Low O2 supply is involved in the poor growth in root-restricted plants of tomato (Lycopersicon esculentum Mill.)

To investigate whether O2 deficit is involved in the depressed shoot growth under root restriction condition, tomato (Lycopersicon esculentum Mill.) plants were grown hydroponically and subjected to root restriction with or without supplemental aeration. Plant biomass, leaf nutrient concentration, dissolved O2 concentration in nutrient solution, root respiration, root hydrolytic ATPase activities, xylem sap abscisic acid (ABA) concentration, water relations, leaf gas exchange, and root cell viability were investigated throughout the experiment. Root restriction significantly depressed root and shoot growth as early as 15 days after treatment and this depressive effect was alleviated by vigorous aeration around the restricted root zone. Growth suppression by root restriction occurred concomitantly with sharp decreases in dissolved O2 concentration in solution together with significant decreases in root total respiration, cytochrome pathway capacity, hydrolytic ATPase activities, and root cell viability. However, no such decreases were found in well aerated root restriction plants. Root restriction-induced growth suppression was independent of nutrients level in leaves and was not primarily related to the decline of leaf water potential or the gas exchange. Root restriction resulted in an increase in xylem sap ABA concentration from day 15 to the end of the experiment but no such effect was observed in well aerated plants. It is likely therefore, that O2 was one of the main limiting factors to the reduced shoot growth under root restriction condition.