DESICCATION TOLERANCE IN SEEDS FROM THREE GENETIC ORIGINS OF COCOA (Th eobroma cacao L.)

Abstract

Recalcitrant seeds such as Theobroma cacao L. die during the drying, but the moisture content at which dying takes place may vary among genotypes within the same species. In this research, desiccation sensitivity to drying was measured in five genotypes of cocoa from three genetic origins, and the physical changes of seeds were observed during the last stage of development. Fruits from ‘Pound 7’ and ‘EET48’ (Forasteros), ‘UF667’ and ‘UF668’ (hybrids) and ‘Carmelo’ (Criollo) were harvested at 5, 6 and 7 months aft er flowering (maf). Variables evaluated were: length, width, thickness, initial moisture content, and seed dry weight. Seeds were dried-down to 300, 200 and 150 g H₂O kg^-1 fw, using fresh-seed samples as controls. The effect of seed drying at the three harvest dates was evaluated through germination rates and structural changes in the embryo axes, radicle apical meristems and caliptre. Results indicated that seed moisture content varied among harvests and genotypes, from 529 g H₂O kg^-1 fw (‘EET48’) to 706 g H₂O kg^-1 fw (‘Carmelo’). Seed dry weight increased by 2.5 g (236 %) from harvest one (5 maf) to harvest three (7 maf), increases which were associated with increments of 0.6 and 4 mm in seed width and thickness. In seeds recently harvested (high moisture) the germination rate reached 94 %, but when seed moisture declined to 300 and 200 g H₂O kg^-1 fw the germination rate decreased to 50 % and 0 %. Th e less sensitive genotypes at 300 g H₂O kg^-1 fw were ‘EET48’ and ‘UF667’. Desiccation modified the cell structure of embryo axes. Genotypes ‘Carmelo’, ‘Pound 7’, and ‘UF668’ showed cellular damages (cytoplasm wrinkle, loss of the affinity of nucleic acids to safranin O, and loss of nucleus) in 50 % of the embryo cells at a moisture of 300 and 200 g H₂O kg^-1 fw. Th e less sensitive genotype was ‘UF668’. It is concluded that cocoa seed sensitivity to drying can be detected among genotypes of the three originis at 300 g H₂O kg^-1 fw, while with dissection to the level of 200 g H₂O kg^-1 fw none of the cocoa genotypes was able to tolerate seed drying.