Genotypes variations in fluorescence parameters among closely related groundnut (Arachis hypogaea L.) lines and their potential for drought screening programs

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, and a 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, drought response indices (SSI and STI), 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). Results showed that yields and maturities of pods of selected lines were higher and more stable across watering condition that those of the parental lines confirming their high adaptive capacity for reallocation of assimilates to the grains. The stress susceptibility index (SSI) did not allow genotypic discrimination and correlated differentially with yield under well-watered and yield under stress whereas stress tolerance index (STI) correlated positively with yield under both treatments showing that STI is more efficient to characterize end-of-season drought resistance. 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. Few measurements of RWC discriminated genotypes and the genotypic differentiation was more marked on the 73-30 derived lines which also showed morphologic dissimilarities. Fluorescence parameters gave greater discrimination among lines especially SFI, which is defined as the resultant between resistance and activity components. Correlations between these physiological traits and yields or drought response indices were highly variable according to lines, treatments and years. However, significant correlations were repeatedly established between SFI and pod yields across conditions indicating that the photochemical response is probably constitutive (independent of stress). 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.