Canopy development and hydraulic function in Eucalyptus tereticornis grown in drought in CO2-enriched atmospheres

Abstract

We report on the relationship between growth, partitioning of shoot biomass and hydraulic development of Eucalyptus tereticornis Sm. grown in glasshouses for six months. Close coordination of stem vascular capacity and shoot architecture is vital for survival of eucalypts, especially as developing trees are increasingly subjected to spasmodic droughts and rising atmospheric CO2 levels. Trees were exposed to constant soil moisture deficits in 45 L pots (30–50% below field capacity), while atmospheric CO2 was raised to 700 μL CO2 L–1 in matched glasshouses using a hierarchical, multi-factorial design. Enrichment with CO2 stimulated shoot growth rates for 12–15 weeks in well-watered trees but after six months of CO2 enrichment, shoot biomasses were not significantly heavier (30% stimulation) in ambient conditions. By contrast, constant drought arrested shoot growth after 20 weeks under ambient conditions, whereas elevated CO2 sustained growth in drought and ultimately doubled the shoot biomass relative to ambient conditions. These growth responses were achieved through an enhancement of lateral branching up to 8-fold due to CO2 enrichment. In spite of larger transpiring canopies, CO2 enrichment also improved the daytime water status of leaves of droughted trees. Stem xylem development was highly regulated, with vessels per unit area and cross sectional area of xylem vessels in stems correlated inversely across all treatments. Furthermore, vessel numbers related to the numbers of leaves on lateral branches, broadly supporting predictions arising from Pipe Model Theory that the area of conducting tissue should correlate with leaf area. Diminished water use of trees in drought coincided with a population of narrower xylem vessels, constraining hydraulic capacity of stems. Commensurate with the positive effects of elevated CO2 on growth, development and leaf water relations of droughted trees, the capacity for long-distance water transport also increased.

Drought × CO2interactions in trees: a test of the low‐intercellularCO2concentration (Ci) mechanism

“…This reduction in leaf area may allow the leaves to continue to function at nonstressed levels of photosynthesis and stomatal conductance. Similar observations of a proportional decrease in leaf area with persistent drought have been reported for both broadleaf deciduous and evergreen species (see Centritto et al ., ; Li & Wang, ; Atwell et al ., ). Thus, longer‐term responses to low water availability differ from short‐term responses.…”

“…() with tropical tree seedlings, found much larger relative responses of whole‐plant biomass to eC a under well‐watered than under drought conditions. Conversely, experiments with Eucalyptus tereticornis (Atwell et al ., ) and Quercus robur (Schulte et al ., ) showed a larger relative response of biomass to e C a under drought than under well‐watered conditions. These contrasting results may be the result of different experimental conditions, such as the drought regime applied (e.g.…”

Elevated [CO2] does not ameliorate the negative effects of elevated temperature on drought‐induced mortality in Eucalyptus radiata seedlings

“…However, studies of [CO 2 ] × drought interactions are inconclusive in that elevated [CO 2 ] has been reported to ameliorate drought stress in some instances (Atwell et al . ; Domec et al . ; Wertin et al .…”

Effects of elevated CO2on plant growth and nutrient partitioning of rice (Oryza sativaL.) at rapid tillering and physiological maturity

“…These organs appear to have a greater elasticity than leaves and therefore act as large sinks for carbohydrates (Drake et al 1997;Gruters 1999;Stitt 1999). Consequently, plants which have true stems from early ontogeny generally partition a substantial proportion of the dry mass (DM) to the stems at elevated CO 2 (Rogers et al 1996;Atwell et al 2007). Therefore, variations in such response to elevated CO 2 within different species could explain the underlying physiological mechanism.…”

“…These morphological adjustments may have implications for light and nutrient capture at elevated CO 2 (Van Oijen et al 1998;Aben et al 1999;Gutierrez et al 2009). Species which can branch profusely have the additional opportunity to respond to elevated CO 2 (Poorter 1993;Stitt and Krapp 1999;Atwell et al 2007). Species which have a limited capacity to increase leaf area for the extra carbohydrate produced at elevated CO 2 , could show large change in carbohydrate partitioning between stems or roots (Seneweera et al 1995;Makino et al 1997).…”