Fig. 8.

Photosynthetic oxygen evolution as function of increasing photon fluence density (P–E curve) in Zygnema sp. B (a, n = 3), Zygnema sp. G (b, n = 3), Zygnema sp. D (c, n = 3) and Zygnema sp. E (d, n = 3) under control conditions in culture medium (white circles) and under osmotic stress condition (black dots, measurements carried out in 800 mM sorbitol solution after an incubation period about 3 h in sorbitol). Curves were fitted according to the hyperbolic tangent light-saturation model of Jassby and Platt (1976).