Physiological, Morphological and Root System Architecture Acclimation Responses to Drought in the African Orphan Millet White Fonio (Digitaria exilis)

Aims White fonio is an ancient West African orphan millet crop. As one of the world's fastest-maturing cereals, it helps smallholders mitigate mid-season hunger. There are no reported studies on drought acclimation responses in fonio to identify traits that can enable breeding for climate change adaptation. Methods Here, two white fonio accessions from wetter (Guinea) and dryer (Mali) environments were grown indoors under three moisture levels in replicated trials. Physiological, morphological and metabolomic phenotyping was undertaken, including root system architecture analysis, culminating in measuring ~ 27,000 root hairs. Results Fonio responded to drought by dramatically upregulating glycine- and alanine-betaine leaf concentrations. Both accessions exhibited increased root:shoot ratio and leaf angle, but reduced shoot branching, leaf width, transpiration, and stomatal conductance. Grain yield most positively correlated with SPAD chlorophyll. Lower grain fill was observed in the Guinea accession, while the Mali accession showed a surprising increase in the harvest index when exposed to extreme drought. In the Mali accession, drought resulted in fewer but longer crown roots, increased lateral root branching, and a greater density and lengthening of root hairs. In particular, leaf width, angle and SPAD were identified as cost- and time effective selection traits. Conclusions This paper has identified above- and below-ground drought acclimation traits in white fonio. These results build a foundation for future efforts to breed this crop to tolerate accelerating climate change, ultimately to assist vulnerable West African farmers.