Variation in drought adaptation among closely related early groundnut lines

Groundnut is the most important oilseed and cash crop in the Sub-Sahel regions. It is mainly cultivated under late-season drought that occurs during the pod-filling phase and regularly causes great reduction in pod yield. Two sets of extra-early closely related groundnut lines (80 days) were developed in Senegal through back-cross between two productive 90 days cultivars, 55-437 and 73-30, anda precocity donor of 75 days, Chico. However reduced genotypic variability of breeding lines is one of the major drawbacks encountered by breeders when improving materials for physiological traits. The objective was to improve selection criteria by establishing relevance of breeding traits based on the extent of their genotypic variation (ability to discriminate among closely related genotypes) and their relationship with yield under both well-watered and late-season drought stress. Yield, earliness, and physiological traits - relative water content (RWC), and parameters of in vivo chlorophyll fluorescence (SFI, "Structure-Function-Index" and Fv/Fm) - were measured on the two sets of lines during two crop seasons (2001 and 2002). The results show that selection done has maintained or improved production under watered and stressed conditions and increased maturity compared to the recurrent female parent for several lines. Yields obtained in 2001 correlated with yields of 2002 but yields under well-watered were not correlated with yields under drought showing the robustness of varietal yielding across years but not across treatments in groundnut. HIs (Harvest Indices) of these lines were high and stable confirming their high adaptive capacity for reallocation of assimilates to the grains. Yield relationships observed for physiological traits varied with watering conditions and genetic background hence confirming that groundnut has various adaptive adjustments capability for maintenance of yield under stress. Data obtained on fluorescence parameters, in particular SFI, traducing the integrity of photochemistry, showed superior susceptibility to drought and genotypic discriminatory capability. This shows that the integrity of the photosystem II centre is likely a key character in the drought adaptation of groundnut. The approach developed in this study provided SFI as potentially relevant selection criteria that can help screening lines because (i) this trait was chosen on the basis of existing variability detected on appropriate genetic materials (ii) it was correlated with yield performance in the field (iii) and it is easy to measure on large collection of genotypes. (Résumé d'auteur