Leaf density, adult vegetative vigor and flushing intensity as relevant criteria for evaluating drought resistance of cocoa trees (Theobroma cacao L.)

In Côte d'Ivoire, one of the most recurrent manifestations of climate change results in long periods of drought lasting more than four consecutive months in the most-affected cocoa production areas. These droughts compromise cacao establishment, productivity and quality, which in turn threaten the longer-term sustainability of Ivorian cocoa farming. To address this constraint, one of the areas of research explored by breeders, is the identification of relevant criteria to assess the resistance of cocoa trees to drought, as a means to developing more climate-resilient genotypes. The purpose of this study is to show how adult vegetative vigor, leaf density and flushing intensity have proven to be relevant criteria for evaluating the resistance of cocoa trees to drought. A trial involving four agro-climatic zones (Bouaflé, Abengourou, Divo and Soubré), two of which experience significant rainfall deficits (Bouaflé and Abengourou), was set up in Côte d'Ivoire. The plant material, planted in two plots per zone, was made up of 15 CNRA families common to each agro-climatic zone and 10 to 15 free progenies, selected in each zone by the cocoa producers for their superior agronomic and technological performance. Eleven criteria were used to evaluate the plant material: i) vegetative vigor measured by the diameter at the collar; ii) trunk circumference at 130 cm above the ground; iii) tree height; iv) foliage density; v) leaf density; vi) flush intensity; vii) drought-sensitivity score ; viii) total number of cherelles per tree; ix) cherelle wilt rate; x) cumulative potential production (2015 to 2018), and xi) bean weight (weight of 100 dry cocoa beans).

The results showed a highly significant zone x family interaction (probability < 0.0001) for all the traits studied. In particular, six hybrid families were ranked in the top ten across all study areas, indicating resilience in the more challenging agro-climatic zones. Of these eleven criteria, those having best characterized the six families thus selected are, in decreasing order of importance: 'i) leaf density; ii) vegetative vigor and iii) flush intensity. The use of these three criteria in the selection of drought-resilient plant material is envisaged.

Keywords: drought resistance, cacao breeding, relevant criteria