Interaction of heat and drought with elevated CO2

By 2050 atmospheric CO2 levels are expected to be around 550 ppm. In C3 species such as wheat and rice, the elevated CO2 level is expected to increase productivity due to the improvement of CO2 diffusion through stomata and a consequent effect on photosynthesis. However, a complex of interactions can arise among plant development, growth and environment variables. Plants that have acclimated to high CO2 and grown new leaves over time (with typically fewer and smaller stomata) do not show the same high photosynthesis rates as a 'normal CO2' plant will under short periods of exposure (Leakey et al., 2009; Parry and Hawkesford, Chapter 8, this volume). Consequently, the observed increases in yield have been only in the order of 10-20% for crops like wheat, when grown in open-top chambers with elevated CO2. Recent open-air experiments for maize have demonstrated no increase in yield in field-level experiments under well-watered conditions and CO2 levels of 550 ppm, although there was substantial reduction in water use (Leakey et al., 2009). These types of findings have implications for irrigation needs in C3 versus C4 crops under elevated CO2: that is, if growth is stimulated in C3 crops, then more water may be required to maintain additional leaf area, and in dry areas, there may be an

Factors

Mechanism

Estimated range of effect (°C) Reference3

Environmental

Agronomic

Genetic

Ambient air temperature Radiation load Rainfall

Relative humidity

Soil depth and water capacity

CO2 level

Planting time

Planting method (e.g. row spacing) Irrigation Tillage system

Residue management

Weed control Pests and diseases

Ground cover and establishment Canopy architecture

Stomatal conductance Root growth Root signalling Pigment composition Epicuticular wax Phenological pattern

Equilibrium with air

Plant organs absorb energy directly

Potential for evapotranspirative cooling

Potential for evapotranspirative cooling

Potential for evapotranspirative cooling

High CO2 can interact with cooling capability via stomatal development and regulation Realized impact of ambient temperatures on development and growth patterns Affects boundary layers and energy balance

Potential for evapotranspirative cooling Affects water infiltration into soil

Residues impact on water fluxes at soil surface

Weeds compete for water Can affect stomatal behaviour

Bare soil heats quickly affecting crown temperature

Area and structure affects energy absorbed and water demand to balance exchange of CO2

Determines rate of evaporative cooling

Area and pattern affects water supply

Affects rate of evaporative cooling

Affects energy absorbed

Affects energy absorbed

E.g. floral structures have lower evapotranspiration rate than leaves

~10 Loomis and Connor (1992)

~10 Ehrler (1973)

~10 Ehrler (1973)

~10 Kirkegaard et al. (2007)

~2 Loomis and Connor (1992)

~10 Ehrler (1973)

~10 Hobbs and Govaerts (Chapter

10, this volume)

~10 Hobbs and Govaerts (Chapter

10, this volume)

~2 Richards (2006)

a Refers to mechanism rather than actual temperature differences, which are estimated by authors.

increased risk of drought impact through the exhaustion of stored soil water compared with 'slower' growing crops.

However, as temperatures increase CO2 solubility declines relative to O2. Thus, for C3 crops the compensation of elevated CO2 can be confounded by photorespiration. Also, elevated temperatures are known to impair Rubisco activase, the enzyme responsible for removing the inhibitory ribulose 1,5-bis-phosphate from a deactivated Rubisco (Parry and Hawkesford, Chapter 8, this volume). As such, some of the apparent benefits of elevated CO2 may be offset by higher temperatures, causing photosynthesis to be energetically more expensive.

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