Sink stimulation of leaf photosynthesis by the carbon costs of rhizobial and arbuscular mycorrhizal fungal symbioses

Key words: biochemical model of leaf photosynthesis; carbon sink strength; chlorophyll fluorescence; harvest index; leaf protein; leaf senescence; legumes; photosynthetic nutrient use efficiency; Pi recycling; source-sink regulation; ureides One of the most fascinating processes in plant physiology and agronomy is the capability of legumes to associate symbiotically with rhizobial bacteria and arbuscular mycorrhizal (AM) fungi. The legumes supply photosynthates in exchange for nitrogen, derived from biological N2 fixation, and soil nutrients mainly phosphate, obtained from foraging of AM fungi from the soil. The rhizobial and arbuscular mycorrhizal symbioses each may use 4-16% of recently fixed photosynthates to maintain their activity, growth and reserves, but in turn, may supply 100% of the plant nutrient requirements. The C costs of the symbioses are often assumed to limit plant productivity due to photosynthate competition between the microsymbiont and the host. In addition, the C costs are often used as an entry point to understand the evolution of the symbioses. It is intriguing that despite of the symbiotic C costs, plants associated with rhizobia and/or AM fungi often produce more biomass and grains than fertilized plants. Increases in plant growth are traditionally attributed to improved plant nutrition and enhanced photosynthesis. This thesis gives evidence that plants – and particularly legumes – are able to overcome any putative C limitation associated with rhizobial and AM fungal symbioses by increasing the rates of photosynthesis due to sink stimulation, over and above the expected nutritional benefits from the symbioses. Sink stimulation of photosynthesis is a consequence of increased C demand from photosynthesis, which increases the export of triose-P from chloroplasts, recycling more inorganic phosphates and activating more photosynthetic enzymes. In the thesis, I report a literature study, which provides a framework for the quantification of sink s