Developmental and Functional Morphology of Crop Root System in Relation to Hydraulic Characteristic

本研究旨在探讨与高粱种子根系吸水导性密切相关的几种根系发育和组织学性状。此外，用硫代胺G（SR）作为外质体示踪剂，测定了水进入部位和吸水速率。然后试图根据这些信息建立根系的水力结构。结果表明，侧根的分枝、伸长和扩张能力、卡斯帕里斯带、外胚层和内胚层发育、木质部尺寸以及与精根轴的维管连接等性状均存在异质性。侧根约占根表面积和根系累积SR总量的三分之二。不同根型和不同部位的种子根轴上单位根面SR积累量不同。SR积累最多的0-15厘米部分从尖端。在侧根中，离种子根轴顶端15cm的侧根的SR积累最为密集。这些结果表明，这部分在根系吸收水分方面最为活跃。但在这一部位，胚根轴上的后叶质部导管（late metaxymlem vessel， LMX）尚未打开，表明轴向水力阻力较大。此外，从这部分开始，积累量急剧下降，而在距离尖端15 cm和20 cm的部分，Casparian条发育和外/内胚层成熟度没有明显差异。这些事实表明，本研究中确定的组织学特征可能不是决定水导率的唯一主要因素，但与年龄密切相关的细胞膜通透性等其他一些特征也可能起重要作用，因此在根系中所占的比例也很重要。

This study aimed to examine several developmental and histological root traits that are closely related with water uptake and conductance of a seminal root system of sorghum seedling. In addition, the site of water entry and water uptake rate were determined by using sulphorhodamine G(SR) as an apoplastic tracer. It was then attempted to establish a hydraulic architecture of the root system based on these information.Results showed that heterorhizy exists in lateral roots traits examined such as branching, elongation and expansion ability, Casparian band, exodermis and endodermis development, xylem dimension and vascular connection to seminal root axis. The lateral roots accounted for approximately two third of both root surface area and total amount of SR accumulated in the root system. The accumulation of SR per unit root surface was different between root types and different portions along the seminal root axis. SR accumulated most on 0-15 cm portion from the tip. Among lateral roots, those emerged 15 cm away from the tip of seminal root axes accumulated SR most densely. These results indicate that this part is most active in water uptake in the root system. However, late metaxylem vessel (LMX) in the seminal root axis was not yet opened at that portion, indicating high axial hydraulic resistance. Furthermore, the accumulation drastically dropped basipetally from that portion on, whereas no clear difference in Casparian strip development and exo/endodermis maturity was detected between 15 cm and 20 cm portion from the tip.These facts suggest that the histological features that were determined in this study may not be the sole major factor to determine the hydraulic conductivity but some other traits like the permeability of cell membrane that is closely related with age and thus portion in a root system may also play important roles.