13.3.5  Drought

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Figure 13.9 CO2 enrichment can enhance leaf water potential in some species. Maranthes corymosa, a tropical monsoon rainforest species of north Australia, maintains a significantly higher leaf water potential throughout the day. (Based on Eamus et al. 1995)

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Figure 13.10 (a) Total area of leaves on lateral stems of 23-week-old Eucalyptus tereticornis grown at ambient and elevated CO2 concentrations and two levels of water supply (50% field capacity – droughted; field capacity – well watered). (b) Total area of leaves on main stem. Treatments as for (a). Values above columns represent percentage increase in leaf area at high CO2. The percentage stimulation in leaf area per plant is largest in droughted plants. (2 × standard errors of means are shown) (Based on B.J Atwell and J.P. Conroy, pers. comm. 1996)

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Figure 13.11 Influence of CO2 enrichment on vessel frequency and mean vessel area of stems of 23-week-old Eucalyptus tereticornis grown under well-watered conditions. (Based on B.J. Atwell and J.P. Conroy, pers. comm. 1996)

One of the primary responses to soil water deficit is a reduction in stomatal conductance. A primary response of many species to CO2 enrichment has also been a reduction in stomatal conductance. A doubling of atmospheric CO2 from 350 to 700 µmol CO2 mol–1 reduces stomatal conductance by about 40%, regardless of the conductance in normal air. This reduction is due partly to reduced stomatal aperture and partly to reduced stomatal density. Furthermore changes in stomatal conductance due to CO2 enrichment are often more pronounced for water-stressed plants than for well-watered ones. As the amount of water lost through transpiration is largely dependent on stomatal aperture, CO2 enrichment can result in maintenance of higher leaf water potentials at any given soil water content (Figure 13.9) as has been observed in Maranthes corymbosa, a tropical tree of northern Australia. However, any improvement in water relations at high CO2 due to reductions in soil water depletion is often counter-balanced in C3 plants by increased leaf area production under high CO2. In C4 plants, lower transpiration at high CO2 is not counterbalanced by a greater leaf area, so that growth can benefit from an improved supply of soil moisture.

In addition to potential advantages from reduced stomatal conductance there are at least two reasons why increased growth may occur in water-stressed plants in response to CO2 enrichment. First, the internal concentration of CO2 (ci) is increased at high atmospheric CO2 concentrations (ca) compared to ambient conditions despite reductions in stomatal conductance as water stress develops. Additional photoassimilates may allow plants to osmoregulate more effectively and to respond more quickly to any alleviation of water stress (compensatory increase in leaf size following rewatering was covered in Section 6.2.7). Second, by slowing down leaf appearance and expansion rates, water deficits induce a reduction in the number of active sinks at any one time. As we have seen, a common response to CO2 enrichment is increased sink strength. In Eucalyptus tereticornis, doubling atmospheric CO2 concentration from 350 to 700 µmol CO2 mol–1 substantially increased total leaf area per plant. The increase in total leaf area is largely a result of decreased apical dominance leading to enhanced lateral branching and a subsequent increase in leaf number per plant. Water-stressed plants show a more dramatic response (Figure 13.10).

CO2 enrichment also enhances sink generation in woody stems, leading to a 30% increase in dry mass (under well-watered conditions). In this instance (Figure 13.11) expression of enhanced sink capacity occurs via an increase in the number of vessels per unit cross-sectional area and a decrease in their average diameter (Figure 13.12). Fast-growing species of Eucalyptus such as E. grandis typically respond in this way to improved growing conditions so that basic wood density does not diminish with site improvement (Bamber et al. 1982). Change in wood properties (Figure 13.12 B, C) is not only important in determining the capacity of eucalypts to respond to CO2 but has implications for carbon cycling in forests: carbon stored in wood remains sequestered until after tree death, and subsequent release will be slowed by higher wood density.

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Figure 13.12 Photomicrographs of transverse sections from Eucalyptus grandis (a) show location of vessels (ve) in secondary xylem in relation to cortex (co) and cambium (ca). Photomicrographs of stem sections from E. tereticornis (b and c) show a greater abundance of narrower vessels in stems grown under CO2-enriched conditions (c) compared with ambient CO2 (b). Remnants of a cambial layer are evident along the top edges of sections in (b) and (c). Scale bar = 100 µm. (Photomicrographs courtesy P.E. Kriedemann (a); B.J. Atwell and J.P Conroy (b and c))

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