Chromoplasts are plastids containing carotenoids.
They lack chlorophyll but synthesize various other coloured pigments.
Carotenoid pigments are responsible for different colours like yellow, orange and red colour imparted to fruits, flowers, old leaves, roots, etc.
Chromoplasts may develop from green chloroplasts. Chlorophyll and thylakoid membranes disappear and carotenoids are accumulated, e.g. during ripening of fruits.
The transition of chloroplasts to chromoplasts is much evident in fruit ripening. There is an extensive synthesis of carotenoids. Thus, chlorophyll and thylakoids are degraded.
The DNA of plastids remains unchanged during transformation but ribosomes and rRNA disappear.
Some chromoplasts are capable of differentiating back to chloroplasts, e.g. chromoplasts of carrot root and citrus fruit, pumpkin, cucumber, etc. They lose carotene pigment and develop photosynthetic apparatus consisting of chlorophyll and thylakoid system.
Redifferentiation to chloroplasts is promoted by gibberellin and nitrates.
Chromoplast development is irreversible in plants like pepper and tomato.
In some plants like papaya, carrot, mango, watermelon chromoplasts are derived from the proplastids or leucoplasts.
Light, temperature and nutrients are important factors in the formation of chromoplasts.
They have plastoglobules and carotenoid-lipoprotein substructures, which store specific carotenoid pigments and specific lipoprotein fibrils.
Carotenoids are divided into two classes: carotene and xanthophylls. Carotene is orange in colour and xanthophyll is yellow.
In xanthophylls, oxygen is present, e.g. fucoxanthin, lutein, etc.
Carotenes only contain carbon and hydrogen, e.g. ꞵ-carotene, lycopene, etc.