Root-stem junctions of a desert monocotyledon and a dicotyledon: hydraulic consequences under wet conditions and during drought.

Junctions between shoot-borne main roots and stems, which are a crucial part of the water-flow pathway in plants, were investigated for the leaf succulent Agave deserti Engelm. and the sympatric stem succulent Ferocactus acanthodes (Lem.) Britton & Rose under wet conditions and during 21 d of drying in soil. During soil drying, the hydraulic conductance per unit pressure gradient (K ) declined dramatically in the junctions and to a lesser extent in the roots, but not in stems. The decline in junction K was particularly important for A. deserti, which lacks vessels in its stem, because even under wet conditions its K was lower in stems and junctions than in roots. For both species, the decline in K was due to embolism in the connective tracheary elements at the junction. Such connective elements may be particularly vulnerable to embolism due to their large areas of unlignified primary cell wall. Because the embolism is reversible, the junctions act as rectifiers. Thus, high K under wet conditions allows for rapid water uptake following rainfall, and low K during drought helps limit water loss from the succulent shoots to a dry soil.