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

• Higher concentrations of Cd were found in the shoots.
• High concentrations of Cd caused ultrastructural changes in progenies of T. cacao.
• Increased expression of genes associated with defense mechanisms has been observed.
• CCN-10×SCA-6 has been observed to be more tolerant than ‘Catongo’בCatongo’.

Seeds from Theobroma cacao progenies derived from the self-pollination of ‘Catongo’בCatongo’ and the crossing between CCN-10×SCA-6 were immersed for 24 h in different Cd solutions (2; 4; 8; 16 and 32 mg L−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’בCatongo’, 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’בCatongo’ 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’בCatongo’, 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’בCatongo’. 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’בCatongo’.