Inhibition of net photosynthesis (Pn) by moderate heat stress has been attributed to an inability of Rubisco activase to maintain Rubisco in an active form. To examine this proposal, the temperature response of Pn, Rubisco activation, chlorophyll fluorescence, and the activities of Rubisco and Rubisco activase were examined in species from contrasting environments. The temperature optimum of Rubisco activation was 10°C higher in the creosote bush ( Larrea tridentata ) compared with the Antarctic hairgrass ( Deschampsia antarctica ), resembling the temperature response of Pn. Pn increased markedly with increasing internal CO 2 concentration in Antarctic hairgrass and creosote bush plants subjected to moderate heat stress even under nonphotorespiratory conditions. Nonphotochemical quenching of chlorophyll fluorescence, the effective quantum yield of photochemical energy conversion (ΔF/F m ′) and the maximum yield of PSII (F v /F m ) were more sensitive to temperature in Antarctic hairgrass and two other species endemic to cold regions (i.e. Lysipomia pumila and spinach ( Spinacea oleracea )) compared with creosote bush and three species (i.e. jojoba ( Simmondsia chinensis ), tobacco ( Nicotiana tabacum ), and cotton ( Gossypium hirsutum )) from warm regions. The temperature response of activity and the rate of catalytic inactivation of Rubisco from creosote bush and Antarctic hairgrass were similar, whereas the optimum for ATP hydrolysis and Rubisco activation by recombinant creosote bush, cotton, and tobacco activase was 8°C to 10°C higher than for Antarctic hairgrass and spinach activase. These results support a role for activase in limiting photosynthesis at high temperature.
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