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Desiccation tolerance and sensitivity of vegetative plant tissue.Sherwin, Heather Wendy. January 1995 (has links)
There is a great deal of work currently being done in the field of desiccation tolerance. Generally workers studying desiccation-tolerant plant tissues have concentrated on the
mechanisms of desiccation tolerance without concomitant studies on why most plants cannot survive desiccation. The present study considers both a desiccation-tolerant plant
as well as a range of desiccation-sensitive plants. The work incorporates physiological, biophysical, biochemical and ultrastructural studies in an attempt to get a holistic picture
of vegetative material as it dries and then rehydrates.
The plant species used in this study are: Craterostigma nanum, a so-called resurrection plant; Garcinia livingstonei, a drought-tolerant small tree; Isoglossa woodii, an
understorey shrub which shows a remarkable ability to recover from wilting; Pisum sativum seedlings, which have a very high water content at full turgor; and finally, Adiantum raddianum, the maiden hair fern, which wilts very quickly and does not recover from wilting. The desiccation-tolerant plant, C. nanum, had an unusual pressure-volume (PV) curve which indicated that while large volume changes were taking place there was little concomitant change in pressure or water potential. The unusual nature of this PV curve
made it difficult to assess the relative water content (RWC) at which turgor was lost. The desiccation-sensitive plants exhibited standard curvi-linear PV curves. The amount of nonfreezable water in the five species was studied and found to show no correlation with the ability to withstand dehydration or with the lethal water content. There were no differences in the melting enthalpy of tissue water between the tolerant and most of the sensitive plants. Isoglossa woodii had a lower melting enthalpy than the tolerant and the other sensitive species. Survival studies showed that the desiccation-sensitive plants all had similar lethal RWCs.
The tolerant plant survived dehydration to as low as 1% RWC, recovering on rehydration within 24 hours. Membrane leakage studies showed that the sensitive plants all exhibited membrane damage at different absolute water contents, but very similar RWCs and water potentials. The increase in leakage corresponded to the lethal RWC for all the sensitive species. The desiccation-tolerant plant recovered from dehydration to very low water contents and did not show an increase in membrane leakage if prior rehydration had taken place. Without prior rehydration this tolerant plant exhibited an increase in leakage at similar RWCs and water potentials to that of the sensitive species. There did not appear to be much difference in the RWC at which damage to membranes occurred whether the material was dried rapidly or slowly. Respiration and chlorophyll fluorescence were studied to determine what effect drying and rehydration have on the electron transport· processes of the leaf. The chlorophyll fluorescence studies gave an indication of damage to the photosynthetic apparatus. Both
qualitative changes as well as quantitative changes in fluorescence parameters were assessed. Characteristics like quantum efficiency (Fv/Fm)remained fairly constant for a
wide range of RWCs until a critical RWC was reached where there was a sharp decline in Fv/Fm. Upon rehydration, C. nanum recovered to pre-stress levels, I. woodii showed no recovery and no further damage on rehydration, whilst the other species exhibited even
more damage on rehydration than they had on dehydration.
Respiration remained fairly constant or increased slightly during drying until a critical RWC was reached at which it suddenly declined. The RWC at which this decline occurred
ranged from 15% and 20% in P. sativum and C. nanum respectively, to 50% for G. livingstonei. On rehydration respiration exceeded the levels measured in dehydrated
material for the sensitive species. Unsuccessful attempts were made to fix material anhydrously for ultrastructural studies so standard fIxation was used. The ultrastructural studies revealed that changes had occurred in the ultrastructure of leaves of the sensitive species dried to 30% RWC particularly in A. raddianum and P. sativum. Drying to 5% RWC revealed extensive ultrastructural
degradation which was worsened on rehydration in the sensitive species. The tolerant species showed ultrastructural changes on drying but these were not as severe as occurred in the sensitive species. The cell walls of the tolerant species folded in on drying. This folding was possibly responsible for the unusual PV curves found in this species. At 5% RWC the cells were closely packed and very irregular in shape. The cell contents were clearly resolved and evenly spread throughout the cell. The large central vacuole appeared to have subdivided into a number of smaller vacuoles. On rehydration the cells regained their shape and the cell contents had moved towards the periphery as the large central vacuole was reformed. Beading of membranes, which was common in the sensitive
species, was not found in the tolerant species suggesting that membrane damage was not as severe in the tolerant species. Western Blot analysis of the proteins present during drying was performed to determine whether a class of desiccation-induced proteins, called dehydrins, were present. These proteins have been suggested to play a protective role in desiccation-tolerant tissue. It was found that C. nanum did, in fact, possess dehydrins, but so did P. sativum. The other three sensitive species did not show any appreciable levels of dehydrin proteins. The presence of dehydrins alone is, therefore, not sufficient to confer desiccation tolerance. While physiologically the damage occurring in the sensitive plants was similar to that of the tolerant plant, at an ultrastructural level the damage appeared less in the tolerant plant. On rehydration from low RWCs damage appeared to become exacerbated in the sensitive plants. This was in contrast to the tolerant plant where damage was apparently repaired. There appears, therefore, to be a combination of protection and repair mechanisms responsible for the ability of C. nanum to tolerate desiccation. The lethal RWC of the sensitive species was higher than that at which protective mechanisms, such as water replacement, might come into play. So it is not just the possible ability to replace tightly
bound water that set the tolerant plant aside. It must also have mechanisms to tolerate damage at the higher RWCs which were damaging and lethal to the sensitive plants. The
lethal damage to sensitive species appeared to be related to a critical volume, thus it is concluded that the tolerant plant had the ability to tolerate or avoid this mechanical damage during drying as well as the ability to remain viable in the dry state. It is hypothesised that the ability of the walls to fold in and the unusual nature of the PV curve may provide some answers to the enigma of desiccation tolerance. / Thesis (Ph.D.)-University of Natal, 1995.
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Measurement of water potential using thermocouple hygrometers.Savage, Michael John. January 1982 (has links)
Theory predicts that the time dependent voltage curve of a thermocouple
psychrometer where there is no change in output voltage with
time during the evaporation cycle defines the wet bulb temperature T[w]
corresponding to the water potential. In practice, a change in
voltage with time does occur and it is convenient to define the
voltage corresponding to the water potential as the maximum point of-
inflection voltage.
A predictive model based on calibration data at a few tempertures
is used to obtain the psychrometer calibration slope at any temperature.
Use of this model indicates that psychrometers differ from
each other and therefore must be individually calibrated if accuracy
better than ±5 % in the measurement of water potential is required.
Dewpoint hygrometers are shown to be less temperature sensitive than
psychrometers and have the added advantage of a voltage sensitivity
nearly twice that of psychrometers, typically -7,0 x 10¯³ μV/kPa compared
to -3,7 x 10¯³ μV/kPa at 25 °C.
The accurate temperature correction of hygrometer calibration curve
slopes is a necessity if field measurements are undertaken using either
psychrometric or dewpoint techniques. In the case of thermocouple psychrometers,
two temperature correction models are proposed, each
based on measurement of the thermojunction radius and calculation of
the theoretical voltage sensitivity to changes in water potential.
The first model relies on calibration at a single temperature and the second at two temperatures. Both these models were more accurate
than the temperature correction models currently in use for four leaf
psychrometers calibrated over a range of temperatures (15 to 38°C).
The model based on calibration at two temperatures is superior to
that based on only one calibration. The model proposed for dewpoint
hygrometers is similar to that for psychrometers. It is based on the
theoretical voltage sensitivity to changes in water potential. Comparison
with empirical data from three dewpoint hygrometers calibrated
at four different temperatures indicates that these instruments
need only be calibrated at, say 25°C, if the calibration slopes are
corrected for temperature.
A model is presented for the calculation of the error in measured
thermocouple hygrometric water potential for individual hygrometers
used in the dewpoint or psychrometric mode. The model is based on
calculation of the relative standard error in measured thermocouple
psychrometric water potential as a function of temperature. Sources
of error in the psychrometric mode were in calibration of the instrument
as a function of water potential and temperature and in voltage
(due to electronic noise and zero offsets) and temperature measurement
in the field. Total error increased as temperature decreased,
approaching a value usually determined by the shape of the thermocouple
junction, electronic noise (at low voltages less than 1 μV)
and errors in temperature measurement. At higher temperatures,
error was a combination of calibration errors, electronic noise and
zero offset voltage. Field calibration data for a number of leaf
psychrometers contained total errors that ranged between 6 (at a °C)
and 2 %(at 45 °C) for the better psychrometers and between 11 (at
0° C) and 5 % (at 45 C) for the worst assuming that the zero offset was
0,5 μV. Zero offset values were less than 0,7 μV at all times. The
dewpoint errors arose from calibration of the dewpoint hygrometer as
a function of water potential, extrapolation of the calibration slope
to other temperatures, setting the dewpoint coefficient and errors in
voltage and temperature measurement. The total error also increased
as temperature decreased, because of the differences in temperature
sensitivity between dewpoint and psychrometric calibration constants.
Consequently, the major source of error in the dewpoint mode arose
from the difficulty in determining the dewpoint coefficient. This
error, which is temperature dependent, contains three subcomponent errors; the temperature dependence, random variation associated with
determining the temperature dependence and error in setting the
correct value. Calibration and extrapolation errors were smaller
than those of the psychrometric technique. Typically, the error in
a dewpoint measurement varied between about 6 and 2 % for the best
hygrometer and between 10 and 3 % for the worst for temperatures
between 0 and 45 °C respectively. At low temperatures, the dewpoint
technique often has no advantage over the psychrometric technique,
in terms of measurement errors.
In a comparative laboratory study, leaf water potentials were
measured using the Scholander pressure chamber, psychrometers and hydraulic
press. Newly mature trifoliates cut from field grown soybean
(Glycine max (L) Merr. cv. Dribi) were turgidified and, after
different degrees of dehydration, leaf water potential measured.
One leaflet from the trifoliate was used for the thermocouple
psychrometer and another for the press while the central leaflet
with its petiolule was retained for use in the pressure chamber.
Significant correlations between measurements using these instruments
were obtained but the slopes for hydraulic press vs psychrometer
measurement curve and hydraulic press vs pressure chamber were 0,742
and 0,775 respectively. Plots of pressure-volume curves indicate
that the point of incipient plasmolysis was the same (statistically)
for the thermocouple psychrometer and the pressure chamber, but
much larger for the hydnaulic press. The above-mentioned differences
between the three instruments emphasize the need for calib rating
the endpoint defined us i ng the press against one or more of the standard
techniques, and, limi ting the use of the press to one person.
Cuticular resistance to water vapour diffusion between the substomatal
cavity and the sensing psychrometer junction is a problem
unique to leaf psychrometry and dewpoint hygrometry; this resistance
is not encountered in soil or solution psychrometry. The cuticular
resistance may introduce error in the leaf water potential measurement.
The effect of abraiding the cuticle of Citrus jambhiri to
reduce its resistance, on the measured leaf water potential was
investigated. Psychrometric measurements of leaf water potential
were compared with simultaneous measurements on nearby leaves using
the Scholander pressure chamber, in a field situation. Leaf surface
damage, due to abrasion, was investigated using scanning electron
microscopy. Thermocouple psychrometers are the only instruments which can
measure the in situ water potential of intact leaves, and which may be
suitable for continuous, non-destructive monitoring of water potential.
Unfortunately, their usefulness is limited by a number of difficulties,
among them fluctuating temperatures and temperature gradients within
the psychrometer, sealing of the psychrometer chamber to the leaf,
shading of the leaf by the psychrometer and resistance to water
vapour diffusion by the cuticle when the stomates are closed. Using
Citrus jambhiri, several psychrometer designs and operational
modifications were tested. In situ psychrometric measurements compared
favourably with simultaneous Scholander pressure chamber
measurements on neighbouring leaves, corrected for the osmotic
potential and the apparent effect of "xylem tension relaxation"
following petiole excision.
It is generally assumed that enclosure of a leaf by an in situ
thermocouple psychrometer substantially modifies the leaf environment,
possibly altering leaf water potential, the quantity to be
measured. Furthermore, the time response of leaf psychrometers to
sudden leaf water potential changes has not been tested under field
conditions. In a laboratory investigation, we found good linear
correlation between in situ leaf psychrometer (sealed over abraided
area) and Scholander pressure chamber measurements (using adjacent
leaves) of leaf water potential, 2 to 200 minutes after excision
of citrus leaves. A field investigation involved psychrometric
measurement prior to petiole excision, and 1 min after excision,
simultaneous pressure chamber measurements on adjacent citrus
leaves immediately prior to the time of excision and then on the psychrometer
leaf about 2 min after excision. Statistical comparisons
indicated that within the first two minutes after excision, psychrometer
measurements compared favourably with pressure chamber measurements.
There was no evidence for a psychrometer leaf water potential
time lag. For the high evaporative demand conditions, water
potential decreased after excision by as much as 700 kPa in the
first minute. Psychrometer field measurements indicated that within
the first 5 min of leaf petiole excision, the decrease in leaf water
potential with time was linear but that within the first 15 s, there
was a temporary increase of the order of a few tens of kilopascal. The thermocouple psychrometer can be used to measure dynamic changes
in leaf water potential non-destructively, with an accuracy that
compares favourably with that of the pressure chamber.
Using in situ thermocouple leaf hygrometers (dewpoint and
psychrometric techniques employed) attached to Citrus jambhiri
leaves, an increase in measured water potential immediately following
petiole excision was observed. The increase ranged between
20 to 80 kPa and occurred 30 s after petiole excision and 100 s
after midrib excisions. No relationship between the actual leaf
water potential and the increase in water potential due to excision,
was found. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1982.
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Internal pressurisation and convective flow in two species of emergent macrophyte; Typha domingensis and Phragmites australis / by Sean D. White.White, Sean D. (Sean Darren) January 1999 (has links)
Bibliography: p. 171-181. / xxiii, 181 leaves : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / The comparision of the pond and field plants suggests both species are eminently more suited to static water regime where adaptations by the plant can be made effectively to help with the problem of oxygen transport. The results also suggest that plants experience a variable and static water regime differently. / Thesis (Ph.D.), University of Adelaide, Dept. of Botany, 1999
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Seminal roots of wheat : manipulation of their geometry to increase the availability of soil water and to improve the efficiency of water use / by Wayne S. MeyerMeyer, Wayne Stewart January 1976 (has links)
xv, 217 leaves : ill., tables, graphs, photos ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Agronomy, 1977
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The development and water use of moisture-stressed and non-stressed sorghum (Sorghum Bicolon (L.) Moench)O'Neill, Michael Kirkbride. January 1982 (has links) (PDF)
Thesis (Ph. D. - Plant Sciences)--University of Arizona, 1982. / Includes bibliographical references (leaves 63-74).
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Leaf senescence and water stress in wheat seedlings /French, Robert John. January 1985 (has links) (PDF)
Thesis (Ph. D.)--University of Adelaide, Dept. of Plant Physiology, 1985. / Includes bibliographical references (leaves 245-271).
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Water stress and disease development in Eucalyptus marginata (jarrah) infected with Phytophthora cinnamomi /Lucas, Anne. January 2003 (has links)
Thesis (Ph.D)--Murdoch University, 2003. / Thesis submitted to the Division of Science and Engineering. Bibliography: leaves 219-235.
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Hydrologic responses of tallgrass prairie ecosystems in the EcoCELLs to seasonal and interannual climate variabilityBatts, Candace. January 2005 (has links)
Thesis (M.S.)--University of Nevada, Reno, 2005. / "May, 2005." Includes bibliographical references. Online version available on the World Wide Web.
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Transpiration as the leak in the carbon factory : a model of self-optimising vegetation /Schymanski, Stanislaus Josef. January 2007 (has links)
Thesis (Ph.D.)--University of Western Australia, 2007.
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The influence of canopy structure and epiphytes on the hydrology of Douglas-fir forests /Pypker, Thomas G. January 1900 (has links)
Thesis (Ph. D.)--Oregon State University, 2005. / Printout. Includes bibliographical references (leaves 165-177). Also available on the World Wide Web.
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