CONCLUSIONS
4. Leaf Characteristics
5.1 Growth and Physiology of Euclayptus clones in response to drought stress
FINAL CONCLUSIONS
Figure 5.1: Summary of results of physiological and morphological parameters that are affected by eucalypt clone, water treatment or tree age in plants grown for 9 or 18 months.
RESULTS SUMMARY
Physiological and Morphological Parameters Are Affected By:
CLONE
• Height (GC > GU)
• Diameter Growth Rate (GUW > GUA &
GC)
• Growth Efficiency (GC > GU)
• Volume (GC & GUW
> GUA)
• An (GUW was signif lower at 6 months)
• Root:shoot (GC >
GU)
• SLA (GUW > GUA
& GC)
• Kh (signif lower in GUA at 9 months)
• Leaf Area (GU > GC) GC: Growth efficiency
GUW: Diameter growth GUA: Leaf area supporting growth
WATER TREATMENT
• Diameter (control >
stress)
• Volume (control >
stress)
• Jmax (↓ at acute wilting point)
• Vcmax (↓ at acute wilting point)
• En + WUE (↓ in chronic treatment)
• gs (↑ in control treatment)
• SLA (acute > control &
chronic)
• Leaf Area (control >
stress)
Control: Diameter; leaf area Chronic: Improved WUE Acute: SLA was greatest
AGE
• An & En (↓ with age and winter)
• gs (↑ with age and winter)
• Biomass allocation (↓
to leaves; ↑ to stems)
• SLA (significant only at 18 months)
• Allocation of resistance to water flow (↑ to leaves; ↓ to roots)
• Leaf Area (significant only at 18 months) At 18 months: change in allocation to biomass and resistance; leaf area maintained is significantly different between clone and water treatment
CONCLUSIONS
Clone: GUW clones maintain greater diameters, improved WUE, and greater above-ground biomass Water treatment: Chronic water stress (small water deficits long-term) improve WUE, maintain
greater diameters, more leaf area than acute water stress (severe, short-term water stress) Age: Leaf area and allocation of biomass and resistance are controlled ontogenetically
Leaf Area is the morphological parameter driving physiological changes
Figure 5.2: Summary of leaf characteristics that are affected or not affected by eucalypt clone or water treatment.
RESULTS SUMMARY Leaf Characteristics
ARE AFFECTED BY:
PARAMETERS NOT AFFECTED
• Stomatal size
• δ15N
CLONE
• R
leaf recovery time (by Day 2 in GC; day 7 in GU)
• R
leaf – contribution of petioles, veins and mesophyll to resistance
• A
n and g
s recovery time (quickest in GC)
• Stomatal density (NO stomata on upper leaf surface of GUA clone)
GC: Recovery time from water stress is rapid for R
leaf, A
n and g
s
GU: Slower to recover, GUA more conservative with water loss (no stomata on upper leaf surface)
WATER TREATMENT
• R
leaf (highest in acute water stress)
• A
n and g
s (lowest in acute water stress)
• δ13C (less negative in chronic water stress treatment)
Control: Ideal but unlikely in reality Chronic: Improved WUE, not significantly less than the control Acute: Most negative impact on leaf productivity and no improved WUE)
CONCLUSIONS
Clone: GC clones recover more rapidly than GU but GUA leaves lose less water due to fewer stomata Water treatment: Chronic stress does not affect leaves as negatively as acute stress. Chronically stressed
leaves show improved WUE compared with the control and acute stress
Both the GU clones (GUA380 and GUW1700) produced more stem biomass than did the GC clone. While plants of the GUW clone achieved greater diameters than those of the GUA clone, the GUA clone produced 30% more stem biomass than the GUW clone after 18 months growth. From personal observations, the GUA clone lost a significant amount of leaf area after an acute water stress event. Leaf dieback in response to water stress could be considered a drought avoidance strategy that prevents further water loss. Beyond a certain point, hydraulic dysfunction from drought can be non-recoverable, and the sacrificial death of disposable plant organs (e.g. leaves) will improve hydraulic conductance and water status in the remaining foliage (Holloway-Phillips and Brodribb, 2011). Recovery of leaf area after water stress is time-consuming, and can take up to 100 days in some woody species (Brodribb and Cochard, 2009), which therefore slows down the maximum diameter growth attained.
The GUW clone produced less stem biomass, but it lost 20% less leaf area than the GUA clone following an acute drought stress event. The GUW clone showed enhanced traits of drought tolerance compared with the GUA clone in response to acute drought stress.
Moderately improved instantaneous water use efficiency (WUE) was also shown in the GUW clone in plants subjected to the chronic water stress treatment. Commercial planting of GU clones should be assessed in conjunction with long-term weather and soil data of the sites to be planted. The GUA380 clone would produce significantly greater stem biomass, provided only that extreme weather conditions (e.g. severe drought) were not experienced or predicted in the long-term. The GUW1700 clone, while less productive, was considerably more drought tolerant and would be a less risky clone choice, at a time where climate change is predicted to cause further extreme weather events.
Clone or water treatments affected some, but not all leaf characteristics (Fig. 5.2). Acute water stress increased the leaf hydraulic resistance to water flow while simultaneously decreasing assimilation rate and stomatal conductance. The leaf recovery time (after re- watering) was however, clone dependent. Leaves of the GC clone recovered from stress (i.e.
Rleaf, An, and gs were not significantly different from the control treatment) as soon as 2 days of re-watering. The GU clone recovered from stress after approximately 7 days after re- watering.
Rapid recovery of Kleaf (inverse of Rleaf), seen in the GC leaves, is likely if the primary cause of leaf hydraulic dysfunction is cell collapse of the minor veins (Blackman et al., 2010).
Beyond a certain point non-recoverable hydraulic dysfunction can occur, and this phenomenon was noted with the GUA clone in response to the acute water stress treatment.
Brodribb and Feild (2010) reported that Kleaf can be related to vein density and hydraulic architecture and there is an influence on gaseous exchange and water flux. Leaf hydraulic recovery could be considered a key functional trait linked to leaf structure, leaf anatomy and ecological tolerance.
One of the fascinating observations from the study was that stomata were not present on the upper surface of the GUA clone unlike the closely related GUW clone. No information could be found regarding stomatal density studies on southern African Eucalyptus grandis clonal hybrids. The absence of stomata on the upper leaf surface could be deemed to be a drought avoidance strategy of the GUA clone, reducing transpiration and preventing leaf dieback during drought stress.