Root Confinement and its Effects on the Water Relations, Growth and Assimilate Partitioning of Tomato (Lycopersicon esculentum Mill)

Although it is well established that the root growth in manyspecies is very sensitive to mechanical impedance or to confinementin small volumes, little is known about the consequent effectson growth of the whole plant and the mechanisms involved. Thiswork investigated the effects of root confinement on the waterrelations, growth and assimilate partitioning of tomato (Lycopersiconesculentum Mill) grown in solution culture. Six-week old plants were transferred to either 4500 ml or 75ml containers filled with nutrient solution, and allowed togrow for 14 d. Transpiration, leaf-air temperature differences,and leaf diffusive resistances were measured frequently. Leaf,stem and shoot dry masses, leaf area and root length, were estimatedwhen the treatments were imposed and at the end of the experiment.After 14 d growth the root and shoot hydraulic resistances wereestimated from measurements of leaf water potential and transpirationrate, using a steady-state technique. Confining root growth to the small containers substantiallyreduced shoot and root growth and increased the proportion oftotal dry matter present in the stems. These effects were dueto drought stress. The hydraulic resistance of the root systemwas greatest in the confined plants. This led to more negativeleaf water potentials, increased leaf diffusive resistance,and reduced the net assimilation rate by a factor of 2.5. Transpirationper unit leaf area was less affected. However, cumulative transpirationwas also reduced by a factor of 2.5. mostly because of the smallerleaf area on the confined plants. Root hydraulic resistivitywas measured at 3.1 x 10¹²s m^–1 in the control treatment,but increased to 3.9 x 10¹² s m^–1 for roots in the smallcontainer. The mechanisms by which root confinement caused drought stressand disrupted the pattern of assimilate partitioning are discussedin detail. Assimilate partitioning, Lycopersicon esculentum, root confinement, plant growth, root growth, root resistance, shoot resistance, tomato, transpiration, water-use efficiency