Branch Morphology of a Mangrove (Avicennia marina (Forsk.) Vierh) Growing in a Per-Arid Area on the Egyptian Red Sea Coast Regulates Water Use of Its Leaves

Abstract

The grey mangrove (Avicennia marina (Forsk.) Vierh.) in per-arid areas on the Red Sea coast is characterized by its tree morphology, as branches tend to extend laterally rather than upward and often carry adventitious roots on their main axis. To test the hypothesis that such branch morphology is linked with the water-use characteristics in leaves, the relationships between morphological parameters of branch and carbon isotope ratio of leaf organic matter at the branch top (δ¹³Com) were examined in 12 trees growing in a 5 × 10 m plot on the shoreline fringe of a stand at Hamata, Egypt. The δ¹³Com was significantly correlated with the sloped branch length between the base and the tip (L) and the adventitious roots and the tip (Lr) when L or Lr < 700 cm, but not with the vertical height of the branch (H). Furthermore, the δ¹³C_om was more strongly correlated with Lr than with L. These results suggest that the intrinsic water-use efficiency of leaves can be linked to the actual path length of water transport from water absorptive roots to leaves and that the leaves of these A. marina trees can enhance intrinsic water-use efficiency in response to decreasing hydraulic conductivity of xylem by elongating branches. Moreover, lateral branches with adventitious roots may also act to improve the hydraulic conductivity of xylem. These two factors account for the survival of A. marina trees growing in per-arid areas along the Red Sea coast.