PARENT SESSION
Poster Session #45: Gas Exchange.
Friday, August 10, 2001. Presentation from 10:30 AM to 12:00 PM. Exhibition Hall

Photosynthesis and drought of an endemic and a widespread Rhamnus species.

Gulias, Javier¹, Flexas, Jaume¹, Medrano, Hipolito¹, ¹

ABSTRACT- Photosynthesis response to drought has been studied in two Mediterranean sclerophyll shrubs: Rhamnus alaternus, a widespread species, and Rhamnus ludovici-salvatoris, an endemic species from the Balearic Islands, in order to determine possible differences between these species that could explain their different distribution. Annual variations of gas exchange were followed in field-grown plants for a two-year period. Measurements were done every two months. In addition, photosynthetic light and CO₂ response curves were performed in leaves of drought and irrigated potted plants. Pigment composition and chlorophyll fluorescence were measured as well. Both species showed similar maximum net photosynthesis (11 mol CO₂ m² s^-1 ) and a strong reduction of gas exchange (stomatal conductance and net photosynthesis) under summer drought conditions (up to 90%). However, R. ludovici-salvatoris showed higher values of respiration in the light than R. alaternus (7.8 and 4.9 mol CO₂ m² s^-1 , respectively). Moreover, R. ludovici-salvatoris presented lower de-epoxidation capacity and thermal dissipation than R. alaternus, thus maintaining high electron transport rate and photorrespiration under severe drought conditions. Thus, the analysis revealed important intrinsic limitations for R. ludovici-salvatoris to survive, since its positive carbon balance is limited by high respiration and photorrespiration in shadowy environments, and by low de-epoxidation capacity and thermal dissipation in sunny environments.

KEY WORDS: balearic endemic species, gas exchange, mediterranean sclerophyll shrubs, photosynthetic capacity