LEAF GAS-EXCHANGE IN ESPELETIA-SCHULTZII WEDD, A GIANT CAULESCENT ROSETTE SPECIES, ALONG AN ALTITUDINAL GRADIENT IN THE VENEZUELAN ANDES

Gas exchange studies along low altitudinal gradients (0-2 500 metres a bove sea level) suggest an increase in plant CO, assimilation with inc reasing altitude. The question arises however, will this hold true for higher altitudinal ranges? The purpose of this work was to characteri ze carbon and water relationships in Espeletia schultzii along an extr eme altitudinal gradient. Gas exchange characteristics under field con ditions during wet and dry seasons were documented for Espeletia schul tzii, a giant caulescent rosette species, along an altitudinal gradien t (2 950. 3 550 and 4 200 m) in the Venezuelan Andes. Significant diff erences in CO2 assimilation rates between sea sons were found for all studied sites. Low assimilation rates obtained during the dry season w ere a consequence of: low leaf conductances to counteract low leaf wat er potentials and/or high vapour pressure differences between in leaf and air, and to high leaf temperatures measured during the dry season. Important differences in A/P-i curve parameters were obtained between wet and dry sea sons. A decrease in A(max) was observed for the two l ower populations, while a similar value was obtained for both wet and dry seasons at the higher altitude. A decrease in the relation P-i/P-a , and an increase in stomatal limitation and leaf temperature were mea sured from wet to dry season. There was a differentiation in CO2 assim ilation with increasing altitude. At 2 950 m, maximum rates were above 5 mu mol . m(-2). s(-1), at 3 550 m 3 mu mol . m(-2). s(-1), while at 4 200 m maximum CO2 assimilation rate was below 3 mu mol . m(-2). s(- 1). This decrease may be explained by: an increase in leaf pubescence, lower leaf conductances and/or lower leaf nitrogen content at higher altitude. (C) Elsevier, Paris.