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Figure 34. Temperature dependence of light-saturated rates of photosynthesis of Plantago major (common plantain) grown at three temperatures. The black line connects measurements at the growth temperatures (after Atkin et al. 2006). Copyright Blackwell Science Ltd.

Bjorkman 1980, Yamori et al. 2005). Below this optimum, enzymatic reaction rates, primarily associated with the ''dark reactions'', are temperature limited. At high temperatures the oxygenating reaction of Rubisco increases more than the carboxylating one so that photorespiration becomes proportionally more important. This is partly because the solubility of CO2 declines with increasing temperature more strongly than does that of O2. Part of the effect of temperature on photosynthesis of C3 plants is due to the effects of temperature on kinetic properties of Rubisco. Vmax increases with increasing temperature, but the Km-values increase also, and more steeply for CO2 than for O2 (Fig. 35). This means that the affinity for CO2 decreases more strongly than that for O2. Additionally, electron transport (Cen & Sage 2005) and gm (Yamori et al. 2006a, Warren 2007) may decline at elevated temperatures. The combined temperature effects on solubility, affinity, and mesophyll conductance cause a proportional increase in photorespiration, resulting in a decline in net photosynthesis at high temperature when electron-transport rates cannot keep up with the increased inefficiency.

Adaptation to high temperature typically causes a shift of the temperature optimum for net photosynthesis to higher temperatures (Fig. 36; Berry & Bjorkman 1980). Similarly, the temperature optimum for photosynthesis shifts to higher temperatures when coastal and desert populations of Atriplex lentiformis acclimate to high temperatures (Pearcy 1977).

Apart from the increase in photorespiration discussed above, there are several other factors important for determining acclimation and adaptation of photosynthesis to temperature. In leaves of Spinacia oleracea (spinach) the Rubisco activation state decreases with increasing temperatures above the optimum temperatures for photosynthesis, irrespective of growth temperature, while the activation state remains high at lower temperatures. Rubisco thermal stabilities of spinach leaves grown at low temperature are lower than those of leaves grown at high temperature. Photosynthetic performance in spinach is largely determined by the Rubisco kinetics at low temperature and by Rubisco kinetics and Rubisco activation state at high temperature (Yamori et al. 2006b). Furthermore, Rubisco can become inactivated at moderately high temperatures. Species adapted to hot environments often show temperature optima for photosynthesis that are quite close to the temperature at which enzymes are inactivated. The lability of Rubisco activase plays a major role in the decline of photosynthesis at high temperatures (Salvucci & Crafts-Brandner 2004b, Hikosaka et al. 2006). Thermal acclimation of Acer rubrum (red

Figure 35. Temperature dependence of Vmax and the Km of (A) the oxygenating and (B) the carboxylating reaction of Rubisco. Vmax is the rate of the carboxylating or oxygenating reaction at a saturating concentration of CO2 and O2, respectively. The Km is the concentration of CO2 and O2 at which the carboxylating and oxygenating reaction, respectively, proceed at the rate which equals 1/2 Vmax. Note that a logarithmic scale is used for the y-axis and that the inverse of the absolute temperature is plotted on the x-axis ("Arrhenius-plot"). In such a graph, the slope gives the activation energy, a measure for the temperature dependence of the reaction (Berry

Figure 35. Temperature dependence of Vmax and the Km of (A) the oxygenating and (B) the carboxylating reaction of Rubisco. Vmax is the rate of the carboxylating or oxygenating reaction at a saturating concentration of CO2 and O2, respectively. The Km is the concentration of CO2 and O2 at which the carboxylating and oxygenating reaction, respectively, proceed at the rate which equals 1/2 Vmax. Note that a logarithmic scale is used for the y-axis and that the inverse of the absolute temperature is plotted on the x-axis ("Arrhenius-plot"). In such a graph, the slope gives the activation energy, a measure for the temperature dependence of the reaction (Berry

& Raison 1981). (C) The combined effects of temperature on kinetic properties as shown in (A) and (B) and relative solubility of O2 and CO2 (O/C) have been modeled, normalized to values at 20°C. (D) Relative rates of the oxygenation and carboxylation reactions of Rubisco (Vo/Vc) and quantum yield (f CO2) modeled using the same parameter values as in (C). For calculation of Vo/ Vc and f CO2, it was assumed that partial pressures of CO2 and O2 in the chloroplast were 27 Pa and 21 kPa, respectively. Kinetic parameters used were calculated from Jordan and Ogren (1981) (courtesy I. Terashima, The University of Tokyo, Japan).

maple) from Florida in comparison with genotypes from Minnesota, US, is associated with maintenance of a high ratio of Rubisco activase to Rubisco (Weston et al. 2007). In Gossypium hirsutum (cotton) expression of the gene encoding Rubisco activase is influenced by post-transcriptional mechanisms that probably contribute to acclimation of photosynthesis during extended periods of heat stress (DeRidder & Salvucci 2007).

High temperatures also require a high degree of saturation of the membrane lipids of the thylakoid for integrated functioning of its components and prevention of leakiness (Sharkey 2005). Therefore, not only Rubisco activity, but also membrane-bound processes of electron transport may be limiting photosynthesis at high temperatures.

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