Photosynthetic Performance of Paulownia tomentosa (Thunb) Steud. Exposed to Heavy Metals Zinc and Cadmium

The present study was carried out to assess the alteration in photosynthetic performance of Paulownia tomentosa seedlings produced in vitro and cultivated under glass house when exposed to trace metals: zinc (Zn) and cadmium (Cd). In this respect, Zn and Cd were added to the substrates of culture at various concentrations: Zn (250, 500, 750 and 1000 µM) and Cd (25, 50 and 75 µM). A non-supplemented substrate with metal salts was served as a control. The photosynthetic activity was evaluated through measurements of chlorophyll fluorescence and the photosynthetic pigments, namely chlorophyll a, chlorophyll b and total chlorophyll as well as the carotenoids. Main results showed that the initial fluorescence (F0) values were higher in plants grown on Zn added substrate compared to those grown in the presence of Cd. However, the Fv/Fm ratio which indicates the efficiency of photosystem II, ranged from 0.78 to 0.82 for all treatments. Additionally, the presence of 75 µM Cd in the substrate stimulates the biosynthesis of chlorophyll pigments by increasing their proportions about approximately 196.77%, in comparison to the control. On the contrary, Zn significantly reduced the contents of these pigments by 9.45% compared to the control. Besides, when the Cd concentrations were 25, 50 and 75 µM and Zn concentration was 250 µM, the carotenoid contents increased up to 115.51%, 253.07%, 239.19% and 87.56% respectively, in comparison to the control, was noted. Overall, results of this study proved the ability of Paulownia tomentosa to maintain its photosystem activity even on Zn and Cd contaminated sites, despite the restrictive effect of Zn on the biosynthesis of photosynthetic pigments when its concentration exceeds 500 µM.