Photosynthesis and associated metabolism during development of a Theobroma cacao hybrid with the lethal factor Luteus-Pa

The recessive lethal character Luteus-Pa, expressed as a yellowing of leaves of young seedlings and followed by death approximately 60 d after emergence, presents a 3:1 segregation in crosses and/or selfpollinated plants. We evaluated quantitatively the fluorescence emission of chlorophyll (Chl), gas exchange, and chemical composition of normal and recessive homozygous cacao seedlings of the cross Pa 121×Pa 169. The characteristics of Chl fluorescence kinetics were studied in stages B2, B3, C, D, and E of leaf development, corresponding to plant ages of 9 to 12, 13 to 15, 16 to 20, 21 to 30, and >30 d, respectively. Gas exchanges were measured in mature leaves of both seedlings. In regular intervals of 3 d beginning at 33 d after emergence, the seedlings were separated into roots, stems, leaves, and cotyledons to determine the contents of saccharides (SAC) and free amino acids (FAA) and variation of the leaf Chl content. The Chl distribution in complexes of the photosynthetic apparatus was analysed by SDS-PAGE in mature leaves of both normal and recessive 32-d-old seedlings. There were variations in Chl fluorescence, gas exchanges and chemical composition of different parts of both types of seedlings. However, no significant differences were found in the Chl distribution through photosynthetic complexes of 32-d-old normal and recessive homozygous seedlings. After that period a decrease in the Chl concentration was observed in the recessive seedlings, and only minimum fluorescence (F0) was found. The F0 values were higher in the recessive seedlings than in the normal ones. The net photosynthetic rate of mature leaves was negative in agreement with low conductance, transpiration rate, and high internal CO2 concentration. These factors might have contributed to a depletion in SAC in different plant parts. Although F0 partially reflects the Chl concentration in leaf tissue, the increase in its value was probably due to a damage in reaction centres of photosystem 2. Therefore, the growth and development of recessive homozygous seedlings depended exclusively on cotyledon reserves, the depletion of which leads to death.