Altitudinal Variation in the Photosynthetic Characteristics of Snow Gum, Eucalyptus pauciflora Sieb. Ex Spreng. III. Temperature Response of Material Grown in Contrasting Thermal Environments

Abstract

Photosynthetic temperature response curves were measured on seedlings of E. pauciflora grown from seed collected at high (1770 m) and low (915 m) elevation sites, in the Snowy Mountains. The material was grown in contrasting day/night temperature regimes (33/28 and 15/10°C) in the Canberra phytotron. The material from the high elevation site showed a temperature optimum at about 20°C when grown at 15/10°C and at about 25°C when grown at 33/28°C. By comparison, the temperature optimum for the low elevation material was near 25° when grown at 15/10°C and shifted to about 30° when grown at 33/28°C. The general form of the temperature response curves was similar for both sets of material, although net photosynthesis of the higher elevation material dropped off more rapidly at temperatures above and below the optimum. When grown at 15/10°C, peak Ievels of net photosynthesis were higher in the high elevation material (66 ng cm^-2 s^-1 v. 54 ng cm^-2 s^-1). When grown at 33/28°C, peak levels were higher in the low elevation material (78 ng cm^-2 s^-1 v. 60 ng cm^-2 s^-1). Similar response patterns were observed in intracellular resistance, r_i, and gas phase resistance, r_i, although there was relatively more change in r_i, and relatively less change in r_i, with respect to growth temperature and material, than in net photosynthesis. The most conservative parameter that was measured was the CO₂ compensation point, Γ. Although it showed a strong dependence on measurement temperature, Γ was not significantly influenced by growth temperature or site location at the levels of probability used.