Photosynthetic response of two tropical liana species grown under different irradiances

We investigated the characteristics of gaseous exchanges and chlorophyll a fluorescence under different irradiances in two liana species Canavalia parviflora Benth. (Fabaceae) and Gouania virgata Reissk (Rhamnaceae), both of a semi-deciduous tropical forest of Southeast Brazil. We used cultivated plants growing under irradiances of 100, 40, 10, and 1.5 % of the photosynthetic photon flux density (PPFD). Higher net photosynthetic rates (P-N) were observed during early morning under full sunlight. After this, reduced P-N values were recorded due to pronounced stomatal closure. In Canavalia, the gas exchange responses diminished concomitant with reduced irradiance. Gouania exhibited a narrower range of response, with high P-N values even at 10 % PPFD. Marked reduction of the effective photochemical yield (Delta F/F-m') near midday was observed, followed by increases in the non-photochemical quenching for both species under full sunlight. Despite the common occurrence of these species in open areas of the forest, both were able to maintain relatively high P-N in shaded environments. We suggest that lianas present an intermediate physiological behaviour between shade and non-shade tolerant species.