A hypersensitive response-like reaction is involved in hybrid weakness in F1 plants of the cross Capsicum annuum × Capsicum chinense.

Hybrid weakness in Capsicum is characterized by the termination of leaf differentiation after the development of several leaves. F1 plants in some crosses between Capsicum annuum and Capsicum chinense show weakness; this phenomenon has not been investigated in detail since first reported. In the present study, we characterized morphologically and physiologically hybrid weakness in Capsicum. F1 plants did not show weaker growth than their parents 20 days after germination (DAG), but at 40 DAG, the hybrid weakness phenotype was evidenced by almost complete arrest of new leaf formation, delayed increase in plant height, and reduced upper internode length. The shoot apical meristem (SAM) of F1 plants exhibited delayed development and an abnormal structure characterized by a flat shape and the presence of fuzzy cell layers on the surface. These abnormal SAMs of F1 plants may lead to dwarfism. Dead cells and accumulation of H2O2 were visually detected in leaves of F1 plants, and cell death was considered to be programmed, as it was accompanied by internucleosomal fragmentation of DNA. The expression of immunity marker genes PR1 and PR2 was upregulated in leaves of F1 plants. These results suggest that a hypersensitive response-like reaction is involved in Capsicum hybrid weakness.