Non-stomatal Limitation of Photosynthesis and Decelerated Vascular Transport in Betula pendula with Carbon Sink Impairment

Xin Zhuang, Teemu Paljakka, Teemu Hölttä, Yann Salmon
INAR, Faculty of Science, P.O. Box 64 (Gustaf Hällströmin katu 2) FI-00014 University of Helsinki, Finland

"Plant photoassimilates are produced by the leaf mesophyll and translocated through the phloem via an osmotically generated pressure difference between carbon sources and sinks. Impaired sink strength can limit canopy photosynthesis and phloem transport. This study aims to investigate how sink limitation affects the phloem loading-transport-unloading process and whether it leads to non-stomatal limitations of photosynthesis.
In a greenhouse experiment, we subjected the roots of 2.5 m tall silver birch (Betula pendula) saplings to a 4°C cooling treatment to reduce their sink activity, using untreated saplings as controls. We continuously monitored plant hydraulic status and canopy gas exchange. When the treatment caused the leaf A/Ci ratio to drop by half, we applied a 30-minute 13CO2 pulse labeling to the canopy and tracked photoassimilate translocation through phloem exudate and stem respiration.
Soil-to-leaf hydraulic conductance indicated that leaf gas exchange was influenced by both carbon sink limitation and decreased root water uptake capacity due to reduced temperature. The treatment impaired carbon sink strength, resulting in significantly decreased quantum yield, photosynthetic capacity, and carbon transport velocity. Additionally, we observed a considerable delay in the appearance of the isotopic signal in phloem sap and stem-respired CO2, suggesting that stem metabolic respiration time should be considered when estimating phloem transport velocity."