At 10°C the primary xylem in pea roots forms late-maturing metaxylem tracheary elements in the center. At 25°C a lysigenous cavity usually forms in their stead, regardless of the cultivation conditions. We hypothesized that other factors besides temperature, such as water and oxygen availability, influenced cavity formation. Pea seedlings were grown three ways: 1) hydroponically at in different aeration levels at 10° and 25°C; 2) in beakers in a moisture series (750-2200ml per 2L vermiculite) at 10° and 25°C; and at 25°C in 2L vermiculite with 750ml water infused with gas mixtures in a series from 2-20% oxygen. Roots were examined microscopically. At 10°C aeration level in the hydroponic system made no difference in root growth or cavity formation (none formed). At 25°C aeration enhanced growth but made no difference in cavity formation (100% formed). In vermiculite root growth was enhanced with increasing water availability up to 1500ml per 2L vermiculite, but became inhibited by water amounts greater than this regardless of temperature. Cavities never formed in the vermiculite system at 10°C, but appeared linearly in the populations from 22-100% formation along the moisture gradient. When seedlings were grown along an oxygen gradient at 25°C in vermiculite at the driest condition, root growth was positively correlated with oxygen availability, but cavity formation was the inverse, showing a 2nd-order decline from 100-30% formation along the gradient. Cavity formation, then, is associated with high water status as well as temperature. The gas manipulation experiment showed that cavities form in response to hypoxic conditions, hence may be akin to cortical aerenchyma which forms in other plants due to flooding. We speculate that respiratory demand in these thick roots exceeds the rate of oxygen diffusion through water at 25°C.

Key words: aerenchyma, cavity formation, hypoxia, pea root, Pisum sativum L. cv. Alaska, water relations