EFFECTS OF TEMPERATURE ON PHOTOSYNTHESIS OF 2 MORPHOLOGICALLY CONTRASTING PLANT-SPECIES ALONG AN ALTITUDINAL GRADIENT IN THE TROPICAL HIGH ANDES

The effects of temperature on photosynthesis of a rosette plant growin g at ground level, Acaena cylindrostachya R. et P., and an herb that g rows 20-50 cm above ground level, Senecio formosus H.B.K., were studie d along an altitudinal gradient in the Venezuelan Andes. These species were chosen in order to determine -in the field and in the laboratory -how differences in leaf temperature, determined by plant form and mic roenvironmental conditions, affect capacity. CO2 assimilation rates de creased with increasing altitude. For Acaena leaves at 2900 m, A reach ed maximum values above 9 mu mol m(-2) s(-1), nearly twice as high as maximum A found at 3550 m (5.2) or at 4200 m (3.9). For Senecio leaves , maximum rates of CO2 uptake were 7.5, 5.8 and 3.6 mu mol m(-2) s(-1) for plants at 2900, 3550 and 4200 m, respectively. Net photosynthesis -leaf temperature relations showed differences in optimum temperature for photosynthesis (A(o.t.)) for both species along the altitudinal gr adient. Acaena showed similar A(o.t.) for the two lower altitudes, wit h 19.1 degrees C at 2900 m and 19.6 degrees C at 3550 m, while it incr eased to 21.7 degrees C at 4200 m. Maximum A for this species at each altitude was similar, between 5.5 and 6.0 mu mol m(-2) s(-1). For the taller Senecio, A(o.t.) was more closely related to air temperatures a nd decreased from 21.7 degrees C at 2900 m, to 19.7 degrees C at 3550 m and 15.5 degrees C at 4200 m. In this species, maximum A was lower w ith increasing altitude (from 6.0 at 2900m to 3.5 mu mol m(-2) s(-1) a t 4200 m). High temperature compensation points for Acaena were simila r at the three altitudes, c. 35 degrees C, but varied in Senecio from 37 degrees C at 2900 m, to 39 degrees C at 3550 m and 28 degrees C at 4200 m. Our results show how photosynthetic characteristics change alo ng the altitudinal gradient for two morphologically contrasting specie s influenced by soil or air temperatures.