Canopy vigor predicts genotype-specific yield collapse under combined potassium excess and heat stress in Chenopodium quinoa

Résumé

Quinoa expansion into heat-prone semi-arid environments increasingly exposes the crop to combined abiotic stresses for which current potassium (K) fertilization guidelines are poorly calibrated. Whether genotype-specific traits modulate crop response to this combined stress remains unknown. We tested five K rates (0–200 kg K 2 O ha - ¹) on three contrasting quinoa genotypes across two Moroccan field sites differing in soil K status (715 vs. 106 mg K 2 O kg - ¹), CEC (18.0 vs. 1.2 meq 100 g - ¹), and thermal regime (Tmax 38.2 °C vs. 27.4 °C during grain filling), complemented by a controlled-temperature pot experiment. At the K-rich, heat-prone site, G41 grain yield collapsed 96% between K120 and K150, whereas at the K-poor, moderate site, the same genotype responded linearly to K200 (AE = 0.46 g g - ¹). The pot experiment showed that identical soil under controlled temperature (Tmax < 28 °C) did not show yield collapse, indicating that heat stress is a critical co-factor in this system. At baseline (K0), G36 exhibited a higher tissue K/Ca ratio (8.28) than G41 (1.95), yet maintained stable yields, indicating that constitutive tissue cation balance alone does not predict vulnerability to yield collapse; instead, G41’s large canopy (LAI 2.50 vs. 0.54) generated far higher absolute Ca² + /Mg² + demand. K fertilization restructured root architecture from absorptive to transport-dominated without changing biomass. We propose demand-side vulnerability, where yield collapse is consistent with the convergence of K-induced cation suppression, heat-amplified Ca² + /Mg² + demand, and genotype-specific canopy scaling. This finding suggests that combined abiotic stress and nutrient disorder can produce outcomes unpredictable from either stressor alone, and that K recommendations must integrate soil K/CEC status, genotype vigor, and expected thermal regime.

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