Soluble carbohydrates are not a limitation in L-Ascorbate upregulation by LEDs in tomato fruit.

MSc-thesis abstract (submitted 13 June 2016): L-Ascorbic acid (AsA; vitamin C) is a water soluble organic compound well-known for its antioxidant properties (free radical scavenger), as an electron donor. AsA levels in higher plants are influenced from multiple factors which include light and carbohydrate availability. A correlation between AsA and sugar content has been reported in various studies. Sugars could act as a substrate for AsA in the main biosynthetic pathway (Smirnoff Wheeler pathway) and as regulator of gene expression related to this pathway. It has been found also that genes related to recycling of AsA were also upregulated due to soluble carbohydrate feeding.

The aim of this study is to investigate the relationship between soluble carbohydrates, photosynthesis and respiration on AsA content.
The objective was to clarify if AsA content is regulated by the soluble carbohydrate content which are primarily the precursor for AsA. Green immature tomato trusses (Solanum Lycopersicum. cv vimoso) were fed with sucrose (20%, 10%, 5%, 1%, 0% w/v) under light (280 μmol m-2s-1) and darkness (0 μmol m-2s-1) for 10 days.
Sucrose feeding increased total soluble carbohydrate content equally in both the light and the dark treatments. Dark respiration was up regulated in both light conditions in the 20% sucrose treatment.
Quantum yield of photosystem system II under high light intensity in the 20 % sucrose treatment was upregulated also. With photosynthesis and respiration upregulated, no effect on AsA concentration was found.
The presence of light stimulated ascorbate accumulation in comparison to the dark treatment where AsA remained constant over time. Therefore, it’s suggested that soluble carbohydrates are not the limiting factor in AsA up regulation by LED light.

Our results propose a signaling function of light in AsA up regulation and that physiological process of photosynthesis and respiration are not affecting directly AsA concentration in tomato fruit.

Keywords: L-Ascorbate, AsA, Photosynthesis, Respiration, Light intensity, D-Man/L Gal, Smirnoff Wheeler pathway, Galacturonate pathway, Salvage pathway