Evans, L. S. (Laboratory of Plant Morphogenesis, Biological Sciences Research Laboratories, Manhattan College, Bronx, NY 10471), Y. Okawa (Laboratory of Plant Morphogenesis, Biological Sciences Research Laboratories, Manhattan College, Bronx, NY 10471) and D. G. Searcy (Department of Biology, University of Massachusetts, Amherst, MA 01003). Anatomy and morphology of red mangrove (Rhizophora mangle) plants in relation to internal airflow. J. Torrey Bot. Soc. 132: 537–550. 2005.—Mangroves cover large areas of shoreline in the tropics and subtropics where they are important components in the productivity and integrity of their ecosystems. Many mangrove plants are rooted (most are stilt roots-adventitious roots derived from stems) directly through standing sea water into anoxic substrate. Under these conditions, shoots must provide O2 to roots. The purpose of this research was (1) to understand the tissues involved with the predominant airflow pattern in red mangroves (Rhizophora mangle) from leaves to roots (2) to understand the anatomical characteristics of air conducting tissues, and (3) to quantify these structures and tissues. Cork warts (density = 0.17 mm−2) on abaxial leaf surfaces are responsible for air uptake. Cork warts deliver air to an aerenchyma tissue near adaxial leaf surfaces. This aerenchyma layer is about 60 μm thick over the entire leaf so that the volume of aerenchyma in a typical leaf is about 0.23 cm3. The aerenchyma in each leaf is linked to the aerenchyma in each petiole (mean cross-sectional area, 0.0024 cm2). Petiole aerenchyma then joins the aerenchyma in the innermost portion of stems (aerenchyma constitutes about 2 to 4% of total stem area). Air descends the stem aerenchyma and then passes to the inner aerenchyma of stilt roots (inner aerenchyma constitutes about 18% of the cross-sectional area of roots). Our results indicate that air descends into the smallest roots growing in anoxic substrates where it may provide O2 to the most rapidly growing portions of the roots. From these growing areas, air ascends the outer aerenchyma of roots (outer aerenchyma constitutes over 70% of such roots) where it may be released through lenticels. Surface densities of lenticels ranged from 0.68 mm−2 in air roots (adventitious roots that have not reached substrate) to 1.07 mm−2 in mud roots (the lower portions of stilt roots in anoxic substrate). Densities of lenticels on mud roots were greater than densities on air roots (P = 0.0001). We believe this report to be the first documentation of the path of airflow within tissues of red mangrove plants including the quantitative anatomical characteristics of these tissues. Taken together these results provide a quantitative assessment of the tissues involved in airflow in R. mangle plants.
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