whole_DavisGordonRichard1988_thesis.pdf (8.34 MB)
Laboratory studies of the effect of soil impedance and soil moisture on root elongation of seedlings of Pinus radiata D.Don.
thesisposted on 2023-05-26, 23:33 authored by Davis, G. R.(Gordon Richard)
In a drying soil root elongation rate often declines. Whether the reduction in elongation is due to reduced soil moisture or increased soil strength is disputed. The objectives of this study were to develop an empirical model of radiata pine root elongation and to determine the relative importance of soil strength and soil moisture content on root elongation, and further, to quantify the parameters of a net-pressure mechanistic model of root elongation for pine and to compare the values with those observed for a more commonly studied crop (peas). The empirical model showed seedling root elongation to be three times more sensitive to increased soil strength than to decreased soil moisture content. The observed pattern of root behavior could be logically explained by a concept of total external stress, where total external stress was a summation of soil moisture potential and an index of penetrometer resistance. Measurement of pine root segment osmotic potential gave values for cell threshold pressure (below which no elongation takes place) and cell wall elasticity consistent with values reported in the literature for other crops. The response of pine and pea seedlings to moisture stress in the absence of soil restraint was recorded. Although root elongation of both species declined as moisture potential decreased, pine seedlings showed no osmotic adjustment to moisture potential and consequently cell turgor decreased as root elongation declined. However, for peas, turgor was maintained but the correlation between turgor and elongation was poor, suggesting that the decline in elongation was due to increased threshold pressure or decreased cell wall elasticity. Measurement of pea root potential, after seedling growth in pots packed with soil of known penetrometer resistance and moisture potential, showed 11% osmotic adjustment to soil restraint and 41% adjustment to moisture potential. The data was consistent with the empirical data for radiata pine root elongation. Root behavior, on growing into a more compact soil layer, was consistent with a net-pressure model of root elongation when studied in situ using a neutron radiography technique. The technique was refined to allow a greater number of exposures for a given reactor run. Soil strength can be considered a general influence on radiata pine root growth except where extremes of soil moisture exist. Any increase in soil strength may reduce the ability of the root to tolerate moisture stress. The general influence of soil impedance on root growth and the slow recovery of some soils after compaction, suggests a better understanding of the likely effects of forest operations on soil strength is needed if productivity losses following harvesting are to be minimized.
Rights statementCopyright 1987 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s). Thesis (M.Sc.)--University of Tasmania, 1988. Bibliography: p. 150-168