Question

apple   Dear Jennifer
my name is Andrea and I work in fruit and veg distribution as QA supervisor. Also I study first year organic horticulture and I am getting ready for presentation about pests and diseases, specially on apples, as nutritional deficiency and so on.
I have samples of apple gala and can't find our what could caused bi colouration on this sample. Apple look a like cut in half, one half is usual gala colour and the second half is in green colour.
Could you please let me know what can be the cause of this?
Thank you in advance
Kind Regards
Andrea

Answer
Hello Andrea,

I will do my best to answer your question, but I'm not sure that there is a perfect answer. Scientists are still studying this sort of thing, but this is what I suspect has happened.

The genes for coloration in apple were recently discovered in 2007.  The genes code for pigments called anthocyanins and when these are present, they give apples a red color.  Scientists have determined that some of these genes are regulated by light.  Apples that are red in full sun may remain green if they are in the shade.  This is why it is common that some apples have spots that remain greenish, even when the rest of the apple is red.  Apple breeders have been selecting for apples that remain fully green or turn completely red even in low light.  They also use the traditional apples that have just a little green.

So why is the apple in your photograph completely half and half?  My guess is that this is related to a genetic mutation that happened early in the fruit production.  Apples start of as one cell with half the genes from the mother plant and half from the pollinator (Daddy plant).  As they fruit grows, the cells divide, first 1, then 2, then 4, then 8, then 16...until there are millions of cells in one apple.  Somewhere early on, it is likely that there was a mutation in the gene that codes for the MYB transcription factor. This is the gene that was discovered in apple and makes the anthocyanin pigment.  Sometimes, when genes have a mutation they "turn off" which means that the proteins are no longer made.  Therefore, that part of the apple was no longer making the pink-purple pigment that gives the apple the red color.  Since other pigments are present in the apple, it does not turn white.  There is chlorophyll in the skin of the fruit which has a green or yellow-green color.  This would explain why 1/2 is green and the other is red.  This would be considered a physiological disorder.

Thanks for asking!