The hydraulic architecture of plants constrains water transport and carbon gain through stomatal limitation to CO₂ absorption. Leaf, stem, and root organs are composed of plant hydraulic architecture, of which the root is the main bottleneck of water transport for a wide range of plant species. The present study aimed to assess the ecophysiological mechanism and importance of the high fraction of root hydraulic resistance. Biomass partitioning and hydraulic conductance of leaves, stems and roots were measured using Japanese knotweed (Fallopia japonica, perennial herb), and Japanese zelkova (Zelkova serrata, deciduous tall tree). Additionally, theoretical analyses examined whether the measured hydraulic architecture and biomass partitioning maximized plant photosynthetic rate, which is the product of leaf area and photosynthetic rate per leaf area. Root hydraulic resistance accounted for 86% and 76% of the total plant resistance for Japanese knotweed and Japanese zelkova trees, respectively. According to comparisons of hydraulic and biomass partitionings, high root-resistance fractions were attributable to low biomass partitioning into root organs rather than high mass-specific root conductance. The measured partitioning of hydraulic resistance closely corresponded to the predicted optimal partitioning maximizing plant photosynthetic rate for the two species. The high fraction of root resistance was still predicted to be optimal with variations in air humidity and soil water potential. These results suggest that the hydraulic architecture of a plant growing in mesic and fertile habitats resulted in a high fraction of root resistance due to small biomass partition into root organ, but contributed to efficient carbon gain.

1. Meta-analyses of root hydraulic conductance measured with a high pressure flow meter method.

2. Hydraulic conductance of leaf, stem and root in Fallopia japonica and Zelkova serrata seedlings with a high pressure flow meter method.

3. Results of the mathmatical analyses using the mechanistic model to estimate the hydraulic architecture optimzing the plant productivity.