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I understand that sun light is needed for fruits to ripen. In the years with less sun light, the fruits are usually not so sweet. So the sun light is needed to make the fruits sweet.

Where and how does the sweetening process occur? Does the sugar in the fruits come from the photosynthesis occurring in the green parts of the plant (leaves) and is then conducted to the fruits, or is the sugar produced in the fruits themselves? Does it matter whether the sun shines directly to the fruits, or is it enough if just the leaves have enough sun light and the fruits become equally sweet even if they are hidden under the leaves?

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It would depend if the fruit has the ability to sense or capture light. This depends on chlorophyll mostly, found in cellular organelles called chloroplasts. If the fruit is incapable of capturing light, the light can still have an indirect impact by being captures on leaves, since plants circulate nutrients - this is why roots can grow without having access to sunlight.

Here's a few excerpts from the following paper (https://www.ncbi.nlm.nih.gov/pubmed/11337414):

"To date, molecular regulation of the role of light in fruit ripening has been studied most thoroughly in tomato, and available evidence suggests that light has its greatest impact on pigmentation, with apparently little effect on additional ripening phenomena."

"The green to red color transition typical of ripening tomato fruit is largely due to the developmental transition of chloroplasts to chromoplasts; as photosynthetic membranes are degraded, chlorophyll is metabolized, and carotenoids, including β-carotene and lycopene, accumulate."

In plain English, the pigment that captures and absorbs sunlight for metabolic reasons (photosynthesis) gets transformed into a pigment which absorbs sunlight for other reasons, such as 'looking tasty' or protection from ionizing radiation. Essentially the same thing happens to leaves in autumn - the green chlorophyll pigment, which absorbs the energy of the sun for the plant to grow, becomes the orange carotene, and the leaf slowly dies off.

Interesting sidenote: biological pigments are very important molecules in ecology and in cell biology. Take a closer look: https://en.wikipedia.org/wiki/Biological_pigment

As for your question about how ripening occurs... Google would do a better job than most.

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  • $\begingroup$ "tomato... light has its greatest impact on pigmentation, with apparently little effect on additional ripening phenomena." Does this mean that if you shaded the tomato fruits agains the sunlight, but left the leaves uncovered, the fruits would become sweet, but remain green? $\endgroup$ – Honza Zidek Jul 12 '18 at 11:17
  • $\begingroup$ Yes. That would be the experiment! $\endgroup$ – S Pr Jul 12 '18 at 11:55

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