NET PHOTOSYNTHESIS-LEAF TEMPERATURE RELATIONS IN PLANT-SPECIES WITH DIFFERENT HEIGHT ALONG AN ALTITUDINAL GRADIENT

The relationship between net photosynthesis and leaf temperature was studied along a gradient in the tropical high Andes in order to determine if there were any differences in CO2 assimilation parameters due to temperature changes along the soil-air continuum. Two acaulescent rosettes, Calandrinia acaulis and Hypochoeris setosus, a shrub, Hypericum laricoides, and a giant rosette, Espeletia schultzii, were chosen for this study at altitudes from 2 950 to 4 200 m. Optimum temperature for photosynthesis decreased from 11.5-degrees-C to 7.5-degrees-C for E. schultzii and from 13.9-degrees-C to 8.3-degrees-C for H. laricoides at 2 950 and 4 000 m, respectively. H. Setosus showed an increase in optimum leaf temperature for photosynthesis from 3 550 to 4 200 m (8.8-degrees-C and 11.7-degrees-C, respectively), and C. acaulis showed no differences between altitudes (approximately 13.0-degrees-C). Net photosynthetic rates were also higher for the two ground level plants (6.5 to 8.0 mumol m-2s-1) compared to the higher level plants (4.1 to 4.6 mumol m-2s-1). All species showed positive assimilation rates at below 0-degrees-C. Optimum temperature for photosynthesis was coupled to air temperatures for the high stratum species, therefore decreasing at higher altitudes. The lower stratum species, strongly influenced by soil temperature, showed similar or even an increase in optimum leaf temperature for photosynthesis with an increase in altitude.