Root submergence enhances respiration and sugar accumulation in the stem of flooded tomato plants

Flooding is a major environmental constraint that obliges plants to adopt plastic responses in order to cope with it. When partially submerged, tomato plants undergo profound changes involving rearrangements in their morphology and metabolism. In this work, we observed that partial submergence markedly dampens root respiration and halts root growth. However, the flooded hypocotyl surprisingly enhances oxygen consumption. Previous results demonstrated that aerenchyma formation in the submerged tomato stem re-establishes internal oxygen tension, making aerobic respiration possible. Indeed, potassium cyanide abruptly stops oxygen uptake, indicating that the cytochrome c pathway is likely to be engaged. Furthermore, we found out that leaf-derived sugars accumulate in large amounts in hypocotyls of flooded plants. Girdling and feeding experiments point to sucrose as the main carbon source for respiration. Consistently, submerged hypocotyls are characterized by high sucrose synthase activity, indicating that sucrose is cleaved and channelled into respiration. Since inhibition of hypocotyl respiration significantly prevents sugar build-up, it is suggested that a high respiration rate is required for sucrose unloading from phloem. As substrate availability increases, respiration is fuelled even more, leading to a maintained allocation of sugars to flooded hypocotyls.