Revisiting Plant Responses to Root Volume Restriction: From Leaf Acclimation to Whole Plant Growth

Soilless cultivation is widely used to avoid soil-borne diseases, promote plant mobility, and enhance resource utilization efficiency. This often results in root restriction, and the physiological mechanism behind the reduction of plant growth under root restriction remains debated. Here, we investigated how root restriction impacts leaf photosynthetic traits, plant growth, and biomass partitioning across various developmental stages in sweet pepper (Capsicum annuum), while controlling for secondary effects such as increased temperature fluctuations, reduced water availability, and altered nutrient availability. Sweet pepper plants were grown for 90 days in different root zone volumes (301 cm³, V300; 1203 cm³, V1200; 4809 cm³, V4800) controlled by different sizes of "inner pots" of nylon mesh but within the same outer pot (volume of 8500 cm³). Smaller root zone volume significantly inhibited the growth of both the aboveground and belowground plant parts (35% reduction in total biomass in V300 compared to V4800), with less dry mass partitioning to leaves and more to fruits. Root zone volume rarely affected leaf photosynthetic rate, except for decreasing the leaf photosynthetic capacity in the middle canopy at the late anthesis stage. A smaller root zone volume reduced the ability of leaves to resist high-light stress. Leaf starch and soluble sugar content were increased, accompanied by a largely decreased leaf area at small rooting zone volume, indicating that root restriction reduced plant growth due to sink limitation. We conclude that growth suppression under root restriction is mainly due to the reduction in leaf area caused by sink limitation, but not due to the changes in leaf-area-based photosynthesis.