THROUGH-FLOW OF WATER IN LEAVES OF A SUBMERGED PLANT IS INFLUENCED BYTHE APICAL OPENING

Submerged aquatic higher plants maintain acropetal water transport to the young leaves in active growth to satisfy their demand for nutrient s and hormones derived from the roots. We here present the first measu rements of hydraulic properties for a submerged plant, the monocotyled on Sparganium emersum Rehman. The hydraulic conductance per unit lengt h, K-h, was measured in leaf segments without the leaf tip and shown t o 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 wi th intact leaf tips, however, K-h was significantly greater in the you ngest leaves, which suggests that the leaf tip with the hydathode infl uences resistance and thus how. Microscopy confirmed that the hydathod al 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 sp ecific 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 fe rns, vines and trees. This finding does not support the traditional co ncept of functionally reduced vascular transport in aquatic plants com pared with their terrestrial counterparts. Our results demonstrate tha t some aquatic plants possess an efficient transport system for acrope tal translocation of inorganic macronutrients and hormones and that th is system is influenced by the developmental stage of the leaf hydatho de.