Physiological characteristics of geophytes in semi-arid Namaqualand, SouthAfrica

摘要

Namaqualand, a semi-arid winter rainfall region of South Africa, supports an exceptional diversity of geophytic species. The survey focused on gas exchange reactions and chlorophyll a fluorescence in geophytes with different leaf orientation in relation to environmental variability. Although the above ground life cycle of geophytes can be extremely short, unlike desert annuals, they are not characterized by a high photosynthetic CO2 uptake. Maximum CO2 uptake ranged from 4 to 20 mu mol CO2 m(-2) s(-1). Temperature optima of photosynthetic CO2 uptake were comparably low and ranged from 12 to 22 degrees C for eleven species tested, with only one species above 19 degrees C. The decrease of CO2 uptake with rising temperatures was associated with a substantial increase of photorespiration. Net photosynthesis was saturated between 500 and 900 mu mol photons m(-2) s(-1) while electron transport through PSII was saturated at higher photon flux densities. At light intensities beyond saturation, a high variability of PSII efficiency occurred. It was highest for horizontal leaves and lowest for upright leaves. However, the maximum quantum yield of PSII (F-v/F-m) remained constant during the course of a day, regardless of leaf orientation. This indicates the absence of photoinhibitory effects and a well protected photosynthetic apparatus. Leaf orientation determined interception of solar radiation and thus leaf temperature which was highest for horizontal leaves. In conclusion, Namaqualand geophytes show photosynthetic characteristics that are well adapted to the mild and generally moist conditions during the growing season.