Morphological, biochemical, molecular and ultrastructural changes induced by Cd toxicity in seedlings of Theobroma cacao L.

Seeds from Theobroma cacao progenies derived from the self-pollination of 'Catongo'×'Catongo' and the crossing between CCN-10×SCA-6 were immersed for 24h in different Cd solutions (2; 4; 8; 16 and 32 mgL(-1)) along with the control treatment (without Cd). Shortly after, the seeds were sown in plastic tubes containing organic substrate and were grown in a greenhouse for 60 days. The treatment with Cd was observed to cause morphological, biochemical, molecular and ultrastructural changes in both progenies of T. cacao. There has been deformation in chloroplasts, nuclear chromatin condensation, and reduction in thickness of the mesophyll. As for 'Catongo'×'Catongo', a decrease in thickness of the epidermis was noted on the abaxial face. There has been increased guaiacol peroxidase activity in the roots of CCN-10×SCA-6, as well as in the''Catongo'×'Catongo' leaves. In the presence of Cd, CCN-10×SCA-6 showed increased expression of the genes associated with the biosynthesis of phytochelatin (PCS-1) and class III peroxidases (PER-1) in leaves, and metallothionein (MT2b), in roots. In 'Catongo'×'Catongo', there has been an increase in the expression of genes associated with the biosynthesis of PER-1 and cytosolic superoxide dismutase dependent on copper and zinc (Cu-Zn SODCyt) in leaves and from MT2b and PCS-1 and roots. There was higher accumulation of Cd in the aerial parts of seedlings from both progenies, whereas the most pronounced accumulation was seen in''Catongo'×'Catongo'. The increase in Cd concentration has led to lower Zn and Fe levels in both progenies. Hence, one may conclude that the different survival strategies used by CCN-10×SCA-6 made such progeny more tolerant to Cd stress when compared to''Catongo'×'Catongo'.