Department of Earth and Environment, Boston University, 675 Commonwealth Ave., Boston, MA 02215, USA.
Tree Physiol. 2013 Feb;33(2):152-60. doi: 10.1093/treephys/tps123. Epub 2013 Jan 7.
Palms are an important component of tropical ecosystems, living alongside dicotyledonous trees, even though they have a very different growth pattern and vascular system. As monocots, vessels in palms are located within vascular bundles and, without a vascular cambium that many dicotyledonous trees possess, palms cannot add additional vessels to their vascular system as they get older and taller. This means that hydraulic architecture in palms is more predetermined, which may require a highly efficient hydraulic system. This preset nature, along with the decoupling of hydraulic and mechanical functioning to different cell types, may allow palms to have a more efficient hydraulic system than dicotyledonous trees. Therefore, this study seeks to determine the efficiency of the hydraulic system in the palm Iriartea deltoidea (Ruiz & Pav.) and compare this efficiency with other tree forms. We measured cross-sectional areas of roots, stems and fronds as well as leaf areas of I. deltoidea saplings. Likewise, cross-sections were made and vessel diameters and frequencies measured. This allowed for the calculation of theoretical specific conductivity (K(S,calc)), theoretical leaf-specific conductivity (K(L,calc)), and vessel diameter and vessel number ratios between distal and proximal locations in the palms. Iriartea deltoidea palms were found to have the largest, least frequent vessels that diverged most from the square packing limit (maximum number of vessels that fit into a given area) compared with other major tree forms, and they therefore invested the least space and carbon into water transport structures. Likewise, conduits tapered by ∼1/3 between ranks (root, bole and petiole), which represents an efficient ratio with regard to the trade-offs between safety and efficiency of the conducting system. Conduits also exhibited a high conservation of the sum of the conduit radii cubed (Σr(3)) across ranks, thereby approximating Murray's law patterning. Therefore, our results indicate that the palm I. deltoidea has a very efficient hydraulic system in terms of maintaining a large conducting capacity with a minimal vascular investment. This efficiency may allow palms to compete well with dicotyledonous trees in tropical and subtropical climates but other developmental factors largely restrict palms from regions that experience prolonged freezing temperatures.
手掌是热带生态系统的重要组成部分，与双子叶树木共存，尽管它们的生长模式和脉管系统非常不同。作为单子叶植物，棕榈树的导管位于维管束内，而且不像许多双子叶树木那样拥有维管形成层，因此随着年龄和高度的增长，它们无法向其脉管系统添加额外的导管。这意味着棕榈树的水力结构更加预先确定，这可能需要一个高效的水力系统。这种预设性质，以及水力和机械功能分离到不同的细胞类型，可能使棕榈树具有比双子叶树木更高效的水力系统。因此，本研究旨在确定手掌科 Iriartea deltoidea（Ruiz & Pav.）水力系统的效率，并将这种效率与其他树木形式进行比较。我们测量了 I. deltoidea 幼苗的根、茎和叶片的横截面积以及叶片面积。同样，制作了横截面并测量了导管直径和频率。这使得可以计算理论比导率（K(S,calc)）、理论叶比导率（K(L,calc)）以及手掌中远、近端导管直径和导管数量的比值。与其他主要树木形式相比，手掌科 I. deltoidea 植物的导管最大、最不频繁，与方形包装限制（适合给定面积的最大导管数量）的偏离最大，因此它们在水运输结构上的投资最小。同样，导管在等级（根、干和叶柄）之间缩小了约 1/3，这代表了在导水系统的安全性和效率之间的权衡方面的有效比例。导管还表现出导管半径立方和（Σr(3))在等级之间的高度保守性，从而近似于 Murray 定律模式。因此，我们的结果表明，手掌科 I. deltoidea 在保持大导水能力的同时，具有最小的脉管投资，从而具有非常高效的水力系统。这种效率可能使棕榈树在热带和亚热带气候下与双子叶树木竞争良好，但其他发育因素在很大程度上限制了棕榈树在经历长时间冻结温度的地区的分布。
