreased with soil penetrometer resistance. Sucker elongation rate increased with net growth pressure, but sucker elongation rate also increased with soil water content at constant net growth pressure. Elongation rate increased with time. Osmotic potential decreased; and turgor pressure and net growth pressure increased with time. The rate of decrease in osmotic potential and the rate of increase in turgor pressure and net growth pressure increased with soil penetrometer resistance, but this effect became less over time. The data suggest that elm root suckers can osmoregulate over time to reach a minimum value of osmotic potential (or maximum value of turgor pressure) which is independent of soil penetrometer resistance, but that the rate at which this minimum osmotic potential (maximum turgor pressure) is achieved increases with soil penetrometer resistance. Elm sucker elongation rate increased as the axis of elongation approached the geotropically preferred axis, ie the vertical upright position. Elm suckers exerted a maximum growth pressure of 0.63 MPa (SE=0.057) in a critical time of 5 to 6 days. Elm suckers exerted a buckling pressure of 0.34 MPa (SE=0.052) in a critical time of 24 to 30 hours. Elm suckers could not detect cracks or holes in a compacted soil by remote-sensing (ie no trematotropism in suckers). Root buds if pre-exposed to cracks had a better chance to develop into suckers than root buds not pre-exposed. Suckers preferentially selected growth pathways in the vertical upright direction provided no pores wider than 2mm existed in its way. If there were pores wider than 2mm diameter existed in its way, the sucker abandoned its vertical growth path preference and selected the wider pores instead even though the wider pores were set at an angle from the vertical.
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