Effects of soil fertility and toxicity on the performance of <i>Chenopodium quinoa</i> (Willd) plants in kin and non-kin interactions

Kin selection theory predicts that closely related organisms may exhibit cooperative behaviors that enhance group fitness despite individual costs. In contrast, the resource partitioning hypothesis posits stronger competition among close relatives due to shared resources and niche overlap. In this study, we tested whether quinoa (<i>Chenopodium quinoa</i> Willd) genotypes differ in performance when grown with kin versus non-kin under different soil fertility and heavy metal toxicity conditions. A two-level, three-factorial experimental design was conducted, including kinship, fertility, and toxicity. Biomass accumulation, allocation patterns, resource acquisition traits, and photosynthetic parameters were measured at the end of the experiment. Kinship and fertility effects were common, but toxicity effects were rare. Biomass accumulation was greater in more fertile soils, and kinship marginally increased biomass. Root allocation was affected by toxicity interactions: kin plants showed greater root allocation under no-toxicity conditions, but this difference was suppressed under metal toxicity. Resource acquisition traits reflected these patterns, with specific taproot length and average leaf mass being higher for kin combinations, while specific stem length and specific leaf area were higher for non-kin combinations. The net assimilation rate, stomatal conductance, transpiration rate, and WUEi were generally higher in non-kin than in kin plants, regardless of soil fertility. These results suggest that quinoa plants may benefit from kin interactions through increased root growth and overall biomass accumulation, but metal toxicity suppresses these benefits, showing that kinship advantages are context dependent and reduced in contaminated soils.