[SS01] Biological transport network: form and function

[SS01-01] Hydraulic architecture and water transport in plants

*Haruhiko Taneda¹ (1. The University of Tokyo (Japan))

Keywords:

water transport,branching system,plant,optimal theory

Plants transpire large amounts of water from their leaves via opening small pores called stomata when they absorb CO₂ for photosynthesis. To maintain leaf water balance, plants absorb water from soil, pull it by strong negative pressure and move through the plant body to transpiring leaves. Water in the plant body passes through less permeable cell membranes in the parenchyma tissues and narrow conduits with the order of 0.01 - 0.1 mm in diameter called “vessel or tracheid” in the vascular tissues, resulting in great hydraulic resistance as water passes through the plant body. Further, large body size of woody plants potentially causes greater hydraulic resistance due to long distances of water transport pathways. Intricately branched root and shoot systems also make it more difficult for water to reach leaves on the distal nodes of the branches. For these reasons, plants must have a hydraulic architecture that can efficiently and evenly deliver water from roots to all transpiring leaves. In my talk, I will introduce our researches focusing on the relationship between hydraulic architecture and photosynthesis mediated by water transport in plants. One of the topics is about the hydraulic architecture to equally deliver water to leaves. We consider a plant with a single main path with several lateral paths bearing at the same interval distance. Our theoretical analysis predicted that the difference in water supply between the distal and basal nodes decreases as the hydraulic resistance of the lateral paths is higher than that of the main path. We tested this prediction using kudzu vine (Pueraria lobata) that has a shoot with ~20 m long stem with ~40 leaves. As a result, leaf lamina was >1000 times higher hydraulic resistance than the stem internode due to much less permeable parenchyma tissue made in leaf lamina, resulting in even rate of water supply and photosynthesis in leaves within the kudzu vine shoot. The other topic is about the optimal hydraulic architecture maximizing photosynthesis. In many plants, root hydraulic resistance accounts for half of the whole-plant hydraulic resistance, so that the roots largely limit plant water use. Our measurements revealed that the large fraction of root resistance was due to small root biomass, and theoretical analysis showed that the large fraction of root resistance with small biomass partition to roots can maximize shoot photosynthetic rate which is a product of photosynthetic rate on leaf area basis and total leaf area in plants. This is because the hydraulic architecture can supply water to leaves enough to open stomata as well as larger total leaf area.