why does fruit need the sun to make it sweet

Fruit needs sun to ripen and when under ripe will taste bitter or sour

There's some chemical reaction occurring in the plant and fruit in sunlight which increases the sugars present in the fruit - I used to know exactly what was going on, but its a long time since I did my training and this isn't something you need to remember in detail. Well, that's my excuse anyway...

the sun's heat speeds up the chemical reactions that change the large sugar molecules into small sugar molecules. basically large sugars are non sweet and small sugar molecules like glucose and fructose [the commonest sugar in fruit] are sweet. thats why growing some plants in the greenhouse speeds up ripenig and maturity.

The bitter taste is another group of chemicals that deter birds and other animals [us] from picking the immature seed and eating them before they are ready.

hope that helps :o)

Welcome to GOY,
The fruit you are growing will not have reached it's mature stage. Many apples are picked before they are fully ripe and ripen in store. As Moongrower says you need sun to get the best results. Bamboo is like me, chemical reaction is very complex and will vary according to the fertilisers/manure used. In the growth of the plants or trees, get the balance wrong and your fruit will be tainted or will never ripen properly.