The weight ratio of Chls a and b to total
carotenoids (a+b)/(x+c) is an indicator of the
greenness of plants. The ratio (a+b)/(x+c) normally
lies between 4.2 and 5 in sun leaves and
sun-exposed plants, and between 5.5 and 7.0 in
shade leaves and shade-exposed plants. Lower
values for the ratio (a+b)/(x+c) are an indicator
of senescence, stress, and damage to the plant
and the photosynthetic apparatus, which is expressed
by a faster breakdown of Chls than
carotenoids. Leaves become more yellowishgreen
and exhibit values for (a+b)/(x+c) of 3.5,
or even as low as 2.5 to 3.0 as senescence
progresses. Also, during chromoplast development
in ripening fruits or fruit scales, which
turn from green to yellow or orange or red, the
ratio (a+b)/(x+c) decreases continuously and
reaches values below 1.0.