Carbodioxide uptake, oxygen evolution and chlorophyll fluorescence of leaves of Lobelia rhynchopetalum Hemsl., a giant rosette plant of the tropical alpine regions of Ethiopia, were studied under field conditions at 4000 m above sea level. Our objective was to investigate the photosynthetic adaptation to the combination of wide fluctuation in diurnal temperature, high photon flux densities (PFD) and low CO2 partial pressure encountered in these regions. At an ambient CO2 partial pressure of c. 17 Pa, maximal rates of CO2 uptake were low, ranging between 4 and 6 μmol m−2 s−1. Such rates, however, required high PFDs and were observed only at levels of 1500 μmol photons m−2 s−1. Carbon dioxide uptake was significantly inhibited when PFD was >2000 μmol photons m−2 s−1. On the other hand, at saturating CO2 levels, maximal photosynthetic oxygen evolution was higher (30 μmol O2 m−2 s−1), saturating at the same PFD as CO2 uptake. Quantum efficiency of CO2 uptake (0·006 mol CO2 mol photons−1, at high altitude and a low CO2 partial pressure of 17 Pa) and even of oxygen evolution under CO2-saturating conditions in the leaf O2 electrode (0·05 mol O2 mol photons−1) indicated reduced photosynthetic efficiency. Electron transport rate (ETR) was strongly correlated with the leaf temperature. Non-photochemical quenching (NPQ) responded inversely to leaf temperature and stomatal conductance.

The results indicated that in the morning, when the sun irradiates the partly frozen leaves with closed stomata, NPQ is the principal mechanism by which Lobelia leaves protect their photosynthetic apparatus. However, during the day, the predominant upright inclination of the leaves significantly contributes to protecting the leaves from excess light absorption. A comparison of the chlorophyll fluorescence of young and old leaves revealed that the former had high ETR and quantum efficiency of photosynthetic electron transport but a lower capacity for NPQ. Extremely high NPQ values but low ETR and low quantum efficiency were recorded for the old leaves. Thus, in the course of maturation the leaves apparently lose photosynthetic efficiency but increase their capability for protective non-photochemical quenching.