74 Figure 3.30: Total hydraulic resistance (Rh) of three Eucalyptus clones (a) clonal effect after 9 months and 18 months of growth; (b) water treatment effect after 9 months of growth and 18 months of growth (Different letters indicate statistical significance between clones or treatments). 103 Figure 4.5: Leaf hydraulic resistance components (petiole, venation and extravascular tissue) of Eucalyptus clonal hybrids ((a) GUA, (b) GUW and (c) GC) in response to water stress and subsequent.
INTRODUCTION
- Eucalyptus
- Eucalyptus in South Africa
- Eucalyptus and the southern African environment
- Eucalyptus and climate change
- Plant water relations
- Cohesion-Tension Mechanism
- Xylem cavitation
- Plant Hydraulic Conductance
- Plant hydraulic conductance and drought stress
- Leaf Hydraulic Characteristics
- Photosynthetic Characteristics
- Photosynthetic capacity and stomatal conductance
- Co-ordination of photosynthetic capacity and hydraulic conductance
- Biomass growth and partitioning in response to environment
- Aims and Objectives of this study
It is also possible to measure the hydraulic conductivity of the roots and locate resistances to water flow throughout the plant using the HPFM (Tyree et al., 1995). Photosynthetic capacity in response to drought stress depends on the severity, rate and duration of soil drying (Rouhi et al., 2007).
MATERIALS AND METHODS
Experimental design
Experimental Site
Water treatments
Methods: Growth and Physiology Measurements
Non-destructive measurements
- Morphological measurements: Tree height and diameter
- Photosynthetic Measurements
- Water relations measurements
Downregulation of photosynthesis in terms of photosynthetic potential is one of the earliest effects of soil drying due to water stress. Measurement of photosynthetic rate and transpiration can be used to calculate instantaneous water use efficiency, which is an indication of the amount of carbon assimilation gained per unit of water lost through transpiration.
Destructive Measurements .1 Biomass at harvest
- Whole Plant Hydraulic measurement
- Leaf Hydraulic Characteristics
Leaves represent a significant part of the resistance to water flow in test-grown trees (up to 38% of the total hydraulic resistance). Investigation of leaf components contributing to resistance to water runoff was carried out using HPFM.
Destructive measurements: Leaf Characteristics
- Leaf stomatal density
- Leaf δ 13 C measurements
A total of 6 leaves of each Eucalyptus clone exposed to each water treatment (9 treatments) were used to determine stomatal density. Stomatal density was determined using cellulose acetate film to produce a "replica" of the leaf surface.
Statistical Analysis
Replicas of both the abaxial and adaxial surfaces were made, ensuring that the replica was taken parallel to the blade midrib, on either side of the blade midrib. The prerequisites for these tests (for both light and CO2 response measurements) require that the normality of the ANOVA residuals and the residuals have the same variance. The normality of the residuals was analyzed using a non-parametric K-S test, and the variance of the residuals was analyzed by Levene's test of equality.
The measured values of leaf hydraulic resistances (intact leaf blade, petiole and lamina with minor veins cut) were subjected to a 2-way ANOVA with the same procedure followed by the assumptions of the test. The components of leaf hydraulic resistance reported as proportions of Rleaf (Rpetiole, Rextravascular tissue, Rvenation) were first √arcsine transformed and then also subjected to a 2-way ANOVA to compare significant differences between treatments. Whole shoot hydraulic resistance measurements (Rtotal leaf, Rstem and Rshoot), biomass measurements (dry weights of leaves, stems and roots), accumulated leaf area, stem diameter and biomass allocation were all also subjected to a 2-way ANOVA and its related assumptions.
The normality of the variables considered for the Pearson correlation was tested using a K-S test to ensure that the correlation was correct.
Eucalyptus Growth and Physiology in response to 18 months drought stress
Non-destructive morphological measurements: Height and Diameter
After approx. At 200 days of growth, the height of the GC clones in the control treatment exceeded the drought stress treatments (Fig. 3.2). The control treatments had the greatest height in all three clones, but the differences were not significant (Figure 3.3 (b); p = 0.127). Imposition of drought stress did not significantly affect the height growth of GUA and GUW clones.
Drought stress reduced the height growth of GC, where the most severe drought stress (acute) showed the least height growth (Figure 3.1). These results indicate that height growth was primarily determined by the clonal hybrid (GC > GU) and not by the imposition of chronic or acute drought stress. It was shown that diameter growth decreased with the introduction of drought stress, and Figure 3.6 (b) shows that the smallest tree diameter was measured in the acute drought stress treatment (mean = 33.2 mm).
The acute stress treatment had the lowest growth, thereby showing that periodic drought stress cycles affected growth more negatively than a chronic (constant, low water availability) water treatment.
- Actual Photosynthetic Rates (Spot Measurements)
The CO2 compensation point (Γ) illustrates the CO2 concentration at which net photosynthesis becomes positive (Fig. 3.15 and 3.16). Neither clone nor water treatment had any impact on the CO2 compensation point of Eucalyptus trees after 6 and 18 months of growth. CO2 compensation point was greater after 18 months of growth, but the variation within water treatments and clones was also greater (Fig.
Carboxylation efficiency (Vcmax, which is the initial slope of the A:Ci curve) was highest in the GC clone after 6 and 18 months of growth (p = 0.063 and p = 0.358, respectively; Fig. As shown in Fig. 3.11 (a) and (b) in terms of Jmax, Vcmax was lower after 18 months of growth compared to 6 months of growth (Fig. Spot measurements of photosynthesis (actual photosynthetic rate, using the top of the LiCor sun/sky chamber) were measured on Eucalyptus leaves at 6, 12 and 18 months.
An did not change by more than 0.5% in the control treatment over 18 months and 3 different seasons (Table 3.1).
Non-destructive measurements: Plant Water Relations .1 Stomatal Conductance
Water use efficiency (WUE, the amount of CO2 fixed per unit of exhaled water) was slightly higher in the GUW Eucalyptus clones, but none of the mean WUEs differed significantly from each other (Figure 3.21). GUA showed a decrease in WUE after 18 months of growth, while the WUE of GC clones increased after 18 months of growth. The chronic water treatment had the highest WUE compared to the control and acute (recovery) treatments (Figure 3.22).
Control and chronic treatments increased WUE with age (6 – 18 months), although acute recovery treatments decreased (Fig. 3.22). Stomatal conductance (gs) and assimilation rate (An) were correlated with each other to determine the relationship between the two. The acute (recovery) treatment had the strongest linear correlation (R2 = 0.551), and the overall correlation between An and gs was significant and positive (p = 0.014).
Destructive measurements: Biomass at harvest (9 and 18 months)
The GUA clone had the greatest leaf and stem biomass after nine and 18 months of growth (Table 3.3). At both harvests, the GC clone had significantly less leaf biomass than the GUA and GUW clones. Leaf biomass did not differ between water treatments after nine months, but was significantly reduced (by 30%) in the acute stress treatment after 18 months of growth (Table 3.3).
The GUA clone had significantly more stem biomass after 18 months compared to GUW and GC (p = 0.041). The control treatment had the greatest stem biomass at nine and eighteen months, but was not significantly higher than chronic or acute treatments for either crop. The acute stress treatment showed significantly less root biomass (25% reduction) than the control and chronic treatments after 18 months of tree growth.
Destructive measurements: Whole Plant Hydraulic Characteristics
The variability within measurements of the same water treatment was relatively low even after nine months, but the variability after 18 months was extremely high. Rh was higher in the control treatment after 18 months, which was the opposite trend compared to trees measured at nine months. At nine months, more than 55% of total Rh resided in the root, in all three clones.
This proportion was similar to that in Figure 3.26 (a), where more than 50% of the total biomass was found in the roots. The distribution of resistance to roots, stems and leaves was not significantly different between clones after 9 and 18 months of growth. The proportion of resistance in above-ground plant components (stems and leaves) increased by 50% up to 18 months (Fig.
There was no difference between water treatments in conferring resistances to plant components in both crops.
Growth and Physiology Characteristics: Correlated Biomass and R h Parameters
Biomass and whole plant hydraulic properties were assessed to determine if there were any relationships between these parameters. Establishing a clear correlation between, for example, biomass and hydraulic conductivity (Kh) will determine whether an increase in the number of hydraulic pathways in a eucalyptus tree will ensure an increase in total biomass in the long term. An increase in biomass increased the total number of hydraulic pathways available for water flow, thereby increasing Kh.
In addition, an increase in hydraulic conductivity could lead to higher leaf water potentials, higher stomatal conductance and higher CO2 assimilation (‘hydraulic limitation hypothesis’). Given that Rh is the inverse of Kh, Figure 3.33 shows that Rh was significantly and negatively associated with total biomass. Root biomass distribution (expressed as a percentage of total biomass) was significantly and negatively correlated with stem biomass distribution (Figure 3.34; p < 0.0001).
Growth and Physiology Characteristics: Correlated Photosynthetic and Hydraulic Parameters
Discussion
RESULTS SUMMARY
CONCLUSIONS
Leaf Characteristics
- Leaf Hydraulic Characteristics
- Leaf hydraulic characteristics and correlation with photosynthetic parameters
- Leaf hydraulic resistance and photosynthetic correlation
- Stomatal Characteristics
- Leaf δ 13 C measurements
- Discussion
- Growth and Physiology of Euclayptus clones in response to drought stress
- Growth and Physiology of Euclayptus clones in response to tree age
- The Implications of Drought Stress Severity and Duration
- Assessing the objectives of the current study
- Future suggestions for research
Leaf hydraulic resistance was also measured in response to drought stress and subsequent recovery from water stress. The water potential in response to water stress becomes more negative, increasing hydraulic tension, cavitation events, and thus hydraulic resistance (Tyree and Sperry, 1989). In GU clones, resistance in the mesophyll tissue (Extravascular) increases significantly in response to water stress (Fig. 4.5).
GU clones, however, show increased hydraulic resistance in the mesophyll tissue in response to water stress and an increase in Rleaf provided a slower recovery of gas exchange after water stress. Both GUA 380 and GUW 1700 leaves showed a drastic increase in mesophyll or extravascular tissue (extravascular) hydraulic resistance in response to water stress. Leaf dieback in response to water deficit could be considered a drought avoidance strategy that prevents further water loss.
Assessing physiological and morphological traits in response to water deficit in the current study yielded some interesting, albeit unexpected, results (Figure 5.1). Acute water stress (severe, short-term cyclical drought, with periods of recovery after re-watering) had a more negative effect on leaf and root biomass reduction compared to chronic stress (mild, long-term water stress). Clone GUA also showed the most pronounced leaf dieback in response to water deficit.
Water stress vulnerability of four Banksia species in contrasting ecohydraological habitats at Gnangara Mound, Western Australia. Photosynthetic limitations during acclimation and recovery from water stress in the drought-adapted Vitis hybrid Richter-110 (V. Response of seedling of two eucalyptus and three deciduous tree species from Ethiopia to severe water stress.
Water stress in Eucalyptus pauciflora: comparison of effects on stomatal conductance with effects on mesophyll capacity for photosynthesis and investigation of the possible involvement of photoinhibition. Photosynthetic responses of guard and mesophyll cells to CO2, O2, light and water stress in a range of related species. Xylem embolism in response to freeze-thaw cycles and water stress in ring-porous, diffuse-porous and conifer species.
Works woody plants near the point of catastrophic xylem dysfunction caused by dynamic water stress.
