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

本研究旨在探讨高粱种子根系的几个发育和组织学特征，这些特征与种子根系的吸水和导水密切相关。此外，水分进入的网站和吸水速率测定使用磺罗丹明G（SR）作为质外体示踪剂。结果表明，在侧根的分枝、伸长和扩展能力、凯氏带、外皮层和内皮层的发育、木质部的大小以及与种子根轴的维管连接等性状上均存在着异质性。侧根约占根系表面积和SR总量的2/3。不同根型和种子根轴沿着不同部位单位根表SR积累量不同。SR在距针尖0-15 cm处积累最多。侧根中，距种子根轴顶端15 cm处的侧根积累SR最多。这些结果表明，这部分是最活跃的根系吸水。然而，在种子根轴的后期后生木质部导管（LMX）尚未开放的部分，表明高轴向水力阻力。而且，从这一部分开始，积累量急剧下降，而凯氏带的发育和exo/结果表明，在离顶端15 ~ 20 cm处，内皮层的成熟度较高，这表明本研究所确定的组织学特征可能不是决定导水率的唯一主要因素，还包括细胞膜透性等其他特征这与年龄密切相关，因此根系中的部分也可能起重要作用。

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.