Stability and instability in any system depend on the balance of competing effects. A steady state of a complex system can be maintained through a set of feedback mechanisms, but changes in conditions can move the system out of its range of stability (e.g., homeostasis breaks down at too high or too low a temperature). With no energy inputs, a system starting out in an unstable state will continue to change until it reaches a stable configuration (e.g., the temperatures of hot and cold objects in contact). Viewed at a given scale, stable systems may appear static or dynamic. Conditions and properties of the objects within a system affect the rates of energy transfer and thus how fast or slowly a process occurs (e.g., heat conduction, the diffusion of particles in a fluid).
Generally, inhibition of photosynthesis is seen as a critical factor in heat stress. Net photosynthesis is typically the first process to be inhibited at high temperatures (Berry and Bjorkman 1980; Allakhverdiev et al. 2008). As temperature rises above optimum, gross photosynthesis is inhibited while respiration and photorespiration increase. The combined effect of these three processes is a marked reduction in net photosynthesis during moderate heat stress (Figure 14.12).
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C4 plants do not suffer from the increase in photorespiration and so can maintain a higher photosynthetic optimum; however, the maximum temperature does not vary to the same extent. The imbalance between photosynthesis and respiration is itself damaging, as carbohydrate reserves can become depleted. As temperature rises further, membrane transport and respiration become inhibited, eventually leading to cell death. Both the light reactions and the Calvin cycle are highly sensitive to moderate heat stress. Injury following severe heat stress is perhaps most acute for the light reactions, with even brief exposure resulting in long-term inhibition of photosystem II (PSII). As the activity of PSII is highly temperature sensitive it can be used as an indicator of heat stress and heat injury; measurements of chlorophyll fluorescence have been widely used for this purpose ().
Economic impacts of climate change on agriculture: The importance of additional climatic variables other than temperature and precipitation
Some C3 species such as snow tussocks have an optimum temperature for CO2 assimilation as low as 5°C, but it is important to note that absolute rates at this temperature may be relatively low (Figure 14.12). Most temperate grasses and cereals as well as many woody species have temperature optima in the range from 15°C to 25°C and within this range many C3 species show only small changes in CO2 uptake. In contrast to temperate C3 species, CO2 assimilation by C4 species increases considerably with a rise in temperature from 15°C to 30°C and optimum temperatures may be greater than this (Figure 14.12). Rice, a subtropical C3 species, has a higher temperature optimum for CO2 assimilation than temperate C3 species. Under high light, C4 species have a characteristically greater photosynthetic rate than the C3 species, but these differences disappear and may be reversed at low temperature. Growth temperatures may also influence the optimum temperature for net photosynthesis, which may therefore vary with season or location. However, modifications leading to an improvement in photosynthesis at high temperatures can result in decreased performance at low temperatures and vice versa.
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Enzyme activity is very responsive to temperature, as is enzyme synthesis, activation and stability. However the response of plant growth to temperature is the result of a number of complex processes involving many enzyme systems, and most likely governed by the response of the enzymes involved in CO2 fixation. The different temperature responses of C3 versus C4 photosynthesis are described next, along with temperature effects on assimilate transport and on the basic concepts of enzyme activity including the Q10: the increase in rate of respiration for a 10°C rise in temperature.
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A combination of at least two factors may be associated with the failure of C3 species to respond more favourably to high temperature in terms of CO2 assimilation. One is the limit placed on photosynthesis by ambient CO2 (Figure 14.13), and a second is the concurrent rise with increasing temperature of light-stimulated photorespiration, which is effectively absent from C4 species. Increased atmospheric CO2 inhibits photo-respiration and results in a much greater uptake of CO2 in response to increased temperature in C3 species.