COLOUR DEVELOPMEMT OF APPLES
When one goes to market to buy fresh apples, he/she will buy that apple which catches their eye best. A key quality parameter of red delicious apple is its skin colour. Colour is one of the important features of apple fruit for consumers in deciding to buy them. Every apple fruit grower is aware of the fact and knows that best colouring apples will get better prices.
Role of Anthocyanins in Red Colour Development
Anthocyanins represent a group of natural flavonoid compounds in plants and are responsible for red colouration. Anthocyanin concentration is the most important factor in determining the colour of apple fruit. Generally the colour development in apples is due to formation of anthocyanin pigments in the apple skin.
FACTORS AFFECTING COLOUR IN APPLES
Exposure of light is required to improve coloration in apples. Sufficient light stimulates anthocyanins accumulation resulting in better colour development. Fruits inside the tree canopy receiving less sunlight reduce anthocyanins formation and fruit colour.
It is inversely proportional to the fruit colour development and anthocyanin formation. Colouring of the fruit is increased by low temperature especially during night. Researchers have found better colouration in Red Delicious when night temperatures were below 15 degree Celsius.
Cooling of apple orchards by overhead sprinklers can be done to enhance anthocyanins formation and reduce the negative effects of hot day temperatures. However, it requires large amounts of water and can promote fungal infection.
Excess Nitrogen in tree results in reduced anthocyanins formation and red colour formation. So foliar as well as soil applications of nitrogen are not recommended close to the harvest. Nitrogen can be applied late in the growing season to some apple cultivars like Granny Smith, where redness is not desired.
Potassium is directly associated with the red colouration. Potassium fertilization enhances anthocyanins formation and hence better red colour. Potassium also compensates for higher nitrogen levels which inhibits anthocyanin formation. So it’s good to fertilize the trees with potassium rich fertilizers in late growing season.
There should be sufficient number of leaves per fruit, more leaves per fruit enhances anthocyanins formation. More leaves results in formation of more sugars by photosynthesis process, which is essential for the formation of anthocyanins.
Spraying ethephon accelerates the ripening process. It slowly releases ethylene which stimulates anthocyanins formation in apples, hence better red colour at harvest. It have negative effects as it reduces the fruit storability and sometimes early fruit drop.