Color Change in Citrus

Fruits on pot-grown oranges (Citrus sinensis)

Tropical Zone.  Citrus fruits like oranges and lemons start out green and remain in that color till maturity.  I remember, having grown up in the tropics, that all the citrus fruits I've seen were green.  When they are mature and harvested their color are still green different only from their immature counterparts by the shine and gloss on the rind.  By the time they turn orange or yellow, they are already over-ripe and cottony and tasteless.  Commercially grown citrus can be treated with ethylene after harvest to induce uniform and timely yellowing for a more appealing look. 

Temperate Zone.  Citrus plants grown in temperate places with mediterrenian-like climate, low temperatures induce a change in color to deep orange or yellow even before the fruits mature.   The color can be deceptive.  It is important to harvest the fruits when they are fully mature, it is when the sugar and the acidity balance is at its best. 

Lemon (Citrus limonia)

Hidden from our eyes are processes that are constantly happening in the plants.  Shifts in these processes are caused by a number of environmental factors.  The green color in unripe citrus plants is a result of the dominant presence of chlorophyll on the rinds.  The yellow or orange color in ripe citrus fruits is a manifestation of the high concentration of carotenoids.  When temperatures go down to a certain critical level, such as in the winter, the rate of chlorophyll production in the plant slows down or completely stops.     Furthermore, the existing chlorophyll undergoes a process of degradation while the biosynthesis of carotenoids increases, particularly in the fruits.

Stroll in your garden... its good for you and your plants.