Seed yield determination of peanut crops under water deficit: Soil strength effects on pod set, the source-sink ratio and radiation use efficiency

Peanut (Arachis hypogaea L.) production in Argentina is affected by frequent and unpredictable periods of water deficit that usually overlap the critical period for pod set of early sown crops. An indirect effect of water deficit is reduced pegging due to increased soil strength promoted by surface soil desiccation. There is no knowledge on the associated effects determined by peg production dynamics and variable plant water status. We evaluated the responses of these traits by means of field experiments (Exp₁: 2002-2003; Exp₂: 2005-2006) that included two peanut cultivars (ASEM 485 INTA and Florman INTA) cropped at different sowing dates and water regimes (IRR: irrigated; WS: water stress). Treatments allowed exploring a range of: (i) evaporative demands, (ii) surface soil strength levels, and (iii) soil water contents (θ). We computed leaf area index (LAI), intercepted photosynthetically active radiation (IPAR), surface soil strength, degree of leaf folding, degree days of stress (SDD), crop (CGR) and pod growth rates (PGR) at critical periods, and radiation use efficiency (RUE). Seed yield and seed yield components (pod number per m², seed number per m² and individual seed weight) were determined at final harvest. WS promoted a significant decline (average of 73%) in seed yield (P <=0.022), which was better explained (r ² =0.98) by the decline in seed and pod numbers than by the decline in individual seed weight (r ² =0.67). Seed number responded chiefly to CGR between R₃ and R₆.₅, but WS plots of Exp₁ departed from the general model fitted to IRR plots (40-53% decrease respect to predicted values). Biomass partitioning to reproductive sinks was also affected in WS plots. Enhanced soil strength promoted by soil drying reduced normal pegging pattern, and a generic bilinear model indicated a soil strength threshold of ca. 2.23±0.10MPa (θ =0.119cm³ cm⁻³) above which peg penetration decreased dramatically (r ² =0.57, P <0.001). WS reduced IPAR accumulation (10-30% reduction) and biomass production (34-67% reduction). The former was affected only by direct WS effects (i.e., tissue expansion, leaf movements). The latter was affected additionally by indirect effects (i.e., those determined by reproductive sink activity). The larger response of biomass production than of cumulative IPAR to WS determined a significant (P <0.05) decline in RUE with increased water deficit.