Through-flow of water in leaves of a submerged plant is influenced by the apical opening

Submerged aquatic higher plants maintain acropetal water transport to the young leaves in active growth to satisfy their demand for nutrients and hormones derived from the roots. We here present the first measurements of hydraulic properties for a submerged plant, the monocotyledon Sparganium emersum Rehman. The hydraulic conductance per unit length, K-h, was measured in leaf segments without the leaf tip and shown to be greater in old, fully developed leaves (1.5.10(-10).m(4).MPa-1.s(-1)) than in young leaves (1.0.10(-10).m(4).MPa-1.s(-1)). In leaves with intact leaf tips, however, K-h was significantly greater in the youngest leaves, which suggests that the leaf tip with the hydathode influences resistance and thus how. Microscopy confirmed that the hydathodal area, which is an apical opening, undergoes structural changes with leaf age; a matrix of microorganisms develops in the older leaves and probably restricts water flow by clogging the hydathodes. The leaf specific conductivity expressing transport capacity relative to the leaf area supplied, of S. emersum (0.1.10(-8) to 9.10(-8).m(2) MPA(-1).s(-1)) was within the same range as for various species of terrestrial ferns, vines and trees. This finding does not support the traditional concept of functionally reduced vascular transport in aquatic plants compared with their terrestrial counterparts. Our results demonstrate that some aquatic plants possess an efficient transport system for acropetal translocation of inorganic macronutrients and hormones and that this system is influenced by the developmental stage of the leaf hydathode.