Elevated photosynthetic potential in Polylepis reticulata trees from the Andean tree line.

Abstract

Polylepis reticulata is a vulnerable Andean páramo endemic tree species that grows at the highest altitude in the world. Despite having abundant available water year-round, P. reticulata trees face extreme environmental conditions, namely low temperatures and frequent persistent clouds and fog, which create extremes of solar irradiance. Yet little is known about how its photosynthetic apparatus adapts to this extreme environment because of Ecuadorian páramo's inaccessibility and paucity of previous research. We collected twigs and measured photosynthesis response curves with a Li-Cor LI-6400XT portable photosynthesis system and a LI-6400-40 Leaf Chamber Fluorometer. We characterized photosynthetic potential according to the Farquhar model, as well as stomatal behaviour and water management strategy. Photosynthesis of P. reticulata saturates at relatively low PAR (340-730 μmol m^-2 s^-1). Additionally, the high Rubisco carboxylation kinetics (V_cmax25 = 83.1 μmol m^-2 s^-1 [73.5-92.7 95% CI]) indicate elevated photosynthetic potential. The low carbon gain per unit water transpired suggests a profligate water use strategy (g₁ = 4.38 ± 0.11). Together, these results suggest that P. reticulata has a highly efficient photosynthetic apparatus with relatively weak stomatal control, taking advantage of the rare high radiation in the páramo and adapting to an energy-limited, rather than water-limited, environment.