Molecular, biochemical, morphological and ultrastructural responses of cacao seedlings to aluminum (Al3+) toxicity

Al3+ toxicity is the main limiting factor of T. cacao sustainability in highly weathered acidic soils in Brazil. However, there is insufficient information on the effects of Al3+ toxicity in cacao. Results showed that, with the increase in Al3+ concentration, ‘Catongo’ × ‘Catongo’ had higher guaiacol peroxidase activity in the leaves, while CCN-10 × SCA-6 had the highest activity in the roots. This resulted in an accumulation of P and K in the stems and K in the roots of ‘Catongo’ × ‘Catongo’, while CCN-10 × SCA-6 accumulated Mg, P and S in the leaves, K in the stems and Fe in the roots. Anatomical analysis of leaf mesophyll showed that CCN-10 × SCA-6 presented increased thickness of the upper epidermis, palisade parenchyma, spongy parenchyma and leaf mesophyll compared to ‘Catongo’ × ‘Catongo’. The increment of Al3+ promotes disruption of leaf cell nuclear membranes, deformity of root epidermis cells, and electrodense material deposits in xylem parenchyma and endodermis cells. Furthermore, rupture of the plasma membrane and vacuole retraction of cortical parenchyma cells (PC) were observed in ‘Catongo’ × ‘Catongo’, while CCN-10 × SCA-6 only experienced rupture of PC cell walls. Furthermore, increased SODcyt expression contributed to the tolerance of CCN-10 × SCA-6 to increased oxidative stress promoted by Al3+. Although an increase in PER-1 gene expression was detected only with the 30 mg Al3+ L−1 dose in ‘Catongo’ × ‘Catongo’ leaves, the increase in GPX activity may have been due to the expression of this gene at a time prior to the collection of plant material for analysis.