Comparison of the effects of flooding vs. low-oxygen gas on pea (Pisum sativum L. cv. 'Alaska') primary roots

Flooding reduces soil oxygen necessary for root growth. In some mesophytes low levels of oxygen are mitigated by the formation of aerenchyma or expansion of intercellular spaces. But root immersion in water may have effects on roots in addition to reducing oxygen levels. At temperatures >15°C Pisum sativum primary roots develop cavities in the centers of their vascular cylinders in response to saturated or flooded conditions. In the present study we compared the response of flooded pea roots to their response to hypoxia without flooding by using an innovative system that allows separation of the gas environment of a root system from that of its shoot system. Seedlings were flooded after 4 d growth and compared to seedlings in unflooded medium and to seedlings in the gas-manipulation experiment. At 25°C in slightly moist vermiculite, roots 4 d after planting were exposed to a gas mixture with 10.5% oxygen and shoots to 20.5% oxygen and compared to normoxic controls. Oxygen levels in all containers were monitored, root growth was measured, and frequency and size of vascular cavities were determined for all treatments. Under flooding and low-oxygen gas, root growth was suppressed and vascular cavity frequency was strongly enhanced compared to controls. Significant differences in root growth responses were not seen between these differing hypoxic conditions, but low-oxygen gas caused larger cavities than flooding, which suggests flooding with water may have subtle effects different than simple hypoxia.