Contrastive response of Phlomis tuberosa to salinity and UV radiation stresses

Abstract

Growth, photosynthetic characteristics and antioxidant defense system were investigated under salinity stress and UV radiation in Phlomis tuberosa (Lamiaceae) grown under environmentally controlled conditions for two weeks. Salinity at 40 mM results in significant reduction of shoot growth up to 20%, while UV radiation at 10 kJ m-2 d-1 did not affect plants dry matter production. Salinity did not influnce PSII photochemistry, while UVA+B radiation caused a significant reduction of maximum quantum yield of PSII. The net photosynthesis rate was inhibited by both salinity and UV stress following reduced stomatal conductance. Leaf osmotic and water potential were decreased by salinity but not UV radiation. Activity of antioxidant enzymes increased under both salinity and UV radiation stress, however, membrane damage was occurred only under UV stress. Our data implied that, high salinity sensitivity in this species was mainly attributable to the salt-induced disturbance in water relations and reduced assimilation rate rather than to other factors such as damage to PSII, oxidative stress or membrane damage. However, PSII photoinhibition, membrane damage and significant reduction of net assimilation rate were not able to affect negatively plants performance under UV stress implying involvement of other factors in high UV stress tolerance in Phlomis tuberosa.