Some epiphytic Hymenophyllaceae are restricted to lower parts of the host (<60 cm; 10–100 μmol photons m-2 s-1) in a secondary forest of Southern Chile; other species occupy the whole host height (≥10 m; max PPFD >1000 μmol photons m-2 s-1). Our aim was to study the photosynthetic light responses of two Hymenophyllaceae species in relation to their contrasting distribution. We determined light tolerance of Hymenoglossum cruentum and Hymenophyllum dentatum by measuring gas exchange, PSI and PSII light energy partitioning, NPQ components, and pigment contents. H. dentatum showed lower maximum photosynthesis rates (Amax) than H. cruentum, but the former species kept its net rates (An) near Amax across a wide light range. In contrast, in the latter one, An declined at PPFDs >60 μmol photons m-2 s-1. H. cruentum, the shadiest plant, showed higher chlorophyll contents than H. dentatum. Differences in energy partitioning at PSI and PSII were consistent with gas exchange results. H. dentatum exhibited a higher light compensation point of the partitioning of absorbed energy between photochemical Y(PSII) and non-photochemical Y(NPQ) processes. Hence, both species allocated energy mainly toward photochemistry instead of heat dissipation at their light saturation points. Above saturation, H. cruentum had higher heat dissipation than H. dentatum. PSI yield (YPSI) remained higher in H. dentatum than H. cruentum in a wider light range. In both species, the main cause of heat dissipation at PSI was a donor side limitation. An early dynamic photo-inhibition of PSII may have caused an over reduction of the Qa+ pool decreasing the efficiency of electron donation to PSI. In H. dentatum, a slight increase in heat dissipation due to acceptor side limitation of PSI was observed above 300 μmol photons m-2s-1. Differences in photosynthetic responses to light suggest that light tolerance and species plasticity could explain their contrasting vertical distribution.

RESUMENSe ha propuesto que los ambientes de alta montaña son especialmente vulnerables al calentamiento global. La interacción de variables como déficit hídrico y las condiciones de temperatura ambiental puede restringir el intercambio gaseoso en zonas de alta-montaña. Debido a las distintas limitaciones abióticas que se encuentran en un gradiente altitudinal, se propone que el aumento de la temperatura afectará positivamente la fotosíntesis de la planta alto-andina Phacelia secunda en elevadas altitudes y negativamente a las plantas de bajas altitudes. En el presente estudio se evaluó el efecto del aumento de la temperatura ambiental sobre el intercambio gaseoso de P. secunda en dos altitudes: 2900 m y 3600 m. En cada altitud se expusieron seis individuos de P. secunda a un sistema de calentamiento pasivo denominado "Open Top Chamber" (OTC) que aumenta la temperatura del aire en ca. 3ºC. Adicionalmente, en cada altitud se seleccionaron otros seis individuos en espacios abiertos para utilizarlos como control y que se encontraban a 2 m de la OTC más cercana. En cada individuo se midió el intercambio gaseoso y el potencial hídrico xilemático. La temperatura ambiental fue en promedio 3,5°C mayor dentro de las OTC en ambas altitudes. En contraste, el potencial hídrico del suelo y de las plantas se redujo un 28% al interior de las OTC, pero sólo a 2900 m. En esta altitud, las tasas de fotosíntesis al interior de las OTC se redujeron un 40,7%, mientras que a 3600 m las tasas aumentaron un 24,4%. Los efectos diferenciales en las tasas de fotosíntesis debido a la temperatura fueron acompañados con cambios en la conductancia estomática. Esto sugiere que los efectos del calentamiento global sobre la fotosíntesis de P. secunda son contrastantes entre ambas altitudes debido a los efectos secundarios que éste tiene sobre la disponibilidad hídrica en cada altitud. PALABRAS CLAVE:Phacelia secunda, calentamiento global, estrés por temperatura, sequía, conductancia estomática. ABSTRACTHigh-alpine environments have been proposed as particularly vulnerable to global warming. The interaction of variables such as water deficit and temperature conditions may restrict gas exchange in high-alpine areas. Due to different abiotic constraints that occur at different elevations in the Andes of central Chile we hypothesize that the temperature increases will positively affect the photosynthesis of the high-Andean plant Phacelia secunda at high elevation but it will be negatively affected by warming at lower elevations. In this study we evaluated the effect of increased environmental temperature on the gas exchange of P. secunda at two contrasting elevations: 2900 m and 3600 m a.s.l. At each elevation we exposed six individuals of P. secunda to a passive warming system with "Open Top Chamber" (OTC) that increased air temperature on ca. 3ºC. Other six individuals we selected in open areas and maintained as control on each elevation. At both elevations, on each selected individual (i.e. within and outside OTCs) we measured gas exchange and xyl...

No abstract