Factors inducing cavity formation in the vascular cylinders of pea roots ( Pisum sativum L., cv. Alaska)

Abstract

When grown at relatively high temperatures pea primary roots ( Pisum sativum L.cv. Alaska) often form long lysigenous cavities in the centers of their vascular cylinders. Factors other than temperature may be involved, however. Pea seedlings were grown at 10 and 25°C in vermiculite in a water-availability series (750–2200 ml water/2 l vermiculite) and hydroponically at various levels of aeration (0, 400, 800 ml air/min). Pea seedlings were also grown at 25°C in vermiculite moistened with 750 ml water/2 l along an oxygen gradient (2–21% O 2). Growth was much slower and vascular cavities never formed at 10°C. At 25°C in vermiculite the rate of cavity formation (22–100%) was positively correlated with water-availability, so water-availability was a significant factor. In the hydroponic system, aeration had little effect on growth at 10°C but increased growth at 25°C. All primary roots grown hydroponically at 25°C contained cavities. As ambient oxygen level was increased so did growth rate, but the reverse was true for cavity formation. Growth rate, therefore, was not a factor in cavity formation but oxygen availability was. Primary roots that did not develop cavities at 25°C had intercellular spaces in parenchymatous tissues of their vascular cylinders, whereas those with cavities did not. These results support the hypothesis that at high temperature elevated respiratory demand exceeds the rate of oxygen diffusion to the center of the mature portions of pea roots that form cavities, and that this situation is aggravated by wet conditions. Therefore, vascular cavities may be functioning as a type of aerenchyma.