Abstract
Salinity and phosphorus (P) deficiency are important environmental factors that decrease plant growth and productivity throughout the world. These two stresses frequently coexist in calcareous salt-affected soils. To better understand how plants adapt to these combined stresses, we investigated the interactive effects of salinity and P availability on photosynthetic activity, leaf pigment, phenolic compounds content and antioxidant activity in Aeluropus littoralis (Gouan) Parl., a promising C-4 monocotyledonous halophyte that usually grows in dry salty areas or marshes. Plants were grown hydroponically under low or sufficient P supply (5 or 180 mM KH2PO4 respectively), with or without 400mM NaCl. When individually applied, salinity and P deficiency stresses significantly restricted shoot and root relative growth rate, with a more marked effect of the former stress. However, the effects of the two stresses combined were non-additive on plant growth. Our results showed that salinity and P deficiency stresses applied individually or combined have no significant effect on CO2 assimilation rate, instantaneous water-use efficiency (WUEi), and leaf malondialdehyde content. Chlorophyll, carotoneoids, anthocyanins and phenolics content increased significantly under both P shortage and salt stresses applied individually or combined. Furthermore, a strong correlation was found between both total antioxidant capacity and DPPH (1,1-diphenyl-2-picrylhydrazyl) radical scavenging activity and shoot phenolics and carotenoids contents. These results suggest that the ability of A. littoralis to cope with both P deficiency and high salt stresses is a result of several mechanisms mainly involved in the conservation of the integrity of the photosynthetic apparatus. Secondary metabolites-mainly phenolic compounds and carotenoids-play an important role in the protection of A. littoralis plants against oxidative damage under combined high salinity and P deficiency stresses.