Morpho-physiological and yield responses of green lettuce to growing media volumes in urban cultivation

Urban agriculture requires efficient production in restricted spaces, making growing medium volume a critical determinant of lettuce performance. This study evaluated the effects of three medium volumes on the morpho-physiological responses and yield of green lettuce (Lactuca sativa L.). The experiment was arranged in a randomized complete block design with three polybag sizes, i.e. 15 × 20 cm (V1), 20 × 25 cm (V2), and 25 × 30 cm (V3), each treatment replicated four times, with six plants per replication. The smallest medium volume (V1) increased midday soil temperature by 34°C and leaf temperature by 33°C, while reducing soil moisture by 20% relative to V3, indicating greater heat and water stress. These conditions reduced canopy development, resulting in 61.6% fewer leaves, smaller leaf dimensions (leaf area decreased by 74.8%), and lower yield (-76.7%). In contrast, the largest medium volume (V3) improved the root zone microclimate and plant water status, producing the highest vegetative growth and yield. Leaf area was reliably predicted non-destructively using the linear model (Y = 0.8019x) (where (Y) is leaf area and (x) is leaf length × width), with a coefficient of determination (R2 = 0.9882). Overall, increasing growing medium volume enhanced root zone conditions, leaf area development and final yield. Small volumes remain usable in urban systems if supported by adequate irrigation to maintain soil moisture.