Effects of Sink Limitation on Betula pendular Carbon Translocation

In plants, photoassimilates are primarily produced by leaf mesophyll, and translocated through phloem under an osmotically generated pressure difference between the sources and sinks. Impaired sink strength may limit canopy photosynthesis and phloem transport. We aim to study how sink limitation regulates phloem loading-transport-unloading process, and whether it causes non-stomatal limitation of photosynthesis. We hypothesize that sugar accumulation causing increasing mesophyll resistance is the bottleneck for photosynthesis and for phloem loading when plants have a restricted carbon sink. Under controlled greenhouse condition, we applied 4°C cooling treatment to 2.5m tall silver birch (Betula pendula) sapling soil and roots to reduce their sink activity, with untreated saplings as controls. We kept monitoring plant hydraulic status by measuring sap flow, soil water content and leaf water potential. When the treatment caused leaf assimilation rate to drop by half after 4-7 days, we performed pulse-chase labelling to the plants’ canopy with a 30-minute exposure to 13C-enriched CO2 and kept tracking the photoassimilate translocation through stem respiration and from phloem exudate. By calculating soil to leaf hydraulic conductance, we were able to separate the effect of sink limitation on leaf gas exchange from the effect of decreased water uptake capacity of the roots due to the reduced temperature. Our preliminary results show that the soil-cooling treatment causes an impaired carbon sink of saplings, leading to delayed phloem loading and decreased photoassimilate transport velocity. The treatment also led to canopy gas exchange decrease and light-saturated assimilation rate decrease. Additionally, we captured a considerable lag of isotopic signal appearing in phloem sap and in stem respired CO2, suggesting the time for stem metabolic respiration to be considered in calculating phloem transport velocity.