Ascorbic acid accumulation in Ribes fruit

Understanding the production of L-ascorbic acid (L-AsA), its cellular roles and its accumulation in fruit has advanced considerably over the last decade. The importance of irradiance on fruit relative to leaves, in tomato, suggests that there is little linkage between leaves and fruits in the supply of L-AsA. While manipulation of kiwi vine temperature supports fruit-based production, others have observed variability in long-distance phloem L-AsA transport from leaves to developing fruit. In apple, fruit L-AsA concentration is dependent on production which declines with maturation, despite L-AsA accumulating with increasing fruit weight. Tomatoes show pectin polymer degradation as a source of precursors for L-AsA synthesis and accumulation via L-galactonic acid. What is clear are species differences in the mechanism by which total fruit L-AsA production is modulated during development; in some fruits, e.g. strawberry, melon and tomato, it remains constant while in others, e.g. apple and orange, it declines). An explanation of how total fruit L-AsA is modulated during fruit development may differ with species. L-AsA is detected in leaf phloem, but what remains unclear is what contribution long-distance transport from potential sources, such as leaves, makes to the pattern and amount of AsA that accumulates in fruit tissues at maturity.

A new paper in Annals of Botany aims to determine the role of green leafy tissues in the development and growth of fruits and how these processes influence L-AsA production and accumulation in fruit. It uses black currants (Ribes nigrum) as a model plant because its fruit have high L-AsA concentrations and there is some knowledge of the pattern of biosynthesis and accumulation of L-AsA over time. What remains unclear is the location of fruit L-AsA synthesis and under what circumstances, if any, does fruit growth compete with L-AsA production.

Linking ascorbic acid production in Ribes nigrum with fruit development and changes in sources and sinks. (2013) Ann Bot 111 (4): 703-712. doi: 10.1093/aob/mct026
Understanding the synthesis of ascorbic acid (L-AsA) in green tissues in model species has advanced considerably; here we focus on its production and accumulation in fruit. In particular, our aim is to understand the links between organs which may be sources of L-AsA (leaves) and those which accumulate it (fruits). The work presented here tests the idea that changes in leaf and fruit number influence the accumulation of L-AsA. The aim was to understand the importance of leaf tissue in the production of L-AsA and to determine how this might provide routes for the manipulation of fruit tissue L-AsA.
The experiments used Ribes nigrum (blackcurrant), predominantly in field experiments, where the source–sink relationship was manipulated to alter potential leaf L-AsA production and fruit growth and accumulation of L-AsA. These manipulations included reductions in reproductive capacity, by raceme removal, and the availability of assimilates by leaf removal and branch phloem girdling. Natural variation in fruit growth and fruit abscission is also described as this influences subsequent experimental design and the interpretation of L-AsA data.
Results show that fruit L-AsA concentration is conserved but total yield of L-AsA per plant is dependent on a number of innate factors many of which relate to raceme attributes. Leaf removal and phloem girdling reduced fruit weight, and a combination of both reduced fruit yields further. It appears that around 50% of assimilates utilized for fruit growth came from apical leaves, while between 20 and 30% came from raceme leaves, with the remainder from ‘storage’.
Despite being able to manipulate leaf area and therefore assimilate availability and stored carbohydrates, along with fruit yields, rarely were effects on fruit L-AsA concentration seen, indicating fruit L-AsA production in Ribes was not directly coupled to assimilate supply. There was no supporting evidence that L-AsA production occurred predominantly in green leaf tissue followed by its transfer to developing fruits. It is concluded that L-AsA production occurs predominantly in the fruit of Ribes nigrum.