The Effect of Xylem on Sucrose Transport in the Phloem: Hydraulic Connectivity or Isolation?

Abstract:

Within plants, the delivery of water, nutrients, sugars and hormones is achieved using two interconnected hydraulic systems. These two systems, the xylem and the phloem, have different functions. When plants photosynthesize, they lose water via transpiration that is supplied under tension via the xylem connecting the soil reservoir to the leaf stomatal cavities. The phloem transports sucrose, a product of photosynthesis that ensures the survival of the cells, from the leaf to different parts of the plant under positive pressure. While extensive research on long-distance xylem water transport and its hydraulic failure has been performed for more than 100 years, sucrose transport inside the phloem remains under-studied. Undoubtedly, connections between xylem and phloem remain a formidable challenge to be confronted, yet necessary for determining risk and plant survival under extensive droughts. The work to be presented here will focus on exploring numerically the interconnection between these two hydraulic systems and investigating the different environmental conditions that might lead to phloem failure due to a low xylem water potential. To do so, the Munch pressure flow mechanism, which is the most accepted hypothesis for describing phloem transport, was used as the basis for this model. It was revisited by including effects of water exchange between xylem and phloem on phloem transport, the distribution of sucrose sinks along the phloem path, and variability of the fluid viscosity due to changes in the sucrose concentration. Two end-member cases are considered - (i) perfect hydraulic coupling between xylem water potential and distributed osmotic pressure, and (ii) perfect hydraulic isolation along the xylem-phloem path except for leaf water potential. The ecological consequences of drought and lowering xylem potential on the efficiency versus safety of the phloem transport are then discussed using bifurcation analysis and catastrophe theory.