Unlocking Drought Tolerance in Quinoa Varieties: Insights from Biochar-Amended Soil on Root and Stomatal Adaptations

Quinoa relies heavily on stomatal regulations to optimize water use and gas-exchange under water-limited conditions. This study investigated the impact of biochar on the stomatal morphology and drought responses of various quinoa varieties. Three quinoa varieties (the Danish Titicaca, Italian Quipu and Pakistan UAFQ7) were grown in pots under greenhouse conditions, using soil without (B0) and with 2% woodchip biochar (B2) and subjected to two successive periods of water stress starting from the 12-leaf stage. Each water stress cycle initiated at field capacity and continued until the soil reached the permanent wilting point. Gas-exchange parameters, water relations and root and stomatal morphology were assessed during the experiment, specifically at the end of each water stress cycle and at the bud initiation phase, at the end of the experiment. Plants grown in biochar-amended soil had significantly higher (less negative) midday leaf water potential (-2.6 MPa) than without biochar (-3.2 MPa). Among the varieties tested, the Italian Quipu had the highest water potential and stomatal density (100 vs. 50), while the Pakistan UAFQ7 demonstrated superior post-stress recovery and larger guard cell dimensions, when grown with biochar. Notably, biochar-amended soil promoted substantial root growth in all varieties, particularly in UAFQ7, with increased root dry weight (0.814 g vs. 0.461 g), projected root area (170 cm(2) vs. 115 cm(2)) and volume (2.592 cm(3) vs. 0.958 cm(3)). In addition to below-ground traits, biochar-treated plants showed a significant increase in above-ground dry biomass, with UAFQ7 plants grown in biochar-amended soil exhibiting a 65% greater biomass compared to those grown in non-amended soil. Additionally, a 35% increase in leaf stomatal density (SD) was observed in plants grown in biochar-amended soil compared to non-amended ones, while a prominent increase by 50% recorded in the Italian Quipu grown in B2 compared to B0. These findings underscore biochar's potential to enhance quinoa's drought tolerance by positively influencing stomatal behavior and root architecture specifically in UAFQ7, thereby contributing to more effective water resource management in water-limited environments.