The response of three Eucalyptus spp. clones (GC550, GU210 and TAG14) to water
availability was assessed in terms of growth, plant water status, leaf gas exchange,
whole plant hydraulic characteristics (both root and shoot), stem xylem vulnerability.
Furthermore, to experimentally assess the influence of hydraulic conductance on leaf
physiology and plant growth, specimens of two of the clones were subjected to long-term
root chilling. Prior to harvesting data were collected on the diurnal variation in
leaf water potential (ΨL), transpiration rate (E), stomatal conductance (gs) and net CO2
assimilation rate (A). Main stem xylem vulnerability was assessed using ultrasonic
acoustic emissions (UAE). Vulnerability of the main stem was assessed as the leaf
water potential corresponding to the maximum rate of acoustic emissions (ΨL,
EPHmax), and as the critical water potential triggering cavitation events, calculated as
the mean of the water potentials of data points lying between 5 and 10% of the total
accumulated emissions (ΨCAV,cUAE,%). Hydraulic conductance was measured on
roots and shoots using the high-pressure flow meter (HPFM). Root data were
expressed per unit root dry mass (Kr/trdw) and per unit leaf areas (Kr/LA), shoot data
expressed per unit shoot dry mass (Ks/tsdw) and per unit leaf area (Ks/LA), and whole
plant conductance was expressed per unit leaf area (KP/LA). Soil-to-leaf hydraulic
conductance was also assessed as the inverse of the slope of the relationship between
leaf water potential and transpiration rate (the evaporative flux, EF, method).
A field study was undertaken on three month old TAGl4 and GU210 plants. Diurnal
values of leaf water potential ΨL, E and gs were consistently higher in TAG14 than
GU210, but A did not differ among the clones. Main stem xylem vulnerability (ΨCAV,
cUAE,%) was higher in TAG14 than GU210. In both clones midday ΨL fell below
ΨcAv,cUAE,%, suggesting lack of stomatal control of xylem cavitation. Kr/LA was
higher in TAGl4 than GU210, whereas, Ks/LA and Ks/tsdw was higher in GU210 than
TAGI4. A greater proportion of hydraulic resistances resided in the roots, particularly
in GU210. Kp/LA was higher in TAGl4 than GU210 clone, although the significance
was marginal (P=0.089). However, all the physiological measurements, were
consistent with the concept of higher hydraulic conductances in TAGl4 leading to
lower leaf level water stress. Above ground biomass was higher in TAG14 than
GU210, in agreement with this concept, although this clone was more vulnerable than
GU210.
Material grown for 14 months in 25 l pots clones showed no differences in ΨSoil
between the high and low watering supply, indicating that even the 'high' supply was
inadequate to prevent water stress. In accordance with this, diurnal values of ΨL, gs, E
and A did not differ significantly between treatments and clones. Early stomatal
closure was apparent, maintaining ΨL constant during the middle of the day. Stem
xylem vulnerability, assessed as both ΨL,EPHrnax and ΨCAV,cUAE,% showed that the
main stem of GC550 was more vulnerable than other two clones, and that low
watered plants were more resistant to xylem cavitation than those receiving high
water. Midday ΨL fell below the vulnerability values assessed by both measures
across treatments and clones, suggesting lack of stomatal control preventing stem
xylem cavitation. There was no relationship between stem xylem cavitation and the
shoot hydraulic conductances. Root pressures did not differ between either treatment
or clones. Kr/LA was marginally higher in high watered plants, and Ks/LA and Ks/tsdw
were higher in low watered plants, possibly by adjustment of leaf hydraulic
architecture, and there were no clonal differences. Kr/LA was much lower than Ks/LA.
Kp/LA did not differ between the watering treatment, but there was a clonal effect.
Growth in dry mass was higher in high watered than low watered plants, but there
were no differences among clones. As KP/LA was not affected by watering treatment
there was no relationship between KP/LA and growth in total biomass.
In plants grown for 21 months in 85 l pots low water treatment decreased midday ΨL,
gs, E and A relative to high watered plants. Interclonal differences occurred at midday.
Stem xylem vulnerability assessed as ΨCAV,cUAE,% and as ΨL,EPHrnax show similar
trends as in the 14 months saplings, clonal differences being significant in ΨL,
EPHmax. There was a 1:1 relationship between minimum leaf water potential and ΨL,
EPHmax, suggesting that the water potential developed was limited by stem
vulnerability. This implies stomatal control to reduce transpiration rates to prevent
extensive cavitation occurring. These plants did not develop positive root pressures,
indicating that recovery from xylem cavitations occurred through some other process.
Kr/LA was higher in high watered plants than those receiving low water, and clonal
differences were observed in Kr/trdw. There was no treatment effect in KS/LA and
KS/tsdw, but a clonal effect was apparent. KP/LA was significantly different between
treatment, and was reduced by low water in two clones, and increased by this in
TAGI4. Reduced water availability reduced biomass production, with a greater effect
on roots than shoots, such that low watering reduced root:shoot ratios. There was a
weak but significant relationship between whole plant hydraulic conductance and
maximum stomatal conductance, and between plant conductance and total biomass
produced; these data are consistent with the concept of some hydraulic limitation to
growth.
Root chilling (achieved through chilling the soil) of two of the clones was used to
experimentally manipulate hydraulic conductance to test the hydraulic limitation
hypothesis. Short-term root chilling decreased both Kr/LA and KP/LA in both clones, but
had marginal effects on leaf gas exchange. With long-term chilling the decrease in
Kr/LA was observed only in GU210, with TAGl4 showing some adjustment to the
treatment. As the roots constitute the major hydraulic resistance, KP/LA largely
reflected those of the roots. Long-term root chilling significantly affected leaf
physiological characteristics, despite the lack of effect on hydraulic conductance in
TAGI4. Long term chilling decreased the whole plant dry mass, but the effect was
smaller in TAGI4, and this clone also showed morphological adjustment, in that root
growth was less adversely affected than shoot growth. The data from GU210 support
the hydraulic limitation hypothesis; because of the morphological and physiological
adjustment to long-term root chilling in TAGI4, the data are unsuitable to directly
assess the hypothesis. / Thesis (Ph.D.)-University of Natal, Durban, 2002.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/4409 |
| Date | January 2002 |
| Creators | Manoharan, Printhan. |
| Contributors | Pammenter, Norman W. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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