Lighting the way for increased vitamin C in tomatoes

L-ascorbate is a phytochemical essential for human health. Light regulates L-ascorbate (vitamin C; ASC) levels in plants with the vast majority of the evidence referring to leaves. In the present study the focus was on the effects of light on ASC levels in tomato fruits. The aim of this work as explained in Chapter 1, was to investigate which physiological processes and how do they mediate the effects of light on ASC levels in tomato fruits. Furthermore, the effects of light over the broader metabolome of the tomato fruit are investigated.

In Chapter 2, a literature review is presented with the aim to highlight the physiological and biochemical network mediating the regulation of ASC by light. Respiration, photosynthesis and soluble carbohydrates are proposed to control ASC levels in plants. Possible interactions between these physiological components and their importance in light regulation of ASC in both leaves and fruits were discussed. Furthermore, a broad biochemical map for ASC biosynthesis, recycling and turnover was presented and the contribution of specific pathways to the ASC pool is discussed. It is concluded that the main biosynthetic pathway (D-man/L-gal) for ASC accounts for the vast majority of the ASC pool in leaves as well as in fruits.

In Chapter 3, the effects of light intensity and spectrum on ASC levels in detached tomato fruits were investigated. ASC levels of fruits increased under light compared to darkness, as long as the fruits were green. Red fruits did not respond to the light treatments. Accumulation of ACS under light was not much affected by the light spectrum. The effect of light on ASC is proposed to be universal for tomato as several tested genotypes were found to be similarly responsive to the same light treatments. As only mature green fruits respond to the light treatments and the rate of photosynthesis correlated with ASC levels, it is put forward that the regulatory effect of light on ASC in tomato fruits is mediated through photosynthesis. No correlation between ASC levels and respiration rates was found.

The precursor for ASC biosynthesis via the D-man/L-gal biosynthetic pathway is glucose. The hypothesis that the availability of soluble carbohydrates regulates ASC levels in tomato fruits was tested in a series of experiments presented in Chapter 4. The correlation between ASC and carbohydrates was tested in different settings: fruits with different ASC levels due to light intensity treatments, fruits with different carbohydrate levels due to fruit pruning treatments and fruits with different carbohydrate levels due to artificial feeding with carbohydrates. In all of the cases no correlation between the ASC levels and soluble carbohydrates was found. This suggests that soluble carbohydrates are not a limitation for ASC accumulation. Galacturonate, the precursor for the secondary/salvage biosynthetic pathway did not increase when ASC was upregulated by light. However, myo-inositol –a precursor of a pathway that was previously thought not to contribute in the ASC pool in non-genetically modified plants- increased when ASC was upregulated by light treatments.

Apart from ASC, a variety of other fruit metabolites respond to the light environment. In two experiments presented in Chapter 5, light versus darkness and spectrum effects on the broad metabolic profile of tomato fruits were tested. Exposure of mature green fruits to light lead to acceleration of ripening as reflected in a more pronounced pigment and flavonoid accumulation and alkaloids reduction. Furthermore, presence of light positively influenced the levels of carotenoids, tocopherols and phenolic compounds. As these metabolites have been proposed to be beneficial for human health, the light treatment compared to darkness produces potentially fruits with improved nutritional value. The light treatment also affected flavour-related compounds such as malic acid, glutamate and GABA. Hence, light may improve the taste of fruits. These results should further be confirmed with sensory studies. Above mentioned results indicate that light treatments can be used to improve and/or fine tune certain quality traits of the tomato fruits.

Chapter 6 summarizes and discusses the findings of Chapters 2 to 5 with the aim to present an overview of the physiological mechanism that mediates the regulation of ASC by light in tomato fruits. Practical concerns for the application of light aiming to improve tomato fruit quality are discussed. Furthermore, suggestions on approaches plant breeding may implement in improving ASC in plants are presented.